- Email: [email protected]
Arsenic speciation by ion-exchange separation and graphite-furnace atomic-absorption spectrophotometry
I
to 938. 1981 All rlghlr rr\sr\cd
Copyright
120935~04102.00/O 0039.9140/81/ 0 1981 Pergamon Press Ltd
ARSENIC SPECIATION BY ION-EXCHANGE SEPARATION AND GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROPHOTOMETRY G. E. PACEY and J. A. FORD Miami
University, (Recritvd
Department 4 February
of Chemistry,
Oxford,
1981. Accepted
Ohio 45056, U.S.A.
6 May
1981)
Summary ~~As(lll), As(V), monomethylarsenic acid (MMA) and dimethylarsenic acid (DMA) were determined dy graphite-furnace atomic-absorption spectrophotometry afier separation of the species by ion-exchange chromatography. The detection limits (ng/ml) were DMA 0.02, MMA 2.0, As(V) 0.4 and total arsenic 4.0. As(III) was determined by difference. This system gave better detection limits and/or shorter analysis times than previously reported systems.
Perez” observed that DMA has a strong affinity for cation-exchange resins in acid medium. Elten and Geigerz3 used this fact to separate MMA and DMA before determination by differential pulse polarography (DPP). Henry er al. reported a method for the determination of As(III) and As(V) and total inorganic arsenic by DPP.24 Later, Henry and Thorpez5 reported a complete separation of all four species, using both anionand cation-exchange columns, and detection with DPP. This separation procedure is superior to all previous systems but suffers from the slowness of the digestions needed to convert the organoarsenicals into As(II1). With this in mind, we have modified this separation system for use in conjunction with graphite-furnace atomic-absorption analysis, and have designed an analysis system capable of giving a comparable detection limit without the time delays caused by the digestions.
The determination of metal species is essential to the understanding of the toxic and carcinogenic nature of metals found in environmental and biological systems. For example, the measurement of total arsenic content does not indicate the true levels of the individual species. To estimate effectively the hazard involved, the variations in toxicity, transport, and bioavailability, which are dependent on the arsenic chemical species, must be taken into account. It has been shown that arsenic can be found in natural systems as As(IIl), As(V), monomethylarsenic acid (MMA) and dimethylarsenic acid (DMA).‘-’ Other organic arsenic species have been reported to occur in some manufacturing processes.’ Analytical methods aimed at differentiating between these species are needed. As would be expected, a variety of techniques have been used for speciation of arsenic. Gas chromatohave been graphy’ lo and spectrophotometry”m14 demonstrated as effective techniques for one or more arsenicals. but neither has been used for mixtures for all four species listed above. Atomic absorption,’ an obvious choice for determination of metals, has been tried in conjunction with the generation and selective volatilization of arsines, which did resolve the four species,15 but the possibilities of molecular rearrangement and poor recoveries at low concentrations have raised doubts about this system.‘b.‘7 Separation by thin-layer chromatography followed by graphite-furnace atomic-absorption analysis has also been reported.‘8,‘9 Some flameless atomic-absorption systems have been reported. One system employed a separation scheme which gave less efficient resolution of species.” Woolson” published a system combining HPLC with graphite-furnace analysis; the limit of detection was 5 ng. So far, the most successful detection system has been electrochemical, following separation on ionexchange columns. Yamamoto” and Dietz and m~28I2
EXPERIMENTAL
-
Apparatus
A Perkin-Elmer model 560 atomic-absorption spectrophotometer and HGA-400 graphite-furnace atomizer were used for all the metal determinations. A new hollow-cathode arsenic lamp was used. Purified and dried argon was used as the sweep gas for the HGA-400. Reagents
High-purity arsenic trioxide was obtained from ROC/ RIC (Belleville. N.J.). All other chemicals were those used by Henry and Thorpe. ” The water was purified with a Barnstead NAN0 pure system followed by a Barnstead glass double-distillation system. All other chemicals were reagent grade. ion-exchange
columns
Dowex 5OW-X8 and AI-X8 resins (5CrlOO mesh) were used. Contaminants were removed from the resins by repeated washing alternately with 0.5M hydrochloric acid and ammonia solution at a flow-rate of 5-10 ml/min. The
b
935
Ci. E. PACEY and
936
exchange columns were slurry-packed. The cationexchanger column was 16 cm long (1 cm diameter) and the anion-exchanger column 10 cm long (1 cm diameter). The cation resin was readily regenerated by passage of l.OM hydrochloric acid through the column. It was necessary to convert the anion resin into the acetate form. This was done by passing 300 ml of 0.5M ammonia solution through the column, followed by 150 ml of l.OM ammonium acetate at a flow rate of 5-10 ml/min. The column was rinsed with water and then with O.lM acetic acid/ammonium acetate buffer (pH 4.7) until the effluent pH was 4.7. The preparation of the anion column was crucial to the success of the separation since careless preparation caused random errors in the results for the standard solutions. Control of the flow-rate was critical.
J. A. FORD
Optimization
of graphite-furnace
conditions
Table 1 shows the atomization parameters for the atomic-absorption determination of arsenic species. The four species can be determined in an essentially identical manner with the exception of the drying temperature used for MMA and As(V). Because of the high acetate concentration it was found necessary to increase the temperature to prevent sputtering and loss of sample. The stopped-flow gas system gave considerably higher absorbance values. with consistently better precision. than miniflow or highflow gas-pur’ge systems. The 193.7-nm line was used for all measurements. Deuterium lamp background correction did not affect the quality of the atomic-absorption measurements and was therefore not used.
ProWdW
RESULTS
The separation and determination of the four arsenic species is-schematically represented in Fig. 1. The suggestion bv. Henrv * and Thorpe” of maintaining - the pH of a sample between 4 and 10 was adopted. Four equal volumes of sample were used. for determination of total arsenic, DMA. MMA and As(V). As(lll) was calculated by difference. To isolate DMA from the matrix 100 ml of sample were mixed with 1.0 ml of 1.75M acetic acid and loaded onto the cation-exchange column at a flow-rate of 5 ml/min, and 70 ml of 0.02M acetic acid were passed through the column at a similar flow-rate to ensure complete separation from MMA. As(fll) and As(V). Then DMA was stripped by passing l.OM ammonia solution through the column at a flow-rate of I.0 ml/min. The Row-rate is critical for successful stripping. The fraction of effluent between 40 and 50 ml was analysed. To isolate MMA 100 ml of sample were mixed with pH 4.7 ammonium acetate/acetic acid buffer (total acetate concentration 0.01.W) and passed through the anion-exchange column, followed by 50 ml of O.OlM buffer of the same pH. The As(ll1) and DMA were eluted in the first 120 ml of effluent but As(V) remained on the column. The MMA was collected by stripping with 0.5iLI acetate buffer of pH 4.7. the fraction of effluent between 10 and 30 ml being collected and analysed. The As(V) did not begin to elute until 65 ml of the 0.5M acetate solution had passed through the column. The next 30 ml were collected and analysed for As(V1. Flow-rates of 1.8-2.0 mljmin were required. As all the MMA was eluted by 40 ml of the 0.5M buffer. a 1M acetate buffer could then be used to strip the As(V) more compactly, the fraction between 20 and 30 of the 1 M acetate eluate ml being sufficiently representative of the As(V) sample.
AND DISCUSSION
The system described provides excellent resolution of DMA, MMA, and As(V). The ion-exchange resins also act as a preconcentrator, thus lowering the overall working detection limits. DMA was retained on the cation-exchange resin in 0.02M acetic acid. A break-through volume of greater than 500 ml was observed. By use of large sample volumes and elution with ammonia an initial DMA concentration as low as 0.02 ng/ml could be determined. The DMA detection limit with the graphitefurnace system was 0.01 ng. This is better by a factor of more than 10 than the 0.13-ng detection limit for total arsenic achievable with the HGA-400 system under the same preconcentration conditions. This enhancement is due to the ammonium ion used to elute the DMA from the column. A similar effect has been reported for several alkali and alkaline-earth metal ions, but the ammonium ion was not mentioned.” Table 2 shows the efficiency of the system for the determination of low levels of DMA. The recovery is good. MMA was retained by the anion-exchange resin. with a break-through volume greater than 150 ml. Again a large volume of sample can be used. A sample with an initial MMA concentration of 2.0 ng/ml can be analysed. The absolute detection limit was 0.5 ng. The enhancement found for DMA was not
SAMPLE
r DMA oliquol
MMA,
I
I As (V)
I
oliquot
I
Anionexchange column
Cation exchange column
Total As oliquot
I
Grophi te furnace
&A
I
Grophite furnace Fig. I. Flow-chart
for the determination
Grophlte furnace
I
As (V)
I
Graphite furnace
.
of total As, As(V), MMA
and DMA.
Arsenic speciation
931
Table 1. Graphite-furnace parameters for determination of arsenic species (stop-flow mode) DMA Drying temperature, -C
loo
Drying ramp time, see Drying hold time, set Charring temperature, -C Charring ramp time, set Charring hold time, set Atomization temperature, ‘C Atomization ramp time, src Atomization hold time. src
MMA
As(V) Total As 140 I 40
loo 1 40
700 30 15 2700 0 5
700 30 15 2700
140
1 40 700 30 15 2700 0 5
1 40 700 30 I5 2700 0 5
0
5
*SO-/t1injections into pyrolytically-coated graphite tubes; argon purge at 60 ml/min.
observed for MMA. Table 3 shows the efficiency of the system for MMA determination; again there is good recovery. Since both As(V) and MMA were retained by the anion-exchange column, it was possible to separate them during the elution. The minimum concentration of As(V) in a sample that could be detected was 0.4 ng/ml and the absolute amount of As(V) detectable in the furnace was 0.2 ng. The As(V) was easily retained and recovered with similar efficiency to that for DMA and MMA. The detection limit for total arsenic concentration was 4.0 t&ml, which is more than adequate in view of
the concentrations in the EPA reference material and the fly-ash slurry sample of Henry and Thorpe.25 At no time was there any evidence that the system allowed any interconversion of species. Therefore, the determination of As(III) by difference was legitimate. Tables 4 and 5 show the recovery of As(V) and As(II1). The concentrations used were 30-80 ng/ml since the naturally occurring range is at that level. The recovery of As(V) is slightly lower than that of either MMA or DMA, because of the strong affinity of As(V) for the anionic resin. The resin usually retains a small but fairly constant amount of As(V) regardless of the pH and ionic strength. The As(III) was calculated by difference. The higher recovery for
this species may be due to impurities in the DMA and MMA and/or the small inherent negative error in the As(V) determination. This system gave detection limits lower than those of the differential pulse method, with the added advantage that the time-consuming digestion process was no longer needed. Variations in the actual detection limits will obviously occur with different graphite furnace systems. The ionic state of each species has not been clearly identified. Dietz and Perez” pointed out that the separation of all four species on a cation-exchange column was nor due to an ion-exchange mechanism, and explained the separation behaviour as due to ionexclusion chromatography. Essentially, it was concluded that at the pH-values used with the cation-exchange column As(V) species were behaving as ionized solutes and were excluded from the resin. The non-ionized nature of the As(II1) species meant that some penetration of the resin would occur but not to the same extent as for MMA or DMA. The pK, values of 3.6 and 6.2 for MMA and DMA respectively fall between those for arsenic and arsenious acids. DMA is strongly bound to a cationexchange column, probably as (CH&As02HNHi in our case. The MMA is strongly held by the anionexchange column, probably as a (CH,)AsO; species.
Table 2. Recovery of DMA on Dowex 5OW-X8
Table 3. Recovery of MMA on Dowex Al-X8 Taken, rig/m//
Found.
ng/ml
Recovery, “jO Taken, ny/n~l
0.080 0.080 0.050 0.050 0.050 0.050 0.030 0.030 0.030 0.030
0.080 0.052 0.054 0.046 0.033 0.030 0.03 1 0.029
Mean lOI”,,; standard deviation 5?:,
95 100 104 100 108 96 110 100 103 96
7.2 7.2 6.0 6.0 6.0 5.0 4.0 4.0 4.0
Found,
ngjml
7.1 7.2 5.9 5.9 6.2 5.2 3.8 4.1 4.2
Mean lOl”,/,: standard deviation 3%
Recovery, “/, 99 100 98 98 103 104 95 103 105
G. E. PACEY and J. A. FORD
938
Table 4. Recovery of As(V) on Dowex Al-X8 Taken, ny/tnl
Found, ng,‘ntl
Recovery, I!,,
80 X0 50 50 50 50 30 30 30 30
16 81 51 50 48 46 30 31 29 28
95 101 102 100 96 92 100 103 97 93
Mean 98“,,: standard deviation 4”,,
Table 5. Recovery of As(lIl) by difference Taken. ny/,nl
Found, ny/rnl
Recovery, “;,
80 80 SO 50 50 50 30 30 30 30
79 82 52 50 54 48 32 30 31 28
99 103 104 100 108 96 107 100 103 93
Mean lOI”,,: standard deviation 5”,,.
The stronger retention of As(V) as an As04H; speties would be consistent with the previously described systems of Dietz and Perez’* or Henry and Thorpe.*’ A[,l\~~o,,,lrdgrrlterlr~Funding for the purchase of the graphite furnace was provided by NSF 69A Grant No. NSF-CDP-800191 I.
REFERENCES I. R. S. Braman and C. C. Foreback. Scienc’e.1973, 182,
1241. 2. F. Challenger, Chrn~.RN., 1945, 36, 315. 3. B. C. McBride and R. S. Wolfe, Biochrtni\rrr, 1971, 10, 4312. 4. D. W. VonEndt. P. C. Kearny and D. D. Kaufman J. Agric. Food Clam.. 1968. 16. 17. 5. E. A. Woolson and P. C. Kearny. Enriron. .%I’.Techt1ol.. 1973, 7, 47. 6. G. G. Ricci. L. S. Shepard. N. Hester and G. Glovos, Paper delivered al 2nd Chemical Congress of the North American Continent. Las Vegas. 1980. 7. A. W. Fickett. E. H. Daughtrey and P. Mushak. Anul. C/tint. Actu. 1975. 79, 93. 8. J. D. Lodmell. Ph.D. Thu.si.~. University of Tennessee. Knoxville, Tenn.. 1973.
9. L. D. Johnson, K. 0. Gerhart and W. A. Aue. Sc,r. Total Enriron.. 1972. I, 108.
10. C. J. Soderquist, D. G. Crosby and J. B. Bowers, 4mrl. C/tern.. 1974, 46, 155. I I. S. A. Peoples, J. Lakso and T. Lais. Proc,. B’c,.\r.Phtrmucwl. sot., 1971. 14. 178. 12. M. G. Haywood and J. P. Riley. Anal. C/tint. 4cro. 1976. 85, 219. 13. T. Kamada, Tuluntu, 1976. 23, 835. 14. S. S. Sandhu, Anul!.st, 1976, 101, 856. 15. Y. Talmi and D. T. Bosik, Amd. Chern.. 1975.47, 2145. 16. J. E. Portman and J. P. Riley. Antrl. Chits. .&ttr, 1964. 31, 509. 17. M. B. Casvalho and D. M. Hercules, Aml. Clwt~.. 1978. 50, 2030. 18. Y. 0. Danard, 0. Matano and S. Goto. Bntt\t& Kay&. 1979. 28, 517. 19. R. R. Stanforth. Etc. Sri. Tech., 1979, 12, 1491. 20. E. A. Woolson and N. Aharowsow, J. ,4ssoc. Qfl. .4rtu/. Chvn~., 1980, 63. 523. 21. M. Yamamoto, Soil Sci. Sot. Am. Proc.. 1975. 39, 859. 22. E. A. Dietz and M, E. Perez. Anal. Chum.. 1976, 48, 1088. 23 R. K. Elton and W. E. Geiger. Jr.. ihid., 1978. 50. 712. 24 F. T. Henry, T. 0. Kirch and T. M. Thorpe. Anctl. Chrrn.. 1979. 51, 215. 25 F. T. Henry and T. M. Thorpe. AIIUI.Chrh~.. 1980. 52, 80.
to 938. 1981 All rlghlr rr\sr\cd
Copyright
120935~04102.00/O 0039.9140/81/ 0 1981 Pergamon Press Ltd
ARSENIC SPECIATION BY ION-EXCHANGE SEPARATION AND GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROPHOTOMETRY G. E. PACEY and J. A. FORD Miami
University, (Recritvd
Department 4 February
of Chemistry,
Oxford,
1981. Accepted
Ohio 45056, U.S.A.
6 May
1981)
Summary ~~As(lll), As(V), monomethylarsenic acid (MMA) and dimethylarsenic acid (DMA) were determined dy graphite-furnace atomic-absorption spectrophotometry afier separation of the species by ion-exchange chromatography. The detection limits (ng/ml) were DMA 0.02, MMA 2.0, As(V) 0.4 and total arsenic 4.0. As(III) was determined by difference. This system gave better detection limits and/or shorter analysis times than previously reported systems.
Perez” observed that DMA has a strong affinity for cation-exchange resins in acid medium. Elten and Geigerz3 used this fact to separate MMA and DMA before determination by differential pulse polarography (DPP). Henry er al. reported a method for the determination of As(III) and As(V) and total inorganic arsenic by DPP.24 Later, Henry and Thorpez5 reported a complete separation of all four species, using both anionand cation-exchange columns, and detection with DPP. This separation procedure is superior to all previous systems but suffers from the slowness of the digestions needed to convert the organoarsenicals into As(II1). With this in mind, we have modified this separation system for use in conjunction with graphite-furnace atomic-absorption analysis, and have designed an analysis system capable of giving a comparable detection limit without the time delays caused by the digestions.
The determination of metal species is essential to the understanding of the toxic and carcinogenic nature of metals found in environmental and biological systems. For example, the measurement of total arsenic content does not indicate the true levels of the individual species. To estimate effectively the hazard involved, the variations in toxicity, transport, and bioavailability, which are dependent on the arsenic chemical species, must be taken into account. It has been shown that arsenic can be found in natural systems as As(IIl), As(V), monomethylarsenic acid (MMA) and dimethylarsenic acid (DMA).‘-’ Other organic arsenic species have been reported to occur in some manufacturing processes.’ Analytical methods aimed at differentiating between these species are needed. As would be expected, a variety of techniques have been used for speciation of arsenic. Gas chromatohave been graphy’ lo and spectrophotometry”m14 demonstrated as effective techniques for one or more arsenicals. but neither has been used for mixtures for all four species listed above. Atomic absorption,’ an obvious choice for determination of metals, has been tried in conjunction with the generation and selective volatilization of arsines, which did resolve the four species,15 but the possibilities of molecular rearrangement and poor recoveries at low concentrations have raised doubts about this system.‘b.‘7 Separation by thin-layer chromatography followed by graphite-furnace atomic-absorption analysis has also been reported.‘8,‘9 Some flameless atomic-absorption systems have been reported. One system employed a separation scheme which gave less efficient resolution of species.” Woolson” published a system combining HPLC with graphite-furnace analysis; the limit of detection was 5 ng. So far, the most successful detection system has been electrochemical, following separation on ionexchange columns. Yamamoto” and Dietz and m~28I2
EXPERIMENTAL
-
Apparatus
A Perkin-Elmer model 560 atomic-absorption spectrophotometer and HGA-400 graphite-furnace atomizer were used for all the metal determinations. A new hollow-cathode arsenic lamp was used. Purified and dried argon was used as the sweep gas for the HGA-400. Reagents
High-purity arsenic trioxide was obtained from ROC/ RIC (Belleville. N.J.). All other chemicals were those used by Henry and Thorpe. ” The water was purified with a Barnstead NAN0 pure system followed by a Barnstead glass double-distillation system. All other chemicals were reagent grade. ion-exchange
columns
Dowex 5OW-X8 and AI-X8 resins (5CrlOO mesh) were used. Contaminants were removed from the resins by repeated washing alternately with 0.5M hydrochloric acid and ammonia solution at a flow-rate of 5-10 ml/min. The
b
935
Ci. E. PACEY and
936
exchange columns were slurry-packed. The cationexchanger column was 16 cm long (1 cm diameter) and the anion-exchanger column 10 cm long (1 cm diameter). The cation resin was readily regenerated by passage of l.OM hydrochloric acid through the column. It was necessary to convert the anion resin into the acetate form. This was done by passing 300 ml of 0.5M ammonia solution through the column, followed by 150 ml of l.OM ammonium acetate at a flow rate of 5-10 ml/min. The column was rinsed with water and then with O.lM acetic acid/ammonium acetate buffer (pH 4.7) until the effluent pH was 4.7. The preparation of the anion column was crucial to the success of the separation since careless preparation caused random errors in the results for the standard solutions. Control of the flow-rate was critical.
J. A. FORD
Optimization
of graphite-furnace
conditions
Table 1 shows the atomization parameters for the atomic-absorption determination of arsenic species. The four species can be determined in an essentially identical manner with the exception of the drying temperature used for MMA and As(V). Because of the high acetate concentration it was found necessary to increase the temperature to prevent sputtering and loss of sample. The stopped-flow gas system gave considerably higher absorbance values. with consistently better precision. than miniflow or highflow gas-pur’ge systems. The 193.7-nm line was used for all measurements. Deuterium lamp background correction did not affect the quality of the atomic-absorption measurements and was therefore not used.
ProWdW
RESULTS
The separation and determination of the four arsenic species is-schematically represented in Fig. 1. The suggestion bv. Henrv * and Thorpe” of maintaining - the pH of a sample between 4 and 10 was adopted. Four equal volumes of sample were used. for determination of total arsenic, DMA. MMA and As(V). As(lll) was calculated by difference. To isolate DMA from the matrix 100 ml of sample were mixed with 1.0 ml of 1.75M acetic acid and loaded onto the cation-exchange column at a flow-rate of 5 ml/min, and 70 ml of 0.02M acetic acid were passed through the column at a similar flow-rate to ensure complete separation from MMA. As(fll) and As(V). Then DMA was stripped by passing l.OM ammonia solution through the column at a flow-rate of I.0 ml/min. The Row-rate is critical for successful stripping. The fraction of effluent between 40 and 50 ml was analysed. To isolate MMA 100 ml of sample were mixed with pH 4.7 ammonium acetate/acetic acid buffer (total acetate concentration 0.01.W) and passed through the anion-exchange column, followed by 50 ml of O.OlM buffer of the same pH. The As(ll1) and DMA were eluted in the first 120 ml of effluent but As(V) remained on the column. The MMA was collected by stripping with 0.5iLI acetate buffer of pH 4.7. the fraction of effluent between 10 and 30 ml being collected and analysed. The As(V) did not begin to elute until 65 ml of the 0.5M acetate solution had passed through the column. The next 30 ml were collected and analysed for As(V1. Flow-rates of 1.8-2.0 mljmin were required. As all the MMA was eluted by 40 ml of the 0.5M buffer. a 1M acetate buffer could then be used to strip the As(V) more compactly, the fraction between 20 and 30 of the 1 M acetate eluate ml being sufficiently representative of the As(V) sample.
AND DISCUSSION
The system described provides excellent resolution of DMA, MMA, and As(V). The ion-exchange resins also act as a preconcentrator, thus lowering the overall working detection limits. DMA was retained on the cation-exchange resin in 0.02M acetic acid. A break-through volume of greater than 500 ml was observed. By use of large sample volumes and elution with ammonia an initial DMA concentration as low as 0.02 ng/ml could be determined. The DMA detection limit with the graphitefurnace system was 0.01 ng. This is better by a factor of more than 10 than the 0.13-ng detection limit for total arsenic achievable with the HGA-400 system under the same preconcentration conditions. This enhancement is due to the ammonium ion used to elute the DMA from the column. A similar effect has been reported for several alkali and alkaline-earth metal ions, but the ammonium ion was not mentioned.” Table 2 shows the efficiency of the system for the determination of low levels of DMA. The recovery is good. MMA was retained by the anion-exchange resin. with a break-through volume greater than 150 ml. Again a large volume of sample can be used. A sample with an initial MMA concentration of 2.0 ng/ml can be analysed. The absolute detection limit was 0.5 ng. The enhancement found for DMA was not
SAMPLE
r DMA oliquol
MMA,
I
I As (V)
I
oliquot
I
Anionexchange column
Cation exchange column
Total As oliquot
I
Grophi te furnace
&A
I
Grophite furnace Fig. I. Flow-chart
for the determination
Grophlte furnace
I
As (V)
I
Graphite furnace
.
of total As, As(V), MMA
and DMA.
Arsenic speciation
931
Table 1. Graphite-furnace parameters for determination of arsenic species (stop-flow mode) DMA Drying temperature, -C
loo
Drying ramp time, see Drying hold time, set Charring temperature, -C Charring ramp time, set Charring hold time, set Atomization temperature, ‘C Atomization ramp time, src Atomization hold time. src
MMA
As(V) Total As 140 I 40
loo 1 40
700 30 15 2700 0 5
700 30 15 2700
140
1 40 700 30 15 2700 0 5
1 40 700 30 I5 2700 0 5
0
5
*SO-/t1injections into pyrolytically-coated graphite tubes; argon purge at 60 ml/min.
observed for MMA. Table 3 shows the efficiency of the system for MMA determination; again there is good recovery. Since both As(V) and MMA were retained by the anion-exchange column, it was possible to separate them during the elution. The minimum concentration of As(V) in a sample that could be detected was 0.4 ng/ml and the absolute amount of As(V) detectable in the furnace was 0.2 ng. The As(V) was easily retained and recovered with similar efficiency to that for DMA and MMA. The detection limit for total arsenic concentration was 4.0 t&ml, which is more than adequate in view of
the concentrations in the EPA reference material and the fly-ash slurry sample of Henry and Thorpe.25 At no time was there any evidence that the system allowed any interconversion of species. Therefore, the determination of As(III) by difference was legitimate. Tables 4 and 5 show the recovery of As(V) and As(II1). The concentrations used were 30-80 ng/ml since the naturally occurring range is at that level. The recovery of As(V) is slightly lower than that of either MMA or DMA, because of the strong affinity of As(V) for the anionic resin. The resin usually retains a small but fairly constant amount of As(V) regardless of the pH and ionic strength. The As(III) was calculated by difference. The higher recovery for
this species may be due to impurities in the DMA and MMA and/or the small inherent negative error in the As(V) determination. This system gave detection limits lower than those of the differential pulse method, with the added advantage that the time-consuming digestion process was no longer needed. Variations in the actual detection limits will obviously occur with different graphite furnace systems. The ionic state of each species has not been clearly identified. Dietz and Perez” pointed out that the separation of all four species on a cation-exchange column was nor due to an ion-exchange mechanism, and explained the separation behaviour as due to ionexclusion chromatography. Essentially, it was concluded that at the pH-values used with the cation-exchange column As(V) species were behaving as ionized solutes and were excluded from the resin. The non-ionized nature of the As(II1) species meant that some penetration of the resin would occur but not to the same extent as for MMA or DMA. The pK, values of 3.6 and 6.2 for MMA and DMA respectively fall between those for arsenic and arsenious acids. DMA is strongly bound to a cationexchange column, probably as (CH&As02HNHi in our case. The MMA is strongly held by the anionexchange column, probably as a (CH,)AsO; species.
Table 2. Recovery of DMA on Dowex 5OW-X8
Table 3. Recovery of MMA on Dowex Al-X8 Taken, rig/m//
Found.
ng/ml
Recovery, “jO Taken, ny/n~l
0.080 0.080 0.050 0.050 0.050 0.050 0.030 0.030 0.030 0.030
0.080 0.052 0.054 0.046 0.033 0.030 0.03 1 0.029
Mean lOI”,,; standard deviation 5?:,
95 100 104 100 108 96 110 100 103 96
7.2 7.2 6.0 6.0 6.0 5.0 4.0 4.0 4.0
Found,
ngjml
7.1 7.2 5.9 5.9 6.2 5.2 3.8 4.1 4.2
Mean lOl”,/,: standard deviation 3%
Recovery, “/, 99 100 98 98 103 104 95 103 105
G. E. PACEY and J. A. FORD
938
Table 4. Recovery of As(V) on Dowex Al-X8 Taken, ny/tnl
Found, ng,‘ntl
Recovery, I!,,
80 X0 50 50 50 50 30 30 30 30
16 81 51 50 48 46 30 31 29 28
95 101 102 100 96 92 100 103 97 93
Mean 98“,,: standard deviation 4”,,
Table 5. Recovery of As(lIl) by difference Taken. ny/,nl
Found, ny/rnl
Recovery, “;,
80 80 SO 50 50 50 30 30 30 30
79 82 52 50 54 48 32 30 31 28
99 103 104 100 108 96 107 100 103 93
Mean lOI”,,: standard deviation 5”,,.
The stronger retention of As(V) as an As04H; speties would be consistent with the previously described systems of Dietz and Perez’* or Henry and Thorpe.*’ A[,l\~~o,,,lrdgrrlterlr~Funding for the purchase of the graphite furnace was provided by NSF 69A Grant No. NSF-CDP-800191 I.
REFERENCES I. R. S. Braman and C. C. Foreback. Scienc’e.1973, 182,
1241. 2. F. Challenger, Chrn~.RN., 1945, 36, 315. 3. B. C. McBride and R. S. Wolfe, Biochrtni\rrr, 1971, 10, 4312. 4. D. W. VonEndt. P. C. Kearny and D. D. Kaufman J. Agric. Food Clam.. 1968. 16. 17. 5. E. A. Woolson and P. C. Kearny. Enriron. .%I’.Techt1ol.. 1973, 7, 47. 6. G. G. Ricci. L. S. Shepard. N. Hester and G. Glovos, Paper delivered al 2nd Chemical Congress of the North American Continent. Las Vegas. 1980. 7. A. W. Fickett. E. H. Daughtrey and P. Mushak. Anul. C/tint. Actu. 1975. 79, 93. 8. J. D. Lodmell. Ph.D. Thu.si.~. University of Tennessee. Knoxville, Tenn.. 1973.
9. L. D. Johnson, K. 0. Gerhart and W. A. Aue. Sc,r. Total Enriron.. 1972. I, 108.
10. C. J. Soderquist, D. G. Crosby and J. B. Bowers, 4mrl. C/tern.. 1974, 46, 155. I I. S. A. Peoples, J. Lakso and T. Lais. Proc,. B’c,.\r.Phtrmucwl. sot., 1971. 14. 178. 12. M. G. Haywood and J. P. Riley. Anal. C/tint. 4cro. 1976. 85, 219. 13. T. Kamada, Tuluntu, 1976. 23, 835. 14. S. S. Sandhu, Anul!.st, 1976, 101, 856. 15. Y. Talmi and D. T. Bosik, Amd. Chern.. 1975.47, 2145. 16. J. E. Portman and J. P. Riley. Antrl. Chits. .&ttr, 1964. 31, 509. 17. M. B. Casvalho and D. M. Hercules, Aml. Clwt~.. 1978. 50, 2030. 18. Y. 0. Danard, 0. Matano and S. Goto. Bntt\t& Kay&. 1979. 28, 517. 19. R. R. Stanforth. Etc. Sri. Tech., 1979, 12, 1491. 20. E. A. Woolson and N. Aharowsow, J. ,4ssoc. Qfl. .4rtu/. Chvn~., 1980, 63. 523. 21. M. Yamamoto, Soil Sci. Sot. Am. Proc.. 1975. 39, 859. 22. E. A. Dietz and M, E. Perez. Anal. Chum.. 1976, 48, 1088. 23 R. K. Elton and W. E. Geiger. Jr.. ihid., 1978. 50. 712. 24 F. T. Henry, T. 0. Kirch and T. M. Thorpe. Anctl. Chrrn.. 1979. 51, 215. 25 F. T. Henry and T. M. Thorpe. AIIUI.Chrh~.. 1980. 52, 80.