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Anal. Chem. 1981, 53, 966-968
Determination of Arsenic(III), Arsenic(V), Monomethylarsonate, and Dimethylarsinate by Ion-Exchange Chromatography with Flameless Atomic Absorption Spectrometric Detection Archibald A. Grabinski Water Chemistry Program, 660 North Park Street, University of Wisconsin, Madison, Wisconsin 53706
Aqueous arsenlc(III), arsenic(V), monomethylarsonate (MMA), and dlmethylarsinate (DMA) were separated with ion-exchange chromatography. The elutlon sequence was as follows: 0.006 M trlchloroacetlc acid (pH 2.5), yielding flrst As(II1) and then MMA; 0.2 M trichloroacetic acid yieldlng As(!!); 1.5 M NH40H followed by 0.2 M trlchloroacetic acid yielding DMA. Detectlon was by flameless atomlc absorptlon spectrometry. Arsenic recoverles (full procedure) ranged from 97% to 104% for typical lake water samples; more erratic but stlll acceptable recoveries (96% to 107%) were ohtained from arsenic contamlnated sedlment interstltlal water. The overall analytlcal detectlon limit was 10 ppb (orlginal sample mixture) for each indlvldual arsenic specles. Relatlve standard deviations ranged from 0.7 % to 1.3 % for lake water and dlstllled deionlzed water replicates spiked at the 500 ppb level.
Large fluxes of inorganic arsenic into the aquatic environment can be traced to geothermal systems ( I ) ,base metal smelter emissions (Z), and localized arsenite treatments for aquatic weed control. The methylated arsenicals have entered the environment either directly as pesticides or by the biological transformation of the inorganic species (3-6). Analytical techniques are needed for arsenic speciation of contaminated natural waters, especially since arsenic species vary considerably in toxicity to humans (7). Existing methods have some analytical shortcomings. Iverson et al. (8)presented a method which separated the inorganic arsenic from each of the organic species using ion-exchange chromatography. Here, further inorganic speciation relies on redox-based colorimetry (9);both the accuracy and precision suffer from the low As(III)/As(V) and As(total)/P ratios normally encountered in the environment. Henry and Thorpe determined these four arsenicals by coupling a digestion and reduction scheme with ion-exchange chromatography ( I O ) . However, the utility of this technique for routine environmental analysis is limited, since the implementation time is substantial. This method also relies on estimating As(V) by arithmetic difference. Braman and Foreback ( I l ) , Andreae (12,13),and others used a method based on hydride reduction at selective pH to obtain arsenic speciation data. Although this technique offers excellent sensitivity, analytical limitations such as molecular rearrangements (14) and incomplete recoveries (15) have been reported. In addition, the accuracy of arsenic speciation data for waters with elevated levels of dissolved ions (such as sediment interstitial water) would require verification, since sufficient ionic levels will prevent completeness of the desired reduction (16, 17). These limitations are circumvented in the work presented here by achieving a complete separation of all four arsenic species on a single column containing both cation- and anion-exchange resins. Flameless atomic absorption spectrometry with a deuterium arc background correction is used as a detection system for this procedure. This detection system was chosen because of its linear response and lack of specificity
for these compounds (8), combined with its resistance to matrix bias in this type of analysis.
EXPERIMENTAL SECTION Instrumentation. Arsenic determinations were made by using a Perkin-Elmer 603 atomic absorption spectrometer equipped with a Model 2100 graphite atomizer containing a tantalum-treated graphite tube (18). Sample injections ranged from 10 to 100 pL. Ramp and drying times were adjusted to 20 and 35 s, respectively, when 100-pLsample volumes were employed. All measurements were made by using a deuterium arc background correction. Argon flow was interrupted during atomization to maximize sensitivity. Other instrumental parameters were previously described (8). Reagents and Materials. Reagent grade sodium arsenate, sodium arsenite, and dimethylarsenic acid were obtained from J. T. Baker Co. (Phillipsburg, NJ). Monomethylarsonic acid (99.9%) was obtained from the Ansul Co. (Weslaco, TX). The cation-exchangeresin, AG 50 W-X8 (100-200 mesh, styrene type, sulfonic acid), and the anion-exchange resin, AG1-X8 (100-200 mesh, styrene type, quaternary ammonium), were obtained from Bio-Rad (Richmond, CA). Remaining chemicals were reagent grade. Distilled, deionized water was used for preparation of conditioning solutions and mobile phases. Column Packing and Conditioning. Nine centimeters of the anion-exchange resin was slurry packed into a glass column (35 cm X 1 cm i.d.) equipped with a 100-mL resevoir and adjustable nitrogen pressure. The cation-exchange resin was similarly packed onto the remaining 26 cm of column. In order to avoid irreversible adsorption or other processes which may prevent quantitative recoveries (19),we exposed the resins to 50 pg of each arsenic species (8) and then washed them several times with alternating solutions of 1.5 M NHIOH (70 mL), 1M HCl(70 mL), and 0.48 M HCl(70 mL) at a flow rate of 5-10 mL min-l. These solutions were also used to regenerate a used column after every chromatogram by passing each one once through the bed. Care was always taken t o avoid drying of the resin bed. Column degradation was not observed, even after several weeks of daily usage. However, in some casks, if a column went unused for an extended period of time, a small visible air pocket was formed at the junction of the two resin beds. Although this problem did not interfere greatly with the function of the column, the used resins were repacked in order to eliminate excess eddy diffusion and consequent peak broadening. Mixing of the resin beds upon removal from the column was avoided, since the resultant mixture could not be repacked without air pocket formation. The entire repacking process took approximately 10 min. Procedure. The sample, at pH 2.5-7, containing 80-4000 ng of total arsenic in a volume not exceeding 2 mL, was gravity loaded onto the resin. The column was eluted with 55 mL of 0.006 M trichloroacetic acid followed by elution with 8 mL of 0.2 M trichloroacetic acid, each at a flow rate of 2 mL mi&. Next, 55 mL of 1.5 M NH40H was eluted, followed by a 50-mL mobile phase of 0.2 M trichloroacetic acid, each at a flow rate of 6 mL min-l. Fractions of known volume (3-20 mL) were collected in Pyrex glassware and treated with HNO, (1%v/v) and Ni(NO& (0.1% wt/v) to prevent arsenic losses upon charring (20). These fractions were analyzed for arsenic by flameless atomic absorption spectrometry.
RESULTS AND DISCUSSION The widely spaced Erst pk values of arsenic and arsenious acid (2.26 and 9.2, respectively) suggested a separation of these species based on an affinity for an anion-exchange resin.
0003-2700/81/0353-0966$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
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Table I. Comparison of Signal Stability (as rsd) with and without Matrix Modification 50 P P ~ 300ppb 50ppb 300ppb untreated, % untreated, % treated, % treated, %
AS(II1)
AS( V) MMA DMA
12.0 10.6 10.7 15.4
1.92 7.83 3.97
5.84
2.36 5.23 5.54 4.08
2.87 3.36 1.37 3.28
Trichloroacetic acid (0.006 M) was chosen as the initial mobile phase since the pH was low enough to completely resolve the As(II1) from the MMA (8)on the cation-exchange resin, while leaving the As(V) as an anionic species. It was found that 23 mL of this mobile phase was sufficient to strip the uncharged As(II1) from the resins, leaving the negatively charged Gs(V) strongly bound to the anion-exchange resin. The 26-55 mL eluent fraction contained the weakly retained MWA. Trichloroacetic acid (0.2 M, 8 mL) provided sufficient [H+]to release the arsenic acid from the anion-exchange resin into the 65-85-mL eluent fraction. NHIOH (1.5 M) was used to strip the strongly retained DMA from the first column stage. Here, the flow rate was increased to 6 mL min-l since difficulties due to peak broadening were not encountered. Finally, DMA release from the anion-exchange resin was aahieved through elution of 0.2 M trichloroacetic acid. Since the detection system responds linearly and nonspecifically to these four arsenic compounds, only one species was needed to prepare standards for calculating quantities of each compound in a chromatographed sample. As(V) standards were used for the measurements reported in this paper. The arsenic compound peaks were identified by comparison with breakthrough volumes of individually run standards. This method was also used as a means for checking degradation of standards. Except for initial peak identification and standard degradation checks, no benefit was derived from running standards through the separation technique. Procedural Optimization. Elution rates were chosen so that base line absorptions between any set of peaks could be observed when 1pg of each species was placed on thg column. The base line absorption criteria was difficult to maintain, regardless of flow rate, when the As(III), MMA, or As(V) quantity was greater than 1pg. Column capacity to resolve DMA (normally a minor constituent in arsenic contaminated natural waters) was not determined. Cation-exchange resin bed length was chosen so that the inorganic fraction was adequately resolved from the MMA and the MMA from the DMA. Anion-exchange resin bed length was chosen so thaf quantitative retention of anionic arsenic species could be achieved while creating minimal dead volume for unchanged species. Ediger’s matrix modification (21), believed to form nonvolatile nickel arsenides in the charring stage of graphite furnace atomic absorption spectrometry, was used for all analyses. This pretreatment was found to consistently enhance the spectrometric signal by 15 f 1.4%. In addition, signal reproducibility was significantly improved (Table I). Attempts to Preconcentrate. A method was unsuccessfully sought which would allow sample preconcentration without changes in relative speciation or spike recoveries. Evaporative preconcentration at pH 10 (as suggested earlier (8)for a two-step speciation) resultea in quantihtive oxidation of As(II1) to As(V), while at pH 2 As(V) was reduced to As(II1) with losses observed for inorganic arsenic and DMA. Interfering Signals. Although deuterium arc background correction was sufficient to eliminate apparent arsenic absorptions or inhibitions from inorganic and organic species normally encountered in natural samples and reagents, measurements near the detection limit required an assessment
Ii
a AsIII b MMA c ASP
IooioAu DMA d
mL
Flgure 1. Sample chromatogram of spiked Lake Mendota water.
Table 11. Arsenic Speciation from Spiked and Unspiked Contaminated Sediment Interstitial Water species As(II1) MMA &(V) interstitial water, ng spike, ng interstitial water + spike, ng recoveryofspike, ng recovery of spike, %
DMA
0 300 310
2278 ,300 2576
300 300 620
18 300 330
310
298
320
312
103
99
107
104
of inorganic arsenic content in the 0.2 M trichloroacetic acid mobile phase. Since this background absorption, which corresponded to approximately 2 ppb arsenic, was only observed at measurements near the detection limit and its effect easily evaluated, further purification of the reagent was not undertaken. Detection Limits. Detection limits were defined as signal to noise ratios of 2 to 1,with noise being the maximum signal variance of replicate determinations. Arsenic leaching from the resins was not observed in blank chromatograms. Applications t o Natural Water Samples. In order to demonstrate the practial utility of this technique, we spiked 0.500 pg of each arsenic species into filtered (0.45 pM) Lake Mendota water (sampled in Madison, WI) and distilled, deionized water. Arsenic recoveries for the entire procedure averaged 104%, loo%, 97%, and 99% for As(III), MMA, A@), and DMA, respectively. Relative standard deviations for replicate determinations ranged from 0.7% for As(II1) and DMA to 1.3% for As(V). A sample chromatogram of Lake Mendota water is shown in Figure 1. Interstitial water from sediment sampled at the mouth of the Menominee River near Marinette, WI, was also analyzed for each arsenic species. The water was contaminated with arsenic (primarily MMA) and had considerable air exposure. The results of this analysis are presented in Table 11. ACKNOWLEDGMENT Sincere gratitude is extended to Robert Stauffer, Robert Stanforth, and Marc Anderson for stimulating conversations and suggestions concerning this research. LITERATURE CITED Stauffer, R. E.; Ball, J. W.; Jenne, E. A. “Chemical Studles of Selected Trace Elements in Hot Spring Drainages of Yeiiowstone National Park:” Geologlcal Survey Professional Paper 1044-F; U.S. Government Printing’Office: Washington, DC, 1980. Creceliqs, E. A. Limnol. Oceanogr. 1075, 20, 441. Andreae M. 0.Anal. Chem. 1977, 49, 820. Challenger, F.; Higgenbottom, C.; Ellis, L. J . Chem. SOC. 1033, 7, 95. Wong. P. T. S.; Chaw, Y. K.; Luton, L.; Bengut, G. A.; Swalne, D. J. Methylatlon of Arsenic in the Aquatic Environment”, Conference Proceedings on Trace Substances In Environmental Health-XI; Hemphill: Universlty of Missouri, 1977. McBride, B. C.; Wolfe, R. S. 6iochemlstty, 1071, 10, 4312.
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Anal. Chem. 1981, 53, 968-971
(7) Webb, J. L. “Enzyme and Metabollc Inhlbitors”; Academic Press: New York, 1966; Vol. 3, Chapter 6. (8) Iverson, D. G.; Anderson, M. A.; Holm, T. R.; Stanforth, R. R. Environ. Sci. Technol. 1979. 13. 1491. (9) Johnson, D. L.; Pilson, M. E. A. Anal. Chim. Acta 1972, 58, 289. (10) Henry, F. T.; Thorpe, T. M. Anal. Chem. 1980, 52, 80. (11) Braman, R. S.; Foreback, C. C. Sclence 1973, 182, 1247. (12) Andreae, M. 0. Anal. Chem. 1977, 49, 820. (13) Andreae, M. 0. Deep-sea Res. 1978, 25, 391. (14) Talml, Y.; Bostik, D. T. Anal. Chem. 1975, 47, 2145. (15) Carvalho, M. B.; Hercules, D. M. Anal. Chem. 1978, 50, 2030. (16) Braman, R. S.;Justen, L. L.;Foreback, C. C. Anal. Chem. 1972, 44, 2195. (17) Pierce, F. D.; Brown, H. R. Anal. Chem. 1976, 48, 693.
(18) Smlth, A. E. Analyst (London) 1975, 100. 300. (19) Zatka, V. J. Ana/. Chem. 1978, 50,538. (20) Zief, M.; Mitchell, J. W. “Contamination Control in Trace Element Analvsis:” Wilev: New York. 1976: Chanter - -r - 6. (21) Edige;, d. At.-j\bso$t. News/.-1975, 14, 127
-.
RECEIVED for review October 27, 1980. Accepted February 17, 1981. This research was supported in part by NOAA, Office of Sea Grant, through an institutional grant to the University of wisconsin, and by a grant ( N ~D, A ~ ~ C-0046) from the U.S. Army Corps of Engineers.
Determination of Alkaline Earth Metals by Ion-Exchange Chromatography with Spectrophotometric Detection Michlo Zenki Department of Chemistty, Okayama University of Science, 1- 1, Rldai-cho, Okayama-shi, 700, Japan
A liquid chromatographlc-spectrophotometric system has been developed to separate magnesium, calclum, strontlum, and barium. The eluent used is 0.7 M sulfosalicylic acid containing chlorophosfonazo 111 as a color-forming reagent. The separated ions after passing through a catlon-exchange column are detected dlrectly by a spectrophotometric detector. The detection limlt Is 50, 2, 6, and 20 ng for magneslum, calclum, strontium, and barium, respectlveiy. The determination of calclum In natural waters was carried out by this method.
The separation and determination of alkaline earth metals have been of great interest in analytical chemistry, because these metals exist widely in nature and occur together in significant amounts. They seem to play an important role in the field of biology. Although atomic absorption spectrophotometry or flame spectrophotometry has been used for sensitive analysis of alkaline earth metals, it is useful for the individual metal but is not available for their simultaneous and automatic determination. Separations of various metals have been achieved by using ion-exchange chromatography, but most separations are still achieved by analyzing the collected fractions from the column. Recently, high-performance liquid chromatographic techniques (HPLC) for the separation and determination of organic compounds has advanced greatly, but ion-exchange chromatography of metal ions has not. The fact that few detectors are suitable for direct and sensitive detection of metal ions is responsible. Fritz and co-workers (1,Z) reported the ion-exchange separation and determination of calcium and magnesium with spectrophotometric detection. A colorforming reagent and buffer solution were added by pump after column separation of sample ions. The reagent, buffer, and eluate were mixed in a mixing coil (postcolumn reaction). Small et al. (3) employed conductance detection after removal of the eluent by a “stripper” column. Freed ( 4 ) used flame emission for detection of calcium, strontium, and barium. The author has investigated the use of bisazochromotropic acid derivatives as sensitive reagents for several metals ( 5 6 ) . The reagents form water-soluble complexes with alkaline earth
metals and have large molar absorptivities (lo4)over a wide pH range. Although Fritz and co-workers ( I , 2) had previously used these reagents in several postcolumn reaction chromatographic studies, no investigation of their use for the determination of alkaline earth metals has been published. This paper reports the application of these reagents for the HPLC separation of alkaline earth metals with subsequent on-line spectrophotometric determination. The postcolumn reaction was not employed in this work, because controlling pH in the color reaction was very difficult. Thus, the bisazochromotropic acid derivatives are mixed with the eluent prior to being passed thraugh the separation column. The effluent is then introduced directly to a visible spectrophotometric detector. The pump which delivers the reagent and buffer solution, and the mixing tee, chamber, or coil are not necessary.
EXPERIMENTAL SECTION Apparatus. Liquid chromatography was performed with a Yanagimoto L-2OOOL (Yanagimoto Co., Kyoto, Japan) and Rheodyne 7125 sample injector (Rheodyne Inc., Berkeley, CA). The analytical column was a 2.6 nun i.d. X 150 mm length s w e s s steel tube fitted with a water jacket. The temperature used in this work was 25 “C. The detector was a Hitachi Model 200 spectrophotometer (Hitachi Scientific Instruments, Tokyo, Japan). Its sample compartment was altered to accommodate a flowthrough cell having a light path of 8 mm and internal volume of 8 pL. An airtight microsyringe (25 pL) was used to inject the samples. Ion-Exchange&sin, A Hitachi custom cation exchange resin, 2613, was slurry packed into a stainless steel column. The resin consisted of porous polymer particles having sulfonic acid ionic group in a styrene-divinylbenzene matrix. The average particle size was about 17 pm. The resin was conditioned with a sufficient amount of eluent solution before use. Reagents. Unless otherwise noted, all chemicals were analytical reagent grade quality and were used as received. Chlorophosfonazo I11 was obtained from Dojindo Laboratories (Kumamoto, Japan) and used without further purification. Other bisazochromotropic acid derivatives were synthesized and purified in our laboratory (6). Metal ion solutions were prepared by appropriate dilution from 1000 ppm standard solutions supplied by Wako Pure Chemicals Co. (Osaka, Japan). Deionized-distilled water was used throughout. Eluent. For the preparation of eluent solution, 152.6 g of sulfosalicylic acid (Wako Pure Chemicals Co.) and 40 mg of
0003-2700/81/0353-0968$01.25/00 1981 American Chemical Society
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