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OCR for page 255
Appendix C:
Determining Traces
of Arsenic in
Natural Materials
This discussion is intended primarily for the consumer of analytic
information, i.e., for the physician, biologist, or ecologist who collects
and selects samples and wishes to obtain the most useful information
from them. The principal paths by which arsenic can be accidentally
added to or lost from the system are mentioned, and the advantages
and disadvantages of the more commonly used analytic techniques are
pointed out, so that the investigator can choose among the available
services and critically evaluate the results. The general approach is that
followed in the recent review by Talmi and Feldman,778 although new
material has been added and some of the less accessible techniques
omitted.
COLLECTION, SUBDIVISION, AND STORAGE OF
SAMPLES
The sample collected should be large enough to represent the material
studied. Because a single mean value is desired for the concentration
of each arsenical species of interest, the sample must be homogenized
and a subsample of suitable size for analysis must be taken. To
minimize contamination, unused sample material should be stored in
255
OCR for page 256
2S6
ARSENIC
closed containers or (depending on sample composition) at a low
temperature.
Choices must sometimes be made regarding what to include in the
sample taken for analysis. Vegetation may be found to be contaminated
with dust; a decision must be made whether to remove the dust or
include it in the sample. Natural water often contains suspended
matter, which must be either filtered out or allowed to remain. If the
particles filtered from an air stream contain volatile forms of arsenic,
consideration must be given to the losses that may occur at the
temperatures and air velocities to which the particles are exposed on
the filter and to the duration of exposure (arsenic trioxide has a vapor
pressure of 0.68 mm Hg at 200 C).453 6s5
Many authors (e.g., Portman and Riley,652 Whitnack and Brophy,855
Al-Sibbai and Foggy, and C. Feldman, personal communication) have
found that acidic, neutral, or basic solutions of inorganic arsenites and
arsenates can be stored without substantial changes in concentration
for several weeks. However, some arsenic compounds present in
natural water are said to disappear rapidly from solution after collec-
tion of the sample (R. S. Braman, personal communication). The
investigator must always be aware of the possibility of losing some of
the species of interest through adsorption on vessel walls or on
suspended matter or through volatilization.
Large liquid samples can be properly divided into aliquots only if
homogeneous, i.e., if the species of interest does not adhere to the
vessel walls and if suspended matter is uniformly distributed before
division. Large solid samples of a mineral nature may, of course, be
subdivided by conventional crushing or impact treatment followed by
mixing and quartering or riffling. Large samples of biologic tissues can
be homogenized in a blender (with the addition of water, if necessary).
If the density of the resulting slurry can be stabilized long enough, the
sample can be subdivided in this manner. Alternatively, the slurry can
be centrifuged and proportionate amounts of residue and supernatant
liquid taken for analysis. Another possibility is lyophilization of the
slurry; the cake obtained is easily pulverized, and the resulting powder
is homogeneous.247 If volatile species are to be determined, the
lyophilization technique may not be appropriate. The amount of han-
dling must always be minimized, in order to minimize contamination.
PRETREATMENT AND DISSOLUTION OF SAMPLES
If organic arsenic compounds are to be determined, the species in
question must be isolated. 779 If total arsenic is to be determined, the
OCR for page 257
Appendix C: Traces of Arsenic in Natural Materials
257
arsenic must be brought into solution and, if necessary, converted to
inorganic form. Regardless of the dissolution procedure used, care
must be taken to ensure that no arsenic is lost by the volatilization of
trivalent arsenic halides. Loss can usually be prevented by boiling the
sample with concentrated nitric acid under reflux early in the proce-
dure.778
The following sample-preparation procedures are typical of those
used in environmental work.
Coal is heated to fumes with concentrated sulfuric acid and treated
with successive small portions of concentrated nitric acid until degra-
dation essentially ceases. Destruction of the remaining nitrogenous
compounds is completed by small additions of fuming concentrated
perchloric acid. The latter step is essential if the arsine generation-arc
emission procedure is to be used for the final determination step.99
The arsenic in fly ash is usually assumed to exist as a surface coating.
All this arsenic can be dissolved with fuming sulfuric acid, as is shown
by comparison with analyses of the same material by neutron-
activation analysis.245 Refluxing such material in boiling water for 1 h
recovers only 13% of the arsenic present (C. Feldman, personal
communication). If the arsenic was deposited from the vapor phase, it
may have been thinly covered by other substances deposited later.
Coal slag is a highly refractory glass and usually contains only small
amounts of arsenic. The arsenic that it does contain cannot be leached
out with ordinary acids. Treatment with hydrofluoric acid in the usual
way would be of dubious value-on the one hand, this reagent may
contain substantial amounts of impurities; on the other, arsenic tri-
fluoride and especially arsenic pentafluoride are rather volatile, so both
contamination and losses might occur. Attack of the slag by fusion is
open to similar objections.
Quartz can be attacked without metallic contamination by vapor-
phase treatment with hydrofluoric acid and nitric acid in a closed
system.89~ This approach was therefore tried with slag, albeit with
some misgivings regarding the volatility of arsenic fluorides. No losses
or contamination seem to have occurred, however, inasmuch as the
results obtained on fly ash agreed well with those obtained with
neutron activation and sulfuric acid leaching.245
Procedures for digesting plant or animal tissues for determining total
arsenic must completely convert the arsenic to inorganic form (prefer-
ably arsenate) and must eliminate any substances that would interfere
with the particular procedure to be used in later determination. Only
the more widely used digestion methods will be mentioned here; others
have been reviewed elsewhere.778 Small samples can be charred with
concentrated sulfuric acid and then subjected to repeated small addi
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258
ARSENIC
lions of concentrated nitric acidly or 30~o7°° or 50~o hydrogen
peroxide. In the latter case, trivalent arsenic will be lost if chloride is
present. Ordinary and fatty tissue weighing up to 5 g can be safely
wet-ached in a volumetric flask by refluxing under a short air con-
denser with appropriate mixtures of sulfuric, nitric, and perchloric
acid, with potassium bichromate as a catalyst.246
PRECONCENTRATION OF ARSENIC SPECIES
To increase the sensitivity and accuracy of analysis, the arsenic-
bearing species is often isolated from its matrix and concentrated. The
principal preconcentration procedures used are coprecipitation,
liquid-liquid extraction, and volatilization.
Coprecipitation with ferric hydroxide, Fe(OH)3, has long been
known to collect pentavalent arsenic quantitatively from solution at
concentrations as low as 2 ng/ml.643 652 The hydroxides of cerium and
zirconium appear to be as effective as ferric hydroxide in this regard.649
Thionalide can collect arsenic efficiently from comparatively large
amounts of seawater, 652 but this reagent apparently does not function
well at low salt concentrations.779
Trivalent arsenic can readily be extracted from 6 N hydrochloric
acid with mixtures of ketone and carbon tetrachloride.276 At lower
acidities (pH, 2-6), it can be precipitated with ammonium pyrrolidine
dithiocarbamate, and the precipitate can be extracted.568 If the arsenic
is originally present in the pentavalent state, this fact can be turned to
advantage: While the arsenic is still pentavalent, other potentially
interfering metals that are extracted under the same conditions can be
extracted and discarded; the arsenic can then be reduced and extracted
without the metals that would otherwise have accompanied it.
Arsenic can also be separated from its matrix by volatilization, as
arsine (boiling point, -55 C) or a substituted arsine. The necessary
reduction can be effected by using zinc and acid in the presence of
stannous chloride or potassium iodide.2522825~ The reducing agent
most commonly used, however, is sodium borohydride, NaBH4. The
properties of this reagent can affect analytic results, especially at low
arsenic concentrations (<1 ppm), and will therefore be discussed
briefly. Sodium borohydride is supplied commercially in the form of
0.20-0.25-g pellets or powder; the grade usually used for analysis is the
same as that used in preparative organic chemistry. The quantity of this
reagent commonly used per determination (0.25 g) often contains
10-20 ng of arsenic;430 the amount varies from portion to portion.
OCR for page 259
Appendix C: Traces of Arsenic in Natural Materials
259
This degree of contamination is of little consequence if the sample
aliquot used in the determination contains several hundred nanograms
of arsenic. But if it does not (e.g., in natural water and small tissue
specimens), both the contamination and the variability of the blank are
sources of error. The kinetics of the reaction between sodium borohy-
dride and arsenic are an additional complicating factor: if a sodium
borohydride pellet is dropped into an arsenic-free acid solution, it
produces a considerably higher blank arsenic reading than if the same
pellet is converted to a 1% solution before being added to the acid
solution. Moreover, this blank response diminishes with the age of the
solution. The fading of the blank response due to arsenic in the sodium
borohydride appears to result from the gradual adsorption of dissolved
arsenic onto suspended impurities in the reagent solution. The ad-
sorbed arsenic is apparently held so tightly that acidification fails to
convert it to arsine. The simplest way to avoid errors from this source
is to use the analytic-grade reagent, which is more expensive, but
usually contains less than 0.5 ng of arsenic per portion (C. Feldman,
personal communication). The efficiency of sodium borohydride in
generating arsine can be impaired by the presence of other substances
that react with sodium borohydride. This effect can be serious.739
METHODS OF DETERMINATION OF TOTAL ARSENIC
Molecular- Ab sorpti on Sp ectroph otometry
Molecular-absorption spectrophotometry in aqueous solution has long
been one of the most reliable methods for determining small quantities
of arsenic. Because of its simplicity and low cost, it will probably
continue to be widely used for all but the lowest concentrations.
Arsenomolybdic acid is formed when arsenate reacts with acidified
molybdate. This heteropolyacid can be partially reduced to give a blue
color, which develops slowly (approximately 30 min), but is stable and
free from interferences.652 The other calorimetric method in common
use involves the bubbling of arsine through a 0.5% solution of the silver
salt of diethyldithiocarbamate in pyridine. An intense red color is
produced; absorption is measured at 533 nm.282 375
Atomic Absorption
Atomic absorption (nebulized sample solution plus argon-hydrogen or
air-acetylene slot burner) is claimed to give sensitivities of 50-100
OCR for page 260
260
ARSENIC
ng/ml.404 In the Blameless atomic-absorption method, a small volume of
sample (1-50 Ill) is deposited in a graphite tube or on a tantalum strip.
Strong heating vaporizes the arsenic and reduces it to As°, which is
then determined by atomic absorption. The absolute and concentra-
tional detection limits of this method are good (40 pg and 10 ng/ml,
respectively) but care is required in controlling sample vaporization
and in dealing with interferences.45 The arsenic can also be introduced
into a gas stream as arsine, with conversion to As° by a flame or a
heated tubei50 430 and detection by atomic absorption. Detection limits
can be reduced to 1.0 and 0.2 ng, respectively, for these two methods
by accumulating the arsine in a cold trap and releasing it quickly.
Atomic-Emission Spectroscopy
Arsenic can be determined by atomic-emission spectroscopy with
various types of excitation. For example, arsine can be accumulated in
a cold trap359 and then introduced into a direct-current glow discharge
in helium (Braman et al.98-~00 and C. Feldman, personal communica-
tion), giving absolute and concentrational detection limits of 0.5 ng and
25 pa/ml, respectively. Other volatile forms of arsenic (e.g.,
triphenylarsine), introduced into a microwave discharge in argon,779
can give an absolute detection limit of 0.02 ng of arsenic. An arsenic-
bearing aerosol, introduced into an induction-coupled radiofrequency
plasma, gives a concentrational detection limit of 40 ng of arsenic per
milliliter.243,429
Neutron-Activation Analysis
Neutron-activation analysis has the advantages of being nondestruc-
tive (in the many cases in which postirradiation radiochemical separa-
tions are not necessary) and of being immune from any danger of
contamination during postirradiation handling. Its absolute sensitivity
is 0.1 ng for a thermal-neutron flux of 10~2 neutrons/cm2-s. In tissue and
mineral samples, however, this sensitivity can seldom be reached. The
activity induced is the 559-keV photopeak of arsenic-76. A relatively
great amount of sodium-24 activity is induced in the sodium present in
such samples, and, although the decay of sodium-24 (half-life, 14.96 h)
is faster than that of arsenic-76 (half-life, 26.5 h), the sodium-24 activity
must be allowed to decay for several days before the arsenic-76 activity
can be counted. This delay does not seriously interfere with the
determination of arsenic at concentrations above a few parts per
million, and the elimination of all chemical treatment of the sample
OCR for page 261
Appendix C: Traces of Arsenic in Natural Materials
261
compensates for the inconvenience. If greater sensitivity is needed
or if radiochemical interferences appear (e.g., bromine or antimony
activities), chemical-group separations can still be performed to isolate
the arsenic-76 activity.350 604
Electrochemical Methods
In the electrochemical methods that have been proposed for determin-
ing traces of arsenic, the arsenic is usually first isolated by volatiliza-
tion or extraction, then converted to the trivalent form and determined
polarographically.27 The most sensitive such technique is differential
pulse polarography, which has a detection limit of about 0.3 ng of
arsenic per milliliter and can be used in the presence of natural
pollutants, such as unfiltered sludge.57~608
Gas Chromatography
Total arsenic can be determined by gas chromatography if the arsenic
is first collected and converted to triphenylarsine. The collection-
conversion procedure is somewhat long, but the absolute limit of
detection is quite low (20 pa) when an atomic-emission detector is
used 777 779
Other Methods
There are other valid methods of determining traces of arsenic, such as
coulombmetry, X-ray fluorescence, atomic optical fluorescence,788 and
ordinary and isotope-dilution mass spectrometry.778
METHODS OF DETERMINATION OF ARSENIC
COMPOUNDS
Most of the analytic work on separating and identifying arsenic com-
pounds has been done with substituted arsines and substituted acids of
arsenic (e.g., methanearsonic and cacodylic). The compounds have
been isolated with paper chromatography, electrophoresis, volatiliza-
tion,99 and (after silylation480 or conversion to the corresponding ar-
sine779 or iodide749) gas chromatography. A specific compound is
identified by its retention characteristics, sometimes in combination
with a specific detector for arsenic. Among the detection methods used
have been autoradiography,507 arc emission,99 and microwave emis-
sion.779 Absolute sensitivities have been in the picogram range.
OCR for page 262
Representative terms from entire chapter:
natural materials
