852
Anal. Chem. 1980, 52, 852-856
Determination of Alkaline Earth and Divalent Transition Metal Cations by Ion Chromatography with SuIfate- Suppr essed Barium and Lead Eluents F. R. Nordmeyer, L. D. Hansen, D. J. Eatough, D. K. Rollins, and J. D. Lamb* Department of Chemistry and Thermochemical Institute, Brigham Young University, Provo, Utah 84602
The catlons MgZ+, Ca2+, Sr2+, Mn2+, Fez+, Co2+, Ni”, Cu2+, 2n2+, and Cd2+ were determined by ion chromatography using Ba(N03)2, BaCI,, or Pb(N03), eluents. The alkaline earth catlons were separated from one another; all transltlon metal Ions except Cu2+ eluted near Ca2+. Background conductivity was suppressed by precipitation of BaSO, or PbSO, In the sulfate-form anion-exchange suppressor column. An H+-exchange post-column inserted between the suppressor and the conductometric detector Increased sensltlvlty by a factor of 5, so that the minimum detectable concentration for alkaline M and for the other cations earth cations was less than was slightly larger than this value. Results obtained with this technique for the concentratlons of Mg2+ and Ca2+ In pond water, soil, and blood serum samples are compared to results obtained with atomic absorption spectrometry.
Ion chromatography has emerged in recent years as an efficient and reliable analytical technique for simultaneous determination of multiple ions in solution (1). The use of a “suppressor” column in this system makes possible the detection of individual cation or anion peaks by conductometric methods in that the background conductivity of eluent ions is removed. Eluent-suppressor systems for use with anions, monovalent cations, and alkaline earth cations have been reported (1) and have found successful application in routine analyses, especially of clinical ( 2 ) and environmental (3-5) samples. However, development of a n eluent-suppressor combinatioli for the separation and detection of divalent cations other than alkaline earth cations has not been previously described. We here report the development of simple eluent-suppressor systems which allow simultaneous ion chromatographic determination of Mg2+, Ca2+,and Sr2+and individual determinations of Mn2+,Fe2+,Co2+,Ni2+, Cu2+,Zn2+,and Cd2+. Aqueous Ba2+and Pb2+which serve as eluting ions are suppressed as insoluble sulfates in a suppressor column initially loaded with S042-. A scheme for regeneration of the suppressor column is described for the Ba2+system, although total replacement of the suppressor resin is quicker and no more costly. I n addition, the concept of an H+-exchange postcolumn is introduced. This column, placed between the suppressor and detector, serves to minimize the effect of p H on the conductometric base line and amplifies divalent cation peak heights by a factor of approximately 5 , thus greatly increasing the sensitivity of the technique. T h e advantage of the eluents and suppression system reported here over other systems proposed for divalent cations ( I ) is that a wide range of eluent p H is available. Thus transition metal cations can be eluted in acidic solution without being exposed to a neutral or alkaline environment at any point in t h e system. However, the present separator system does not allow effective separation of the transition metal cations from each other or from Ca2+. Work is in 0003-2700/80/0352-0852$01.OO/O
progress to effect such separations.
EXPERIMENTAL Equipment. A Dionex model 10 ion chromatograph with 0.1-mL sample loop was used for all experiments. Standard 3 X 150 mm and 6 X 250 mm cation separator columns (provided with the instrument) were prepared for use with Ba2+or Pb2+ eluents by conditioning with 0.1 M solutions of the nitrate salt of the respective cation for 30 min at 50% pump rate followed by a 15-min rinse with water. Flow rate in mL/min may be determined by multiplying % pump rate by 0.0767. As the work progressed, it was shown that the separator columns could be conditioned with just three 0.1-mL injections of 0.1 M BaClz or Pb(N03)zwith eluent flowing followed by a 5-min water rinse. Similarly, the separator could be restored to the normal H+ form by using three 0.1-mL injections of 3 M HNO,. The sulfate suppressor was a standard Dionex 9 X 250 mm cation suppressor or the same size Altex column packed with Bio-Rad AG 1-X10 (200-400 mesh, chloride form). Where specifically noted, a 6 X 250 mm suppressor column was used. The packed suppressor column was conditioned with 1.2 M Na2S04for 30 min at 60% pump rate followed by a 10-min rinse with water. The H+-exchange post-column was a 3 X 250 mm Altex column packed with Dowex 5OW-X12 (100-200 mesh, H+ form) or Bio-Rad AG 50WX16 (20(r400 mesh, hydrogen form). A Hewlett-Packard model l700B recorder with 1-V span was used to record ion chromatographic output and on occasions to record pH simultaneously. The pH of the exit stream was recorded from a Sargent-Welch Model LS pH meter. Chemicals. All solutions were prepared in distilled water using reagent grade chemicals. All standard solutions were prepared by weight. Solutions of Ba(NO&, BaCl,, and Pb(N03)2,used as eluents, were adjusted to the desired pH using the acid of the corresponding anion. RESULTS AND DISCUSSION Suppressor Regeneration. The suppressor reaction involved the precipitation of BaS0, or P b S 0 4 and the replacement of S042-ions on the suppressor resin with NO3or C1- ions from the eluent stream. At high eluent Ba2+ concentration (2.0 mM), and low pump rate (15%),precipitation of BaS0, in the suppressor could be visually monitored as the boundary between the BaSO, band and the unused portion of the column moved down the column. At lower Ba2+ concentrations (1.0 mM and below) and the commensurately higher pump rates necessary to keep retention times at comparable values, the boundary was often difficult to discern. The 9 X 250 mm suppressor columns used a t the beginning of this study were replaced after approximately 24 h of continuous running either a t 1 mM Ba2+and 30% pump rate or a t 2 mM Ba2+ and 15% pump rate. I t was later found that the 6 X 250 mm suppressors would give 30 h of service. For the Ba2+eluent system, the following scheme was devised for regeneration of the suppressor column by removal of BaS04 precipitate. The suppressor column was flushed for 4 h at 60% pump rate with 0.5 M tetrasodium ethylenediaminetetraacetate followed by a 1-h water rinse. The line leading from the regenerant reservoir was rinsed with water. The column was subsequently flushed for 80 min with 0.77 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6. MAY 1980
M Alz(S04)3 and again rinsed with water for 10 min. The Ba2+ eluent or a H N 0 3 solution of the same p H was pumped through the acidic (probably with HSOJ suppressor until a stable base line was achieved (90 to 120 min for eluent a t p H 4 ; overnight for unbuffered eluent a t p H 5.1-5.8). Alternatively, the resin was emptied from the column into a beaker and the p H adjusted with addition of NaOH and H N 0 3 solutions. As a more efficient means of adopting the above procedure, it was found that used resin could be removed from the column, stored, and regenerated in large quantities. The major part of BaS04 in used resin was physically separated by stirring, after which chemical regeneration such as that outlined above was performed. No attempt was made to develop a chemical suppressor regeneration scheme for Pb2+ eluent. Rather, exhausted suppressor resin was replaced as outlined below. Replacing the suppressor column packing material with fresh Bio-Rad AG 1-X10 (200-400 mesh, chloride form) was no more costly and far less time consuming than the regeneration procedure described. In addition, it was suspected t h a t incomplete removal of EDTA used in chemical regeneration resulted in loss in sensitivity among transition metal cations. For these reasons, resin replacement rather than regeneration is recommended. During resin replacement, fresh suppressor resin was seeded with 1 mL of previously used resin to facilitate precipitation of metal sulfate in the newly packed column. Suppressor Chemistry. In order to make passage of transition metal cations through the system possible, it was anticipated t h a t acidic eluents were desirable. Therefore, eluents of various p H values were studied to determine the optimum p H for analysis of different cations. These eluents were pumped through the suppressor column (with no separator) while the pH of the exit stream was monitored. Table I lists the peak heights (normalized to Ca = 100) for the cations studied a t several eluent exit stream p H values. One millimolar solutions were used for all cations. As expected, no significant effect of pH on the peak heights of Mg2+,Ca2+,or Sr2+was observed. The peak heights of the transition metal cations Mn2+,Fe", Co2+,Ni2+,Zn2+,and Cd2+reached their highest values in the p H range 5.2-5.8 (which is the p H of Ba(N03)zor BaClZsolutions with no added acid). The Cu2+ peak was largest a t pH 4.0, although Cu2+peaks in general were lower and broader than those of the other divalent transition metal cations or for the alkaline earth cations. At p H 4.0, A13+ and Cr3+ were also eluted; however, Fe3+ and HgClz were not detected. Under normal running conditions, with the separator column in line, the peak heights increased with eluent pH. A detailed study of the effect of p H on 1 mM M g ( N 0 J 2 peak heights was made by producing a series of chromatograms a t various suppressor effluent pH values using 1 mM Ba(N03)z eluent a t 30% pump rate. These p H values were obtained by adjusting the p H of the suppressor resin with NaOH before the column was packed. Effluent p H remained constant a t p H values greater than 5 when acid-free eluent was used. The peak heights (in pmhos) corresponding to the eluent pH values were: 0.5 (pH 3.1), 9.4 (pH 3.81,11.0 (pH 4.0), 11.6 (pH 4.1), 12.1 (pH 6.2), 17 (pH 7 ) . The same trend was observed for all other cations studied. Inclusion of 0.1 mM and 1 mM N a N 0 3 in the eluent showed little or no effect on Mg2+peak heights. Analysis of the effluent by anion chromatography showed S042-to be the only anion present in the exhaust stream from the suppressor column. Therefore, sample and eluent anions are trapped by the suppressor resin. I t was concluded that the described peak height effect was a function of eluent p H and virtually independent of the metal ion injected in the sample or of ions other than H + in the eluent.
+
m
&
i
m 3
I I
I
l o o
l
l
0 0 0 0 0
0 0 0 0 0 irlrlrlrl
rx
0
2
x
d
853
854
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
In an attempt to better understand suppressor chemistry, the p H of the effluent stream was continuously monitored. It was found that a drop in [ W ] appeared simultaneously with each sample peak. These negative H+ peaks accompanied all sample peaks under all the following conditions: (i) using Ba(N03)2eluent with the separator and suppressor; (ii) using Ba(N03)2eluent with the suppressor only; and (iii) using water or HNOBeluent and the suppressor only. The H+ peaks were not present, however, in the absence of the suppressor column, or when an H + post-column (described below) followed the suppressor. T h e following explanation is offered for the peak height dependence on p H and for the negative peaks in [H+]. An equilibrium involving the anions bound to the suppressor resin, R, and those free in solution can be expressed as follows: 2RHS04 = R2S04 2H+,, S042-Bq (1)
+
+
A sample of 1mM Mg(N03), would be converted to MgS04aq in the suppressor column. Within the band of MgSO,,, passing through the suppressor a common-ion effect would lead t o a lowered H + concentration according to the above equilibrium. This accounts for the drop in [H+]which always accompanies the elution of a cation peak. This effect would be the same for any sample salt injected. The drop in [H+] which accompanies each cation band moving through the suppressor column and subsequently through the conductometric detector tends to offset the conductivity produced by the sample ions. Thus all peaks are lower than they would be in the absence of the equilibrium described in Equation 1. This common-ion effect is more pronounced at higher [H+] and so gives rise to decreased peak heights a t lower pH values. This loss in sensitivity can be minimized using the H+-exchange post-column described below. In addition to affecting peak heights, the equilibrium described in Equation l also causes retardation of movement of H t through the suppressor column. Any H+ moving into the suppressor, whether from sample or eluent, participates in the equilibrium reaction. A t eluent pH values used in this work, most of the H+ ions are in the RHS04 form bound to the stationary phase. Thus, H+ is strongly retarded by the suppressor whereas most other cations pass through the suppressor unretained. As a result, the peak associated with H+ present in a sample is greatly delayed and broadened. Almost any sample regardless of concentration will perturb the otherwise constant [H’] entering the suppressor and the result of this perturbation will be seen 60 to 90 min later (at pH 4) in the form of a wave with a height of 0.1 +mho and width of about 12 min. These H+ “retention” times decrease as the suppressor becomes exhausted. In some cases an H+ wave such as those described above can cause severe interference. For instance, a wave consisting of a positive then negative excursion of the base line of about 10 pmho magnitude occurs between 1 and 2 h after flow of p H 4 eluent begins through the suppressor column. This “start-up” wave is less pronounced in magnitude if the flow has only been off for a few minutes rather than for hours. The start-up wave is best avoided by allowing flow of eluent through the suppressor for 2 h before the samples are to be run. Similar problems arise from samples with 20.01 M H+ which can cause interfering waves for subsequent samples. In general, pH waves are retained for shorter times, are greater in magnitude and hence more troublesome a t lower eluent pH values. This is to be expected since at low eluent pH, a greater fraction of the H+ is in the mobile phase and the retention time of H+ decreases. H+-Exchange Post-Column. To reduce the variation of peak height with pH and to increase the sensitivity of the instrument, an H+-exchange column, as described in the
Figure 1. Retention times for divalent cations under various conditions (with 3 X 150 mm and 6 X 250 mm separator columns and 9 X 250 mm suppressor column, eluent at pH 4): (a) 0.3 mM Ba(NO&, 50% pump rate; (b) 1.O mM Ba(NO&, 30% pump rate; (c) 2.0 mM &(NO3)*, 15% pump rate; (d) 0.3 mM BaCI2, 50% pump rate; (e) 1.0 mM BaCI,, 30% pump rate; (f) 1.O mM Pb(N03),, 30 Yo pump rate
Experimental section, was introduced between the suppressor and the detector. This column, hereafter called the H+-exchange post-column, or simply post-column, replaces all metal cations in the eluent stream with protons, which have much higher intrinsic conductivity in water. Introduction of this column helps to minimize the H+ equilibrium effects in the suppressor which cause peak height variations. With the post-column in line, we obtained the following Mg2+ peak heights (in pmho) a t the indicated pH values: 60 (pH 3-11, 52 (pH 4.1), 37 (pH 6.0), 65 (pH 7.3). The post-column also increased the sensitivity of the instrument by a factor of 3 to 5 at pH values down to 4 and by a much larger factor below pH 4. Separations among cations were unaffected by the presence of the post-column. Base-line fluctuations due to pH waves as described above were not amplified by this column. Thus the effective ratio of signal to pH “noise” was improved considerably by this method. Cation Separations. Retention times of divalent cations using several Ba2+and Pb2+eluents with the cation separator (including a “precolumn” separator to protect the main separator) and suppressor are illustrated in Figure 1. These data exhibit several interesting features: (i) the eluent anion accompanying BaZ+ (C1- or NO3-) has little effect on cation retention times; (ii) Mg2+,Ca2+,and SrZ+are well separated with all eluents; (iii) the divalent transition metal cations (except Cu2+)always fall in a group preceding or close to Ca2+; and (iv) the retention time of Cu2+varies widely and is often much longer than those of other transition metal cations. In mixtures it was found that peaks of the transition metal cations, except in the case of Cu2+and sometimes of Zn2+,were not always separated from the Mg2+ and Ca2+peaks. The cations Mg2+,Ca2+,and Sr2+eluted in mixtures a t the same times they did individually. The degree of separation of transition metal ions such as Co2+ and Zn2+from the peaks of Mg2+ and Ca2+ was greatest with 3 x lo-* M BaC12, M HC1 eluent a t 50% pump rate. While all other peaks (Mg2+,Ca”, Sr2+,Mn”, Fez+,Co“, Ni2+,Zn2+,Cd2+)were sharp, the Cu2+peaks were invariably broad and tailed. Variability of Cu2+retention time was found not to correlate with eluent but with suppressor resin age and/or exhaustion. Retention times are not reported in Figure 1 for A13+ or Cr3’ which did not elute under the conditions listed. The retention times of Zn2+and Cd2+show some variability among eluents though not as much as Cu2+. It was noted that the presence of acetate anion in a solution of Zn2+caused the
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
855
Table 11. Relative Error Least Squares Fit ( 6 ) of Calibration Data for Several Divalent Cations Using 1.0 mM Ba(NO,), Eluent
eluenta pH
salt
5.4
Mg(NO,)a
concn. range,b m ,c M x lo6 (Mlpmho) x 10'
b,'
Mx
lo7
0 PC
%
minimum detectable concn.,d M X 10'
( a ) N o Post-Column
Sr( NO 3 ),
800-8 800-8 800-8
8.64 10.4 14.4
0.38 -4.9 -9.0
3.3 3.5 3.3
35 42 58
(b) With 3 X 1 5 0 mm Post-Column
5.2 5.1 4.0
320-1 2000-1 2000-3 800-8 800-8 800-8 800-2 800-8 800-2 800-8 800-2 800-8 2000-16 2000-16 2000-16 2000-16 2000-16 2000-16 2000-16 2000-16 2000-32 2000-32 2000-128 2000-128 2000-16 2000-16
2.77 3.04 4.05 1.13 1.66 2.04 1.35 1.37 1.87 1.87 2.60 2.60 2.34 3.38 3.21 4.11 3.15 3.85 3.17 3.59 13.5 4.38 3.91 3.56 3.95 3.29
-5.7 2.7 -0.2 -1.0 3.6 11.1
-1.4 0.1 -3.2 - 3.7 - 1.6 1.7 12.1 -7.8 76 10.4 21 - 10
6.4 14.1 8.3 4.9 6.0 7 .O 3.8 3.8 4.4 5.7 7.3 5.9 3.7 6.7 9.5 4.6 7.9 12.5 11.5 6.9 10.6
- -.
- -. - -.
22 24 32 10 14 17 4.1 4.1 5.6 5.6 7.8 7.8 7.0 10 9.4
9.5 60 -64 1770 1000 -713 18.1 433 5.4 16 -282 1.5 12 18.3 9.3 -14.7 24.7 a Pump rate = 30% (2.3 mL/min) with 3 X 1 5 0 mm and 6 X 250 mm separator columns and a 9 X 250 suppressor column unless otherwise noted. A series of dilutions of the solution of highest concentration b y a factor of 2 or of 2.5. Relative error least squares fit ( 6 ) of the data t o the equation [cation] = rn x (peak height or peak area) + b. u is the relative standard deviation of the fit. The concentration which gives a peak height twice the height of base-line noise. e A 6 x 250 mm suppressor column was used. f 0.01 M NaCl added to these samples. g Peak areas (determined by weighing the strip chart paper) used, rather than peak heights as in all other cases. Therefore units of m are (M/pmho-min) x l o 5 . Zn2+peak to be retained &IO% longer than if the acetate were not present or if the acetate were neutralized with 1.5 equivalents of HCl. Response and Sensitivity. Table I1 lists the slopes and intercepts of response curves for Mg2+,Ca2+,Sr2+,Mn2+,Fe2+, Co2+,Ni2+,Znz+,and Cd2+using Ba(N03)2eluent at pH 5 with and without the post-column and a t p H 4 with the postcolumn. The data in Table I1 are from linear fits by a relative error least squares technique (6) using peak heights and/or peak areas. Points were taken over the concentration range indicated in the table by successive dilutions of the salt solution by factors of 2.0 or 2.5. The average deviation from the linear fit is given by u. The minimum detectable concentration which gives a peak height greater than twice the base-line noise is given for each element in Table 11. Data are presented for the pH 5 eluent both with and without the post-column. The data indicate that use of the post-column increases sensitivity for all elements, as indicated by the slope m, by a factor of approximately 5. Consequently, the limit of detectability is lowered. The experiments a t pH 5 also show that the narrower bore 6 x 250 mm suppressor leads to greater peak heights and a twofold increase in sensitivity. T h e Mg2+,Ca2+,and Sr2+response curves were repeated a t p H 4 with 0.010 M NaCl present in the samples in order to determine the effect on response of high alkali metal background concentrations in samples. These data, shown in Table 11, indicate that no significant change in response
occurs with NaCl present a t this concentration. Since under these conditions all alkali metal cations elute together, it is concluded that this procedure for measuring divalent cations is reliable for samples, such as biological samples, in which relatively high background alkali metal concentrations are expected. The chromatograms of transition metal samples were analyzed both by measuring peak heights (in pmho) and peak areas (in lmho-min). Judged by the values of u, in some cases the linear fit was better using peak heights; in other cases, it was better using peak areas. We see no reproducible evidence that reliability of the technique is improved by measuring peak areas as opposed to peak heights. No response data are presented for Pb2+ eluent. At the sample concentrations studied, it was found that response for all cations with this eluent was very similar to that with the Ba2+ eluent. Applications. Use of the ion chromatographic technique described above in the analysis of Mg2+ and Ca2+ in pond water samples is illustrated in Table 111. The samples were analyzed both by ion chromatography using the 1 mM Ba(NO& eluent a t pH 4 with the post-column in line and by atomic absorption spectrometry. The same standards were used on both instruments and the results from the two techniques never differed by more than 3%. In Table IV are listed Mg2+and Ca2+concentrations determined for several soil samples by the BaZf eluent ion chromatographic technique and by atomic absorption spec-
858
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
Table 111. Comparison of Mg" and Caz+Concentrations in Environmental Water Samples Measured by Ion Chromatography Using 1.0 mM Ba(NO,), Eluent (pH 4 ) and by Atomic Absorption Spectrometry Mg" concn., ppm Ca2+concn., ppm IC= A A ~ IC' A A ~
sample PWlC PW2C PW3C PW4C
389 149 98 150
EPA 1174Bd
399 145 97 146
7.40
57 2 381 214 340
7.29
561 37 0 210 334
24.3
23.8
Pump rate = 30% (2.3 mL/min) with 3 X 150 mm and 250 mm separator columns, 6 X 250 mm suppressor, and 3 X 150 mm post-column; analysis by peak height. Perkin-Elmer Model 603 Atomic Absorption Spectrophotometer. From a 25-fold dilution of local pond water sources. d EPA standard: 7.20 ppm Mg2+;22.2 ppm Ca2+. a 6X
Table IV. Comparison of Mg2+and Ca2+Concentrations in Environmental Soil Samples Measured by Ion Chromatography Using 1.0 mM Ba(NO,), Eluent ( p H 4 ) and by Atomic Absorption Spectrometry
sample lb(USU120) Xb(USU121) 3C(M.L. W) 4C(M.L. Bu) 5C(M.L.Gi) GC(M.L. H) 7C(M.L.P1)
Mg2+concn., PPm IC' AAa
Caz+concn., PPm ICa AA'
777 520 431 187 114 568 174
2130 1740 1330 1960 7 60 1430 7 30
705 467 384 175 134 579 205
2200 1780 1320 2200 860 1570 8 30
' See footnotes a, b in Table 111. Soil samples obtained from Utah State University, Logan, Utah. Soil samples obtained from Soiltest Farm Consultants, Moses Lake, Wash. ~
~~~
Table V . Comparison of Mg" and Ca2+Concentrations in Filtered Blood Serum Samples Measured by Ion Chromatography Using 1.0 mM Ba(NO,), Eluent (pH 4 ) and by Atomic Absorption Spectrometry samplea pH 1.05 1.17
Mg" concn., mg % Ca2+concn., mg % ICb A A ~ ICb A A ~ 2.16 1.99
2.2 2.1
11.5 11
12.2 12.1
a Moni-TrolI1.X Chemistry Control reagent; pH adjusted using HC1. See footnotes a, b in Table 111.
trometry. Soil samples were extracted by the standard method (7) as modified by V. Hunsaker (8): 5 g of soil were stirred with 25 mL of 0.8 M KC1 for 15 min, allowed to settle, filtered, a n d diluted by 100-fold. Concentrations determined by ion chromatography using peak heights were in good agreement with those from atomic absorption analysis. T h e application of this ion chromatographic technique to the determination of Mg2+and Ca2+in blood serum was tested
0 - 0 minuter
Figure 2. Representative chromatogram of filtered blood serum using 1.0 m M Ba(N03)*eluent (pH 4),3 0 % pump rate (2.3 mL/min), with 3 X 150 mm and 6 X 250 mm separator columns, 6 X 250 mm suppressor column, and 3 X 150 mm post-column. Full scale is about 15 pmhos
using Moni-trol 1I.X Chemistry Control reagent (lot no. XPT-9593) freeze-dried blood serum reconstituted with 10 mL of HCl solution of desired pH. This solution was centrifuged for 5 min in an IEC PR-6000 refrigerated centrifuge (25 f 1 OC) using type CF25 centriflo membrane cones (Amicon) to remove species of molecular weight >25000. T h e resulting filtrate was analyzed both by Ba2+eluent ion chromatography (with post-column) and by atomic absorption spectrometry. The results of these analyses appear in Table V and a typical chromatogram is shown in Figure 2. Agreement was good between results from the two analytical techniques. The variation in measured Mg2+and Ca2+levels with the serum p H at the time of filtration will be the subject of a future publication. ACKNOWLEDGMENT We are indebted to S. J. Rehfeld of Veteran's Administration Hospital, San Francisco, Calif., for helpful discussions and for providing filtered blood serum samples; t o R. E. Lamborn, Utah State University, Logan, Utah, and to V. Hunsaker, Soiltest Farm Consultants, Moses Lake, Wash., for providing soil samples; to L. B. Merritt of Brigham Young University for providing pond water samples; and to S. Swain, G. Patch, and S. Jones for assistance in some of the experimental determinations. LITERATURE CITED (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801-1809. (2) Anderson, C. Clin. Chem. 1976, 22, 1424-1426. (3) "Ion Chromatographic Analysis of Environmental Pollutants", Sawicki, E., Mulik, J. D., Wittgenstein, E., Eds., Ann Arbor Science: Ann Arbor, Mich., 1978. (4) Lee, M. L.; Later, D. W.; Rollins, D. K.; Eatough, D. J.; Hansen, L. D. Science 1980, 207, 186-188. (5) Hansen, L. D.; Richter, B. E.; Roliins, D.K . ; Lamb, J. D.; Eatough, D. J. Anal. Chem. 1979, 51, 633-637. (6) Anderson, K. P.; Snow, R. L. J . Chem. Educ. 1967, 4 4 , 756-757. (7) Held, W. R. In "Methods of Soil Analysis. Chemical and Microbiological Properties", Black, C. A,. Ed.; American Society of Agronomy: Geneva, N.Y., 1965; p 1000. (8) Hunsaker, V., Soiltest Farm Consultants, Moses Lake, Wash., personal communication, November 20, 1979.
RECEIVED for review November 28, 1979. Accepted January 18, 1980.
