~~~
~~
~
Table 111. Aluminum Assay of Normal Urine Subject 3b
/*E
Allml
Std dev
0.13 0.03 0.06 0.02 2" 0.03 0.03 46 0.02 0.02 5" 0.02 0.01 6c 0.01 0.01 " n = 8. n = 9. c n = 6. 1"
h sample
Std dev
A1 resin retention yield, %
222.5 60.3 127.4 40.8 33.5 15.0
40.9 16.1 119.0 29.0 13.1 15.0
84.5 92.6 86.4 86.7 84.4 90.2
/*g A1124
values. It is interesting to note that the range in aluminum concentration varied from near zero with a relative standard deviation of f100% to 0.13 pg/ml with a relative standard deviation of f23%. Employing urine samples which have been spiked with 5 pg of aluminum per ml of urine, we find, in general, relative standard deviations of less than f5%. In the range 0.05 to 0.30 pg Al/ml urine, the relative standard deviation is relatively constant a t -20%. Below these amounts, the relative standard deviation increases to 100%. We have assigned the value 0.05 pg Al/ml of urine as the lower detection limit in urine. Several reasons as to why the precision decreases with decreasing concentration of aluminum are as follows: (a) the statistical nature of radioactivity. For a sample containing 0.5 /*gof A1 or 0.05 pg Al/ml, the relative standard deviation resulting from the statistics of radioactive decay is 7%; (b) all of the reagents, including the resin, contain trace quantities of aluminum. Even employing the best grade reagents, the reagent blank is still significant. As the aluminum level in the urine approaches that of the reagent blank, the relative precision and accuracy, of course, will suffer; and (c) nonsystematic contamination of samples by aluminum contained in dust particles. I t is our contention that trace levels of aluminum, not only in urine but in other biological specimens such as tissue, bone, and serum, now can be routinely analyzed by the procedure described in this paper. In our experience in routinely analyzing biological specimens, no modification of this technique will be required. However, it is important to observe strict environmental aluminum decontamination techniques in the sampling procedure for biological material.
LITERATURE CITED (1) G. M. Berlyne, J. Ben-Ari, D. Pest, J. Weinberger, M. Stern, G. R. Giimore, and R. Levine, Lancet, 494 (1970).
F
8
-
IO
I
I
12
14
AI 16
MOLARITY OF NITRIC ACID FINAL WASH
Figure 2. Aluminum and sodium resin retention yields from urine or water using varying molarities of nitric acid as a final wash 0 (a) 28Alretention (water). 0 (6)'*AI retention (urine). A (c) 24Naretention (water). A (d) "Na retention (urine). 28AI resin retention from urine at zero molarity represents an experiment where water was used instead of "03 for the entire procedure
(2) 0. M. Wrong and J. D. Swales, Lancet, 1130 (1970). (3) H. Thurston and J. D. Swales, Br. Med. J., 490 (1971). (4) G. M. Berlyne and R. Yagil, Lancet, 47 (1972). (5) G. M. Berlyne, R. Yagil, J. Ben-Ari, C. Weinberger, E. Knopf. and G. M. Danovitch, Lancet, 564 (1972). (6) H. Thruston, G. R. Gilmore, and J. D. Swales, Lancet, 881 (1972). (7) G. M. Berlyne, R. Yagil, J. Ben-Ari, and G. M. Danovitch, Lancet, 1070 (1972). (8) G. M. Berlyne. Lancet, 1253 (1970). (9) D. Waidron-Edward, P. Chan, and C. Skoryna, Can. Med. Assoc. J., 1297 (1971). (10) D. Smith and M. E. Mchin. Jr., Radlochem.,Radioanal.Left, 16, 89 (1974). (11) K. Fritze and R. Robertson, J. Radioanal. Chem., 7, 213 (1971). (12) G. R. Gilmore and B. L. Goodwin. Radiochem. Radioanal. Lett., 10 (4) 217 (1972). (13) F. Girardi. R. Pietra, and E. Sabbioni, J. Radioanal. Chem., 5, 141 (1970). (14) 0. U. Anders, Nucl. lnstrum. Methods, 68, 205 (1969). (15) A. Wyttenbach, J. Radioanal. Chem., 8, 335 (1971). (16) A. J. Blotcky, D. M. Duven, W. M. Grauer, and E. P. Rack, Anal. Chem., 46, 838 (1974).
RECEIVEDfor review October 6, 1975. Accepted March 11, 1976. This research was supported by the Omaha Veterans Administration Hospital (MRIS 7319-01 and 7365) and the US. Energy Research and Development Agency. This is ERDA document number COO-1617-43.
Purification and Analysis Methods for Methylarsonic Acid and Hydroxydimethy Iars ine Oxide Edward A. Dietz, Jr.,* and Maria E. Perez The Ansul Co., P.O. Box 1165, Weslaco, Texas 78596
Behavior of arsenous, arsenic, methylarsonlc acids and hydroxydimethylarsine oxide (cacodylic acid) with a strong-acid cation-exchange resin was investigated. The chromatographic separation of these compounds using this resln and only water as eluting solvent was utilized in two analyses: for determining concentrations of inorganic arsenicals in samples of methylarsonic acid; and for determining concentrations of methylarsonlc acid, in addition to inorganic arsenicals, In samples 1088
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
of cacodylic acid. Application of the analytical methods as a purification process has afforded 100.0% pure reference samples of methylarsonic acid and cacodylic acid.
Current concern about the effects and fate of arsenical pesticides in the environment has led to numerous investigations. For an excellent review of past and present efforts in
this field, a recent publication is available ( I ) . A significant portion of this work has been aimed at development of specific methods for differentiating and determining arsenicals. Since arsenic acid, arsenous acid, methylarsonic acid (MAA), and cacodylic acid (CA), are of major concern, determination procedures for these compounds have been worked out (1-8). In spite of these efforts, no reports have appeared describing methods for analyzing purified reference samples of MAA or CA. When possible, analytical procedures should be related to standard samples whose purity and composition are accurately known. Primary standard arsenic trioxide is available for total arsenic analyses. For studies involving MAA or CA, researchers obtain reference samples from chemical supply houses, arsenical manufacturers, or purify technical grade material. Commercial or laboratory syntheses of these compounds generally rely on the Meyer reaction (9): Na3AsO3
-
+ CHBX
NaX
+ CH3AsO(ONa)2
(1)
+ NaX + (CH3)2AsO(ONa)
(2)
where X = C1- or Br-. 1) SOZIHCI 2) KaOH 3) CH3X
CH3AsO(ONa)Z -Na2S04
Acidification of these reaction mixtures with HC1 or HzS04 followed by appropriate isolation steps provides impure MAA and CA. The acids thus obtained may contain NaCl (or NaBr), Na2S04, and arsenous acid; CA contains MAA as an additional component. Arsenic acid also is present in both products because of air oxidation of arsenite or from an intentional oxidation step conducted during commercial manufacture. Even though impure MAA and CA are purified to yield technical grade samples, varying amounts of salt and arsenical impurities are not removed. Further purification of technical material can reduce the impurity concentrations; however, since exact assay procedures and methods for establishing the arsenical contaminants in these purified samples have previously not been available, they can only be called purified (or reference samples). Total arsenic analyses and acid titrations sometimes are used to provide an estimate of purities, but these methods are accurate 9 n l y if arsenical c o n t a m i n a n t s in t h e respective samples have been removed. An estimated sample purity may be sufficient for some studies; for analytical methods development, metabolism studies, or toxicology investigations, concentrations of arsenical impurities should be taken into account. We have looked at available arsenical determination methods for application in analyzing arsenical contaminants in purified samples of MAA and CA. The only methods currently in use involve arsine generation reactions and require instrumentation that is expensive and unavailable to some researchers. Also, these arsine generation reactions are accompanied by molecular rearrangements (8) which would introduce significant errors in the analytical results for very pure MAA or CA samples. In fact, the need for certified reference samples for investigating rearrangement reactions and in developing one analytical procedure has been pointed out (7, 8).Therefore, we have developed an alternate technique which has general applicability. Our technique relies on a unique method for separating arsenic acid, MAA, and CA with a strong-acid cation-exchange resin. This separation also has afforded a method for obtaining very pure samples of MAA and CA.
EXPERIMENTAL Instrumentation. Colorimetric determinations were conducted using a Bausch and Lomb Spectronic 20 spectrophotometer which
employs 0.5-in. i.d. test tube cuvettes. Arsenic determinations by atomic absorption spectrometry were carried out with a Varian Techtron Model 1200 instrument. Determinations were made using a nitrogen-hydrogen-entrained air flame a t a wavelength of 1937 nm. Potentiometric and acid-base titrations were performed on a Metrohm Herisau Model E-536 recording potentiograph with an E-535 Dosimat equipped with a 50-ml autoburet. This instrument was used in the differential recording mode (dEldV) for chloride and acid titrations. For establishing end-point pH values needed in calculation of carbonate-corrected base normalities, the standard recording mode (dpH/dV) was employed. Reagents. Samples of technical and reference purity MAA and CA were available from The Ansul Co., Weslaco Technical Center, P.O. Drawer 1165, Weslaco, Texas 78596. All other chemicals were of reagent grade quality. All water originated from a central adsorption deionization purification system. Prior to use, this water was passed through another series of activated carbon and mixed-bed ion-exchange columns. Sodium hydroxide solution (0.25N) was prepared and standardized immediately before use. For titrations of MAA, the base was standardized to a pH value of 10.7; for CA the base was standardized to a pH value of 9.5. The strong-acid cation-exchange resin used in this work was AG 50W-X8 (100-200 mesh, hydrogen form) and was purchased from Bio-Rad Laboratories. For preparations of pure MAA and CA, this resin was purified before use by washing with water, 6 N HC1, water, methanol, water, 6 N HCI, and finally water. For analytical determinations, the resin was washed with water, 6 N HC1, and water before use. In all cases, resin weights refer to the moist resin (-50% water) prior to any washing steps. Procedures for Analyzing Methylarsonic Acid and Cacodylic Acid Samples. Apparent Acid Purity. Titrate approximately 1.5-g samples of either CA or MAA with standardized NaOH. From the titration results, calculate an apparent sample purity value. Chloride. From potentiometric titrations with AgN03, determine the NaCl content of either MAA or CA. Arsenous Acid. From redox titrations with BrO3- (methyl orange indicator), determine the arsenous acid concentrations in samples of CA or MAA. Sulfate. Place 1.7 g of MAA or 2.0 g of CA in a 5-ml screwcap vial. Dissolve the sample in 3.5 ml of water, then add 0.5 ml of 0.2 N BaC12. Cap the vial, agitate it for 1 min, then allow it to stand for 15 min. By visual inspection, compare turbidity in each vial to the turbidity produced by standard samples containing 0,40,50, etc. pg of NaZS04. Total Inorganic Arsenicals in Methylarsonic Acid. Dissolve 1 g of MAA in 4 ml of water. To oxidize trivalent arsenic, add bromine water until a perceptible yellow color remains. Remove excess bromine by heating, cool the solution, then place a 2-ml solution aliquot onto a chromatograph column (18-mm i.d.1 containing 200 g of ion-exchange resin. Elute the column with water (2 ml/min) and collect the arsenic acid fraction (80-120 ml). Determine the quantity of separated arsenic acid by atomic absorption spectrometry or the standard colorimetric silver diethyldithiocarbamate method (10). Total Inorganic Arsenicals and Methylarsonic Acid in Cacodylic Acid. Dissolve 1 g of CA in 5 ml of water and oxidize trivalent arsenic as described above. Place the solution onto a chromatograph column (18-mm i.d.) containing 10 g of ion-exchange resin. Elute the column with water ( 2 ml/min) and collect 100 ml of eluate. Following the colorimetric procedure of Peoples et al. ( 4 , 5 ) , determine the arsenic acid and MAA concentrations in the eluate. Chromatographic Purification of Methylarsonic Acid. In a typical preparation, 5 g of technical grade MAA was dissolved in 10 ml of water and oxidized with bromine as described above. The solution was placed onto a chromatograph column (18-mm i.d.) containing 200 g of ion-exchange resin, then eluted (2-3 ml/min) from the column with water. The eluate between 200-450 ml was collected then evaporated using a rotary vacuum evaporator to recover the MAA. The recovered MAA was titurated in 50 ml of acetone, oven dried at 110 OC for 1h, and stored in a vacuum desiccator for several hours. The final product (4.3 g) had a melting point of 160-161 OC. Chromatographic Purification of Cacodylic Acid. In a typical preparation, 50 g of impure CA (98% with 0.17% NaC1, 1.3% MAA, 0.6% H3As04) was dissolved in 150 ml of water and oxidized with bromine water as described above. While collecting eluate, (2-3 ml/min), the solution was added to a chromatograph column (18-mm i.d.) containing 200 g of ion-exchange resin. The first 750 ml of eluate was discarded; a t this point, a yellow color band due to absorbed CA had reached the bottom of the column. Collection of eluate containing CA was begun, and an ammonia solution (22 g of 30% NHz in 100 ml ANALYTICAL CHEMISTRY, VOL. 48, NO. 7 , JUNE 1976
1089
of water) was placed onto the column. After all the ammonia solution was on the column, elution with water was continued.From this elution, 350 ml of eluate was collected. Cacodylic acid was recovered from the eluate by evaporating the water using a rotary vacuum evaporator. The recovered CA then was titurated with 75 ml of acetone, oven dried at 110 "C for 1 h, and stored in a vacuum desiccator for several hours. The CA obtained from this process (27 g) had a melting point of 199-200 "C. A second fraction (250 ml) of eluate was collected and contained 4.6 g of CA. A small sample of the CA was dissolved in a few milliliters of water, then several pellets of NaOH were added. Presence of NH4+ was shown by a moist piece of pH paper which was held above the solution. A similar test with the CA for the first 350-ml fraction indicated no NH4+.
RESULTS A N D DISCUSSION In 1960, Baumgartel (11)briefly mentioned that CA was retained on a strong-acid ion-exchange resin. While completing this manuscript, work by Yamamoto has appeared: Soil Sci. SOC.Am. Proc., 39,859 (1975). He observed the retention of MAA and CA on a cation-exchange resin and applied it in separating arsenate MAA and CA in soil and pond water. These results and our findings that arsenous, arsenic, methylarsonic, and cacodylic acids have different retention volumes when chromatographed on a column of sulfonic acid resin is unusual because no true ion-exchange mechanism can prevail. In fact, these four protonic acids would not be expected to diffuse into the resin matrix (Donnan Membrane Theory) and, therefore, should elute from a chromatograph column after one void volume displacement. However, many non-ion-exchange interactions may occur when a solute passes through a resin bed (12). Evaluation of volume partition coefficients for these arsenicals in relation to their chemical properties suggests an explanation for the observed absorption differences. From Equation 3, in which V, is the elution volume of a component, V, is the external liquid volume of a column, Vi is the internal liquid volume of a column, and Kd is the volume partition coefficient, values of Kd have been calculated for the arsenicals. (3) When compared to HCl, which is assigned a zero value, a Kd for arsenic acid of about 0.05 is found. Thus, arsenic acid like HCl behaves as an ionized solute and is excluded from the acidic interior of the resin particles. This is expected since arsenic acid has a pK value of about 2.3 (13).The Kd value for arsenous acid is about 0.6; a value similar to that reported for methanol (12). The nonionic nature of arsenous acid, which is indicated by its pK value of about 9 (13),allows it to partially penetrate the liquid volume within the resin matrix. In this way, the inorganic arsenicals are separated on a column of resin. Since the pK values (9) for MAA and CA (3.6 and 6.2, respectively) are between those for the inorganic arsenicals, values of Kd near 0.6 might be anticipated. Surprisingly, the Kd value of MAA is 3; CA is so strongly bound (very large Kd) that a specific Kd is not meaningful. The strong retention of CA and moderate retention of MAA must result from a unique mechanism. The amphoteric nature of MAA and CA (9) provides an explanation for their resin sorption. For CA, an equilibrium constant of 37 (14) has been established for the protonation reaction:
+
(CH~)~ASOZH Hf
-
( C H ~ ) ~ A S O ~ H ~ +(4)
Although MAA is amphoteric, its basicity is no doubt less than that of CA. The functioning of these arsenicals as bases toward the very acidic resin particle probably results in their observed resin sorption. An alternative rationalization could be attraction of the nonpolar methyl groups of the arsenicals for 1090
ANALYTICAL
CHEMISTRY, VOL.
48, NO. 7, JUNE 1976
Table I. Apparent Purities of Methylarsonic Acid Samples Which Contain Arsenic Acid Sample compositiona
% MAA
-
Apparent purity of Apparent purity of MAA(%)from calcd MAA(%)from exptl % AAb total arsenic acidity titration
99.57 0.43 100.00 99.99 99.05 0.95 99.98 100.11 95.29 4.71 99.94 100.23 a MAA was of chromatographic purity; H3As04 was prepared from ASZOS. AA represents arsenic acid.
n
I 0
I 100
r-------4
I 200
I
I
I
300
400
500
M L OF E L U A T E Figure 1. Separation of (A) 1 mg of H3As04, (B) 1 mg of H3As03, and (C) 0.1 g of MAA on a 200-9 column (18-mm i.d.)of AG 50W-X8 using
water eluent. Eluate fractions were analyzed by arsenic AAS the hydrocarbon resin matrix. Nonpolar sorption of solutes is common. For example, phenol is attracted to the hydrocarbon skeleton of the H-form of sulfonic acid resins. This attraction is strong enough that it exists for the phenolate anion on the Na-form of the resin (12). If a nonpolar sorption of MAA and CA onto the resin predominates, then sodium salts of these arsenicals should be retained to some extent on the Na-form of the resin. However, we have found Kd values of zero for CH3As03Na2 and (CH3)zAsOzNa.This complete lack of retention demonstrates that a nonpolar sorption process is not significant: the mechanism based on arsenical amphoterism is supported. Analysis of Methylarsonic Acid Samples. Calculations reveal that arsenic acid can be present in MAA and yet not be noted in a total arsenic determination. A similar situation is shown by experimental acid titration results (Table I). Neither of these analyses is useful unless a correction is made for the arsenic acid component. Thus, separation and determination of arsenic acid in an MAA sample is needed to obtain a true purity rather than an apparent purity. The elution profile for a mixture of arsenous acid, arsenic acid, and MAA on AG 50W-X8(H)is shown in Figure 1.This resolution of components would appear satisfactory for use in analyses of MAA for arsenous and arsenic acids. Recovery studies for the individual components (1000,2000,and 4000 hg) on a column containing 20 g of resin resulted in 100 f 2% recoveries for MAA and arsenic acid; unfortunately, variable (80-100%) recovery levels were observed for arsenous acid.
n
I \
1'
I ril
Lc ffi
I
d
1
I
0
50
100
M L OF
\
150
t
200
I
I 100
0
ELUATE
200
I 300
I
400
1
500
M L OF E L U A T E
Figure 2. Elution curve for 1 mg of H3As03on a 200-9 column (18-mm i.d.) of AG 50W-X8 using water eluent. Eluate fractions were analyzed by arsenic AAS The sharp elution profile seen in Figure 2 rules out a loss of arsenous acid from peak tailing. The variable recoveries suggest a physical retention process, e.g., trapping of arsenous acid in properly sized resin pores. This problem was not significant for samples dealt with here, since they were of commercial origin and contained very small quantities of arsenous acid. However, for general application, an oxidation step with bromine is employed to convert any arsenous acid to arsenic acid (Figure 3). In this way, complete separation of inorganic arsenicals from MAA is possible. An arsenic acid analysis in conjunction with determinations for chloride, sulfate, arsenous acid, and total acidity allow the purity of MAA samples to be established. Using these analytical procedures, the results listed in Table I1 were obtained. Chromatographic P r e p a r a t i o n of MAA. The preparation of pure MAA closely follows the analytical method except for a larger sample size (5 8). Although 5 g is a small quantity compared to the 200 g of resin used for the chromatographed
Figure 3. Chromatogram of (A) H3As04 and (B) MAA after a sample ~ ~0.1 g of MAA was containing 1 mg of H3As04, 1 mg of H ~ A s Oand oxidized. The column (18-mm i.d.) contained 200 g of AG 50W-X8, eluent was water, and eluate fractions were analyzed by arsenic A A S column, the same column can be used repeatedly for preparing considerable quantities of pure MAA. In addition to removing inorganic arsenicals from MAA, this chromatographic method also eliminates any salt contaminants by typical ion-exchange mechanisms. Thus, any NaCl is converted to HCl and eluted from the column of resin prior to collection of any MAA. Analyses of MAA purified by chromatography are shown in Table I1 along with comparative analyses for samples purified by methanol recrystallization. Analysis of Cacodylic Acid Samples. Calculations show that arsenic acid and/or MAA may be present in CA samples and yet not be detected by total arsenic analyses. On the other hand, acid titrations of CA (Table 111) are sensitive to contamination from these compounds. However, the acid titration results can indicate only that contamination exists but not specify the particular arsenical. Also, very small concentra-
Table 11. Comparative Analyses of Methylarsonic Acid and Cacodylic Acid Samples Apparent purity (%) from Material acid titration N a ~ S 0 4 NaCl ~ (ppm)'
H ~ A s O ~ H3A~04 (ppmId (ppm)' MAA (ppm)
Technical Purity MAA 99.9 f 0.1 n.d.a 1700 13 477 ... Reference Purity MAA 100.1 f 0.1 n.d. 200 13 85 ... Chromatographic Purity MAA 100.0 f 0.1 n.d. n.d. n.d. n.d. ... Technical Purity CA 100.5 f 0.1 n.d. n.d. n.d. 1560 4710 Reference Purity CA 100.1 f 0.1 n.d. n.d. n.d. 420 1380 Chromatographic Purity CA 100.00 f 0.05 n.d. n.d. n.d. 0.8 7 Detection limit is 25 ppm for MAA samples and 20 ppm for CA samples. ' Detection limit is 10 ppm. a n.d. is not detected. Detection limit is 5 ppm. e Detection limit is 0.5 ppm.
Table 111. Apparent Purities of Cacodylic Acid Samples Which Contain Methylarsonic Acid and/or Arsenic Acid Sample compositiona %
YO
(CH~)~ASO~H 99.51 99.03 99.51 99.03 99.02 a
-
-
%
C H ~ A S O ~ H-~ 0.49 0.97 0.00 0.00 0.49
-
H~AsO~ 0.00 0.00 0.49 0.97 0.49
Apparent purity of CA(%) from calcd total arsenic
Apparent purity of CA(%) from exptl acidity titration
100.00 99.98 100.00 99.98 99.98
100.49 100.96 100.31 100.69 100.85
CA and MAA were of chromatographic purity; H3As04 was prepared from As2Oj. ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
1091
tions would not be detectable within the experimental titration error. As with MAA, determination of the exact purity of CA samples requires a procedure for separating arsenic acid and MAA from the CA. After oxidation of the sample solution to eliminate arsenous acid, rapid separation of the arsenic acid and MAA from samples of CA is readily accomplished on a short column of resin because of the strong sorption of CA. Determination of the MAA and arsenic acid along with determinations for chloride, sulfate, arsenous acid, and acidity allow the purity of CA samples to be established. Using these procedures, the results listed in Table 11were obtained. Chromatographic Preparation of Cacodylic Acid. Even though CA is held very strongly on the resin, it eventually does elute using only water as eluent (Figure 4). However, as a preparative procedure, the use of water elution is very timeconsuming and produces excessive volumes of water that must be evaporated for CA recovery. If only 1 or 2 g of pure CA is needed, a small preparation can be conducted. For example, we chromatographed 4 g of technical grade CA on a 20-g column of resin. The first 100 ml of eluate contained the arsenic acid and MAA. A yellow color band produced by the sorbed CA then slowly migrated down the column and began to elute after another 100 ml of elution. The elution was continued and lo00 ml of eluate collected. The CA recovered from this eluate was 2.5 g. To prepare larger quantities of pure CA, however, ammonia solution was used as the eluent. The strong affinity of NH4+ for the sulfonic acid resin (RS03H) allows CA to be liberated: R S O ~ ( C H ~ ) ~ A S+ONH3 ~ H ~ RS03NH4 + ( C H ~ Z A S O Z H( 5 ) +
A t the top of the column this liberated CA would also be neutralized by ammonia: NH3
+ ( C H ~ ) ~ A S O ~(HC H ~ ) ~ A S O ~ N H ~(6) +
Migration of the ammonium cacodylate down the column results in exchange of NH4+ for H+:
+
( C H ~ ) ~ A S O ~ NRHS ~O ~ ( C H ~ ) ~ A S O ~ H ~ -.+ 2(CHz)As02H RS03NH4
+
(7)
The net result of Equations 5,6, and 7 is elution of pure CA. T o ensure no elution of any NH4+, only 80%of the ammonia needed to completely neutralize all the re_sin is used. This markedly reduces the yields of CA, but was needed since extensive elution does result in NH4+ contamination (see Experimental). Also, when CA is first placed in the column of resin, any NaCl or NazS04 will exchange to HC1 and HzS04 and be eluted from the column before CA collection. The sodium ions will be retained on the resin and will be displaced later by NH4+. Therefore, unneutralized resin a t the bottom of column is needed so that any liberated Na+ will be trapped a t the bottom of the resin bed. Analyses of CA purified by chromatography are shown in Table 11, along with comparative analyses for samples purified by methanol recrystallization. We found that elution with ammonia generates a noticeable heating of the resin due to the neutralization reactions. This heating causes a very slight resin "color throw" which imparts a faint color to the final CA. Although this impurity is exceedingly small and was not detected in analysis results (Table 11),we chose to recrystallize the CA from methanol to remove this color. In spite of sample loss from recrystallization (30%)
1002
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
\ I
L: 0
z d
T
' a 0 irA
I 0
I
I 10
I 20
I
1
30
40
FRACTION NUMBER Figure 4. Elution curve for 0.1 g of CA o n a 10-g column (18-mm i.d.) of AG 50W-X8 using water eluent. Each 100-ml fraction was analyzed by arsenic AAS. Curve A is for undiluted eluate: curve B represents a tenfold dilution of eluate
and the reduced yield of the initial chromatographic step, 20 g (40%) of pure CA was obtained from the overall process.
CONCLUSIONS With the analytical methods described here, purities of MAA and CA samples can be established. As seen in Table 11, highly purified reference material can be obtained by simple methanol recrystallizations of technical grade samples. Chromatographically purified samples, although required in this study, probably are not needed for most research. The precision of these analytical methods will depend on the reliability of the atomic absorption or colorimetric method employed. Our concern was the column efficiency in separating arsenical components. We obtained 100 f 2%recoveries of contaminants for spiked samples of MAA (chrom. pure MAA with 250 ppm H3As04) and CA (chrom. pure CA with 500 ppm MAA and 250 ppm H3As04) as determined by atomic absorption spectrometry.
LITERATURE CITED (1) E. A. Woolson, "Arsenical Pesticides", ACS Symposium Series, No. 7 (1975). (2) C. J. Soderquist, D. G. Crosby, and J. Bowers, Anal. Chem., 46, 155 (1974). (3) R. M. Sachs, J. L. Michael, F. B. Anastasia, and W. A. Wells, Weed Sci.. 19, 412 (1971). (4) S . A. Peoples, J. V. Lakso, and T. Lais, Proc. West. Pharmacol. SOC.,14, 178 (1971). ( 5 ) J. V. Lakso. S.A. Peoples, and D. E. Bayer, WeedSci.,21, 166 (1973). (6) D. W. Von Endt, P. C. Kearney, and 0. D. Kaufman, J. Agric. Food Chem., 16, 17 (1968). (7) Y. Talmi and D. T. Bostick, J. Chromatogr. Sci., 13, 231 (1975). (8) Y. Talmi and D. T.Bostick. Anal. Chem., 47, 2145 (1975). (9) G. 0. Doak and L. 0. Freedman, "Organometallic Compounds of Arsenic, Antimony, and Bismuth", John Wiley and Sons, New York, N.Y. 1970, pp 22-32. ( I O ) W. Horwitz, Ed. "Official Methods of Analysis", 1l t h ed., Association of Official Analytical Chemists, Washington, D.C., 1970, Sec. 25. (11) E. Baumgartei. Naturwissenschaften.. 47, 468 (1960). (12) J. X. Khym, "Analytical IonIxchange Procedures in Chemistry, and Biology; Theory,Equipment, Technique", Prentice-Hall, Englewwd Cliffs, N.J., 1974. (13) W. L. Jolly, "The Chemistry of the Non-Metals", Prentice-Hall. E n g l e w d Cliffs, N.J.. 1966, pp 100-101. (14) M. L. Kilpatrick, J. Am. Chem., SOC.,71, 2607(1949).
RECEIVEDfor review January 19,1976. Accepted March 15, 1976.
