Electrostatic Ion Chromatography. 2. Partitioning Behaviors of Analyte

Anal. Chem. 1994, 66, 2514-2520

Electrostatic Ion Chromatography. 2. Partitioning Behaviors of Analyte Cations and Anions Wenrhi Hu,’*+Hiroaki Tao,t and Hirokl Haraguchi* National Institute for Resources and Environment 16-3 OnogaWaf Tsukuba, Ibaraki 305, Japan, and Department of Applied Chemistry, School of Engineeringf Nagoya University Furo-cho, Chikusa-ku, Nagoya 464-0 1, Japan

Partitioning behaviors of analyte cations and anions in “electrostatic ion chromatography (electrostatic IC)” were investigated. Detectors of ICP-AES (inductively coupled plasma atomic emission spectrometry), UV absorption, and conductivity were connected on-line to the electrostatic IC and used for the selective detection of cations, anions, and “ionpairing-like forms” (both cations and anions), respectively. When a mixed electrolytic aqueous solution containing cations A, C, ..., Y, and anions B, D, Z ,was passed through the electrostatic IC column, the analyte ions, which dissociated from the original salts AB, CD, and YZ, were each redistributed and forced into a new state. This new state was termed an ion-pairing-like form because of the simultaneous electrostatic attraction and repulsion interactions. The same species of anion or cation are found in more than one ionpairing-like form; however, they occur in different volumes. What is more, each ion-pairing-likeform has a different priority of formation. Born’s equation was used to evaluate that priority. Retention time of the ion-pairing-like form was dependent on the species of the anion and the charge of the cation. All ion-pairing-like forms can be simultaneously separated with the exception of those having the same anion species as well as the same cation charge. Ion chromatography using a strong/ strong positive/negative charged zwitterionic stationary phase with pure water as the mobile phase was termed electrostatic ion chromatography.l

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Electrostatic ion chromatography (electrostatic IC), reported in our previous paper,’ is a method for the separation of ions, based on the simultaneous electrostaticattraction and repulsion interactions between the same and opposite charged particles and using pure water as the mobile phase. The partitioning behaviors of the analyte ions in electrostatic IC were initially investigated using UV absorption and conductivity detectors. It was demonstrated that the analyte cations and anions were eluted together in a new state of simultaneous electrostaticattraction and repulsion termed an “ion-pairinglike form”.’ Most of our previous studies focused on the analyte anions, not the cations. This was because a selective detector for the cations was not available. For the investigation of the partitioning behaviors of the analyte cations, an ICP-AES was introduced as an on-line detector, for the recognition of cations, in the present study. The UV absorption detector + National Institute for t Nagoya University.

Resources and Environment.

(1) Hu, W.; Takeuchi, T.; Haraguchi, H. A n d . Chem. 1993, 65, 2204.

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and the conductivity detector were used to investigate further the analyte anions and the ion-pairing-like forms. The present experimental results indicate that complicated partitioning behaviors of analyte ions occur in electrostatic IC. It is also demonstrated that the retention time of the ion-pairing-like form is decided not only by the species of anion but also by the charge of the cation.

EXPERIMENTAL SECTION Apparatus. Two HPLC systems were used in this study. The first was a Shimadzu (Kyoto, Japan) LC-6A equipped with a LC-6A pump, a sample injector (Rheodyne 7161, California) with a 20-pL loop, a SPD-6A UV absorption detector, and a CDD-6A conductivity detector. The ICPAES (Model 075 Plasma Atomcomp MKII, Thermo JarrellAsh, Franklin, MA) was also on-line with this HPLC, as the cation detector. The other HPLC system was a Shimadzu LC-7A equipped with a LC-7A pump, a SIL-6A autoinjector (sample volumes, 20 pL), a SCL-6A system controller, a photodiode array UV-vis detector (SPD-M6A), and a conductivity detector (CDD-6A). The only column used throughout this study was the ODS column (L-Column, 4.6 X 250 mm, Chemical Inspection and Testing Institute, Tokyo, Japan) coated with strong/strong positive/negative charged zwitterionic bile micelles. The procedure for the preparation of the strong/strong positive/ negative charged zwitterionic stationary phase was the same as described in the previous paper.’ Reagents. The strong/strong positive/negative charged zwitterionic surfactant reagents, such as 3- [(3-cholamidopropy1)dimethylammoniol- 1-propanesulfonate (CHAPS) and 3- [ (3-cholamidopropyl)dimethylammonio] -2-hydroxy- 1-propanesulfonate (CHAPSO), were obtained from Dojin (Kumamoto, Japan). Inorganic metal salts, used as the analytes, were purchased from Wako (Osaka, Japan). These reagents were used as received. Purified water was prepared in the laboratory using a Milli-Q system (Nihon Millipore Kogyo, Tokyo, Japan). Calibration Graphs. Standard aqueous solutions containing a single salt species with a concentration range of 0.1-10 mM were initially examined by electrostatic IC, to derive the calibration graphs (peak areas vs concentrations). The calibration graphs, obtained using a conductivity detector, were used to calculate the concentrations of the ion-pairinglike forms. The calibration graphs obtained using ICP-AES and a UV-absorption detector were used for comparison. Good agreement of results was achieved. 0003-2700/94/0366-25 14$04.50/0

0 1994 American Chemical Society

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Fbure 1. Chromatogram of an aqueous solutlon containing 10 mM KBr and 10 mM NaSCN: column, ODs column (250 X 4.8 mm) coated with CHAPS0 micelles; mobile phase, pure water; flow rate, 0.7 mL/ min; detection, UV absorption at 230 nm (A), ICP-AES (B), and conducthrity (C); (1) Na+-Br, (2) K+-Br, (3) Na+-SCN-, and (4) K+SCN-.

RESULTS AND DISCUSSION 1. Ion Redistribution. Choosing Analytes. In order to clearly illustrate the partitioning behaviors of the analyte ions, inorganic salts used as the analytes were carefully selected using the following parameters: (1) inorganic salts which completely dissociate to ions in water, i.e., the strong electrolytes; (2) inorganic salts which can be base-line separated by electrostatic IC (refer to the separations done in the previous studies’). Mixed Electrolytic Aqueous Solution: Cations (+) and Anions (-). An aqueous solution containing 10 mM KBr and 10 mM NaSCN was separated by electrostatic IC. UVabsorbing anions ( B r and SCN-) were detected by a UVabsorption detector (Figure 1A). The countercations (K+ and Na+) were detected by an ICP-AES detector (Figure 1B). Both ions, termed an ion-pairing-like form were detected by a conductivity detector (Figure 1C). As can be seen, K+ions,

Flgure 2. Chromatogram of an aqueous solution containing 10 mM NaSCN and 5 mM BaC12: detection, UV absorption at 230 nm (A), ICP-AES (B), and conductlvity (C); (1) Na+-Ci-, (2) Ba2+-2C!-, (3) Ne+SCN-, and (4) Ba2+-2SCN-. Separation conditions are the same as in Figure 1.

dissociated from KBr, were redistributed. Some eluted with the SCN- ions, dissociated from NaSCN, and some eluted with their original counteranions (Br). The Na+ ions, dissociated from NaSCN have the same partitioning behaviors as the K+ ions. The concentrations of K+ ions in the ionpairing-like form K+-Br and K+-SCN- were 4.81 and 5.19 mM, respectively. The concentrationsof Na+ ions in the ionpairing-like form Na+-Br and N a + S C N - were 5.19 and 4.81 mM, respectively. These were calculated from the calibration graphs of the standard analytes, obtained using the ICP-AES detector. Mixed Electrolytic Aqueous Solution: Cations (+, 2+) and Anions (-). An aqueous solution containing 5 mM BaC12 and 10 mM NaSCN was prepared and separated by electrostatic IC under the same separation conditions as shown in Figure 1. Figure 2A shows the chromatogram of SCN- ions obtained using the UV-absorption detector. Figure 2B shows the chromatogram of Na+ and Ba2+ions obtained using the ICPAES detector. The chromatogram of ion-pairing-like forms shown in Figure 2C was obtained using the conductivity Analytical Chemistry, Vol. 66,No. 15, August 1, 1994

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detector. Four peaks, corresponding to the ion-pairing-like forms Na+-C1-, Ba2+-2C1-, Na+-SCN- and Ba2+-2SCNwere observed. The experimental results shown in Figure 2 indicate that ion redistribution occurred not only in cations but also in anions. We suggest the following model

to express the ion redistribution, where e is the charge of the cation. The concentrations of Na+-C1-, Ba2+-2C1-, Na+SCN- and Ba2+-2SCN- were calculated to be 8.41, 0.795, 1.59,and 4.205 mM, respectively, from the calibration graphs of the standard analytes. Most SCN- ions were redistributed to Ba2+ions rather than their original countercation (Na+). However, most C1- ions, dissociated from BaC12, were redistributed to Na+ ions. Mixed Electrolytic Aqueous Solution: Cations (+, 2+, 3+) and Anions (-). Figure 3A shows a chromatogram of an aqueous solution containing 5 mM CeC13, 5 mM BaC12, and 10 mM NaSCN, obtained using a UV-absorption detector (a peak corresponding to Ce3+ions is also found in Figure 3A because the Ce3+ ions are UV absorbed). Chromatograms obtained using ICP-AES and conductivity detectors are shown in parts B and C of Figure 3, respectively. The cations (Na+, Ba2+,Ce3+)and anions (Cl-, SCN-), which dissociated from NaSCN, BaC12, and CeC13, were redistributed as Na+-Cl-, Ba2+-2C1-, Ce3+-3Cl-, Na+-SCN-, Ba2+-2SCN-, and Ce3+3SCN-. The process of the ion redistribution can also be expressed as follows: Na'CI'

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The concentrations of Na+-Cl-, Ba2+-2C1-, Ce3+-3C1-, Na+SCN-, Ba2+-2SCN-, and Ce3+-3SCN-were found to be 9.44, 4.48, 2.20, 0.56, 0.52, and 2.80 mM, respectively, from the calibration graphs. The ion-pairing-like form N a + S C N - was alsodetected in a chromatogram obtained using a conductivity detector with increased sensitivity (chromatogram not shown). Na+ ions which eluted with SCN- ions could not be detected by the ICP-AES. This is because the concentration of Na+ ions in Na+-SCN- was lower than the detection limit of ICPAES. Most of the SCN-ions, dissociated from NaSCN, were redistributed to the Ce3+ and Ba2+ ions; fewer of the SCNions were retained by their original countercation (Na+). On the contrary, a large proportion of the C1- ions, dissociated from BaCl2 and CeC13, were redistributed to the Na+ ions. The ion redistribution that occurred in the initial separation (Figure 1 ) can also be expressed as follows:

However, the ion-pairing-like form K+-Br- and Na+-Breluted at the same time, as did K+-SCN- and Na+SCN-. 2516

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Flgure 3. Chromatogram of an aqueous solution contalning 10 mM NaSCN and 5 mM BaC12and 5 mM M I 3 : detection, UV absorption at 230 nm (A), ICP-AES (B), and conductivity (C); (1) Na+-CI-, (2) Ba2+-2CI-, (3) Ce3+-3Cl-, (4) Na+-SCN-, (5) Ba2+-2SCN-, and (6) Ce3+-3SCN-. Separation conditions are the same as in Figure 1.

According to the experimental results shown in Figures 1-3, the partitioning behaviors of analyte cations and anions in electrostatic IC may be summarized as follows. (1) Ion redistribution occurs among all analyte ions, in other words, all possible combinations of the anions with the cations (ion-pairing-like forms) will be observed. (2) The process of ion redistribution in the formation of ion-pairing-like forms can be expressed as the product of the matrices one ( mrows, 1 column; m is the cation species) and two (1 row, n columns; n is the anion species). (3) The distribution ratio of cations and anions to each ion-pairing-like form was different and depended on a priority of formation. 2. Priority Among the Ion-Pairinglike Forms. Aqueous solutions where the ion species were the same but the ion concentrations were different, were also prepared as the analytes. Five aqueous solutions containing 2 mM NaSCN and 2 mM CaClz (i), 4 mM NaSCN and 2 mM CaC12 (ii),

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Flgure 4. Chromatograms of NaSCN/CaCI2 aqueous solutions: NaSCN/CaCI2 = 2/2 (A), NaSCN/CaCI2 = 412 (B), NaSCN/CaCI2 = 612 (C), NaSCN/CaC12= 812 (D), and NaSCN/CaC12 = 1012 mM/mM (E). Conditions column, ODS column coated with CHAPS micelles: mobile phase, pure water; flow rate, 0.7 mL/min; detection, conductivity (top) and photodiodearray UV-vis (bottom): (a) Na+-CI-, (b) Ca2+-2Cl-, (c) Na+-SCN-, and (d) Ca2+-2SCN-.

6 mM NaSCN and 2 mM CaC12 (iii), 8mM NaSCN and 2 mM CaC12 (iv), and 10 mM NaSCN and 2 mM CaC12 (v) were respectively separated under the same conditions.

Chromatogramsare shown in Figure 4A-E. Four ion-pairinglike forms, i.e., Na+-CI-, Ca2+-2C1-, Na+SCN-, and Ca2+2SCN-, were anticipated. The separation results however, Ana!yticalChemistry, Vol. 66, No. 15, August 1, 1994

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5 Retention Time (min)

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Flgure 5. Chromatogram of an aqueous solution COntainlng 2.857 mM each of Na2S04, NaCI, NaBr, NaI, NaSCN, CaI2, and Ca(SCN)2. Separatlon conditions and detection (conductlvky) are the same as in Figure 4. Key: (1) 2Na+-S04", (2) Na+-CI-, (3) Na+-Br, (4) Ca2+2Br, (5) Na+-I-, (6) Ca2+-21-, (7) Ca2+-2SCN-.

show that the ion-pairinglike forms N a + S C N - and Ca2+2C1- are not always apparent. The occurrence of these two ion-pairing-like forms was ultimately decided by the concentration ratio of NaSCN/CaC12. On the contrary, the occurrence of the ion-pairing-like forms Na+-Cl- and Ca2+2SCN- was independent of that ratio. These experimental results indicate that the ion-pairing-like forms have a priority in their formation. The occurrence and hence detection of an ion-pairing-like form which has a lower priority depends on the concentrationratio of the analytes in the original solution. When an aqueous solution containing many ion species is separated by electrostatic IC, several ion-pairing-like forms might not be observed because of their low priority of formation. A mixed aqueous solution containing 2.857 mM each of Na2S04, NaCl, NaBr, NaI, NaSCN, CaI2, and Ca(SCN)2, and a mixed aqueous solution containing 3.333 mM each of Na2S04, NaCl, NaBr, NaI, NaSCN, and Ca(SCN)2 were prepared and separated under the same separation conditions, as described in Figure 4. Ten ion-pairing-like forms, Le., 2Na+-S0d2-, Ca2+S042-,Na+-C1-, Ca2+-2C1-, Na+-Br-, ea2+-2Br, Na+-I-, Ca2+-21-, N a + S C N - , and Ca2+-2SCN-, were anticipated. However, the three ionpairing-like forms Ca2+-S042-, Ca2+-2C1-, and Na+-SCNwere not observed in the first separation(chromatogramshown in Figure S), and the three ion-pairing-like forms Ca2+S042-, Ca2+-2Cl-, and Ca2+-21- were not observed in the latter separation (chromatogram shown in Figure 6). Determining Factors in the Priority of Ion-Pairing-like Forms. In the separation of a mixed electrolytic aqueous solution containing cations A, C, ..., Y , and anions B, D, ..., Z by electrostatic IC, the Occurrence of ion-pairing-like forms can be predicted as follows:

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However, some of these ion-pairing-like forms might not be observed in the real separation because of their low priority of formation. 2518

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Flgure 8. Chromatogramof an aqueous solution contalnlng3.333 mM each of Na804,NaCI, NaBr, NaI, NaSCN, and Ca(SCN)2. Separatlon conditions and detection (conductMty) are the same as in Figure 4. Key: (1) 2Na+-S042-, (2) Na+-Cl-, (3) Nat-Br, (4) Ca2+-2Br, (5) Na+-I-, (6') Na+-SCN-, and (7) Ca2+-2SCN-.

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In order to evaluate the priority of a given ion-pairinglike form, its molal energies (AG) of its ions, which are decided by the charge and the hydrated radius, were taken as determining factors. The molal energy (AG) of a sphere of charge e and radius r immersed in a medium of dielectric constant e, can be calculated from Born's equation.* Four mixed aqueous solutions of CaIz/NaBr (4/4, 4/8, 4/ 12, and 4/ 16, mM/mM) were prepared and separated by electrostatic IC. According to Born's equation, the order of the molal energies (AC) for cations is A G a + > AGcaa+, and for anions is AGI- > AGB,-. Ion-pairing-like forms Na+-Br (AG--catim + AGmin.*doo) and Ca2+-21- (AGmin-cation + AGmax-anion) were observed in these separations. The other possible ion-pairing-like form Na+-I- (AGmax-cation + AGmx-anion) and Ca2+-2Br (AG-+tiOn + AGdn-anion) were not observed. The concentrations of ion-pairinglike forms Na+-Br and Ca2+-21- were calculated from the calibration graphs and plotted as a function of NaBr concentration in the original solution (Figure 7). Aqueous solutions of CaBrz/NaCl (4/4, 418, 4/12, and 4/16, mM/mM) in which the order of the molal energies

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Flgure 8. Concentrationsof ion-pakinglike formsvs NaClconcentration of NaCI/CaBr2 mixed aqueous solutions.

(AG) for the cations is > AGCaz+, and for anions is AGB,- > AGcl-, were prepared and separated. Ion-pairinglike forms Na+-Cl- (AGmax-Mtion + AGdn-anion) and Ca2+2 B r (AGmin-cation + AGmax-anion) were observed. The possible ion-pairing-like forms Na+-Br (AGmax-mtion + AGmax-anion) and Ca2+-2C1- (AGmin--tion + AGmin-mion) were not observed. The concentrations of ion-pairing-like forms Na+-C1- and Ca2+-2Br were calculated from the calibration graphs and plotted as a function of NaCl concentration in the original solution (Figure 8). We may conclude from the above experimental separations of aqueous solutions containing cations (A, C, ..., Y ) and anions (B, D, ..., Z ), where the orders of the molal energies (AG) are AGA > AGC > ... > AGY for the cations and AGB > AGD > ... > AGZ for the anions, that the ion-pairing-like forms AZ (AGmax-cation + AGmin-anion) and YB (AGmin-cation + AGmax-anion) will have top priority and hence will undoubtedly be observed. The other ion-pairing-like forms, particularly AB (AGmax-cation + AGmax-anion) and YZ (AGmin-cation + AGmin-anion), may not be observed, depending on the concentration ratios of the analytes in the original solution. When any chemical interaction occurs between the cations and anions in the mixed electrolytic aqueous solutions, the priority of formation of ion-pairing-likeforms may be changed. Aqueous solutions of CaC12/Na2S04 (414, 418, 4/12, and 4/16, mM/mM) in which the order of the molal energies (AG) for cations is A G N a + > AGca2+,and for anions is AGa> AGs0,2-, were prepared and separated by electrostatic IC. Ion-pairing-like forms Na+-Cl- (AGmax-cation+ AGmax-anion) were observed together with 2Na+-S042- (AGmax-cation + AGmin-anion) and Ca2+-2C1- (AGmin-cation + AGmax-anion). This is because a precipitation interaction occurred between Ca2+ (AGmin-cation) and so42(AGmin-anion) ions, in the CaCla/NazSO4aqueous solution. Some S04” ions were lost to the Ca2+ ions. As a result, the remaining Na+ ions had to combine with the remaining C1- ions; Le., the concentration of Na+C1- was equal to the concentration of CaS04(s). The concentrations of Na+-Cl-, 2Na+-SOd2-, and Ca2+-2C1- were calculated from thecalibrationgraphs and plotted as a function of Na2S04 concentration in the original solution (Figure 9). 3. Species of Anion and Charge of Cation as Determining Factors in the Retention Time of the Ion-Pairing-like Form. Parameters which determine the TR (retention time) of an

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Figure 9. Concentrations of ion-pairing-like forms vs Na2S04 concentration of Na2S04/CaCi2mixed aqueous solutions.

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Figure 10. Chromatogram of an aqueous solution containing 0.5 mM each of NaCi, CaCi2, and CeCi3: (1) Na+-CI-, (2) Ca2+-2CI-, and (3) Ce3+-3CI-. Separation conditions and detection (conductivity) are the same as in Figure 4.

ion-pairing-like form were investigated. An aqueous solution containing 0.5 mM each of NaCl, CaC12, and CeCl3 was separated by electrostatic IC (chromatogram is shown in Figure 10). As can be seen, the ion-pairing-like forms Na+C1-, Ca2+-2Cl-, and Ce3+-3Cl- were separated even though they all had the same anion (Cl-). This means that the TR of the ion-pairing-like forms is decided not only by the species of anion but also by the cation. A mixed aqueous solution containing chloride salts of the alkali metal group (Li, Na, K, Rb, and Cs) and the alkaline earth metal group (Mg, Ca, Sr, and Ba) was prepared and separated by electrostatic IC. Ion-pairing-like forms Li+-Cl-, Na+-Cl-, K+-Cl-, Rb+-Cl-, and Cs+-Cl- were eluted together (could not be separated). Ion-pairing-like forms Mg2+-2C1-, Ca2+-2C1-, Sr2+-2C1-, and Ba2+-2C1- were also eluted together. However, the ionpairing-like forms of the alkali metal group separated from those of the alkaline earth metal group (chromatograms are not shown). This result indicates further that the TR of the ion-pairing-like form is determined by the charge of the cation. In conclusion, ion-pairing-like forms can be separated by electrostatic IC when the species of anion and/or the charge of the cation are different. However, ion-pairing-like forms Analytical Chemistty, Vol. 66, No. 15, August 1, 1994

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cannot be separated when the species of anion and the charge of the cation are the same. The separation of ions by electrostatic IC was achieved by simultaneouselectrostatic attraction and repulsion interactions between the same and opposite charges of the analyte ions and the zwitterionic stationary phase. In principle, every possible ion-pairing-like forms or ions should be separated because the combined effect of electrostatic attraction and repulsion interactions on an analyte ion is determined by the charge and the radius of the analyte ion when other conditions are fixed. However, different species of cation when they have the same charge, cannot be separated. Bernal and Fowler2 reported that the orientation of water molecules in aqueous solutions is different with respect to the proximity of positive and negative ions. The different electrostatic behaviors of the hydrated cation and anion in the separations described here, might be due to the different orientations of water molecules. It has been reported that when a crown ether-coated stationary phase is employed, ion separations can also be (2) Bernal, J. D.; Fowler, R. H. J . Chem. Phys. 1933, I , 535. (3) Blasius, E,; Janzen, K.-P.;Adrian, W.; Klautke, G.; Lorscheider, R.; Maurer, PA.;Nguyen, V. B.; Nguyen Tien, T.; Scholten, G.; Stockemer, J. Z . Anal. Chem. 1977, 284, 337. (4) Blasius, E.; Janzen, K.-P.; Luxenburger, H.; Nguyen, V. B.; Klotz, H.; Stockemer, J. J. Chromatogr. 1978, 167, 307. (5) Kimura, K.; Nakajima, M.; Shono, T. Anal. Lett. 1980, 13 (CA9). 741. (6) Igawa, M.; Saito, K.;Tuskamoto, J.; Tanaka, M. Anal. Chem. 1981,53,1944. (7) Pedersen, C. J. J . Am. Chem. SOC.1967, 89, 7017. (8) Born’s equation -AG = ( 1 - l/t)e2/2r: Robinson, R. A,; Stokes, R. H. Electrolyte Solutions; Butterworth & Co. Publishers Ltd.: London, 1959; p 69.

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achieved with water eluent.34 Blasius et aL3 reported that LiC1, NaC1, and KCl were separated by ion chromatography (not HPLC) with water eluent when a crown ether-coated stationary phase was employed. In this case, the cations were separated even though they had the same charge. This is because the separation mechanisms occurring in the crown ether-coated stationary phase and the strong/strong positive/ negative charged zwitterionic stationary phase are different. Separation of ions using the crown ether-coated stationary phase is due to a cavity of crown ether which selectively encapsules the cations;’ Le., the separation is determined by the cations. However, in our work, the ion separations achieved using the zwitterionic stationary phase are due to simultaneous electrostatic attraction and repulsion interactions. In other words, the separation is determined by both the cation and anion. Igawa et a1.6 also reported that S042-,F-, C1-, B r , N O 3 - , and SCN- could be separated by HPLC using a crown ethercoated stationary phase with water eluent. However, the separation coefficient for anions in electrostatic IC is superior to that achieved by HPLC using the crown ether-coated stationary phase.

ACKNOWLEDGMENT The authors thank Mr. Geoffrey Bland for his valuable assistance in correcting the manuscript. Received for review October 1, 1993. Accepted April 8, 1994.’ *Abstract published in Aduance ACS Abstracts, June 1, 1994.

Electrostatic Ion Chromatography. 2. Partitioning Behaviors of Analyte

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