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Anal. Chem. 1981, 53, 749-750
Iodate-Suppressed Lead Eluent for Ion Chromatographic Determination of Divalent Cations J. D. Lamb, L. D. Hansen, G. G. Patch, and F. R. Nordmeyer" Department of Chemistty, Thermochemical Institute, Brigham Young University, Provo, Utah 84602
We recently reported the use of sulfate-suppressed barium and lead eluents to determine alkaline earth and divalent transition metal cations by ion chromatography (I). We have since developed a similar iodate suppressor system for Pb2+ eluents. This system has the same analytical capabilities as the sulfate-suppressed barium system including determination of divalent transition metal cations under acidic conditions. The iodate suppressor has two advantages over the sulfate suppressor: first, Ba2+can be determined; and second, the sensitivity for Cu2+is incresaed, the peaks being sharper and the retention time reduced. We have also investigated the use of an OH- secondary suppressor column for alkaline earth cation determinations. This column has the same peak-amplifying effect as the H+-exchange postcolumn introduced earlier ( I ) , but functions on a wholly different principle. The OH- secondary suppressor provides better base-line stability than does the H+postcolumn. The H+ postcolumn may also be used in the iodate-suppressed lead eluent system for determination of transition-metal cations.
EXPERIMENTAL SECTION The separator columns of the Dionex Model 10 ion chromatograph and the H+-exchangepostcolumn were prepared as reported previously (1). The iodate suppressor was an Altex 9 X 250 mm or 6 X 250 mm column packed with Bio-Rad AG 1-X8 or AG 1-X10 (no longer available) (200-400 mesh, chloride form), conditioned with 0.33 M KI03 for 30 rnin at 60% pump rate, followed by a 99 min water rinse, Some runs were made by using Dowex 1-X10in the suppressor column similarly conditioned with KI03 However, it was found that this suppressor soon plugged with Pb(IO& If the suppressor column containing Dowex 1-X10 was instead conditioned with a solution of 0.16 M KI03and 0.16 M KC2H302for 30 min at 60% pump rate followed by a 99 min water rinse, this problem was alleviated due to dilution of IO3-
on the column. For convenience, the Bio-Rad AG1-X8 suppressor was used for most runs and all data were collected using this suppressor unless otherwise indicated. The OH--exchange secondary suppressor was a 3 X 150 mm Altex column packed with Bio-Rad AG 1-X8(200-400 mesh, chloride form) or Dowex 1-X10 (200-400 mesh, chloride form) which was conditioned with 1.0 M NaOH for 10 min at 60% pump rate followed by a 30-min water rinse. All solutions, including 1.0 mM Pb(N03)2/0.1mM "03 eluent and divalent cation standards, were prepared as reported previously (1). Lead nitrate eluent at pH 4 was used for all the runs reported herein.
RESULTS AND DISCUSSION The Pb(N0J2 eluent was suppressed by precipitation of white Pb(I03)2which could be visually monitored in the suppressor column. At 30% pump rate, the 9 X 250 mm suppressor column had a useful life expectancy in excess of 40 h. For this reason a regeneration scheme for the iodate suppressor was not developed. Rather, the suppressor resin was replaced after suppressor exhaustion. As with the sulfate suppressor resin (11, the iodate suppressor resin retarded the passage of hydronium ion. The mechanism for hydronium ion retention in the iodate suppressor is probably the formation of an H(I03)2-anion (2). A startup wave (1)which was associated with hydronium ion retention in the sulfate suppressor system was absent with the iodate suppressor. However, in the iodate system, a moderate, regular drift in base line and sensitivity which lasted approximately 1 h after the flow of eluent began was noticed. An H+-exchange postcolumn (1) was used between the suppressor and detector with the iodate-suppressed lead nitrate eluent system. The mechanism of operation of this column in amplifying peak heights and in compensating for suppressor pH effects has been described (1). With the iodate
Table I. Relative Error Least-Squares Fit of Calibration Data for Several Divalent Cations Using Iodate-Suppressed 1.0 mM Pb(N0,)JO.l mM HNO, Eluenta salt
concn range,b M X IO6
mC
( M / ~ ~ x- 105 I)
b,C M x 106
0 ,c
%
rnin detectable concn,d M x lo6
(a) With H' Postcolumn 3.11 -5.2 1.6 2 4.09 1.3 1.3 3 5-663 5.72 3.0 7.8 3 Sr(NO 312 16-1000 Ba(N03)2 6 2-1 00 0 13.3 1.7 4.3 8 MnCl, 31-1000 5.07 -4.9 6.3 3 FeSO, 16-1000 3.89 6.2 3.1 4 COCI, 16-1000 3.81 -4.2 6.2 3 NiCl, 16-1000 4.65 -2.6 4.1 2 CUCl, 16-1000 4.21 6.2 12.2 4 z n (NO312 16-1000 4.22 0.3 8.8 3 Cd(NO,), 16-1000 5.17 4.7 5.1 6 ( b ) With OH- Secondary Suppressore Mg(N03)z 5-656 4.08 -1.1 3.9 0.6 Ca(NO3)2 5-663 4.48 -0.6 2.3 0.7 a Pump rate = 30% (2.5 mL/min) with 3 X 150 mm precolumn, a 6 X 250 mm separator column, and a 9 X 250 suppressor column. A series of dilutions of the solution of highest concentration by a factor of 2 or of 2.5 ( 6 points run in duplicate). Relative error least-squares fit (see ref 3 ) of the data to the equation [cation] = m x (peak height) + b. u is the relative standard deviation of the observed points, The concentration which gives a peak height twice the height of base-line noise. e Dowex 1-XlO used (OH'-form); for these runs, the suppressor column was charged with mixed iodate/ acetate solution, as described in the Experimental Section. Mg(NO,),
5-6 56
0003-2700/81/0353-0749$01.25/0
0 1981 American Chemical Society
750
Anal. Chem. 1981, 53, 750-751
Table 11. Comparison of Mg*+and Ca*+Concentrations in Pond Water (PW) and Filtered Blood Serum (FBS) Samples Measured by Ion Chromatography and by Atomic Absorption Spectrometry Mgz+concn, Ca2+concn, PPm PPm sample IC AAS IC AAS PW1 PW2 PW3
FBS, pH 1.05 FBS, pH 1.17
381 147 97 21 22
399 145 97 22 21
575 384 225 134 138
561 370 210 122 121
suppressor system, a peak amplification factor of about 20 (compared to 5 for the sulfate suppressor system) was observed with the postcolumn present. This large amplification factor does not imply greater sensitivity of the iodate system over the sulfate system. Rather, it resulted largely from the fact that in the absence of the postcolumn, peaks were very small. The OH--exchange secondary suppressor column was used only when analyzing for alkaline earth cations since transition-metal ions are precipitated in this column. In fact, this column may be used when analyzing mixed samples containing both alkaline earth cations and transition-metal cations to eliminate transition-metal ion peaks which may interfere with the Ca2+peak. Like the H+-exchangepostcolumn, this column served to increase the sensitivity of the method. However, unlike the H+ postcolumn, the OH- secondary suppressor operated by removing the background H+ conductivity, effectively lowering the base-line conductivity above which cation peaks were detected. This column gave a smoother, more stable base line than did the H+postcolumn. This latter observation confirmed the hypothesis ( I ) that much of the base line noise associated with these eluent-suppressor systems is a result of pH fluctuations. The cation retention times for the iodate system were similar to those for the sulfate system ( I ) except in the case of Cu2+which eluted with the other transition-metal cations at 8.2 min. The Cu2+peak was not broadened as it was with the sulfate suppressor system. The sensitivity in M/@ of the iodate system (Table I) was comparable to that of the sulfate system for all cations except Cu2+,for which the iodate system was approximately twice as sensitive as the sulfate system. By use of the iodate system, Ba2+could be determined along with the other alkaline earth cations. The retention time for Ba2+was 19 min. The Ba2+peaks were symmetrical and not much broader than those for Sr2+,although sensitivity for Ba2+
Fl7l-T5
0
10
minutes Flgure 1. Chromatogram of pond water sample no. 3 using 1.0 mM Pb(NO& eluent (pH 4), 30% pump rate (2.3mllmin), with 3 X 150 mm and 6 X 250 mm separator columns, 6 X 250 mm suppressor, and 3 X 150 mm postcolumn. Full scale is about 15 pa-’. The large
peak before Mg2+and Ca2+is due to monovalent cations.
was not as great as for the other alkaline earth cations (Table I\
1).
Blood serum samples, prepared as outlined previously ( I ) , as well as pond water samples were analyzed for Mg2+and Ca2+using the iodate system. The results of these analyses are given in Table I1 and a representative chromatogram appears in Figure 1. Good agreement was found between results obtained by ion chromatography and by atomic absorption spectrophotometry.
ACKNOWLEDGMENT We are greateful to S. Jerry Rehfeld of the Veteran’s Administration Hospital, San Francisco, CA, for helpful discussions and for providing filtered blood serum samples and to L. B. Merritt of Brigham Young University for providing pond water samples.
LITERATURE CITED (1) Nordmeyer, F. R.; Hansen, L. D.; Eatough, D. J.; Rollins, D. K; Lamb, J. D. Anal. Chem. 1980, 52, 852-856. (2) Pethybridge, A. d.; Prue, J. E. Trans. Faraday SOC.1967, 63, 2019-2033. (3) Anderson, K. P.; Snow, R. L. J . Chem. Educ. 1967, 44, 756-757.
RECEIVED for review October 14,1980. Accepted November 24, 1980.
Erlenmeyer Flask-Reflux Cap for Acld Sample Decomposition Darryl D. Slemer” and Harry G. Brinkley Exxon Nuclear Idaho Company, Box 2800, Idaho Falls, Idaho 8340 1
When acid decomposing organic or biological samples (e.g., filter papers) for subsequent chemical analysis, it is still common practice to recommend either watch glass covered beakers or Kjeldahl flasks as digestion vessels (I). The beaker-watch glass combination has the disadvantage that considerable sample and spattered acid tend to “hang up” and dry out on the watch glass and the rim of the beaker. This necessitates an awkward, time-consuming, and possibly dan0003-2700/81/0353-0750$01.25/0
gerous rinsing step to get everything back into the beaker before proceeding with the decomposition. Very gentle, prolonged, heating of the digestion vessel is often recommended instead of a fast, vigorous, sample attack, to avoid this extra work. The Kjeldahl flask system requires careful heating with a special heating apparatus and manifold assembly in order to avoid sample expulsion from the neck of the flask. This 0 1981 American Chemical Society
