700
Anal. Chem. 7907, 59,708-712
(24) Fritz, J. S . ; Sutton, S. A. Anal. Chem. 1956, 28, 1300-1303. (25) ShR, Y. T.; Carr, P. W. Talanta 1981, 28, 411-414. (26) Vespelec, R.; Neca, J. J . Chromtogr. 1983, 287, 35-47. (27) Verzele, M. LC Mag. 1983, 1(4),217-216. (28) Rivler, J. E. J . Liq. Chromatogr. 1978, 7 , 343,366. (29) Hearn, M. T.: Greco. B. J . Chromatoor. 1983. 255. 125-136. (30) Hearn, M. T.; Grego, B. J . Chromato&. 1983; 266; 75-87. (31) Schoenmakers, P. J.; Billlet, J. A.; DeGalan, L . J . Chromatogr. 1981, 218, 261-284. (32) Browning, E. Toxicity of Industrial Metals ; Butterworth: London, 1969.
(33) Luckey, T. D.; Venugopal, B. Metal Toxicity in Mammals; Plenum Press: New York, 1977. (34) Karger, 6. L.; Giese, R. W.; Snyder, L. R. Trends Anal. Chem. 1983, 2(5). 106-109.
RECEIVED for review June 2, 1986. Accepted November 1, 1986. The financial support provided by the Maytag Company, Newton, IA, is gratefully acknowledged.
Determination of Bicarbonate by Ion-Exclusion Chromatography with Ion-Exchange Enhancement of Conductivity Detection Kazuhiko Tanaka’ and James S. Fritz*
Ames Laboratory-DOE
and Department of Chemistry, Iowa State University, Ames, Iowa 50011
Carbon dloxlde and blcarbonate are determlned In aqueous samples by lon-excluslonchromatography uslng water as the eluent and a conductlvky detector. The sensltlvtty of detectlon Is Improved approxlmately 10-fold by the use of two ion-exchange “enhancement” cdumns Inserted In S e r b between the separatlng column and the detector. The flrst enhancement column converts carbonic acld to potasdum blcarbonate and the second enhancement column converts the potasslum bicarbonate to potasslum hydroxlde. The method provides a fast, selectlve, and sensttlve way to determlne carbon dioxlde or bicarbonate. Appllcatlon to several types of aqueous samples Is demonstrated.
The quantitative determination of carbon dioxide and bicarbonate is a very important analytical problem, especially when low concentrations are to be measured. Recently, a simple method for the determination of CO2/HCO3- based on a conductometric sensor with a gas-permeable membrane has been developed (1, 2). Although the method is very sensitive to COz/HC03-, the detector response to concentration is not linear. An acidic or basic gas, such as SO2 or NH3, interferes in the C 0 2 determination. Ion-exclusion chromatography provides a convenient way to separate molecular acids from highly ionized substances. The separation column is packed with a cation exchange resin in the H+ form so that salts are converted to the corresponding acid. Ionized acids pass rapidly through the column while molecular acids are held up to varying degrees. A conductivity detector is commonly used. Carboxylic acids have been separated by using water, a dilute mineral acid, or a dilute benzoic or succinic acid as the eluent (3-6). Carbon dioxide or bicarbonate has been determined by ion-exclusion chromatography with water as the eluent and a coulometric detection of H+ from H2C03(7). The method is reasonably selective for C02/bicarbonate and has a linear calibration curve but is somewhat lacking in sensitivity. In the present work, methods for determination of the sum of carbon dioxide and sodium bicarbonate that use ion-exclusion chromatography and a conductivity detector are Present address: Government Industrial Research Institute,
Nagoya, 1-1,Hirate-cho, Kita-ku, Nagoya-shi, Aichi, 462, Japan.
presented. The simplest procedure uses a cation-exchange separation column with purified distilled water as the eluent. However, the use of ion-exchange “enhancement” columns connected in series with the separation column greatly improves the sensitivity of detection and gives a linear calibration curve over a large concentration range.
EXPERIMENTAL SECTION Apparatus. A rather conventional chromatographic system was used, which consisted of the following components: eluent reservoir, pump, pulse dampener, precolumn for removing COz gas from the eluent, a 100-pL sample loop, separating column, first and second enhancement ion-exchange columns, a Wescan 213 conductivity detector, and a strip-chart recorder. A flow rate of 1.0 mL/min was used for all of the work reported. The plastic separating column was 7.5 x 100 mm and was packed with a cation-exchange resin in the H+ form TSK SCX, 5 pm (TSK, Tokyo, Japan). The first enhancement column was constructed of plastic (4.6X 50 mm) and packed with a cationexchange resin in the K+ form (TSK SCX, 5 pm;TSK IC-Cation for cation chromatography use, 10 pm; TSK SP-5 PW for HPLC use, 10 pm; or TSK IC-Cation SW for cation chromatography use, 5 pm). The second enhancement column was constructed of plastic (4.6 X 50 mm) and packed with an anion-exchange resin in the OH- form, TSK SAX (5 pm). The precolumn was constructed of plastic (7.5 x 100 mm) and packed with an anion-exchange resin in the OH- form (TSK SAX, 5 pm). Reagent and Chemicals. Standard solutions of bicarbonate were prepared by using NaHC03 of reagent grade. Other standard solutions used were of reagent grade. All standard solutions were prepared by using distilled, deionized water from a Mi&-Q reagent grade water system (Millipore, Bedford, MA). Sample Preparation. All actual samples were filtered with a 0.45-pm poly(tetrafluoroethy1ene) (PTFE)membrane filter before injection into the column. RESULTS AND DISCUSSION Determination of Carbon Dioxide or Bicarbonate by Ion Exclusion. A 100-pL sample containing 1.0 mM NaHC03and 1.0 mh4 KCl was injected to test the separation ability of the system used. Distilled water as the eluent and conductivity detection were employed. The separation column contains a cation exchanager of high capacity and thus converts the sodium bicarbonate to carbonic acid and the potassium chloride to hydrochloric acid. Good resolution of the H+-C1- and H2C03was obtained, the retention times being approximately 2 min and 5 min, respectively. However, HzC03
0003-2700/87/0359-0708$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, A
n
B
NO.5, MARCH 1, 1987
709
C
1
I
I
1.128pScm-'
032pScrn-
i, i
k2
li.
Flgure 1. Effect of type of first enhancement column (K+ form, 4.6 X 50 mm): (A) TSK SCX, polystyrene-diinylbenzene copolymer with high cation-exchange capacity (4.2 mequiv/g), 5 pm; (B) TSK ICCation, polystyrene-divinylbenzene copolymer with low cation-exchange capacity for cation chromatography use (30 pequivlg), 10 pm; (C) TSK SP-5PW, hydroxylated, polyether with low cation-exchange capacity, for HPLC use (0.3 mequivlg), 10 pm: (D) TSK ICCatlon SW, silica gel with low cation-exchange capacity for cation chromatography use (0.45 mequiv/g), 5 pm. Peak 1 is CI- and peak 2 is HCOB-. Conditions: separating column is TSK SCX (H+ form), 5 pm, 7.5 X 100 mm. Eluent is water (1 mL/min). Sample Is a mixture of 1 mM KCI and 1 mM NaHCO, (0.1 mL).
is a very weak acid (pK1 = 6.4) and the conductance of the carbonic acid peak is consequently low. Use of the Cation-Exchange Enhancement Column. In order to obtain a more sensitive detection of carbonic acid, a cation-exchange column was inserted between the cationexchange separation column and the detector. This is called the first enhancement column. Its purpose is to convert the carbonic acid to a more highly ionized form and thereby to increase the conductivity. When the enhancement column is in the K+ form, the exchange reaction is as follows:
R,-K+
+ H&03
-
R,-H+
+ K'HC03-
To evaluate the performance of the first enhancement column in the K+ form, four kinds of cation-exchange columns with different cation-enhancement capacities and matrices were used and compared. The TSK SCX column is a polystyrene-divinylbenzene copolymer-based material with a high cation-exchange capacity (4.2 mequiv/g). The TSK IC-Cation column is a polystyrene-divinylbenzene copolymer-based material with a low cation-exchange capacity (30 pequivlg) for single column cation chromatography use. The TSK SP-5PW is a hydroxylated, polyether-based material with a low cation-exchange capacity (0.3 mequiv/g) for HPLC use. The TSK IC-Cation SW is a silica gel based material with a low cation-exchange capacity (0.45 mequiv/g) for single column cation chromatography use. The order of detector response of HC03- on these first enhancement columns is TSK SP-5PW > TSK SCX > TSK IC-Cation SW >> TSK IC-Cation, as shown in Figure 1. Although the highest detedor response of HCO, was obtained on the TSK SP-5PW column with low cation-exchange capacity, the lifetime of this column is shorter than that of the TSK SCX with high cation-exchange capacity. Therefore, the
z
i
Flgure 2. Effect of dimension of separating column (TSK SCX, H+ form) on separation of strong acid anion (CI-) and HCO,-: (A) 4.6 X 50 mm; (B) 7.5 X 100 mm; (C) 8 X 200 mm. Conditions: First enhancement column is TSK SCX, K+ form, 4.6 X 50 mm. eluent is water (1 mL/min). Sample is a mixture of 1 mM KCI and 1 mM NaHCO, (0.1 mL).
TSK SCX column in the K+ form was chosen as the most suitable first enhancement column. Several ionic forms were tried for the cation-exchange enhancement column with TSK SCX. The cation-exchange resin in the K+ form was higher than that on the cation-exchange resin in the other ionic forms because the ionic equivalent conductance of K+ (73.5) is higher than those of Na+ (50.1), Mg2+ (53.1), and Ca2+ (59.5). Although ionic equivalent conductance of NH4+(73.6) has almost the same value as that of K+, the detector response obtained on the cation-exchange resin in the NH4+form was considerably lower than that on the cation-exchange resin in the K+ form. This could be due to the unstable nature of NH4HC03,which is formed in the cation-exchange reaction. From these results, a cation exchanger in the K+ form was judged to be the most effective for the first enhancement column. With this column, a 1.0 mM standard sample of bicarbonate gave a conductance of 0.504 pS cm-l, which is approximately 5.5 times greater than that obtained with no enhancement column. Effect of the Dimension of the First Enhancement Column. The column dimensions of the first enhancement column affect the efficiency of conversion by cation exchange, the column lifetime, and the peak resolution. First enhancement columns with the following dimensions were studied: 4.6 X 50 mm, 4.6 X 100 mm, 7.5 X 100 mm. The most efficient separation and highest detector response were achieved on a small column with dimensions of 4.6 X 5.0 mm. Effect of the Dimension of the Separation Column. In ion-exclusion chromatography a considerable amount of the resin is absolutely necessary to get a reasonable separation (3). Figure 2 shows the effect of the dimension of the separating column on the resolution of a strong acid anion (Cl-) and HC03-. The difference in retention times of these two anions is increased by increasing the dimension of the separating column. Good resolution was obtained in about 7 min on the 7.5 x 100 mm separation column. If the concentration of chloride is low, a smaller separating column (4.6 X 50 mm) will be usable. But if the concentration of chloride is high, a larger separating column (8 X 200 mm) must be used.
710
ANALYTICAL CHEMISTRY, VOL. 59, NO 5, MARCH 1, 1987
I
16pScm'
0.05 0.1 0.2 0.5 1 2 Concn. of H C 0 3 - , mM
2 Zero-
H 5mln
Flgure 3. Comparison of ion-exclusion chromatograms of HC03- with
and without first and second enhancement columns and precolumn: (A) Separating column alone (H' form) (no enhancement);(6) separating column (H' form) first enhancement column (K' form); (C) separating column (H+form) + first enhancement column (K' form) -tsecond enhancement column (OH- form); (D) (C) with precolumn (OH- form) for removal of COP gas in water eluent. Conditions: First enhancement column is TSK SCX, 5 pm, 4.6 X 50 mm. Separating column is TSK SCX (H+ form), 5 hm, 7.5 X 100 mm. Eluent is water (1 ml/min). Sample is a mixture of 1 mM KCI and 1 mM NaHCO, (0.1
+
mL).
For other common strong acid anions such as Br-, I-, NO
