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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

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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

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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.

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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

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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

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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.

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