Anal. Chem. 1985, 57, 2257-2263
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Theoretical Approach of Dual-Column Ion Chromatography Michele Doury-Berthed,* Pierre Giampaoli, Helmut Pitsch, Catherine Sella, and Claude Poitrenaud Institut National des Sciences et Techniques Nucle'aires, Centre d'Etudes Nucle'aires de Saclay, 91191 Gif-sur- Yvette, France
The analytical response In dual-column Ion chromatography coupled wtth conductometricdetection appears to be the sum of the slmple conductometric contrlbutlon of the solute and of the complex eluent. A general mathematical expresslon for the detector response Is established whlch allows derlvatlon of the theoretical callbratlon law based on peak helght. I n the general case this law Is not a proportional function of the Injected sample concentratlon. I t Is experlmentally verlfled for chloride, nltrate, phosphate, and sulfate samples wlth the Dlonex-carbonate system and for chlorlde samples wlth the Vydac-phthalate and Vydac-benzoate dual-column process. The IlmttaUon of this law Is discussed and Its appllcatlon range Is glven for the three systems under study. A method can be proposed to mathernatlcally get the whole callbratlon graph, by means of only one standard, for any solute, eluent buffer, and separator column.
Ion chromatography using conductometric detection is a sensitive method, introduced by Small et al. in 1975 ( I ) , for the determination of inorganic anions in mixtures. Coupling a suppressor column in the H+ form to the separator improves the senisitivity of the process. The major feature of this dual-column system is that the eluent anions are neutralized to their weak acid form and the sample species is converted to its corresponding acid. This one is either weak or strong, depending on the species nature and, in the former case, completely or partially dissociated according to the pH of the eluate, i.e., the nature of the eluent buffer. As a rule, the presence of the sample species increases the concentration of the H+ ions in the eluate and thus involves a shift of the dissociation equilibrium of the weak acid eluent. Consequently, the contribution of the eluent to the conductivity is not constant but decreases with increasing of the sample Concentration. As a result, the calibration graphs whether based on peak heights or peak areas cannot be expected to be linear. Slanina et al. and Van Os et al. (2,3)already studied this phenomenon. They recently attempted to derive an equation allowing calculation of the sample species concentration in the eluate from the conductance measurement ( 4 ) . However their equation is not general as it may be only applied to strong monoacids anions eluted with solutions of divalent anion salts. Furthermore, it is partially incorrect for two reasons. First, the authors consider that, during the elution process, all of the sample ions contained in the last section of the separator column leave the column instead of the amount present in the mobile phase only. The second reason is that the authors do not discriminate phenomena due to the sample injection from those relative to the elution step. In this paper, a general formula is established which correlates the measured conductance to the species concentration in the eluate for any species and eluent composition in the case of the dual-column system. This formula clearly displays all of the parameters affecting the conductometric response of the detector. The equation giving the peak height vs. the species concentration in the injected sample is then derived by applying the classical expression of the elution peak.
THEORY General Expression of the Conductance. Let us consider the general case of the injection of a species H,X("-,)belonging to the acidobasic system of a weak polyacid H,X, eluted by a mixture of HL- and L2- anions. The m value depends on the acidobasic properties of the acid H,X and on the pH of the eluent buffer, i,e., on the nature of the eluent anions and their respective concentrations. The sample injection at the top of the separator column leads the species to be fixed in the stationary phase through both equilibria
and
where the bars symbolize the stationary phase. As a result, the sample fixation produces a displacement from the stationary phase toward the mobile phase of an ionic equivalent amount of L2-and HL- anions. If the sample matrix has the same composition as the eluent, this phenomenon induces a local increase of the total concentration of the eluent anions in the mobile phase and a concomitant decrease of the concentration of the same buffer anions in the stationary phase. This anion excess, which is accompanied by an equivalent cation excess, moves faster in the chromatographic system than the sample species, producing a positive chromatographic peak which appears before the species (excepted for F- which is very slightly retained in the separator). A detailed investigation of this effect and its consequences upon the analysis of slightly retained solutes was previously reported (5). When this excess is separated from the sample species in the separator column, the transport of the latter one through the column according to eq 1and 2 is accompanied by a deficit compared to the eluent of L2- and HL- buffer anions. This deficit is distributed concomitantlywith the species H,X(")between both phases of the separator and emerges in the eluate from the suppressor column as a deficit of H2L at the same time as the neutralized sample anions (assuming that the H2Ldeficiency is not specifically retarded in the H+ section of the suppressor column by a Donnan equilibrium). If a fraction @ of H,X("-,)- anions is exchanged in the separator column with the eluent anions L2- and a fraction (1 - p) exchanged with the HL- form, then the contributions of the H2L deficiency of both anions L2- and HL- are, respectively (@/2)(n - m)C,and (1- P)(n - m)C,, C, being the local sample concentration in the eluate. So the total deficiency ACH,L which locally occurs in the eluate is related to C, by
where @,n, and m are constant for a given chromatographic system and solute. Consequently, the concentration of the eluent species H2Lwhich enters into the detector together with the sample species is given by
0003-2700/85/0357-2257$01.50/00 1985 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
(3)
CEbeing the H2Lconcentration in the eluate containing a zero sample concentration. By arriving at the H+ section of the suppressor column, the sample anions H,X(,-,)- are neutralized into the weak acid H,X, just as the eluent turns into H2L. The pH of the mobile phase then decreases and the H,X acid is dissociated into more protonated species than HmX(n-m)-. Suppose H,tX(n-m’)is the most probable acid form of the sample species in the eluate from the suppressor column. This species can be the center of the following acidobasic dissociation equilibria: H,,X(n-m’)- + H+ Hm’+l X(n-m’-1)and
Hm,Xb-m’)- + H+ + Hm,-lX(n-m’+l)In the same manner, the weak acid H2Lis partially dissociated according to the equilibrium
H2L
the ions in the eluate is generally low (
