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Arsenic in Drinking Water (1999)

Chapter: 3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials

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Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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3
Chemistry and Analysis of Arsenic Species in Water, Food, Urine, Blood, Hair, and Nails

In this chapter, the subcommittee describes the chemistry of arsenic and its analysis in water and biological materials. The chapter is divided into four sections. The first section, Arsenic Compounds in Water and Food, provides the reader with general information on the various arsenic species that are now known to be present in food and water and that could be of concern in assessing normal human exposure (i.e., nonoccupational exposure) to arsenic. However, it should be emphasized that many unidentified arsenic species are probably present in the environment, including many in living organisms. It is not an easy task to detect and identify low concentrations of arsenicals, and methods to do so have been developed only in recent years.

The second section, Relevant Chemical Considerations, provides a brief account of arsenic's chemistry that is relevant to considerations of toxicity and carcinogenicity. In the third section, Analysis of Arsenic Compounds, general methods that have been used to analyze arsenic and its species are outlined. The results of applying these methods to the analysis of water and food, respectively, are presented in the sections Arsenic in Water and Arsenic in Food. A separate section discusses the application of these methods to the analysis of arsenic in urine, blood, hair, and nails.

Summary Of Arsenic Compounds In Water And Food

Table 3-1 lists the most important arsenic compounds and species known to be present in water and food consumed by humans. The identified compounds that are not listed are (1) the volatile arsines MexAsH3-x

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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TABLE 3-1 Some Arsenic Compounds and Species Known to be Present in Water and Food Consumed by Humans

Name

Abbreviation

Chemical Formula

Arsenous acid

As(III)

H3AsO3

Arsenic acid

As(V)

H3AsO4

Oxythioarsenic acid

 

H3AsO3S

Monomethylarsonic acid

MMA

CH3AsO(OH)2

Methylarsonous acid

MMA(III)

CH3As(OH)2[CH3AsO]n

Dimethylarsinic acid

DMA

(CH3)2AsO(OH)

Dimethylarsinous acid

DMA(III)

(CH3)2AsOH[((CH3)2As)2O]

Trimethylarsine

TMA

(CH3)3As

Trimethylarsine oxide

TMAO

(CH3)3AsO

Tetramethylarsonium ion

Me4As+

(CH3)4As+

Arsenocholine

AsC

(CH3)3As+CH2CH2OH

Arsenobetaine

AsB

(CH3)3As+CH2COO-

Arsenic-containing ribo-

Arsenosugar X-XVa

 

sides

Arsenolipidb

 

image

aArsenosugar

     
 

R

X

Y

X

(CH3)2As(O)-

-OH

-OH

XI

(CH3)As(O)-

-OH

-OPO3HCH2CH(OH)CH2OH

XII

(CH3)2As(O)-

-OH

-SO3H

XIII

(CH3)2As(O)-

-OH

-OSO3H

XIV

(CH3)2As(O)-

-NH2

-SO3H

XV

(CH3)3As+-

-OH

-OSO3H

bArsenolipid

     
 

R

X

Y

 

(CH3)2AsO-

-OH

-OPO3HCH2CH(OPalm)CH2OPalm

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 29

(Me =CH3, x = 0-3) produced naturally by the action of microorganisms on available arsenicals (Cullen and Reimer 1989); (2) the ethylmethylarsines EtxAsMe3-x (Et=C2H5, x = 1-3) found in natural gas (Irgolic et al. 1991); (3) phenylarsonic acid, C6H5AsO(OH)2, found in shale oil and retort water (Fish et al. 1982); and (4) the undoubtedly numerous arsenicals not yet discovered.

The structures of the arylarsenicals that are approved as animal-food additives are shown in Figure 3-1. 4-Hydroxy-3-nitrophenylarsonic acid (3-NHPAA) and p-arsanilic acid (p-ASA) are approved for poultry and swine. 4-Nitrophenylarsonic acid (4-NPAA) and p-ureidophenylarsonic acid (p-UPAA) are approved only for controlling blackhead disease in turkeys (Ledet and Buck 1978; Adams et al. 1994). Melarsoprol, a related arylarsenical, is still the drug of choice for treating secondary trypanosomiasis in humans in rural Africa (Berger and Fairlamb 1994).

image

FIGURE 3-1
Structures of arylarsenicals approved as animal-food additives. 3-NHPAA, roxarsone, 4-hydroxy-3-nitrophenylarsonic acid;
 p-ASA,  p-arsanilic acid; 4-NPAA, 4-nitrophenylarsonic acid;  p-UPAA, p-unridophenylarsonic acid

As(III) (pKa (negative logarithm of equilibrium constant for dissociation) = 9.23, 12.13, 13.4), As(V) (pKa = 2.22, 6.98, 11.53), MMA (pKa = 4.1, 8.7), DMA (pKa = 6.2), TMA, and TMAO are usually associated with the terrestrial environment, As(III) and As(V) being dominant in water. Unidentified species ("hidden species" not detected by using hydride generation; see section Hidden Arsenic Species), however, can reach up to 22% of total arsenic in river water (Sturgeon et al. 1989), and methylarsenicals can reach up to 59% of total arsenic in lake water (Anderson and Burland 1991). The recent discoveries of AsB, AsC, and Me4As+ in mushrooms (Byrne et al. 1995; Kuehnelt et al. 1997a,b); arsenosugar X in algae (Lai et al. 1997); and oxythioarsenic acid and methylarsenic(III) species MMA(III) and DMA(III) in water (Bright et al. 1994; Hasegawa 1996, 1997; Schwedt and Reickoff 1996a,b) extend the range of identified species.

In the marine environment, all the compounds shown in Table 3-1, except

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 30

the oxythioarsenate, have been identified. The most important arsenic compounds in seawater are the inorganic species As(III) and As(V). Those species are usually associated with much lower concentrations of DMA and MMA. In fish, the principal arsenic species is AsB, which is regarded as being ubiquitous. The telost fish Silver Drummer, however, does not contain any AsB, and its principal arsenic species is TMAO (Edmonds et al. 1997). Small amounts of AsC, DMA, MMA, As(V), TMAO, Me4As+, and arsenosugars also are found in marine animals (e.g., Larsen et al. 1993a), although the last two arsenic species can be important in some bivalves. High concentrations of unknown arsenicals have been identified in the abalone (Edmonds et al. 1997). Marine algae contain arsenosugars, principally X-XIII, and 15 arsenosugars have been identified to date. In addition, marine algae contain small amounts of inorganic arsenic and DMA (Francesconi and Edmonds 1997). High concentrations of inorganic arsenic (38-61% of total arsenic) are found in some marine algae, notably Sargassum muticum and Hizikia fusiforme (Morita and Shibata 1990; Francesconi and Edmonds 1997). The arsenolipid (Table 3-1) is a minor component of the brown alga Undaria pinnatifida, an edible seaweed known as Wakame (Shibata et al. 1992), but lipid-soluble arsenicals can reach high concentrations in some species.

Relevant Chemical Considerations

Is Arsenic Similar to Phosphorus?

Arsenic is situated in the Periodic Table in Group 15 (old Group V) below nitrogen and phosphorus. The oxidation state of arsenic in compounds found in the environment is either III or V (Table 3-1), and much of the chemistry of those compounds results from the easy conversion between those two states. The two-electron reduction of arsenate As(V) to arsenite As(III) is favored in acidic solution (E° (standard reduction potential) = 0.56 volts), whereas the reverse is true in basic solution (Eº = -0.67 volts) (Latimer and Hildebrand 1951). In contrast, phosphorus(V) compounds are difficult to reduce.

Another major difference between arsenic and phosphorus is the stability of the esters of phosphoric acid to hydrolysis, allowing the existence of, for example, DNA and adenosine 5'-triphosphate (ATP). Esters of As(V) acids are easily hydrolyzed; the half-life in neutral pH is about 30 min. If As(V)OR has a good leaving group such as -P(V) or C(O)R', the half-life falls to seconds. Enzymes can accept arsenate to incorporate into other compounds, such as ATP, but the analogues formed hydrolyze immediately. Thus, arsenate uncouples oxidative metabolism from ATP biosynthesis. That phenome-

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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non is believed to account for some of the toxicity of arsenate (Dixon 1997; see Chapter 7).

Many As(III) compounds are formulated as RAsX or (R2As)2X (X = 0, S). In the solid state, some of those compounds can be polymeric (e.g., (CH3AsO)3 and (CH3AsS)3), but CH3As(OH)2 and (CH3)2AsOH seem to exist in dilute aqueous solution (Hasegawa 1996, 1997).

Affinity Of Arsenic For Sulfur

The affinity of arsenic for sulfur is revealed in any list of natural arsenic-containing minerals. Many are sulfides and include As4S4 (realgar), As4S6 (orpiment), and FeAsS (arsenical pyrites, mispickel). That affinity also has been invoked to account for the toxicity of As(III) compounds through the interaction with protein thiols, as shown in Equation 3-1 in Figure 3-2 (see also Chapter 7). Such binding to proteins might inhibit the function of such enzymes as pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase (Knowles and Benson 1983a,b; Dixon 1997).

image

FIGURE 3-2
Affinity of arsenic for sulfur. BAL, British Anti-Lewisite (dimercaprol).

The action of dimercaprol, British Anti-Lewisite (BAL), in aiding the elimination of arsenic species from humans, is believed to result from the displacement of bound arsenic from a protein because of the formation of a more stable complex (Equation 3-2). However that action does not necessarily mean that the initial postulate of Equation 3-1 is correct. Binding of the arsenic to BAL could restore the function of an enzyme regardless of how the arsenic was originally bound. Li and Pickart (1995) pointed out that little evidence supports the proposal that the binding of As(III) compounds to

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 32

enzymes is solely or even predominantly that represented in Equation 3-1. Li and Pickart (1995) found that the binding of phenylarsenoxide, (C6H5AsO)n, (which is possibly C6H5As(OH)2 in dilute aqueous solution) to Arg [Arginine]-tRNA protein transferase does not involve vicinal thiols. However, Cys [Cysteine]-31 and Cys-184 seem to be implicated in lecithin-cholesterolacyl transferase (Jauhiainen et al. 1988).

The reduction of As(V) compounds by thiols has been well documented (Cullen et al. 1984), but sulfhydryl groups in enzymes do not always affect the reduction (Dixon 1997), presumably because this reductive interaction with sulfhydryl groups requires that more than one sulfhydryl group reach the same arsenic atom (i.e., reduction is a two-electron process).

The As(III)-sulfur bond is much more resistant to hydrolysis than the As(III)-oxygen moiety (Sagan et al. 1972; Zingaro and Thomson 1973).

Biomethylation of Arsenic

Endogenous thiols probably play a critical role in the metabolic conversion of As(III) and As(V) species. It is likely that glutathione (GSH) acts as a reducing agent for As(V) species; the resulting As(III) species can then accept a methyl group from S-adenosylmethionine (SAM) to produce the methylarsenic(V) species in an oxidative-addition reaction, as illustrated in Figure 33 (Cullen and Reimer 1989). This cycle of reduction followed by oxidative addition of a methyl group can be continued, and the end product seems to depend on the organism. This cycle is based on the pioneering studies of Challenger (1951). The end products can be trimethylarsine oxide or trimethylarsine for fungi, the tetramethylarsonium ion for clams, and probably DMA for humans (Cullen and Reimer 1989; Cullen et al. 1994) (see Chapter 5).

image

FIGURE 3-3
Biomethylation of arsenic: SAM as methyldonor, GSH as reducing agent. GSH, glutathione.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 33

SAM is probably the source of the adenosyl group that is found in the arsenosugars (Francesconi and Edmonds 1997). The As(III) derivatives seem to have the unique ability to accept all three groups that are attached to sulfur in SAM, as illustrated in Figure 3-4.

image

FIGURE 3-4
Reaction of As(III) derivatives with SAM. SAM, S-adenosylemthionine.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 34

The As(III) species that are intermediates in the biotransformation of arsenic might well be toxic (Hocking and Jaffer 1969; Sheridan et al. 1973; Cullen et al. 1989b) (see Chapter 7). For example, glutathione reductase (GR) is a key enzyme in the metabolism of GSH and is inhibited by the methylarsenic(III) and As(III) species (Delnomdedieu et al. 1994). The action of GR is critical in maintaining the redox status of cells.

Some Geochemical Considerations: Absorption and Redox

As(III) exists in most natural water as As(OH)3 (pKa = 9.2) and is more mobile than As(V) because it is less strongly absorbed on most mineral surfaces than the negatively charged As(V) oxyanions (H3AsO4; pKa = 2.22, 6.98, 11.53). Iron(III) oxy species are well known to have a high affinity for As(V) (Waychunas et al. 1993; Lumsdon et al. 1984), and As(III) also seems to be adsorbed on some iron(III) surfaces (Sun and Doner 1996).

Little is known about the adsorption of As(III) on the terrestrially abundant aluminum oxides and aluminosilicate minerals. Activated alumina has a twofold higher affinity for As(V) than for As(III) at pH 7 (Ghosh and Yuan 1987); negligible removal of As(III) from drinking water is achieved by coagulation with alum (Hering et al. 1997). Kaolinite and montmorillonite also have higher affinities for As(V) than for As(III) (Frost and Griffin 1977). Abiotic oxidation of As(III) is enhanced in the presence of the clay minerals kaolinite and illite, a process that results in strongly bound As(V) species (Manning and Goldberg 1997; Scott and Morgan 1995). Thus, long-term modeling of arsenic mobility in soils and aquifers must consider the effects of pH and mineral conditions, which will influence both adsorption and abiotic oxidation of As(III).

Little is known about the adsorption behavior of the organic arsenic species listed in Table 3-1 in spite of the use of the methylarsenicals MMA and DMA and their salts as pesticides, herbicides, and defoliants (Vallee 1973; Nriagu and Azcue 1990).

Microbial Activity and Arsenic Mobilization

Direct microbial reduction of arsenate to the more mobile arsenite is known for bacterial, algal, and fungal species (Cullen and Reimer 1989; Silver et al. 1993; Diorio et al. 1995). Microbial activity has been implicated in arsenic mobilization from sediments. Iron-reducing bacteria might cause arsenate dissociation from sediment that is solid as a consequence of iron

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 35

oxide dissolution (Lovley et al. 1991). Sulfate-reducing bacteria produce hydrogen sulfide, which might promote arsenate reduction. In recent years, some arsenate-respiring bacteria have been isolated; those include the dissimilatory arsenate-reducer strain MIT-13 (Ahmann et al. 1994) and Chrysiogenes arsenatis native to gold-mine waters (Macy et al. 1996).

Ahmann et al. (1997) showed that arsenic-rich anoxic sediments from the Halls Brook storage area are mobilized by native microbial activity. In particular, strain MIT-13 is likely to catalyze that activity. In related experiments, Ahmann et al. (1997) found that microbial activity catalyzed rapid dissolution of arsenic, as As(III), from Fe(II) and Fe(III) arsenates.

Free Radical and Peroxy Species

It has been suggested that the observed tumor promotional activity of DMA might be due to the action of active oxygen-containing species, such as the dimethylarsenic peroxyl radical (CH3)2As-O-O (Rin et al. 1995; Yamanaka et al. 1996) (see Chapter 7). In fact, not much is known about peroxy species of arsenic, but the proposed As(III) species does not seem to be a plausible entity. Phenylarsonic acid, C6H5AsO(OH)2, can act as an oxygen-transfer agent by reacting with peroxide to form the intermediate peroxy acid C6H5AsO(OH)(OOH) (Jacobson et al. 1979a,b). One compound that was formulated as (CH3)2As-S-S-As(CH3)2, close in structure to the proposed peroxyl radical, turned out to have the structure (CH3)2As(S)-S-As(CH3)2, which has arsenic in two oxidation states (Camerman and Trotter 1964). On that basis and because of the ease of oxidation of DMA(III), (CH3)2As(O)-O or even (CH3)2As(O)-O-O might be more likely candidates for active oxygen-containing species. Related antimony peroxy species have been isolated and characterized (Dodd et al. 1992).

Analysis Of Arsenic Compounds

The discovery by Scheele in 1775 that arsenic compounds could react under reducing conditions to produce a volatile gas, arsine (AsH3) (Partington 1962), provided a tool to counteract the use of arsenic, usually as the oxide As2O3, for homicide; that use had reached epidemic proportions during the Middle Ages. The discovery led to the development of the Marsh test for arsenic, in which arsine is volatalized out of the reaction mixture and is detected, for example, by decomposition to an arsenic mirror. Alternatively, in the Gutzeit modification, the arsine is brought into contact with a filter

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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paper soaked with either silver nitrate or mercuric halide to produce a colored deposit. Both procedures can be made semiquantitative. They are the first applications of an analytical method now referred to as ''hydride generation" that is widely applicable to the analysis of elements that form volatile hydrides when treated with an appropriate reducing agent, usually sodium borohydride (Vallee 1973).

In older studies, zinc and hydrochloric acid were used as reducing agents. The Gutzeit method was used in a recently published Japanese study (Tsuda et al. 1995) in which well water was found to contain arsenic in the parts-per-million range—the measurements were made in 1959. A variation of this method, Natelson's method (Natelson 1961), was used in the early study of arsenic in the well water of Taiwan (Tseng et al. 1968). That method uses colorimetric detection and is said to detect arsenic at 40 µg/L. Arsine produced from  arsenate and arsenite is sometimes reacted with silver diethyldithiocarbamate solution to produce a red solution of undetermined chemical nature that can be measured colorimetrically (Vallee 1973; Irgolic 1994; see also Table 3-3). The colorimetric method is easy to use, is inexpensive in terms of equipment and operator costs, and is commonly used by the water-supply industry. For example, Saha (1995) used the colorimetric method for his work on the wells of Bengal, and the method was used in the Lane County, Oregon, study (Morton et al. 1976); however, it has limitations with regard to sensitivity and arsenic speciation.

Total-arsenic determination commonly involves oxidation of the sample, by using digestion or ashing, with a mixture of chemicals, including HNO3-H2SO4-H2O2 or HNO3-H2SO4-HCIO4 for wet digestion and MgO-MgNO3 for dry ashing (Irgolic et al. 1995). The arsenic is then determined by using one of a number of methods ranging from hydride generation (colorimetric or spectroscopic detection or neutron activation) to spectrophotometry (e.g., graphite-furnace atomic absorption (GFAA) or inductively coupled plasmaatomic emission spectrometry (ICP-AES)).

Hydride Generation with Speciation

The commonly used hydride-generation method that is currently applied to the determination of arsenic species is outlined in Figure 3-5. Volatile arsines are produced from a range of inorganic and methylarsenicals in both oxidation states. The four arsines, AsH3 and MexAsH3-x (x = 1-3) are produced from their appropriate precursors and are then quantified following separation if necessary. Bramen and Foreback (1973) and Andreae (1977) were among the first to use this method for arsenic speciation in natural systems.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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image

FIGURE 3-5
Hydride generation. Arsenic speciation by using hydride generation. UV, ultraviolet.

The method involves a derivatization technique, and the number of methyl groups in the evolved arsine, MexAsH3-x, is generally the same as that in the arsenic species before reduction (Andreae 1977). The precursors to the arsines are also generally assumed to be the oxyspecies shown in Table 3-1. That is not necessarily correct. For example, in some sulfur-rich environments, the precursors to the arsines might be such compounds as MeAs(SR)2 (Bright et al. 1994, 1996).

An important feature of the hydride-generation reaction is its pH dependence. All the As(III) compounds produce arsines at about pH 6 (Andreae 1977; Bright et al. 1994; Hasegawa 1997). At about pH 1, all the arsenicals are reduced to arsines. By using that difference in reactivity, As(III) and As(V) can be determined in a sample. In the absence of other confounding factors, at intermediate pHs (e.g., in acetic acid) further differentiation between species and oxidation state can be obtained (Cullen et al. 1994).

When a mixture of arsines is obtained following hydride generation, the arsines must be separated before quantification can be achieved. Often, in the usual batch-type operation mode, the arsines are trapped at liquid nitrogen temperature (cryofocused) and then separated by using gas chromatography. Alternatively, the cold trapped arsines are allowed to warm up slowly, and the differences in boiling points result in sequential presentation of the four compounds to the detector. In another variation, the hydrides are trapped in cold solvent, and aliquots of the resulting solution are injected into a gas chromatograph.

Many detectors have been used for the analysis of the separated arsines.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
×

Page 38

Element-specific detection is preferred, although flame ionization and electron capture have been used. The arsines are passed into a flame, a quartz-tube atomizer, or a graphite furnace for atomic absorption detection. A mass spectrometer or ICP-MS (inductively coupled plasma-mass spectroscopy) can be used as can ICP-AES. The detectors and their detection limits for AsH3 are listed in Table 3-2. The detectors respond differently to different arsines, so separate calibration curves are usually needed. However, by adjusting such factors as pH and flow rate in a flow system, compromise conditions for hydride generation-atomic absorption (HG-AA) can be found so that the different arsines produce the same response (Le et al. 1994a) and separate calibration curves are not needed.

Hidden Arsenic Species

Figure 3-5 indicates that apart from the methylarsenicals and inorganic arsenic species listed in Table 3-1, all other arsenic species likely to be found in the environment do not form hydrides that can be quantified by using the usual technique. AsB is in that category. Compounds that are not detected by hydride generation are sometimes referred to as "hidden arsenic" species. That term originated in the era when hydride generation was the only method readily available for arsenic speciation. As indicated in Figure 3-3, all arsenicals can be decomposed to arsenate for analysis. Decomposition via ultraviolet (UV) irradiation (Cullen and Dodd 1988), microwave-assisted persulfate oxidation (Le et al. 1992), or UV irradiation combined with persulfate oxidation (Zhang et al. 1996a) are singled out because of their application to speciation methodology.

Arsenic(III) Species

Occasionally, specific As(III) species are extracted from aqueous solution by use of sodium diethyldithiocarbamate before determination by HG-AA, for example. (Chatterjee et al. 1995; Hasegawa 1996). This procedure involves a preconcentration step using chloroform.

Chromatographic Separation of Involatile Arsenic Species

As mentioned previously, hydride generation is a method that involves derivatization; hence, assumptions must be made about the nature of the

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 39

TABLE 3-2 Methods for the Determination of Total Arsenic in Drinking Water

 

Detection Limit

Instrument cost,

Operator

Applicable Arsenic Limit

Method

Absolute

Concentration

$ x 10-3

Skill Level

50 µg/L

20 µg/L

2 µg/L

2 ng/L

Hydride Generation

               

Colorimetrica

1 µg

10 µg/L (100 mL)

5

Low

Yes

No

No

No

Thermal conductivity

 

10 µg/L

40

High

Yes

Yes

Yes

No

Atomic fluorescence

 

20 ng/L

20

High

Yes

Yes

Yes

No

Graphite furnace AAS

0.2 ng

2 ng (100 mL)

60

High

Yes

Yes

Yes

No

Quartz-tube AAS

0.5 ng

32 ng/L (15 mL)

60

High

Yes

Yes

Yes

No

Helium discharge

0.1 ng

1 ng/L (100 mL)

20

High

Yes

Yes

Yes

No

Microwave plasma

20 pg

1.4 ng/L (15 mL)

20

High

Yes

Yes

Yes

No

DC-plasma

0.4 µg

4 µg/L (100 mL)

60

Very high

Yes

Yes

No

No

ICP emission

2 ng

20 ng/L (100 mL)

80

Very high

Yes

Yes

Yes

No

ICP-MS

10 pg

0.1 ng/L (100 mL)

200

Very high

Yes

Yes

Yes

No

Electrochemical

               

Differential pulse

20 ng

1 µg/L (20 mL)

20

High

Yes

Yes

No

No

polarography

               

Anodic stripping

6 pg

0.3 ng/L (20 mL)

30

High

Yes

Yes

Yes

Yes

voltammetry

               

Spectroscopic

               

Flame atomic absorption

 

1 mg/L

20

Low

No

No

No

No

Graphite furnace AAS

 

1 µg/L

80

High

Yes

Yes

No

No

DCP or ICP-AES

 

10 mg/L

80

Very high

Yes

Yes

No

No

ICP-MS

 

0.05 µg/L

200

Very high

Yes

Yes

Yes

No

aSilver diethyldithiocarbamate.

Abbreviations: AAS, atomic absorption spectroscopy; DC, direct current; ICP-MS, inductively coupled plasma-mass spectrometry; DCP, direct current plasma; AES, atomic emission spectroscopy.

Source: Adapted from Irgolic 1994.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 40

analyte species. To minimize problems and examine all soluble arsenic species, some form of chromatography has been used for the initial separation of the arsenicals, and high-performance (pressure) liquid chromatography (HPLC) is rapidly becoming the method of choice. Ion exchange (cation and anion), size exclusion, and reversed-phase (with ion pairing) columns are used (Morita and Shibata 1990; Sheppard et al. 1992; Vela and Caruso 1993; Inoue et al. 1994; Kawabata et al. 1994; Kumar et al. 1995; Pergantis et al. 1995). Hydride-generation methods can be used to quantify the fractions separated from conventional chromatography columns, a procedure that has been used in many urine studies, as described below; however, investigators are now using HPLC for the separation coupled with an arsenic-specific detector, such as an ICP-MS or ICP-AES, for quantification. Hyphenated techniques, such as HPLC-ICP-MS or HPLC-ICP-AES, have the advantage in that, in principle, all species in the sample can be separated and detected with selectivity and sensitivity in the subnanogram-to-subpicogram range. Of the two detectors, the ICP-MS is more sensitive than the ICP-AES. The ICP-MS also has a wide linear range and high precision, but it is expensive. One inexpensive HPLC detector for arsenic species consists of a chamber in which the eluate is combusted in a hydrogen-oxygen flame, and then the products are entrained into a silica T tube situated in the light path of an AA spectrometer. Detection limits are in the subnanogram region (Monplaisir et al. 1994).

One promising development, in which hydride generation is still used, was published by Le et al. (1996). HPLC is used to separate the arsenicals, which are then decomposed on line to arsenate by using microwave-assisted persulfate oxidation. Arsenate is then converted to arsine, which is detected by using atomic fluorescence. This detector is much less expensive than the ICPMS alternative, and the method has a detection limit for arsenic species in urine of 10 parts per billion (ppb).

Mass Spectrometry for Arsenic Speciation

The most recent development in speciation methodology has been to combine the separation capability of HPLC with the molecular-recognition capability of mass spectrometry. Electrospray mass spectrometry appears to offer important advantages (Siu et al. 1988; Siu et al. 1991; Florêncio et al. 1997; Pergantis et al. 1997a,b). In the most complete study to date, 10 arsenicals were separated and detected by using a microbore reversed-phase column coupled to an electrospray triple-quadrupole mass spectrometer (µ-HPLC-ESMS-MS). The electrospray mass spectrum of the compounds listed in Table 3-3 essentially consists of only protonated species (if not cationic to start with) or the cation (if cationic). The collision-induced dissociation (CID)

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 41

fragment ions listed in Table 3-3 were obtained by using argon as the collision gas in the second quadrupole. The monitored CID reaction allows the analysis of the species even if the species is not separated completely by the chromatography. Thus, discrimination is achieved within the mass spectrometer. The detection limits are also listed in Table 3-3; these limits are becoming comparable to those obtainable with ICP-MS (Table 3-2); the instrument cost, however, is higher than the ICP-MS cost. AsB has been measured in a standard reference material, although matrix problems have been encountered (Pergantis et al. 1997b). A closely related ionization mode, ionspray (IS), has been incorporated into the hyphenated method HPLC-IS-MS-MS (Corr and Larsen 1996) with equally impressive results: arsenicals, including AsB and arsenosugars, were verified in flatfish and oyster tissues.

Arsenic In Water

In 1994, Irgolic published a review of the methods for determining arsenic and arsenic species in drinking water (see also Irgolic et al. 1995). Irgolic concluded that drinking water normally contains arsenic as arsenate and, if the water is anaerobic, some arsenite. Methylated species (MMA and DMA) would rarely be present in water supplies. Unless special circumstances, such as pollution by arsenical herbicides or high biological activity, exist, Irgolic argued that determination of arsenic species in water supplies is unnecessary; knowledge of the inorganic arsenic content is sufficient for regulatory purposes. The subcommittee evaluated that conclusion, as described below.

Arsenic in Groundwater

Although it is widely believed that arsenate is the major water-soluble species in groundwater, there is increasing evidence (Korte and Fernando 1991) that arsenite might be more prevalent than anticipated. Improved methods of sampling, sample preservation, and analysis have contributed to that conclusion. For example, Anderson and Bruland (1991) found it necessary to trap AsH3 generated from As(III) in the field and to store the sample in liquid nitrogen for later laboratory analysis. Significant differences were found between those samples and water samples collected, immediately frozen (-196°C), and later analyzed in the laboratory. Andreae (1977) claimed that speciation is unchanged in samples stored at -15°C or under dry ice, although an initial loss of arsenite (approximately 0.02 ppb) is experienced. Korte and Fernando (1991) listed other procedures that have been used for sample preservation.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 42

TABLE 3-3 CID Fragment Ions and Selected Reactions Monitored for 10 Organic Arsenic Compounds

   

Precursor

 

CID Reaction

Detection

Compound

Mna

Ion

CID Fragment Ionsb

Monitored

Limit (ppm)c

Methylarsonic acid

140

141

141(100); 123(15); 109(7); 91(5)

141-123

0.3

p-Arsanilic acid

217

218

218(100); 109(37); 108(13); 92(13)

218-109

0.1

4-Hydroxyphenylarsonic acid

218

219

219(100); 201(16); 110(31); 93(10)

219-110

0.3

Dimethylarsinic acid

138

139

139(100); 121(27); 108(8); 91(8)

139-121

0.07

3-Nitro-4-hydroxy-phenylarsonic acid

263

264

264(100); 246(12)

264-246

0.2

4-Nitrophenylarsonic acid

247

248

248(100); 230(12); 202(16); 122(5)

248-202

0.1

Arsenobetaine

178

179

179(100); 120(18)

179-120

0.002

Trimethylarsine oxide

136

137

137(100); 122(17); 107(23)

137-107

0.04

Arsenocholine

165

165

165(100); 121(5); 45(9)

165-45

0.02

Tetramethylarsonium ion

135

135

135(100); 120(44); 105(15)

135-120

0.02

aNominal relative molecular mass; the monoisotopic molecular mass rounded to the nearest whole number.

bNumbers in parentheses correspond to percent relative intensity.

cSelected reaction monitoring, 1 µL injections.

Abbreviation: CID, collision-induced dissociation.

Source: Adapted from Pergantis et al. 1997b.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
×

Page 43

The use of excess Fe(II) and acidic pH at less than 2 has been suggested to suppress the oxidation of As(III) (Borho and Wilderer 1997). However, samples from different bodies of water usually have different redox potential and therefore require different amounts of Fe(II). Thus, this approach might be difficult to apply to all water samples without prior knowledge of the sample.

Yalcin and Le (1998) recently developed an interesting method in which As(III) and As(V) were separated on-site by using disposable cartridges. Immediately after collection, a measured volume of a water sample was pumped through a resin-based strong cation-exchange cartridge followed by a silica-based strong anion-exchange cartridge. The cation-exchange cartridge retained DMA and allowed other arsenic species to pass through. The anion-exchange cartridge retained MMA and As(V), and As(III) remained in the solution. The cartridges and the solution containing separate arsenic species were brought to a laboratory for analysis. The unretained arsenic in the solution was a measure of As(III) in the original water sample. Arsenic eluted from the cation-exchange cartridge with 1 M HC1 was quantified for DMA. The anion-exchange cartridge was first eluted with 0.06 M acetic acid for the determination of MMA and then with 1 M HC1 to elute As(V). If only inorganic As(III) and As(V) were present as the major arsenic species, the procedure could be simplified by using only the anion-exchange cartridge.

Chatterjee et al. (1995) also found that As(III) is about 50% of the total arsenic in the groundwater of many wells in six districts of West Bengal, India.  (As(III) was separated from As(V) by extraction with sodium diethyldithiocarbamate.) No MMA or DMA was found. Total-arsenic concentrations ranged from 10 to 1,095 µg/L.

Hydride generation was used to speciate arsenic in groundwaters from Taiwan (Chen et al. 1994). In the blackfoot-disease (BFD) area, the average arsenic, predominantly As(III), concentration in three wells was 671 ± 149 µg/L. The ratio of As(III) to As(V) was 2.6:1. Outside the BFD area, the arsenic concentration dropped to 0.7 µg/L. No methylarsenic species were present (at less than 1 µg/L), and hidden arsenic species were not significant. Insoluble suspended arsenic accounted for about 3 % of the total arsenic in the water from the three wells, and ultrafiltration revealed that about 11% of the soluble arsenic, mainly As(III), was associated with molecules of molecular mass greater than 300,000 daltons. Possibly, the material was humic. Much has been written on the possible connection between BFD, humic material, and arsenic (e.g., Lu 1990; Lu and Lee 1992; Lu et al. 1991, 1994; Yang et al. 1994).

The arsenic species detected in ground water by hydride generation generally are believed to be the oxy species MexAsO(OH)3-x or As(OH)3.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 44

However, that might not always be the case, especially in a reducing environment where arsenic and sulfur compounds would be expected to dominate (Cullen and Reimer 1989). The presence of the oxythioarsenate H3AsO3S in arsenic-rich terrain has been established, and one base extract contained arsenate at 2,616 µg/kg of soil and the thio analogue at 115 µg/kg, as determined by capillary electrophoresis (Schwedt and Reickhoff 1996a,b).

Arsenic in Fresh Surface Water

An important study of the arsenic species in the Ottawa River, Canada (Sturgeon et al. 1989), opens with the statement, "Few data exist on the species distribution of arsenic in natural waters. " At the time of publication, no reports had hinted at the possible presence of nonhydride-active organic arsenic species (hidden arsenic) in water. The Ottawa River samples were used in the preparation of the river-water reference material for trace metals, the Standard Laboratory Reference Sample (SLRS-1), for the National Research Council of Canada (NRCC 1986). Careful analysis of the standard gave the following results (species and concentration in micrograms per liter): As(III), 0.16; As(V), 0.18; MMA, less than 0.02; DMA, 0.05; AsC and TMAs, less than 0.01; nonhydride and inactive (hidden) species, 0.12; organically bound, less than 0.01; total arsenic, 0.52 ± 0.03. The certified value for that reference water was 0.55 ± 0.08 µg/L. The nonhydride and active species constituted 22 % of the total arsenic, and the authors suggested that the compound or compounds are similar to AsB. However, the workup of the sample, which involved the use of a strong cation-exchange column, could have changed the species originally present. Arsenic speciation in the sterile water sample (pH 1.6) was stable for at least 13 months.

In a study of a number of U.S. lakes and estuaries (mostly Californian), Anderson and Bruland (1991) used the HG-AA method (Andreae 1977). Measurable amounts of methylarsenicals were detected, amounting to 1-59 % of the total arsenic. With the exception of two highly alkaline lakes, Mono and Pyramid Lakes, the methylated forms constituted on average 24% of the total dissolved arsenic in lake surface water. In the Davis Creek Reservoir, a seasonally anoxic lake, DMA became the dominant surface arsenic species during the late er and fall. The study indicated that the DMA can be demethylated later in the yearly cycle. The total-arsenic concentration varied with the season, and arsenate was generally the predominant inorganic species, although appreciable amounts of arsenite were detected. For example, in October 1988, the ratio of total arsenic to As(V) to As(III) to DMA was 128:45:12:58. No attempt was made to look for hidden arsenic species in those waters.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 45

Studies conducted on Lake Biwa, Japan (Hasegawa 1997; Sohrin et al. 1997), in which a more sophisticated form of hydride generation was used, revealed the presence of methylarsenic(III) species, possibly MeAs(OH)2 and Me2AsOH, in low concentrations. The authors also found that arsenic speciation and concentration varied with the season, particularly in the euphotic southern basin of the lake where DMA can become the dominant species. Seasonal variation in methylarsenic(III) and (V) species was also found in coastal waters (Hasegawa 1996). Similar compounds were seen in sediment pore water from Yellowknife, Canada (Bright et al. 1994, 1996), but hidden arsenic species were also present and were the major species in some samples.

Arsenic in Estuarine Water

Hydride generation was used for arsenic speciation in coastal waters of southern England (Howard and Comber 1989). The overall concentration of arsenic increased by approximately 25 % when samples were irradiated with UV light, indicating that arsenicals undetectable by hydride generation become detectable following irradiation. In the opinion of the authors, those hidden arsenic species might be arsenosugars but are probably not AsB. Their maximum concentration occurred when biological activity was high. Targus Estuary, Portugal, also was found to contain hidden arsenic amounting to 19-25 % of the total (de Bettencourt et al. 1994).

For most of the year, Howard and Apte (1989) found As(V) to be the only detectable arsenic species in the Itchen Estuary in southern England. In saline areas, the total-arsenic concentration was 0.7-1.0 µg/L and was 0.2 µg/L at a freshwater site. DMA was found in the river only in May and June. In the estuary, 30% of the total arsenic was in the form of methylated species when productivity was at a maximum (temperatures more than 12°C). At lower temperatures, the absence of methyl species might have been due to (1) lower general turnover of arsenic; (2) lower ability, or need, of organisms to excrete methylarsenicals; or (3) excretion of arsenicals not detected by hydride generation.

Practical Quantification Level for Arsenic in Drinking Water

A recent study (Eaton 1994) attempted to determine the practical quantification level (PQL) for total arsenic in drinking water. Twenty-five laboratories, commercial and utility, were involved. The laboratories used approved analytical methods: HG-AA and GFAA. The data indicated that a reasonable value for a minimum interlaboratory PQL for arsenic is 4 µg/L. At that concentration, a laboratory technician should be able to measure arsenic to an

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 46

accuracy of better than ±25 % for 95 % confidence limits; individual laboratories might be capable of achieving much greater accuracy. The PQL of 4 µg/L is about the same as that expected from the data given in Table 3-2 (Irgolic 1994) for hydride generation but is lower than that estimated for the less-sensitive direct spectroscopic GFAA method.  The detection limits suggested by Irgolic are conservative but realistic, and if the arsenic maximum contamination level (MCL) is set at 2 µg/L, GFAA will not be capable of reaching that target. HG-AA will still be of use, and the method has the advantage of being able to supply some speciation information if required. Hydride generation can achieve low-picogram arsenic detection limits with improved detection by ICP-MS. GFAA is limited in both sensitivity and speciation capabilities. Eaton (1994) found that below 4 µg/L, precision and bias degrade substantially, indicating that reliable interlaboratory measurements are not routinely possible below that concentration.

Arsenic In Food

Total-Arsenic Concentrations

One of the most comprehensive studies of arsenic in food was published in 1993 (Dabeka et al. 1993). Food collected in Canadian cities in the years 1985-1988 was analyzed for total arsenic, and the food groups containing the highest mean arsenic concentrations were fish (1,662 ng/g), meat and poultry (24.3 ng/g), bakery goods and cereals (24.5 ng/g), and fats and oils (19.0 ng/g). The average daily dietary ingestion of total arsenic by Canadians was estimated to be 38.1 µg (48.5 µg for adults) and varied from 14.9 µg for children 1-4 years old to 59.2 µg for males 20-39 years old. Previous estimates for Canadians were 16.7 µg per day and 30 µg per day (Dabeka et al. 1987).

Those figures are comparable with data from other countries: United States, 62 µg per day (Gartrell et al. 1985); United Kingdom, 89 µg per day (Food Additives and Contaminants Committee 1984); Austria, 27 µg per day (Pfannhauser and Pechanek 1977); and New Zealand, 55 µg per day (Dick et al. 1978). Assuming that those figures have general application, it is apparent that individuals receive a considerable daily dose of arsenic from food.

On the basis of the U.S. Food and Drug Administration (FDA) Total Diet Study, Adams et al. (1994) provided the most thorough analysis of arsenic in the U.S. diet to date. Out of about 5,000 foods regularly consumed in the United States, 234 were selected as being representative, and excess caloric diets were constructed from those foods for total diet studies. (The diet consisted of 82 foods from 11 food groups in quantities for a 14-day intake

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 47

with excessive energy intake of 4,200 kcal per day.) The results indicate that food contributes 93 % of the total intake of arsenic, and seafood contributes 90 % of that 93 % (see Table 3-4). Mean arsenic intakes by groups of individuals were estimated in 1982-1991, and those results are given in Table 3-5. The results from the latter years are generally lower than those from the peak period 1984-1986. Nriagu and Azcue (1990) arized the data from a number of studies on total dietary arsenic, and Jelinek and Corneliussen (1977) reported similar EPA data for the years 1967-1974.

Results of the most recent FDA Total Diet Study are shown in Table 3-6 (Tao and Bolger 1998). The total arsenic intake is divided into contributions from seafood and others. The estimates given in the final column are based on the assumption that 10% of the arsenic in seafood is inorganic and that 100 % of the arsenic in the rest of the food is inorganic. The estimates of 10 % and 100% are high but nevertheless set an upper bound. For example, a 25-30 year old 79-kg male will ingest 9.89 g of inorganic arsenic per day from food. The significance of the intake of inorganic arsenic from food increases as the concentration of arsenic in water decreases. If water contains 50 µg/L of inorganic arsenic, arsenic in food might not be significant. However, if water contains 5 µg/L of arsenic and 2 L per day is consumed, the contribution of inorganic arsenic from diet and water are comparable.

The average arsenic intakes reported in Table 3-6 are generally higher than those reported for earlier years (Table 3-5). The new numbers probably reflect an increase in the consumption of seafood (Table 3-4).

Studies of Arsenic Speciation in Food

In view of the arsenic concentrations in some foods, the nature of the arsenic species present is an important consideration. The simplest method that has been used for distinguishing between so-called ''organic" arsenic in food and "inorganic" arsenic is to use hydrochloric acid as an extractant and reagent for isolating an arsenic-containing fraction. The HC1 fraction is distillable from the remainder of the extract and is regarded as containing the inorganic arsenic originally present in the food. The question has been raised whether organic arsenic can be broken down to inorganic arsenic species during the HC1 distillation (Mushak and Crocetti 1995), thus increasing the proportion of inorganic species in the sample. That seems unlikely: the arsenic-carbon bond in methylarsenicals is robust, and DMA, for example, is stable in hot nitric acid.

HC1 distillation has been widely used for marine-animal arsenic speciation. Edmonds and Francesconi (1993) collected much of the available

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 48

TABLE 3-4 Foods Containing Arsenic at Concentrations above 50 ppb as Determined by the FDA Total Diet Study for Market Baskets Collected from 1990 through 1991

Fish

Meat

Vegetable

Tuna in oil

Turkey breast

Rice, cooked

Fish sticks

 

Rice cereal

Haddock, fried

 

Mushrooms, raw

Shrimp, boiled

 

Olive/safflower oil

Tuna noodle casserole

   

Fish sandwich

   

Clam chowder

   

Source: Adams et al. 1994.

TABLE 3-5 Mean Total Daily Arsenic Intakes (µg/d) as Determined by the FDA Total Diet Study for Market Baskets Collected from April 1982 through April 1991

Population-

 

Group Age

Arsenic Intake (µg/d) and Years of Collection (Number of Collections)

(Sex)a

1982-84 (8)b

1984-86 (8)c

1986-88 (8)c

1988-90 (8)c

1990-91 (5)c

6-11 mo

4.9

7.4

6.2

3.6

3.2

2 yr

12.2

15.8

12.8

9.3

9.0

14-16 yr (F)

23.0

29.3

23.5

17.5

16.9

14-16 yr (M) 27.7

35.7

28.7

20.6

20.3

 

25-30 yr (F) 31.0

39.5

31.3

23.7

23.3

 

25-30 yr (M) 45.3

58.1

45.6

34.0

34.6

 

60-65 yr (F) 35.2

45.3

34.8

26.7

26.6

 

60-65 yr (M) 43.8

56.6

43.1

32.7

33.4

 

aF, female; M, male.

bIn calculating intakes, "trace" concentrations were assigned a value of one-half the limit of quantitation; the "trace" concentration assigned to arsenic during those years was 0.01 mg/kg of diet.

cIn calculating intakes for this time period, "trace" concentrations were estimated by the analyst.

Source: Adams et al. 1994.

information, and regression analysis of the data indicates that the proportion of inorganic arsenic falls from about 1% at very low total-arsenic concentrations to about 0.5% at total-arsenic concentrations of 20 mg/kg. These proportions contrast with the 2-10% proposed by WHO (1989) and GESAMP (1986) for the inorganic arsenic content of marine animals. The distillation method seems to provide minimum but useful and reliable information on speciation of arsenic in marine-animal food for routine work, but Edmonds and Francesconi (1993) suggested that more sophisticated methods should be used to establish the arsenic status of particular marine foods, such as algae, before approval for consumption.
Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 49

TABLE 3-6 Estimated Average Inorganic Arsenic Intakes for Various Age and Sex Groups as Determined by the FDA Total Diet Study for Market Baskets Collected in 1991-1997

Population Group

 

Inorganic Arsenic In-

Total Inorganic

 

Weight

Total Arsenic

takea (g/d)

 

Arsenic Average

Age (Sex)b

(kg)c

Intake (µg/d)

Seafood

Others

Intake (µg/d)

6-11 mo

7

2.15

0.09

1.25

1.34

2 yr

13

23.4

2.11

2.30

4.41

6 yr

22

20.3

1.74

2.90

4.64

10yr

64

13.3

1.01

3.20

4.21

14-16 yr (F)

53

21.8

1.85

3.30

5.15

14-16 yr(M)

64

15.4

1.21

3.30

4.51

25-30 yr (F)

62

27.5

2.46

2.90

5.36

25-30 yr (M)

79

56.6

5.19

4.70

9.89

40-45 yr (F)

67

36.8

3.38

3.00

6.38

40-45 yr(M)

81

46.8

4.28

4.00

8.28

60-65 yr (F)

67

72.1

6.93

2.80

9.73

60-65 yr(M)

81

92.1

8.84

3.70

12.54

70+ (F)

62

45.4

4.25

2.90

7.15

70+ (M)

74

69.4

6.63

3.10

9.70

aBased on the assumption that 10 % and 100% of the total arsenic is inorganic arsenic in seafood and all other foods, respectively. Seafood inclues seven TDS food items (tuna, fish sticks, haddock, shrimp, tuna noodle casserole, clam chowder, and fish sandwich).

bF, female; M, male.

cSelf-reported weights from the data tapes of the U.S. Department of Agriculture.

Source: Tao and Bolger 1998.

The HC1 extraction-distillation method was applied in two recent studies that included a range of food classes. In the first (Yost et al. 1998), 15 food samples from  Canada were examined for inorganic and organic arsenic (arsenic in the residue after the distillation step) and total arsenic (arsenic in a wet digest, H2SO4-HNO3-HClO4, of the sample). The data used by Yost et al. (1998) was previously available as a report from the Ontario Ministry of the Environment (OME 1987). Yost and co-workers (1998) claimed that the data were misinterpreted when estimates of dietary arsenic were made for EPA's 1988 risk assessment. For example, although no potatoes or vegetables were analyzed, the data have been cited as indicating that arsenic in yams (EPA 1988) and vegetables (Mushak and Crocetti 1995) occurs primarily in organic form. Fifteen food groups were examined (OME 1987), but the study can only be regarded as preliminary (quality-assurance and quality-control (QA-QC) procedures were not described); only one sample per food group was analyzed, and some mass balances were poor. For example, 50% of the

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
×

Page 50

arsenic in bread, with arsenic at 0.024 mg/kg, is inorganic, yet the organic arsenic was undetectable. In the study, the HCI distillate was further treated with HNO3 and H2SO4 before HG-AA analysis.  No reason is given for following that procedure; perhaps the investigators believed it might be necessary to convert any methylarsenicals in the distillate to arsenate (a possibility that should be investigated) (see Table 3-7). As mentioned above, the organic arsenic was determined as the total arsenic in the residue left after the HC1 distillation step. Some of the data are shown in Table 3-7. As expected, canned shrimp had a high total-arsenic content, but the inorganic content seems very high in view of the report of Edmonds and Francesconi (1993) described above. The concentrations of arsenic in rice seem low in view of the study described below. The results given in Table 3-7 together with those of Dabeka et al. (1993), Gunderson (1995), and Borum (1992) were used by Yost et al. (1998) to estimate daily dietary intakes of inorganic arsenic ranging from 8.3 to 14 µg per day in the United States and from 4.8 to 12.7 µg per day in Canada, on the basis that inorganic arsenic constitutes 20-40% of the total dietary intake. A previous estimate (EPA 1988) of 17 or 18 µg per day for inorganic arsenic includes arsenic from food, drinking water, and beverages.

TABLE 3-7 Arsenic Concentration in Food and Cigarettes (Values in ppm, Wet Weight)

Food (Sample No.)

Total

Inorganic

Organic

% Inorganic

Vanilla ice cream (1)

0.016

0.0042

<0.002

26

Cured pork (1)

0.013

0.018

<0.007

144

Pastrami (1)

0.024

0.024

<0.009

99

Chicken (1)

0.022

0.0090

0.012

41

Sole (1)

4

0.022

4.4

1

Tuna (1)

1.1

0.025

1.2

2

Pickerel (1)

0.14

0.022

0.086

15

Shrimp (1)

0.65

0.10

0.52

16

Rice (1)

0.24

0.1

0.16

43

Special K cereal (1)

0.27

0.070

0.15

26

Bread (1)

0.024

0.012

<0.006

50

Flour (1)

0.011

0.0076

<0.005

69

Apple juice (1)

0.012

0.0088

<0.002

73

Tea (1)

0.035

0.0091

0.025

26

Cigarettes (1)

0.18

0.11

0.03

61

Source: Adapted from Yost et al. 1998.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
×

Page 51

Schoof et al. (1998) concentrated on the arsenic species in Taiwanese rice and yams and used two methods of extraction claimed to yield similar results. In the first, 2 molar (M) HC1 was used (12 hr, 80°C), and the extract was analyzed without further distillation for arsenic speciation by using HG-AA. In the second method, the sample was simply extracted with water (12 hr, 25°C) followed by HPLC-ICP-MS for speciation (an example of a higher-level speciation study; see following section). Table 3-8 presents some of the data from that study.

TABLE 3-8 Arsenic Species in Rice and Yams from Taiwan (Average Values in ppm, Dry Weight, 1995 Samples)


Extract (Sample No.)


As(T)a


MMA


DMA

As(V) and As(lll) (% of total)


Recovery (%)

Rice

         

HCI (>5)

0.13

0.021

0.021

0.083 (72%)

105

Water (5)

0.15

0.013

0.013

0.11 (69%)

86

Yam

         

HCI (>15)

0.081

0.020

ND

0.058 (76%)

86

Water (1)

0.40

0.21

ND

0.13 (33%)

87

aSum of species, not determined.

Abbreviations: As(T), arsenic total; ND, no data; MMA, monomethylarsonic acid; DMA, dimethylarsinic acid.

Source: Adapted from Schoof et al. 1998.

The average arsenic concentrations in rice and yams were 0.15 mg/kg and 0.11 mg/kg, respectively, for samples collected in 1995 and 1993. In contrast to Yost et al. (1998), who defined organic arsenic as the arsenic content of the residues after extraction on the basis of the OME (1987) data, Schoof et al. (1998) defined the organic arsenic fraction as the sum of MMA and DMA, species that are extracted and ignored the residues. Although the data vary from sample to sample (not an unusual finding; see Dabeka et al. 1993), they were used to estimate mean values of 31 µg per day and 19 µg per day for total inorganic intake from yams and rice, respectively, for a total of 50 µg per day within a range of estimates of 15-211 µg per day. That is considerably higher than previous estimates of 2 µg per day (EPA 1988; Abernathy et al. 1989).

In an earlier study from Taiwan (Li et al. 1979), most of the crop (95%) in 1975 was found to contain detectable amounts of arsenic mainly in the 0.1-0.7 mg/kg (total arsenic) range. The concentrations in some samples were said to exceed the "tolerance level" of 0.76 mg/kg and ranged up to 1.43 mg/kg. The rice was grown in arsenic-rich soil that had probably been treated with arsenical pesticides of unstated composition.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Higher-Level Speciation Studies of Food

Most dietary arsenic originates from fish, shellfish, and algal products. The arsenic speciation in those products from the marine environment has been comprehensively arized (Francesconi and Edmonds 1997). In general, the major arsenical found in the parts of fish that are usually eaten is AsB (see Table 3-9), which is believed to have low-to-negligible toxicity on the basis of studies with mice (the lethal dose for 50 % of the test animals (LD50) was more than 10 g/kg) (Kaise and Fukui 1992), and with humans (Vahter 1994). The arsenic in marine algae and algal products is mainly determined to be one or more of the four arsenosugars X, XI, XII and XIII (see Table 3-1). Little work has been done on the toxicity of those compounds, although the consensus seems to be that they are similar to AsB in that regard. However, arsenosugars are metabolized by humans, largely to DMA (Le et al. 1994b, 1996); therefore, the question of the toxicity of DMA should be addressed (e.g., Yamanaka et al. 1996).

In the case of marine shellfish, early studies of arsenic speciation indicated that AsB and the tetramethylarsonium ion were the main arsenicals present (Cullen and Reimer 1989). That understanding changed with the development of improved analytical methods and sampling procedures. Now, arsenosugars are found in molluscs, which are commonly eaten in North America, (Shibata et al. 1992; Reimer et al. 1994) at concentrations that are comparable to those of AsB.

Although total-arsenic concentrations are known to be lower in freshwater fish than in marine animals, little is known about arsenic speciation in freshwater fish (Cullen and Reimer 1989). Freshwater fish, including pike (arsenic at 0.05 µg/g), bass (at 0.12 µg/g), carp (at 0.18 µg/g), pickerel (at 0.04 µg/g), and white fish (at 0.02 µg/g), do not contain AsB or AsC but do contain soluble unidentified arsenic species amounting to approximately 70% of the total arsenic (Lawrence et al. 1986); however, in a recent abstract, Koch et al. (1998) report that AsB is found in all the freshwater fish studied and that arsenosugars are the major species in freshwater mussels, AsB being absent. Inorganic arsenic species were either not found or were present at trace concentrations. In salmon (with arsenic at 0.31 µg/g), approximately 50% of the soluble arsenic (90 % of the total) is AsB and the rest is unknown (Lawrence et al. 1986). The concentrations are given on a fresh weight basis.

The major arsenic species in cultured rainbow trout (Salmo gairdneri) and wild Japanese smelt (Hypomesus nipponensis) is AsB. The AsB in the trout might originate from the commercial feed; that in the smelt is naturally derived. Of the total arsenic in the trout (1.46 µg/g) and smelt (1.08 µg/g), more than 95% was water soluble and was mainly AsB (Shiomi et al. 1995).

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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TABLE 3-9 Arsenobetaine in Marine Animals


Animal (No. of Species)

Arsenic Concentration
(mg/kg, Wet Wt.)

Arsenobetaine Content
(% of Total Arsenic)

Fish

   

Elasmobranchs (7)

3.1-44.3

³94

Teleosts (17)

0.1-166

48 to >95

Crustaceans

   

Lobsters (4)

4.7-26

77 to >95

Prawns (5)

5.5-20.8

55 to >95

Crabs (6)

3.5-8.6

79 to >95

Molluscs

   

Bivalves (4)

0.7-2.8

44-88

Gastropods (6)

3.1-116.5

58 to >95

Cephalopods (3)

49

72 to >95

Source: Francesconi and Edmonds 1997. Reprinted with permission from Advances in Inorganic Chemistry; copyright 1997, Academic Press.

Much is known about the ability of freshwater microalgae to grow in water containing high arsenic concentrations and to accumulate arsenic mainly as inorganic species with minor amounts of DMA and MMA (Maeda 1994). In contrast, the freshwater edible Nostoc species contain arsenic at a concentration of 3 ppm (dry-weight basis), but only 34% of that is extractable into water-methanol. For most marine algae, extraction efficiency is approximately 90% or better (Morita and Shibata 1990). Seasonal variations have been observed, however (Lai et al. 1998). The extracted arsenic in the Nostoc species consisted mainly of arsenosugar X (93 %), the rest being DMA as determined by HPLC-ICP-MS.

That same method has been applied to determine arsenic species in mushrooms, the only other type of food for which significant speciation information is available. Most mushroom species contain arsenic at less than 0.1 ppm on a dry weight basis (Vetter 1994). Arsenic-accumulating mushrooms contain a range of arsenicals, depending on the fungus (Byrne et al. 1995). For example, Sarcosphaera coronaria (accumulating arsenic at up to 2,000 µg/g of dry weight) contained only MMA, and Entoloma lividum (with arsenic at 40 µg/g) contained only As(III) (8%) and As(V) (92 %). Two Agaricus species (arsenic at 8 ppm) contained AsB as the major arsenic species. In the edible mushroom Laccaria amethystina, the arsenic (40 µg/g) was mainly in the form of DMA. Samples for the Byrne et al. (1995) study came from Switzerland and Slovenia. Kuehnelt et al. (1997a,b) showed that the arsenic species in Collybia maculata (30 ppm) is mainly AsB, as it is in Collybia butracea (11 ppm). AsB and AsC were found in the toxic mushroom Amanita muscaria (15-22 ppm and approximately 730 ppm in samples found growing on a contaminated site); AsB and

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 54

AsC were found at equal concentrations (15% each of the total arsenic). In addition, several unidentified arsenicals were present, amounting to 60% of the total arsenic. Those findings are the first reports of AsC and AsB in the terrestrial environment. In contrast to mushrooms, the concentration of AsC in marine animals is much less than that of AsB (Francesconi and Edmonds 1997). The mushroom results and others (Lai et al. 1997) suggest that AsB in the terrestrial environment might not originate from an arsenosugar precursor. That suggestion is contrary to the widely held view that the arsenosugars are precursors to AsB in the marine environment.

In meat and poultry, some of the high total-arsenic concentrations might result from the use of arylarsonic acids as growth promoters (Ledet and Buck 1978; Adams et al. 1994). It is also possible that the arsenic originates from fish meal present in the feed. Animal feeds often contain substantial amounts of arsenic (more than 0.1 ppm) even without supplements (Nriagu and Azcue 1990). When the arsenicals used as food supplements are administered orally, a considerable percentage is eliminated in the feces. Limited biotransformation can occur; for example, the nitro group of 3-NHPAA (Figure 3-1) can be reduced to an amino group (Dean et al. 1994). Retention of those arsenicals might be partly responsible for increased total-arsenic concentrations found in some poultry, but Dean et al. (1994) found that if the supplement is stopped for 7 days before slaughter (the time required by regulations), no arsenical can be detected in the muscle of chicken. That study used enzymatic digestion with trypsin at pH 8 to release the arsenical, 4-NHPAA, Roxarsone, from the flesh and used HPLC-ICP-MS for the analysis. Sensitive HPLC-MS and µ-HPLCESMS-MS methods are now available for that and other analyses (Pergantis et al. 1997a,b,c).

One study of arsenic speciation in rice was described above (Schoof et al. 1998). Odanaka et al. (1985) grew rice hydroponically in the presence of DMA (arsenic at 500 µg as the sodium salt in 500 mL of nutrient solution). A monomethylated species accumulated in the plants, mainly in the roots, at a concentration of up to 777 ppm. Translocation to the shoots seemed to be restricted-arsenic at 18 ppm accumulated as the monomethyl species, but little DMA was found. The loss of a methyl group is uncommon. Part of the monomethylated product seems to be complexed in the plant, as reported for other plants (e.g., Sckerl and Frans 1969; Pyles and Woolson 1982). In addition, part of the product might be a reduced form of MMA, as described by Nissen and Benson (1982). The arsenicals were extracted with methanol and water and identified with HG-AA.

Pyles and Woolson (1982) used methanol and water to extract arsenic from garden vegetables grown in arsenate-amended soil (arsenic at 100 ppm). The average arsenic recovery was only 28%. The most recalcitrant plants were

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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beets, tomato, swiss chard, and potato flesh. Total arsenic in the edible plants ranged from trace amounts in cabbage and corn to 3.0 ppm in potato peel. MMA and As(V) were found in the extracts of broccoli, lettuce, potato flesh, potato peel, and swiss chard. Some extracts contained hidden arsenic that required digestion with hot 2 M NaOH before detection with HG-AA was possible. Most of the arsenic in the methanol-water phase appeared to be a complex organic arsenical. Nissen and Benson (1982) found that terrestrial plants, such as Lycopersicon esculentum, metabolize arsenic differently from freshwater plants, such as Lemna minima. The higher terrestrial plants take up and reduce arsenate, producing methylated arsenicals only when grown in nutrient-deficient conditions. Freshwater plants produce not only methylarsenic species but also more elaborate compounds that might be arsenosugars.

The fourth major food group with high arsenic content comprises fats and oils (see Total-Arsenic Concentrations above). Margarine and peanut butter have high concentrations of arsenic, but unfortunately, nothing is known about speciation in those types of foods.

The Nature of Arsenic in Plants

In many instances, recovering arsenic species from plants of terrestrial origin has proved to be difficult, explaining why most studies have been concerned with measuring total arsenic. That difficulty has given rise to speculation that arsenic in terrestrial plants is bound up in an organic form that is not bioavailable to humans as is the AsB found in seafood. However, it is worth considering some relevant chemistry. First, if the arsenic is truly an organometallic compound and contains As-C bonds, the organic moieties are most likely to be the same as or similar to those already known and listed in Table 3-1. Apart from arsenolipids, all those compounds are water soluble and should be easily extracted if they are not bound in some way to bigger organic moieties. Such bonding would likely be via oxygen or sulfur. As discussed previously, the As-OR links are not hydrolytically stable. Although the As-S bond is more stable than the As-OR bond, it is not particularly robust, especially in the As(V) state (Zingaro and Thomson 1973; Sagan et al. 1972). AsSe bonds are more robust. The arsenic-containing moieties in such compounds should be available to mild extractants unless there is some physical barrier, such as a cell wall, to the extractant. On the same basis, inorganic arsenic species should be extractable unless there is a barrier. Alternatively, the difficulty in the extraction could arise because the arsenic is bound up in a lipid-soluble species, possibly membrane located. The species could be related to the well-characterized arsenolipid shown in Table 3-1 where the tail of the

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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arsenolipid is made up of palmitolate groups. Choline-based lipids are well known, so it is certainly possible that lipids could be based on AsC as the head group. Some investigators have postulated that lipid-like arsenic species are present in plants (e.g., Nissen and Benson 1982). Studies involving arsenical uptake by plant cell cultures of the Madagascan periwinkle found that after uptake methylarsenicals became invisible to NMR spectroscopy; that finding suggests that they become bound to large molecules or cell walls (Cullen and Hettipathirana 1993; Cullen et al. 1989a). The same studies found that arsenate is methylated by the cells but that repeated extraction was necessary to recover the species. The suggestion was made that the plant stores the unwanted arsenicals in the vacuole.

The use of enzymes for digestion of difficult samples should be explored (e.g., Dean et al. 1994). Microwave irradiation in the presence of tetramethylammonium hydroxide has proved effective for the extraction of related organotin compounds (Schmitt et al. 1997).

Arsenic In Urine, Blood, Hair, And Nails

Arsenic in Urine

As described in Chapters 5 and 6, some arsenic species are more toxic than others, so it is important to obtain information about the speciation of the arsenic absorbed (e.g., from food, water, and particulates), eliminated (e.g., by urine, feces, nails, and hair), and stored (e.g., in liver and kidney). Some information is available on the speciation of arsenic that is ingested (as described in the section Arsenic in Food) and eliminated (as described below), but little is known about what forms of arsenic might be stored in the body (e.g., Vahter et al. 1984; see Chapter 5).

Total Arsenic in Urine

Some foods, especially those of marine origin, have high concentrations of arsenic, and consumption results in a surge in the concentration of arsenic in the urine. That might not be of toxicological significance if the arsenic species (e.g., AsB) is nontoxic and is excreted rapidly unchanged in the urine (Vahter 1994; Le et al. 1994b). However, arsenosugars, which are present in algal products and many shellfish, are metabolized by humans to DMA and other arsenicals, presumably sugars (Le et al. 1994b). Thus, high urinary arsenic concentrations in an individual can indicate exposure to high concentrations of

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Page 57

toxic inorganic arsenic species or recent consumption of certain foods. Depending on the nature and amount of arsenic species found in the food, recent consumption of certain foods might not represent a toxicological concern.

The concentration of total arsenic in urine is generally accepted as a good indicator of the absorbed amount of arsenicals, because excretion via urine is the primary route for elimination of most arsenic species from most animals (Buchet and Lauwerys 1983; Vahter 1994). However, ingestion of arsenic compounds might not be detected (see Chapter 5). Arsenic was not detected in the urine of a number of individuals following ingestion of As(III) sulfide in a herbal preparation for asthma (Tay and Seah 1975), possibly because of the low rate of absorption. Arsenic concentrations in urine samples from a group of volunteers usually increased following the ingestion of Nori, an algal food product containing arsenosugars (Le et al. 1994b). However, the amount of the increase and the rate of the elimination varied greatly for each subject. In the most extreme case, a 20-year-old female showed no increase in urine arsenic content (Le et al. 1994b). In other studies, mice fed partially purified arsenosugars eliminated most of the arsenic via the feces (86% of the dose eliminated in 48 hr) (Shiomi et al. 1990).

Repeated exposure of human volunteers to As(V) for 10 days (66 µg per day) established a constant speciation pattern, but only 48% of the dose was recovered over 18 days (Johnson and Farmer 1991). Generally, 40-60% of the daily intake of inorganic arsenic is excreted each day in urine (Buchet et al. 1981; Farmer and Johnson 1990).

Arsenic Species in Urine

In a recent study by Buchet et al. (1996), two methods were used to determine arsenic speciation. The first method was the same as that used by Foà et al. (1984) in a study in which 148 Italian male subjects who were not occupationally exposed to arsenic and reportedly did not eat seafood during the study period (Foà et al. 1984). Total arsenic was determined by HG-AA following dry ashing (MgNO3 and MgO at 600°C). Arsenic species in the urine were isolated on a strong anion-exchange column. Elution with 0.5 M HC1, H2O, NH4OH (5 %), and NH4OH (20%) yielded in each fraction, respectively, As(V) and As(III), MMA, other forms (OF) of arsenic, and DMA. The OF fraction is said to represent dietary arsenic, such as AsB, that is not metabolized. That fraction can be analyzed following dry ashing but is usually ignored or determined by difference. The known species in the isolated fractions were determined by HG-AA. The QA-QC procedures were described and seem adequate. The results for this reference population are shown in Table 3-10. More than 60% of the urinary arsenic is present in the OF frac-

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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tion. Only the concentration of DMA in urine correlated with blood arsenic concentrations (Foà et al. 1984). The second method (Buchet et al. 1996) used hydride generation to liberate the arsenic species, which were then separated on the basis of volatility. Total arsenic was determined following dry ashing, and OF arsenic was determined by difference. There was good agreement between the two methods, which were used in different laboratories, although detection limits were not reported. The results for subjects who refrained from eating seafood are given in Table 3-11. The results for smokers and nonsmokers do not differ. The OF fraction is again significant and is reflected in the data from 15 subjects who reportedly had not eaten seafood for 5 days before the 24-hr sampling: As(V) and As(III), mean value 0.6 µg per 24 hr (standard deviation (SD) 0.3); MMA, 0.5 µg (SD 0.2); DMA, 3.8 µg (SD 1.8); and total arsenic, 20 µg (SD 16). Here, OF arsenic represents 76% of the total arsenic excreted. Buchet et al. (1996) found increases in DMA concentrations following ingestion of mussels. That is not surprising in view of the high arsenosugar content in mussels (Le et al. 1994a,b).

TABLE 3-10 Total Arsenic in Blood and Arsenic Species in Urine in the Foà et al. (1984) Studya

   

Arsenic Species in Urine (µg/L)


Measure

Total Arsenic in Blood (µg/L)


As(V) and As(III)


MMA


DMA


OF


As(T)b

Mean

5.1

1.9

1.9

2.1

11.3

17.2

SD

6.9

1.2

1.4

1.5

10.1

11.1

Range

0.5-32

0.5-10

0.5-9

0.5-10

0-43

0.5-48

aSeafood was not consumed during the study.

bSum of species, not determined.

Abbreviations: DMA, dimethylarsinic acid; MMA, monomethylarsonic acid; OF, other forms of arsenic determined by difference in this case; As(T), arsenic total; SD, standard deviation.

The first major U.S. study of arsenic species in urine was reported by Kalman et al. (1990), and some results are given in Table 3-12. In all, about 3,000 samples were collected from residents of a community surrounding an arsenic-emitting copper smelter. The work was carefully done, and HG-AA was used for speciation with a quantitation limit of 0.7 ppb. Unfortunately, total arsenic was not determined. The authors estimated that with no seafood consumption, the sum of the species should equal the total-arsenic output and be about 10 µg/L. The results are in agreement with that value.

Cation-exchange chromatography followed by HG-AA was used in a urinary study of patients with blackfoot disease (Lin and Huang 1995). A standard urine sample was used for QA-QC (total arsenic only; no certified

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
×

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TABLE 3-11 Arsenic Species in Urine in the Buchet et al. (1996) Studya

 

Arsenic Species in Urine (mg/g of Creatinine)

Measure

As(V) and As(III)

MMA

DMA

OF

As(T)b

Mean

1.8

0.6

2.8

6.14

10.8

 

(2.1)

(0.5)

(5.1)

 

(27.7)

SD

0.7

0.4

1.5

 

6.2

 

(1.4)

(0.2)

(3.1)

 

(56.2)

Range

0.7-3.0

0.2-1.3

1.0-5.3

 

2.4-21.9

 

(0.6-5.8)

(0.2-0.9)

(1.6-12.4)

(4.5-196.5)

 

aSeafood was not consumed during the study; 10 did not smoke and 11 smoked

tobacco products. Values in parentheses are from smokers.

bSum of species, not determined.

Abbreviations: DMA, dimethylarsinic acid; MMA, monomethylarsonic acid; OF,

other forms of arsenic determined by difference in this case; As(T), arsenic total; SD,

standard deviation.

TABLE 3-12 Control-Group Data on Arsenic Species in Urine in the Kalman et al. (1990) Study

 

Arsenic Species in Urine (µg/L)

Measure(Sample No.)

As(V) and As(III)

MMA

DMA

As(T)

Mean (696)b

1.3

1.6

6.4

9.2

SD

1.1

1.3

5.8

7.5

Mean (140)c

1.5

1.7

11.3

14.5

SD

1.2

1.4

9.4

11.2

aSum of species, not determined.

bSamples with reported seafood consumption excluded.

cSamples with reported seafood consumption only.

Abbreviations: DMA, dimethylarsinic acid; MMA, monomethylarsonic acid; OF,

other forms of arsenic determined by difference in this case; As(T), arsenic total (sum

of species and is not determined); SD, standard deviation.

standards are available for speciation). The results for 30 patients and 30 controls are listed in Table 3-13. The authors stated that the OF data indicated that the high total-arsenic values were probably the result of diet.

Table 3-14 presents a comparison of those data (Lin and Huang 1995) with the data from studies in Japan (Yamauchi et al. 1989) and Italy (Foà et al. 1984); also included are data from studies in Argentina (Vahter et al. 1995). The similarity between the results of the Taiwanese and European studies is strong. The high values in the Japanese study are almost certainly associated with seafood consumption, and arsenosugars are a likely source of the DMA; however, the inorganic concentrations are noteworthy, and demethylation during storage or analyses could be a cause.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
×

Page 60

TABLE 3-13 Arsenic Species in Urine in the Lin and Huang (1995) Study

 

Arsenic Species in Urine (µg/g of Creatinine)

 

Measure(Subject No.)

As(V) and As(III)

MMA

DMA

OF

AsT

Patients with BFD (30)

         

Mean

6.1

6.2

7.2

36.1

55.6

SD

2.8

2.3

4.5

10.5

13.1

Controls (30)

         

Mean

3.4

3.9

6.4

26.3

40.1

SD

2.3

1.9

4.1

6.7

8.2

Abbreviations: DMA, dimethylarsinic acid; MMA, monomethylarsonic acid; OF,

other forms of arsenic determined by difference in this case; AsT, arsenic total; BFD,

blackfoot disease; SD, standard deviation.

TABLE 3-14 Comparison of Arsenic Species in Urine in Four Studies

   

Arsenic Species in Urine (Mean µg/L ± SD)

Study

 

As(V) and

       

(Subject No.)

Location

As(III)

MMA

DMA

OF

AsT

Lin and Huang

Taiwan

1.7 ± 1.1

2.0 ± 1.0

3.3 ± 2.5

13.7

20.7 ± 7.0

1995 (30)

           

Foà et al. 1984

Europe

1.9 ± 1.2

1.9 ± 1.4

2.1 ± 1.5

11.3

17.2 ± 11.2

(148)

           

Yamauchietal.

Japan

11.4±5.9

3.6 ± 2.8

35.0± 20.8

71.0

121 ±101

1989 (102)

           

Vahteretal.

Argentina

66±41

7.1 ± 12

185 ± 110

13

274 ±98

1995 (11)

           

Abbreviations: DMA, dimethylarsinic acid; MMA, monomethylarsonic acid; OF, other forms of arsenic determined by difference in this case; AsT, arsenic total; SD, standard deviation.

The urinary arsenic concentrations are much higher in native Andean women exposed to high concentrations of arsenic in drinking water (Vahter et al. 1995, Table 3-14). Ion exchange chromatography in combination with HG-AA was used for speciation and for determination of total-arsenic concentration following dry ashing. A standard reference water (National Institute of Standards and Technology, USA, No. 1643a) was used for QA-QC, and the speciation method was validated in an interlaboratory comparison exercise (Crecelius and Yager 1997). A similar method was used in a study that involved Mexican individuals exposed to high arsenic concentrations in drinking water (408 µg/L) and controls (31 µg/L) (Del Razo et al. 1997) (Table 3-15). In both studies, the sum of the As(III) and As(V), MMA, and DMA concentrations is close to the total-arsenic concentrations, presumably because of diets without seafood.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
×

Page 61

Chatterjee et al. (1995) also used ion exchange, both cation and anion, and flow injection-HG-AA in a study of the urine of individuals living in the arsenic-rich region of West Bengal (up to 3,700 µg/L in well water and about 50% As(III)). In addition to MMA and DMA, inorganic As(V) and As(III) were found in the urine samples. The sum of those species is close to the measured total-arsenic values.

TABLE 3-15 Arsenic in Urine of Subjects in the Del Razo et al. (1997) Study

   

Arsenic Species in Urine (µg/g of Creatinine)

 

Exposure Group

 

As(V) and

     

(Subject No.)

Measure

As(III)

MMA

DMA

AsT

High (35)

Mean

171.8

63.5

303.6

561.3

 

Confidence

       
 

intervals

135.8-217.3

46.8-86.2

239.4-385.0

449.8-700.5

Low (35)

Mean

1.8

1.4

16.1

20.6

 

Confidence

       
 

intervals

1.3-2.5

1.1-1.7

11.5-22.6

15.1-28.0

Abbreviations: AsT, arsenic total; MMA, monomethylarsonic acid; DMA, dimethylarsinic acid.

The maximal total-arsenic concentration of 956 µg/g of creatinine was measured (hydride generation) in a group of occupationally exposed British workers (Farmer and Johnson 1990).  For the most-exposed group, the average speciation pattern was 1-6% As(V), 11-14% As(III), 14-18% MMA, and 63-70% DMA. Significant concentrations of hidden arsenic were noted in some urine samples and were attributed to seafood consumption.

The development of modern techniques based on HPLC separation has enabled the identification of more arsenic species in urine, although no extensive studies are available to date. Some results for the urine standard-reference-material (SRM 2670, high concentration) species are as follows: AsB, 15 ± 3 µg/L; DMA, 49 ± 3 µg/L; MMA, 7 ± 2 µg/L; As(V), 443 ± 20 µg/L; total, 514 ± 23 µg/L. The certified value for total arsenic is 480 ± 100 µg/L (Le and Ma 1998).

An analytical method based on microwave decomposition and flow-injection analysis coupled to HG-AA has been used to differentiate between total arsenic in urine and hydride-producing and nonhydride-producing species (Le et al. 1993).

The nonhydride-producing species (OF in Tables 3-10, 3-11, 3-13 and 314) are generally assumed to be organic arsenicals of dietary origin (e.g., AsB from fish or mushrooms) rather than metabolites of inorganic arsenic. A recent intercomparison of analytical methods for arsenic speciation in

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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urine (Crecelius and Yager 1997) found that both accuracy and precision are relatively poor for arsenic concentrations of less than 5 µg/L. Hydride generation was used by five of the seven participating laboratories. It should be noted that this limit is just above the practical quantification level of 4 µg/L for inorganic arsenic in drinking water, reflecting the increased difficulty of the analyses (see previous section Practical Quantification Level for Arsenic in Drinking Water).

Collection and Storage of Urine Samples for Arsenic Speciation Analysis

Important considerations for collection and storage of urine samples are the prevention of contamination and the minimization of loss of trace amounts of analytes.  An additional prerequisite for obtaining reliable speciation information is to maintain the stability of chemical species in the sample.

Polyethylene containers are normally preferred to glass containers because the former is less adsorptive to arsenic (Schaller et al. 1991). To maintain the stability of arsenic species in the sample, refrigeration and frozen conditions have been repeatedly shown to be most effective for urine-sample storage (Schaller et al. 1991; Palacios et al. 1997; Le et al. 1998). Among the most common urinary arsenic species, DMA, MMA, and AsB are generally more stable than As(III) and As(V), which can interconvert (Larsen et al. 1993b; Palacios et al. 1997; Le et al. 1998).

Palacios et al. (1997) found that As(V), MMA, DMA, and AsB (each at 200 µg/L) in urine were stable for the entire test period of 67 days at 4°C without using any additives. AsC was oxidized to AsB. In dried urine residue, all of the five arsenic species tested were stable during the same testing period at 4 °C and at ambient temperature. This study did not test the stability of As(III) because other work (Larsen et al. 1993b) had shown a rapid oxidation of As(III) to As(V).

Le et al. (1998) compared the effects of the following storage conditions on the stability of As(III), As(V), MMA, DMA, and AsB (each at 50 µg/L): temperature (25°C, 4°C, and -20°C), storage time (1, 2, 4, and 8 months), and the use of additives (hydrochloric acid, sodium  azide, benzoic acid, benzyltrimethylammonium  chloride, and cetylpyridinium  chloride).  In replicate studies that used urine samples from male and female volunteers and from commercial sources, the authors found that all five arsenic species were stable for up to 2 months when urine samples were stored at 4°C and -20°C without additives. For 4 and 8 months of storage, the stability of arsenic species was dependent on urine matrices. Although the five arsenic species

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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in some urine samples were stable for the entire 8 months at 4°C and -20°C, in other samples stored under identical conditions, there were significant changes in speciation. The use of additives did not improve the stability of arsenic species in urine. The addition of 0.1 M HC1 to urine samples resulted in alterations of inorganic arsenite and arsenate concentrations and in some demethylation. Thus, an acidification procedure is not appropriate for speciation analysis, even though dilute acetic, hydrochloric, and nitric acids have traditionally been added to samples to minimize possible adsorption of trace elements onto sample containers.

Arsenic Species in Blood

Blood is a more difficult matrix than urine for chemical analysis, so until very recently, only total-arsenic concentrations in whole blood were reported. The sample was first decomposed by using wet or dry ashing, prior to analysis by conventional methods. However, a neutron-activation method, which involved a radiochemical separation and re-irradiation, was used in a study of the blood, plasma, cells, and whole blood of blackfoot-disease patients and others (Heydorn 1970) (see Table 3-16).

Dry ashing followed by HG-AA was used in a study by Vahter et al. (1995). They reported a mean arsenic concentration of 8.0 µg/L (a range of 2.7 to 18.3) in the blood of exposed Andean women and 1.5 µg/L (a range of 1.1 to 2.4) in controls. The mean value for blood arsenic in a reference population of 148 subjects was 5.1 µg/L (a range of 0.5 to 32) (Foà et al. 1984). Blood arsenic in 85 patients undergoing hemodialysis treatment had a mean value of 8.5 ± 1.8 µg/L versus that in 25 controls who had a mean value of 10.6 ± 1.3 µg/L (Mayer et al. 1993).

TABLE 3-16 Arsenic in Blood of Families with Blackfoot-Disease Patients and Control Subjects in the Heydorn (1970) Study

 

Arsenic in Blood (µg/L)

 

BFD Families

Danish Controls

Taiwanese Controls

 

Tissue

Measure

(Subject No.)

(Subject No.)

(Subject No.)

Plasma

Mean

38.1 (47)

2.4 (16)

15.4 (17)

 

SD

1.90

1.9

1.39

Red blood cells

Mean

93 (11)

2.7 (7)

32.7 (5)

 

SD

1.84

1.3

1.84

Whole blood

Mean

60 (11)

2.5

21.6(5)

 

SD

1.70

-

1.67

Abbreviations: BFD, blackfoot disease; SD, standard deviation.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Shibata et al. (1994) used HPLC-ICP-MS to detect, for the first time, AsB in human blood. The limit of detection was 0.3 µg/L, and the results for a 29-year-old male were as follows: plasma, 3.3 µg/L; serum, 4.6 µg/L; red blood cells, 10.1 µg/L. Only one additional high-molecular-weight compound was detected, but not identified, by gel-permeation chromatography. The HPLC method was developed in Japan and is described by Shibata et al. (1992). A less expensive but still sensitive method has been used to confirm the presence of AsB in human serum (Zhang et al. 1996a,b). Trace amounts of DMA are also found. The method uses ion-exchange HPLC to separate arsenicals. The next step is photo-oxidation (ultraviolet with persulfate) to arsenate, followed by HG-AA. (The method is similar to that described above for urine speciation (Le et al. 1996).) The detection limits in blood were 1.0, 1.3, 1.5, and 1.4 µg/L for MMA, DMA, AsB, and AsC, respectively.

The rat accumulates arsenic in red blood cells as DMA (Lerman and Clarkson 1983). The speciation analysis involved the use of electrophoresis following dosing with radiolabeled arsenate and thin-layer chromatography (Odanaka et al. 1980).

Arsenic in Hair and Fingernails

Arsenic is believed to accumulate in hair and fingernails more than other tissues because of the high content of keratin (and the corresponding high content of cysteine) in those tissues. Consequently, arsenic is assumed to be bound as As(III). There are reports (Yamauchi and Yamamura 1984; Yamato 1988) that DMA(V) is present in both hair and nails, but the oxidation state is assumed; the species could well be DMA(III) (see Affinity of Arsenic for Sulfur, above).

Arsenic in hair and nails might be increased as a result of surface contamination. Harrison and Clemena (1972) and Agahian et al. (1990) developed elaborate schemes for the removal of arsenic on hair surfaces. The procedure was found to remove 98% of exogenous arsenic. However, it is not clear whether such procedures can remove arsenic that is bound to the surface of hair and nails as a result of contact with arsenic in the water. A method of determining externally bound arsenic on hair and nails on the basis of the generation of arsine (Gutzeit method) from undigested samples was proposed by Pirl et al. (1983). The authors showed that soaking hair in arsenate or arsenite (0.2 % solution) results in the surface uptake of arsenic that cannot be removed by washing with water or 4 M HCI.

Hindmarsh (1998) claims that the source of arsenic on the outer surface of hair is both ingestion and external contamination, and that the two sources

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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cannot be differentiated. External contamination can produce hair arsenic concentrations of several thousand parts per million. This work was done by using a nondestructive method, the proton-induced X-ray emission. According to this abstract, hair arsenic concentrations of up to 100 ppm are found in live patients with clinical arsenic poisoning even though concentrations of about 45 ppm can be associated with death. Hindmarsh (1998) recommends that hair samples should consist of at least 1 g of hair cut close to the scalp and derived from several sites on the head, and the whole sample should be analyzed.

Smith (1964) determined that the arsenic concentration in human hair was 0.62 ppm in a group of males and 0.37 ppm in a group of females. Direct neutron-activation analysis (Pan et al. 1993) of carefully washed samples of hair from 28 healthy subjects from Taiwan revealed an arsenic content of 0.27 ± 0.33 ppm. It is interesting that the antimony content of the hair of healthy Taiwanese and of patients with BFD was the same, about 0.08 ppm, and that for some individuals, the concentration of antimony was greater than that of arsenic. The crustal abundance of antimony is appreciably less than that of arsenic (Latimer and Hildebrand 1951).

In Hungary, 2,095 hair samples were washed and analyzed by wet digestion and flame atomic absorption (FAA) (Bozsai et al. 1989). The FAA method is not sensitive for arsenic, although the authors claimed a detection limit of 0.10 ppm for a 1-g sample of arsenic. Hair concentrations of 3 ppm were found in some individuals. In the United States, Kalman et al. (1990) found a mean hair arsenic concentration of 6.8 µg/g, median 0.5 µg/g (SD 27.7), in 32 individuals in the study's Tacoma control subgroup. Higher values (mean 15.2 µg/g, median 3.7 µg/g, SD 35.0) were found in 40 individuals living in Ruston, close to an arsenic-emitting copper smelter.

In a study in Japan, Yamato (1988) digested unwashed hair with 2 M NaOH at 95°C for 3 hr. That procedure was designed to extract the arsenicals with minimal change in speciation. HG-AA revealed the presence of inorganic arsenic and DMA. (That is likely to be DMA(III) as pointed out above.) The mean arsenic concentration was 0.08 ppm, ranging from 0.04 to 0.33 ppm in 100 samples. Inorganic arsenic accounted for 73% of the total, and DMA accounted for 27%. DMA was also found in the hair of hamsters (Yamauchi and Yamamura 1984).

Liebscher and Smith (1968) used neutron-activation analysis on the fingernails from a normal population of 124 subjects and established that total-arsenic concentrations are in the range of 0.02 to 2.90 ppm. In a more recent study (Agahian et al. 1990) in which acid digestion and HG-AA were used, the limit of detection was 1.5 µg/g. The results were as follows: no exposure, more than 1.5 ppm; low exposure, 2.0-3.0 ppm; medium exposure, 2.58.6 ppm; high exposure, 10.6-16.0 ppm.

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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Summary And Conclusions

A large number of arsenic-containing chemical compounds are known to be present in the terrestrial environment: they include the inorganic species arsenate, or As(V), and arsenite, or As(III), in addition to organic derivatives that usually contain As-methyl units. The brief account of the chemistry and biochemistry of the species emphasizes that their chemical and physical properties are greatly species dependent. Analytical methods used to identify arsenic species are described. Early investigations used a derivatization technique, hydride generation, that could be used to quantitate a limited number of water-soluble species. Recent developments permit the analysis of a much wider range of species, and examples of the use of HPLC-ICP-MS (separation with element-specific detection) and HPLC-ESMS-MS (separation with molecule-specific detection) are given.

The arsenic species present in groundwater and surface water are largely arsenate and arsenite, and the latter can amount to around 50% of the total arsenic present, as found in West Bengal and to a greater extent in Taiwan. In some surface water methylated arsenic species can achieve high concentrations presumably as a result of the biological conversion of the inorganic species. The practical quantitation limit (PQL) of the water industry for arsenic in drinking water apparently is 4 ppb. The reliable analysis of arsenic at lower concentrations will probably require skilled operators and more expensive instruments.

The highest concentrations of arsenic compounds in food (at more than 1 ppm) are found in products that originate from the marine environment. In such food as fish and shrimp, the major arsenical is arsenobetaine (AsB), an apparently nontoxic species that is not metabolized by humans and is rapidly excreted in the urine. A range of arsenic species that exist in shellfish include AsB, arsenosugar derivatives, and the tetramethylarsonium ion in varying proportions. Arsenosugars are the principal arsenic species present in most marine algal products, although up to 50% of the arsenic in certain algae is the more toxic arsenate. Arsenosugars are metabolized by humans mainly to DMA.

Fish and algal products from terrestrial sources contain low arsenic concentrations (less than 1 ppm), but little is known about the species content. AsB is present but is by no means ubiquitous. The available data indicate that, as in marine organisms, concentrations of inorganic arsenic in freshwater fish are probably low.

Mushrooms (total arsenic at more than 1 ppm) contain a wide range of arsenic species. Most of the arsenic in an edible mushroom species is found as DMA; AsB and AsC are present in a poisonous mushroom. The arsenic

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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content of other foods is less than 1 ppm, but little is known about speciation. The arsenic species are usually difficult to extract intact, possibly because they are difficult to access or are bound to cellular components. This ''food arsenic" is likely to be based largely on inorganic species. Crude separations into so-called "organic" and "inorganic" fractions have been based on methods that have been shown to be applicable to some food, such as fish, but the value of those separations when applied to other foods, such as meat and grain, is moot.

Many studies have been conducted on the arsenic species present in urine, because humans eliminate most ingested arsenic (e.g., 40-80% of ingested arsenite or arsenate) by this route. Speciation studies have mainly concentrated on the determination of inorganic species, MMA, DMA, and total arsenic, largely because of available hydride-generation methods. There are problems with both accuracy and precision of these measurements at low concentrations of arsenic species (less than 5 µg/L). In some populations (e.g., those in Mexico and Argentina), MMA, DMA, and inorganic arsenic are essentially the only species present, and the mean total-arsenic concentrations can be as much 561.3 µg/g of creatinine in an exposed population and 20.6 µg/g in controls. In other populations, high concentrations of additional species are found, for example, in Taiwan, where the mean total-arsenic concentration in the urine of one exposed group, 55.6 µg/g includes unidentified arsenic species at 36.1 µg/g. The additional species are believed to be largely AsB, which is not metabolized and which reflects consumption of seafood; however, unidentified species can be found even in the urine of individuals who claim not to have eaten seafood before the study. Individuals who eat algal products and shellfish seem to have increased concentrations of DMA, probably because the arsenosugars present in the food are metabolized to DMA. In general, total urinary arsenic concentrations and the concentrations of the arsenic species increase with exposure; however, in a given population, a wide range is seen in the concentration of each species and in the total-arsenic concentrations. More modern methods are now being applied to determine all arsenic species in urine, but at present, application is only at the research level.

The range of total-arsenic concentrations in human blood is smaller (up to 100 ppm) than that found in urine. Blood is a more-difficult matrix for speciation studies; however, DMA, AsB, and inorganic arsenic species have been identified in serum.

Arsenic concentrations in hair and nails also reflect exposure, but there are difficulties in ensuring that the arsenic is not adsorbed on the surface as a result of external exposure. According to a recent abstract (Hindmarsh 1998), the upper range of arsenic concentrations in hair and nails after excessive

Suggested Citation:"3 Chemistry and Analysis of Arsenic Species in Water and Biological Materials." National Research Council. 1999. Arsenic in Drinking Water. Washington, DC: The National Academies Press. doi: 10.17226/6444.
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arsenic ingestion may be higher than previously reported (e.g., 100 versus 10 ppm).

Recommendations

Analytical techniques are needed to determine the arsenic species in various media (e.g. urine), including biological tissues. Quality-control data and certified standards for arsenic speciation are also needed to help legitimize interlaboratory and intralaboratory studies.

More comprehensive studies should be undertaken to determine the arsenic content in food, especially after food processing. These studies should focus on growth conditions, speciation, availability to humans, residence time in humans, and mass balances.

Other studies of less critical importance for assessing the risk of arsenic from drinking-water exposures but nonetheless important to fill critical data gaps include the following:

— Fundamental studies on the transport of arsenic species through membranes and cell walls.

— Studies investigating low-cost and easy-to-use methods for routine measurement of all arsenic species at concentrations below the current PLQ of 4 ppb in water.

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The U.S. Environmental Protection Agency (EPA) has been considering a more stringent regulation of arsenic in water. A significant reduction in the maximum contaminant level (MCL) could increase compliance costs for water utilities. This book discusses the adequacy of the current EPA MCL for protecting human health in the context of stated EPA policy and provides an unbiased scientific basis for deriving the arsenic standard for drinking water and surface water.

Arsenic in Drinking Water evaluates epidemiological data on the carcinogenic and noncarcinogenic health effects of arsenic exposure of Taiwanese populations and compares those effects with the effects of arsenic exposure demonstrated in other countries—including the United States.

The book also reviews data on toxicokinetics, metabolism, and mechanism and mode of action of arsenic to ascertain how these data could assist in assessing human health risks from arsenic exposures. This volume recommends specific changes to improve the toxicity analyses and risk characterization. The implications of the changes for EPA's current MCL for arsenic are also described.

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