80
Anal. Chem. 1980, 52, 80-83
(18) Blaedel, W J.; Wang, J. Anal. Cbem. 1979, 57,1724. (19) Jordan, J.; Javick, R. A. Electrochim. Acta 1962, 6, 23. (20) Blaedel, W. J.; Jenkins, R. A . Anal. Cbem. 1975, 4 7 , 1337. (21) Blaedel, W. J.; Schieffer, G.W. J . Electroanal. Chem. 1977, 80, 259. (22) Thirst. H. R.: Harrison. J. A. " A Guae to the Studv of Electrode Kinetics": Academic Press: New York, 1972; pp 86-87: Laser, D.; Ariel, M. J . flectroanal. Cbem. 1974, 52, 291. Blaedel, W. J.; Mabbott, G. A. Anal. Cbem. 1978, 50, 933. Blaedel, W. J.; Jenkins, R. A. Anal. Chem. 1974, 46, 1952. Bogotskaya, I. A . Dokl. Akad. Nauk SSSR 1952, 85, 7057. Engstrom, R. C.; Blaedel, W. J. Cbem., Biomed., Environ. Instrum. 1979, 9 ,61.
(28) Davis, G. C.; Holland, K. L.; Kissinger, P. R. J . Liquid Cbromatogr. 1979, 2, 663. (29) Kryger, L.; Jagner, D. Anal. Chim. Acta 1975, 78, 251.
RECEIVED for review August 3: 1979. Accepted October 15, 1979. This work was funded in part by the University Sea Grant Program under a grant from the Office of Sea Grant, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and by t h e State of Wisconsin.
Determination of Arsenic(III), Arsenic(V), Monomethylarsonate, and Dimethylarsinate by Differential Pulse Polarography after Separation by Ion Exchange Chromatography F. T. Henry"' and T. M. Thorpe' Department of Chemistry, Miami University, Oxford, Ohio 45056
Monomethylarsonate(MMA) and dimethylarsinate (DMA) were determined at trace levels by differential pulse polarography (DPP). The arsenicals were isolated through interactions with ionexchange resins and digested in perchloric acld. Reduction by SO, allowed quantitation as As( 111). Recoveries averaged 98% for MMA and 100% for DMA with relative standard deviations of less than 5 % in each case. The detection limits were 18 ppb and 8 ppb, respectively. Inorganic As(II1) was determined directly either in 1.0 M HCI or in 1.0 M HC104. Total Inorganic arsenic (As(tot)) was measured by DPP after the reduction of As(V) to As(II1) with SO,. The concentration of Inorganic As(V) was evaluated as the difference between the results for As(tot) and inorganic As(II1).
Interest in determining the levels of individual compounds of arsenic in environmental samples arises from the recognition of their toxicity and possible carcinogenicity. Measurement of total arsenic content does not establish a n effective base from which to estimate hazard because it fails to reflect variations in toxicity, extent of transport, and bioavailability which occur with respect to chemical form. I t has been shown ( I ) t h a t arsenic(V), arsenic(III), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA) are present in natural water systems. (The structures of DMA and MMA are shown in Figure 1.) Moreover, a dynamic relationship exists whereby oxidation-reduction and biological methylation-demethylation reactions (2-5) provide pathways for the interconversions of the arsenicals. Analytical methods capable of distinguishing between the predominant species of arsenic are necessary if immediate and potential impacts are to be accurately assessed. A variety of techniques have been used to obtain speciation d a t a for t h e aforementioned forms of arsenic. Spectrophotometry (6-9) and gas chromatography (10-13) have been Present address, Ross Laboratories, 625 Cleveland Avenue, Columbus, Ohio 43216. 'Present address, The Procter & Gamble Company, Sharon Woods Technical Center, 11530 Reed Hartman Highway, Cincinnati, Ohio 45241. 0003-2700/80/0352-0080$01 .OO/O
useful for one or more of the arsenicals, b u t have failed t o determine all four species. The procedure developed by Braman and co-workers ( I ) for generation and selective volatilization of arsines resolved the four forms of arsenic. However, molecular rearrangements (14) and incomplete recoveries a t low concentrations (15, 16) have been reported. Yamamoto (17) and Dietz and Perez (18) observed that DMA has a strong affinity for acid-charged cation-exchange resins. Elton and Geiger (19) used this fact to separate MMA and DMA prior t o determinations of the organoarsenicals by differential pulse polarography (DPP). The authors reported detection limits of 0.1 Fg/mL and 0.3 kg/mL, respectively. Henry, Kirch, and Thorpe (20) reported a method for the determination of As(III), As(V), and total inorganic arsenic by DPP. As(II1) was measured directly in 1 M HCIOl or 1 M HC1 (21). Total inorganic arsenic was determined in either of these supporting electrolytes after the reduction of electroinactive As(V) with aqueous sulfur dioxide. As(V) was evaluated by difference. Sulfur dioxide was selected because it reduced h ( V ) rapidly and quantitatively, and excess reagent was readily removed from t h e reaction mixture. This paper describes a procedure for the determination of As(V), As(III), DMA, and MMA. As(V) and As(1II) are determined by t h e method of Henry, Kirch, and Thorpe (20). DMA and MMA are isolated through interactions with ionexchange resins, digested in hot, concentrated perchloric acid, reduced by sulfur dioxide, and quantitated as As(II1) by DPP. EXPERIMENTAL Instrumentation. A Princeton Applied Research Corporation (Princeton, N.J.) Model 174A polarographic analyzer and a Hewlett-Packard (Avondale, Pa.) Model 7040A X-Y recorder were
used for all APP determinations. The flow rate of the DME was 0.845 mg/s. A 2.0-s drop time was employed for all measurements. Other instrumental parameters are as previously described (20). Reagents. High purity arsenic trioxide was obtained from ROC/RIC (Belleville, N.J.). Reference purity dimethylarsinic and monomethylarsonic acids were provided by the Ansul Company (Weslaco, Tex.). All other chemicals were reagent grade. Triply distilled, deionized water was used to prepare all solutions. Preparation of Ion-Exchange Columns. Cation, Dowex 50 W-X8, and anion, AG 1-X8, exchange resins were obtained from 0 1979 American
Chemical Society
ANALYTICAL CHEMISTRY, VOL.
0 H 0 - t -~ CH3 0 H
0 H3C -As-CH3 0
52, NO. 1,
Efficiency of Digestion of DMA with Perchloric Acid determined, PPb
DMA MMA Figure 1. Structures of dimethyarsinic and munomethylarsonic acids
400
Bio-Rad (Richmond, Calif.). To remove potential interferents, the 5Cb100 mesh resins were extensively washed with alternating 0.5 M solutions of HCl and NaOH at flow rates of 5-10 mL/min. The cation-exchange resin, in the H+ form, was slurry packed into a 1.0-cm i.d. glass column to a height of 16 cm. After use, the column was regenerated by reaction with 1.0 M HC1. The anion-exchange resin was slurry packed into a 0.8-cm i.d. glass column to a height of 11 cm. The resin was converted to the acetate form by passing 250 mL of 0.5 M NaOH over the column followed by 100 mL of 1.0 M NaC2H3O2,both at flow rates of 5-10 mL/min. The column was rinsed with triply distilled water and a mobile phase that was 0.1 M in total acetate concentration and had a pH of 4.7 was passed over the column until the pH of the effluent was also 4.7. Optimized Procedure. Figure 2 depicts the analytical procedure for separation and the subsequent determination of each of the arsenicals. The pH of the sample is adjusted to between 4 and 10, and the sample is divided into 4 aliquots. Two of these are used to determine As(II1) and total inorganic arsenic as previously described (20). The concentration of As(V) is calculated from the difference between these two values. A third, 200-mL, aliquot is mixed with 2.0 mL of 1.74 M acetic acid and passed through the cation-exchange resin at a flow rate of 5 mL/min to isolate the DMA from the matrix. The sample is followed by a mobile phase consisting of 0.02 M acetic acid. As(III), As(V), and MMA elute within 70 mL. DMA is recovered by stripping the column with 1.0 M NaOH at 1.0 mL/min. Eluent collected from 31-42 mL after addition of the basic mobile phase contains the DMA. The fraction of eluent containing DMA is added to 7 mL of 70% HCIOl contained in a flask fitted with an air condenser (22). The resulting solution is evaporated to fumes and heated to 200 "C for 2.5 h in a stainless steel hood. The solution is cooled and diluted with 10 mL of triply distilled water before it is removed from the hood. The As(V) produced in this digestion is reduced and determined as As(II1) according to the procedure of Henry, Kirch, and Thorpe (20). MMA is isolated using the anion-exchange column. A 50.0-mL aliquot of the sample is mixed with sodium acetate/acetic acid buffer to obtain a total acetate concentration of 0.01 M and a pH of 4.7. The resulting solution is loaded onto the column and followed with 50 mL of a mobile phase of similar composition. As(II1) and DMA will elute within this 100 mL (sample + eluent).
400
380 400 37 0 380 310 260 150
400 400 290 270 150 120
120
110
110
59
61
DM A
1
CATION EXCHANGE COLUMN
1
DIGESTION
1
recovery, 'b
95 100
92 95 107 104 100
100 100 103
mean = 100% S.D. = 4.6%
N = 10 Recovery of DMA froin Separation and Digestion Procedure arsenic added, ppb
arsenic found, ppb
final recovery, %
396 99 89 89 69 49 25
376 107 86 86 68 51
95 108 97 97 99 104
25
100
mean = 100% S.D. = 4.5% AT= 7 MMA and As(V) are retained on the resin. 'The flow rate is constant at 1-2 mL/min throughout the procedure. To resolve MMA from As(V), the total acetate concentration of the mobile phase is increased to 0.1 M while maintaining a constant pH. Eluent containing MMA is collected between 20 and 40 mL from the introduction of the more concentrated phase. As(V) begins to appear after 75 mL from the same reference point. The fraction containing MMA is digested in perchloric acid under the same conditions as is DMA. The reaction proceeds at a faster rate, however, and reaches completion within 30 min. The As(V) produced in this reaction is determined as previously cited (20).
RESULTS A N D D I S C U S S I O N Following separation, DMA and MMA may be directly determined by DPP (19, 23). Limits of detection are sub-
SAMPLE
ALIOUOT
81
Table I added, PPb
H
JANUARY 1980
As!Ill) ALIQUOT
INORGANIC ALIOUOT
DETERMINATION as As(lIl)
REDUCTION
ANIOPI EXCHANGE COLUNN
DETERMINATION
DIGESTION
1
1
REDUCTION
4
DETERMINATION as As(lll)
Figure 2. Flow chart for the determination of As(III), As(V), MMA, and DMA
I
as A s ( l l l )
MMA A L IO UCIT
1
1 1
REDUCTION
4
DETERMINATION as As(lli)
82
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
Table I1 Efficiency of Digestion of MMA with Perchloric Acid added, ppb
determined, ppb recovery, %
1640 1640 1640 1640 821 821 737 125 85 43
1680
102
16'70
102
1640 1670 783 843 790 113 80 43
100 102
95 103 107 90 94 100
mean = 100% S.D. = 5.1%
N = 10
Recovery of MMA from Separation and Digestion Procedure added, ppb
determined, ppb recovery, 70
1640 737 266 177
1560 7 29 27 7 168 86
88 88
95 99 104 95
98 97
a5
mean = 98% S.D. = 3.0% N=6 stantially improved, however, if the organoarsenicals are first oxidized to As(V) and subsequently reduced to As(II1). The demonstrated success in the degradation of a wide range of refractory organic compounds by hot, concentrated perchloric acid (22) indicated potential in this application. Determination of DMA. Under the conditions described, DMA is strongly retained by the cation-exchange resin; a break-through volume in excess of 500 mL was observed for this system. By loading large sample volumes and eluting with basic mobile phases, direct polarographic measurements of DMA (23)in the eluent were accomplished a t initial sample concentrations as low as 200 ppb. For more dilute solutions, it was necessary to convert DMA to As(II1) before determination. Studies were conducted to determine the length of time required to complete the decomposition of DMA at the boiling point of the perchloric acid-water azeotrope (203 O C ) . Digestion was found to be incomplete if the reaction time was
less than 2.5 h. The data in Table I were obtained from experiments in which this length of time was exceeded, and conversion to inorganic arsenic is seen to have been complete. Efforts to reduce reaction time by addition of catalytic amounts of vanadium(1V) produced an interfering response a t a peak potential of 4 . 4 8 vs. SCE. This corresponds to the peak potential of V(II1) (24) which was produced during the SO2reduction step. The air condenser utilized in this procedure serves to slow evaporation and to reflux the azeotrope, allowing the oxidation potential of the systems to rise gradually as the temperature increases and as the perchloric acid becomes more concentrated. Although this minimizes the hazards involved in the analysis of samples containing low levels of organic matter, uncontrolled reactions are possible in other matrices and necessitate special precautions (22). The efficiency of the perchloric acid digestion and the recovery of DMA from the entire procedure were measured and the data are presented in Table I. The concentrations investigated ranged from low ppm to low ppb in both cases. The mean recoveries and standard deviations indicate the procedures are quantitative and reproducible. The detection limit for DMA, calculated as previously reported (20),is 8 ppb. Determination of MMA. While Yamamoto (17) was able to separate MMA using a cation-exchange column, the affinity of this arsenical for the resin was much lower than in the case of DMA. This, in conjunction with the low levels of MMA commonly encountered in environmental samples, restricted the amount of MMA that could be loaded onto the column. In view of the difficulties involved in recovering sub-microgram amounts from conventional ion-exchange resins (25) and the desire for preconcentration, an alternative method was sought. The range of pK, values exhibited by the arsenicals suggested that the extent of ionization, and hence the selectivity of an ion-exchange separation, could be controlled by variations in pH. At a p H of 4.7, As(V) and MMA exist largely as singly-charged anions capable of undergoing typical ionexchange equilibria, while DMA and As(II1) are predominantly protonated, neutral species and, thus, have little affinity for the resin. Separation of As(II1) and DMA from As(V) and MMA was accomplished on the anion-exchange column using a mobile phase composed of 0.01 M acetate a t the aforementioned pH. In this medium, the break-through volumes of the retained species exceeded 150 mL, allowing loading of large sample volumes and, consequently, microgram amounts of analyte. Furthermore, after elution of As(II1) and DMA, the acetate concentration of the mobile phase was increased to 0.1 M while maintaining a constant p H , thereby shifting
Table 111. Results of the Analysis of an EPA Standard Referenc:e Samplea
a
species
concentration present
As( 111) As( V)
MMA
26 82 83
DMA
63
determination 3
1
2
25 85 72 69
33
26
81 86 71
76 85 63
1
average standard determination deviation
34 76 79 65
30
3.7 d.4
80 80
6.5
67
3.7
Values in ppb arsenic.
Table IV. Ash Basin Slurry Sample initially species
As(II1) As( V ) DMA MMA Not detectable.
present, ppb
63 46 24 N.D.a
spiked, PPb 76 73
62 85
total present, ppb
139 119 86 85
found, PPb
134 121 83
93
recovered, %
96
102 97 109
ANALYTICAL CHEMISTRY, VOL.
the distribution coefficients of As(V) and MMA so that MMA was eluted rapidly and was resolved from the more strongly retained As(V). Table I1 presents data obtained from studies of the efficiency of the perchloric acid digestion of MMA and recovery results for MMA standard solutions when subjected to the entire procedure. The digestion of MMA occurred much more quickly than that of DMA, reaching completion in less than 30 min. The mean efficiency and the standard deviation show the reaction was both quantitative and reproducible. Recovery of known amounts of MMA is also seen to have been complete. T h e detection limit for MMA was 18 ppb. Speciation of Arsenic. Although the measurements of MMA and DMA may be performed independently, a more comprehensive knowledge of the sample may be acquired if these methods are combined with those for the determinations of As(II1) and As(V) as depicted in Figure 2. By dividing the sample into aliquots and treating each in the appropriate manner, the four environmentally significant arsenicals may be speciated a t trace levels. The accuracy and precision of the speciation procedure were examined in a complex matrix by performing replicate analyses on a U S . Environmental Protection Agency water reference standard. While the reference standard provides potential interferents (i.e., P b , V, Se) in concentrations comparable to natural waters, neither of the organoarsenicals is present and previous analyses (20) revealed only small amounts of As(V). Consequently, it was necessary to add known amounts of these species and to measure recoveries. Table I11 compares the result of each determination to the level of the arsenical present in the prepared sample. The mean values and standard deviations of the determinations indicate the method would be effective in the analysis of samples of similar complexity. T o demonstrate the applicability of the method to systems of more practical interest, samples were obtained from a holding pond which received an aqueous slurry of fly ash from a coal-fired power plant. The small volume of sample available precluded multiple determinations, but the results of a single analysis of one such sample are presented in Table IV. The
52, NO. 1, JANLIARY 1980
83
recoveries of the known additions of As(III), As(V), MMA and DMA substantiate the accuracy of the procedure and these results demonstrate the utility of the method for the speciation of arsenicals in complex aqueous matrices.
ACKNOWLEDGMENT Reference purity DMA and MMA were supplied through the courtesy of Edward Dietz of Hooker Chemical Company and James Warkentin of Ansul Company. LITERATURE CITED R. S. Braman and C. C. Foreback, Science, 182, 1247 (1973). F. Challenger, Chem. Rev., 36, 315 (1945). B. C. McBride and R. S. Wolfe, Biochemistry, IO, 4312 (1971). D. W. VonEndt, P. C. Kearny, and D. D. Kaufman, J . Agric. Food Chem., 18, 17 (1968). E. A. Woolson and P. C. Kearny, Environ. Sci. Techno/., 7, 47 (1973). S. A. Peoples, J. Lakso, and T. Lais, Proc. West. Pharmacal. Soc., 14, 178 (1971). M. G. Haywood and J. P. Riley, Anal. Chim. Acta, 85, 219 (1976). T. Kamada, Telanta, 23, 835 (1976). S.S. Sandhu, Ana/yst(London), 101, 856 (1976). A. W. Fickett, E. H. Daughtrey, and P. Mushak, Anal. Chim. Acta, 79, 93 (1975). J. D. Lodmell, Ph.D. Thesis, University of Tennessee, Knoxville, Tenn., 1973. L. D. Johnson, K . 0. Gerhart, and W. A. Aue, Sci. TotalEnviron., 1, 108 (1972). C. J. Soderquist, D. G. Crosby, and J. B. Bowers, Anal. Chem., 48. 155 (1974). Y. Talmi and D. T. Bostik, Anal. Chem., 47, 2145 (1975). J. E. Portman and J. P. Riley, Anal. Chim. Acta, 31, 509 (1964). M. 8 . Casvalho and D. M. Hercules, Anal. Chem., 50, 2030 (1978). M. Yamamoto, Soil Sci. Soc. Am. Proc., 39, 859 (1975). E. A. Dietz and M. E. Perez, Anal. Chem., 48, 1088 (1976). R. K. Elton and W. E. Geiger, Jr., Anal. Chem., 50, 712 (1978). F. T. Henry, T. 0.Kirch, and T. M. Thorpe, Anal. Chem., 51, 215 (1979). D. J. Myers and J. Osteryoung, Anal. Chem., 45, 267 (1973). G. F. Smith, "The Wet Chemical Oxidation of Organic Compositions Employing Perchloric Acid", G. Fredric Smith Chemical Co., Inc., Columbus, Ohio, 1965. R. K . Elton and W. E. Geiger, Jr., Anal. Lett., 9, 665 (1976). J. J. Lingane, J . Am. Chem. Soc., 67, 182 (1945). M. Zief and J. W. Mitchell, "Contamination Control in Trace Element Analysis", John Wiley 8 Sons, New York, 1976; Chapter 6.
RECEIVED for review June 13, 1979. Accepted September 28, 1979. Portions of this research were presented a t the 30th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1979.
