Determination of Arsenic Species in Natural Waters Meinrat 0. Andreae Scripps Institution of Oceanography, University of California at San Diego, La Jolia , California 92093
A method Is described for the determination of arsenate, arsenite, mono-, dl-, and trimethyl arsine, monomethylarsonic and dlmethylarsinlc acid, and trlmethylarsine oxide in natural waters with detection limits of several ng/L. The arsines are volatilized from the sample by gas stripping; the other species are then selectively reduced to the corresponding arslnes and voiatiilzed. The arslnes are collected in a cold trap cooled with liquid nitrogen. They are then separated by slow warming of the trap or by gas chromatography, and measured with atomlc absorptlon, electron capture, and/or flame lonlzatlon detectors. Four arsenic species have been found and measured In natural waters.
For our studies on the behavior of arsenic in natural waters, a method was needed that allows determination of arsenic species down to the part per trillion level. Existing methods suffered from a variety of inadequacies. The more sensitive methods require extensive sample pretreatment, e.g., neutron activation analysis (1);others do not give adequate sensitivity, e.g., the dc-discharge emission technique of Braman and Foreback (2). A method proposed by Talmi and Bostick (3) using gas chromatography with a microwave emission spectrometric detection system requires equipment not usually available in analytical laboratories. A review on the determination of arsenic has been published recently by Talmi and Bostick (4). This article describes a new analytical method for the determination of As(III), As(V), monomethylarsonic acid (MMAA),dimethylarsinic acid (DMAA),trimethylarsine oxide (TMAO), and the corresponding mono-, di-, and trimethyl arsines. The steps of (1)isolation of the species from the sample, (2) separation of the species, and (3) detection of the separated species are performed by interchangeable units. The combination of these units is tailored to the demands imposed by the sample type. The method is based on the sequential volatilization technique described by Braman and Foreback (2),where the species are volatilized as arsines. The arsines are then separated either by fractional volatilization or by gas chromatography, and detected either by atomic absorption, electron capture, or flame ionization.
EXPERIMENTAL Apparatus. The apparatus for the volatilization and trapping of the arsines (Figure 1) is constructed from Pyrex glass, with Teflon stopcocks and tubing, and with Nylon Swagelok connectors. The sample trap consists of a 6-mm 0.d. Pyrex U-tube of ca.15-cm length, filled with silane treated glass wool. The interior parts of the six-way valve which interfaces the volatilization system with the gas chromatograph are made of Teflon and stainless steel. The gas chromatograph (Hewlett-Packard Research Chromatograph HP 5750B) is equipped with a 63Nielectron capture detector mounted in parallel with a flame ionization detector and an auxiliary vent by the use of a column effluent splitter. The outputs of the two detectors are recorded on a two-pen strip chart recorder. The separation is performed on a 4.8-mm o.d., 6-m long stainless steel column, packed with 16.5% silicone oil DC-550 on 80-100 mesh Chromosorb W AW DMCS. The He carrier gas flow rate is 80 mL/min. 820
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
The electron capture detector (Hewlett-Packard 2-6195, with a 63Nisource) is operated in the constant pulse mode with a pulse interval of 50 j s . The atomic absorption detection system consists of a Varian AA5 with a hollow cathode arsenic lamp; the standard burner head is replaced by a 9-mm i.d. quartz burner cuvette (Figure 2), modified after a design by Chau, Wong, and Goulden (5).
Standards and Reagents. Arsenate and arsenite standards were prepared from Baker Analyzed NaH2As04and “Primary Standard” arsenic trioxide. Monomethylarsonicacid (99.9% pure) and dimethylarsinic acid (99.87% pure) standards were obtained from The Ansul Corporation, Weslaco, Texas. Mono- and dimethylarsine standards were prepared by reduction from the corresponding acids and purified by fractional distillation. Trimethylamine was obtained from Ventron Corp., Danvers, Mass. Stock solutions with 1000 ppm As were prepared for each of the acids and diluted daily before use. Arsine standards were prepared in oxygen-free methanol and/or toluene in sealed containers immediately before use. Sodium borohydride is obtained from Ventron. From blank runs, this reagent did not contain detectable amounts of As(II1) or organoarsenicals. The As(V) content varied between 1 to 30 ppb from lot to lot. Baker Analyzed Reagent HC1 and Sigma Reagent Grade Tris and Tris-HC1 were used for the buffers. The Tris buffer solution is 2.5 N in Tris and 2.475 N in HC1, giving a pH of 6.2 after dilution to 0.05 N. Methods. Zsolation of the Arsenic Species. The sample, 1-50 mL, is introduced into the gas stripper with a hypodermic syringe through the injection port. If an air-free transfer is necessary, the septum is replaced by a fitting, which connects to a four-way valve with a sampling loop. The system is first flushed with helium, and then the sample is injected by turning the valve. Any volatile arsines in the sample are stripped out by bubbling a helium stream through the sample. Then 1mL of the Tris buffer solution for each 50-mL sample is added, giving an initial pH of about 6. Into this solution 1.2 mL of 4 % NaBH4 solution is injected while continuously stripping with He. After about 6-10 min, the As(II1) is converted to arsine and stripped from the solution. The pH at the end of this period is about 8. Then 2 mL of 6 N HCl is added, which brings the pH to about 1. The addition of three aliquots of 2 mL of 4% NaBH4solution during 10 min reduces As(V), monomethylarsonic acid, dimethylarsinic acid, and trimethylarsine oxide to the corresponding arsines, which are swept out of the solution by the He stream and the evolved hydrogen. Drying of the Gas Stream. It was found necessary to dry the helium gas stream coming from the reaction vessel, because water clogged the gas trap and contaminated the electron capture detector. Various chemical drying agents were tried (CaC12, K2C03, Mg(C104)2,silica gel, Drierite). These materials either failed to adequately remove the water or irreversiblytrapped the arsines. Effective drying was obtained with a 28-cm long Pyrex U-tube, 7-mm id., immersed in a dry ice-isopropyl alcohol bath. Trapping and Separation of the Arsines. For direct detection by the atomic absorption technique, the stripping gas stream can be used to carry the sample into the burner. The arsines are isolated by immersing the gas trap in liquid nitrogen and released by slowly warming it up to room temperature. This slow warming results in a sequential separation of the arsines on the basis of their boiling points as described by Braman and Foreback (2). If the gases are to be separated by gas chromatography, the sample trap is attached to a six-way valve, which allows it to be switched from the stripping gas stream of the generating apparatus to the carrier stream of the gas chromatograph (Figure 1). The sample is first trapped at liquid nitrogen temperature from the stripping gas, then the valve is switched to the carrier stream and the arsines
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Calibration curves for As(II1)and As(V) with the AA detector,
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I V O L A T I L I Z AT ION
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. SEPARATION
Figure 1. Apparatus for the volatilization, trapping, and separation of the arsines burner cuvette.
Figure 2. Quartz cuvette burner head. The cylindrical base of the cuvette holder fits into the standard AA burner socket
are rapidly evaporated by immersing the trap in hot water. Detection. Three different detection devices have been investigated: a flame ionization detector, an electron capture detector, and an atomic absorption detector. Any combination of them can be attached to the outlet of the gas chromatographic column. The atomic absorption detector can also be used without column separation. The gas flow rates are: for the FID 25 mL/min He-carrier, 28 mL/min Hz, 240 mL/min air; auxiliary 30 mL/min He; for the ECD, 30 mL/min He-carrier, 25 mL/min P-10; (10% methane in argon); for the AAD, 250 mL/min H2,150 mL/min air, 50-200 mL/min carrier He.
DISCUSSION Volatilization. The efficiencies of the stripping and trapping process for the arsines were checked by comparison of their direct injection onto the column vs. their addition to distilled water in the stripper. They were found to be 92%, 89%, and 100% for the mono-, di-, and trimethylarsines, respectively. The reduction yield of the arsenic acids is a function of the pH (2,6)and can be used to separate arsenate and arsenite. Whereas the reduction of arsenite proceeds quantitatively from pH 0 to about 10, the reduction yield of arsenate drops to near zero at pH 5. It was found that about 0.2% of the arsenate was reduced at pH 4, the pH of the potassium hydrogen phthalate buffer suggested by Braman and Foreback (2). A phosphate buffer gave quantitative reduction at pH 7 in distilled water, but could not be used for seawater because of the precipitation of calcium phosphate. Therefore a Tris-HC1 buffer system which buffers the solution initially at pH 6.2, was used. As the borohydride reagent is alkaline, the pH rises after its addition to 7-8, depending on the amount of reagent used. This buffer system has the advantages of quantitative reduction, zero reagent blank, and
convenient handling. Further, the high solubility of Tris-HC1 allows the preparation of 2.5 N stock solutions. For the reduction of arsenate, HC1 is preferred as an acid to oxalic acid as it does not produce a precipitate with seawater. The yields for mono- and dimethylarsinic acids as compared to direct injection of the arsines were not completely quantitative, (94% and 91%, respectively), but highly reproducible (a 3% relative standard deviation at 5 ng). Interference of a number of metal ions with the reduction process has been noted (7-9). The metal concentrations at which interference occurs are, however, far above those usually found in natural waters. If the presence of interfering ions is to be suspected for a sample, standard additions should be used to check the reduction yields. Trapping. A number of trap packings were investigated. Chromatographic packings (silicone oil on Chromosorb W, Porapak Q, Molecular Sieves) trapped the arsines completely, but the elevated temperatures necessary for elution caused high losses of the arsines. Glass beads (Alltech Associates, Arlington Heights, Ill.) irreversibly captured the arsines; after a pretreatment of an hour of boiling in concentrated HC1, these beads still gave recoveries of only 80%. Chromatographic quality glass wool (silanized) eventually gave a quantitative recovery, when used as packing in a 6-mm 0.d. glass U-tube of about 15-cm length. Separation. The simplest way to separate the arsines is by slowly warming up the cold trap, whereupon the arsines vaporize in the sequence of their boiling points. This method works well for the separation of AsH3 from the methylated arsines, and for the methylated arsines from each other in the range from 2 to 20 ng. Because of the increase of peak width from AsH3to trimethylarsine, the detection limit of the atomic absorption dector decreases from 0.1 to 2 ng. Gas chromatographic separation reduces the peak broadening and makes the use of the highly sensitive electron capture detector possible. The corrected retention times at 55 "C are 29,96, 176, and 206 s for AsH3,MMA, DMA, and TMA, respectively. Detection. Atomic Absorption Detector. In the AA detector, the arsines are converted to arsenic atoms in a H2-rich hydrogen/air flame. Optimal sensitivity is obtained at an Hz/Oz ratio of about 5, but does not change appreciably upon small changes (
