Anal. Chem. 1995,67,3713-371 6
Direct Determination of Inorganic Ions at Sublppb Levels by Ion Chromatography Using Water as a Mobile Phase Wenzhi Hu,*~tAkin Miyazaki,t Hiroaki Tao,t Akihide Itoh,* Tomonari Umemura,* and Hiroki Haraguchi* National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, lbaraki 305, Japan, and Department of Applied Chemistty, School of Engineering, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
Electrostatic ion chromatography (EIC) was used for determinationof trace level inorganic ions. EIC is a new method of separating ions based on simultaneous electrostatic attraction and repulsion interactions between analyte ions and fixed positive/negative charges of a stationary phase, having the special advantage of using only water as the mobile phase. Initial results showing two elutions of the same analyte gave new insights into the mechanism of EIC. It suggests that the zwitterionic stationary phase, like a single charged stationary phase, has a Stern layer and a diffuse layer. The first elution is from the di€t’use layer, and the second is from the Stem layer, As simpler analysis is facilitated by a single elution, a new species of inorganic salt with a longer elution time was added to the original sample solution in order to release analyte ions from the Stern layer to the diffuse layer, The newly introduced salt is called a s a c s c e species, Without preconcentration, inorganic ions at subppb levels were successfully detected by this method. The desirability of an ion chromatography (IC) technique using only water as the mobile phase is summarized by Small:’ ‘There are a number of ion chromatographic techniques that, though not widely used, deserve mention because of their simplicity and their potential for further development. All use water or other polar solvent as the sole component of the mobile phase. This not only eliminates the need for electrolytes and precise eluent make-up, but it avoids many of the detection problems that ionic eluents can impose. Detection can be expected to be very sensitive since the background is essentially deionized water.” In an earlier paper dealing with the results of ion separation achieved using water as the mobile phase, published in 1977; the stationary phase used was a crown ether bonded one. Later, Small et al.3 reported that inorganic ions could also be separated using water as the mobile phase when a very weak ion exchange resin stationary phase was used. The separations achieved using both of these methods, however, were poor compared with the results of the separation of the same ions using the conventional
’ National Institute for Resources and Environment.
* Nagoya University Furocho. (1) Small, H. Ion Chromatography; Plenum: New York, 1989;p 132. (2) Blasius, E.; Janzen, IC-P.; Adrian, W.; Klautke, G.; Lorscheider, R; Maurer, P.-G.; Nguyen, V. B.; NguyenTien,T.; Scholten, G.; Stockemer, J. 2.Anal. Chem. 1977,284, 337. (3) Small, H.; Soderquist, M. E.; Pischke, J. W. U S . Patent 4,732,686, 1988. 0003-2700/95/0367-3713$9.00/0 0 1995 American Chemical Society
ICs (using an ion exchange stationary phase with a mobile phase containing the replacing ions). A new approach for separating ions, also using water as the mobile phase but employing a zwitterionic stationary phase, has been developed by Hu et al.4 When a small amount of aqueous solution containing an analyte (cations and anions) is passed through a zwitterionic stationary phase, neither the analyte cations nor the analyte anions can get close to the opposite charge k e d on the stationary phase, because another charge on the same molecule, k e d on the stationary phase, repels the analyte ions simultaneously. The analyte cations and anions are forced into a new state of simultaneous electrostatic attraction and repulsion interaction in the column. This was termed an “ion-pairing-like form”. This method of separation was termed electrostatic ion ~,~ chromatography @IC) .4 The previous ~ t u d i e s demonstrated that the separation of inorganic ions (with the exception of cations having the same charge) using EIC is comparable to separations of the same ions obtained using conventional ICs. In previous studie~,4>~ samples having high concentrations (mmoVL) of analyte ions were used in order to understand the mechanism. In those studies, samples with low concentrations of ions were not investigated. In this study, attention was turned to the determination of trace level inorganic ions using EIC. Initial results showing separate elution times for the same analyte gave new insights into the mechanism of EIC and led to the development of a new technique for simpler determination of trace level inorganic ions. EXPERIMENTAL SECTION Apparatus. The HPLC system used in this study was the same system used previ~usly.~ It was a Shimadzu (Kyoto, Japan) LC-6A system equipped with a Shimadzu LC-7A pump, a SIL6A autoinjector, a SCL6A system controller, and a CDD-GA conductivity detector connected with a SPD-M6Aphotodiode array Wvis detector. An ICP-AES (Model 075 Plasma Atomcomp MKII; Thermo Jarrell-Ash, Franklin, MA) was also connected to this HPLC system for the detection of the cations. Two ODSpacked columns &Column, 4.6 x 250 mm; Chemical Inspection and Testing Institute, Tokyo, Japan), the first coated with CHAPS micelles and the second coated with Zwittergent-3-14 micelles, were prepared and used as the separation columns. The proceH. Anal. Chem. 1993,65, 2204 (5) Hu, W.; Tao, H.; Haraguchi, H. Anal. Chem. 1994,66, 2514.
(4) Hu, W.; Takeuchi, T.; Haraguchi,
Analytical Chemistry, Vol. 67, No. 20, October 15, 1995 3713
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Figure 1. Chromatogram of an aqueous solution containing 1.OpM each of NaCl and CaC12. Column, ODs-packed column (250 x 4.6 mm) coated with Zwittergent-3-14 micelles; mobile phase, pure water; flow rate, 1.O mumin; sample injection, 1OOpL;detection,conductivity. (1, 1') Na+-CI- and (2, 2') Ca2+-2CI-.
dure for the preparation of the columns was described in previous paper^.^^^ Reagents. The zwitterionic detergents of CHAPS {3-[(3cholamidopropyl)dimethylammoniol-l-propanesulfonate}and Zwittergent-3-14 were obtained from Dojin (Kumamoto, Japan). Inorganic salts used as the analytes were purchased from Wako (Osaka, Japan). All of these reagents were used as received. Pure water used as the mobile phase and to dissolve the reagents was prepared in the laboratory using a Milli-Q puri6cation system (Millipore, Bedford, MA), All of the samples and the pure water used as the mobile phase were stored in clear polypropylene bottles (wall thickness, 0.7 mm). RESULTS AND DISCUSSION An aqueous solution containing 1.0 pmol/L each of NaCl and CaClz was prepared and separated using EIC. The chromatogram shows main peaks (1 and 2) and secondary peaks (1' and 21 for both Na+-Cl- and Ca2+-2C1- (Figure 1). These secondary peaks have not been observed in previous studies. Furthermore, when concentrations of NaCl and CaClz in the sample solution were lower than 0.4 pmol/L, the main peaks (1 and 2) were never observed, while the secondary peaks (1'and 21 were always observed. This phenomenon provides new insights into the mechanism of EIC. To investigate this phenomenon further, this EIC system was connected with photodiode array W-vis and ICP-AES detectors. Standard solutions containing barium nitrate with a concentration range of 1.0 pmol/L-10.0 mmol/L were prepared and analyzed using the Zwittergent-3-14 micelle-coated column and the CHAPS micelle-coated column. Barium nitrate was chosen because barium is a very sensitive element for the ICP-AES and nitrate ions are easily identified using the W-vis detector. A main peak (peak A) and a secondary peak beak A') were observed when a Zwittergent-3-14 micelle-coated column and a CHAPS micellecoated column were used. The retention time of peak A is shorter than that of peak A'. The results obtained using these three detectors indicate that both main and secondary peaks correspond to the same ionic association of Ba2+-2N03-. The peak areas of (6) Hu, W.; Tao, H.; Tominaga, M.; Miyazaki, A; Haraguchi, H. Anal. Chim. Acta 1994,299, 249.
3714 Analytical Chemistry, Vol. 67, No. 20, October 15, 1995
A and A' are directly proportional to the concentrationof Ba(NO3)z in the original sample solution. But the peak areas of A' become constant when the concentration of Ba(N03)2 is larger than 120 pmollL. When the concentration of Ba(NO3)z is relatively high (>0.3mmol/L), the ratio of A'/A beak areas) is very small (
