Mechanism of ion stacking in aqueous partition chromatographic processes

Abstract

It has been reported that ion enrichment phenomena are observed in liquid chromatographic processes with an aqueous mobile phase on the columns packed with nonionic materials. However, the mechanism of the ion enrichment is not at all well understood. In this study, we investigated the retention and enrichment behaviors of simple inorganic anions on a C18-bonded silica column and a cross-linked hydroxylated methacrylic polymer gel column with pure aqueous mobile phases containing various electrolytes. We show that the stacking of ionic solutes can successfully be accounted for by the ion partition model, and it takes place due to the effect of the background coion in the eluent and/or sample solution on the distribution of the ions between the bulk water and the water incorporated in the packing material, which acts as the stationary phase. Using the ion exclusion effect of fixed anionic charges on a packing material as well as the ion stacking by partition, we developed a simple and versatile method for effective enrichment of anionic solutes in aqueous solutions. The enrichment factor and the elution time of the stacked ion zone can be predicted by the ion partition model.

Partition/Ion-Exclusion Chromatographic Ion Stacking for the Analysis of Trace Anions in Water and Salt Samples by Ion Chromatography

“…Therefore it is desirable that the component anion of the background electrolyte has as small a distribution coefficient as possible in order to achieve higher enrichment of the target anions by PIEC stacking. In a previous study, 23 we demonstrated that sulfate ion exhibits the smallest distribution coefficient (D Na2SO4 SO4 = 0.32) among common inorganic anions, and that the overall stacking can be achieved for anions having the D A Na2SO4 larger than 1.78, such as nitrate, iodide, thiocyanate with the TSKgel G2500PWXL column (Vs = 3.42 mL), even when 5.00 mL of the sample solution was injected. We have thus adopted Na2SO4 as the background electrolyte to be added to the sample solution and validated the on-line PIEC ion stacking-IC method for the accuracy and reproducibility by determining nitrate ion in tap water.…”

“…We have shown that anionic solutes are excluded from the interior of the polymer gel particles in the TSKgel G2500PWXL column by the ion-exclusion effect of fixed anionic charges at an ionic strength of 1 mM or below, whereas the ion-exclusion effect is suppressed at 50 mM or above. 23 Accordingly, the concentration of the background electrolyte was set at 0.1 M.…”

“…We have recently developed a unique method for the enrichment of trace amounts of anions using an ion stacking phenomenon in a partition chromatographic process. 23 The effective concentration of anionic solutes in aqueous solutions can be performed only by introducing the sample solution into a hydrophilic polymer column after the addition of a suitable electrolyte to the solution, and then eluting it with pure water; the anions exhibiting weaker retention than the anion of the added electrolyte are stacked at the leading edge of the added electrolyte zone, while the other anions showing stronger retention are accumulated at the trailing edge. This ion-stacking phenomenon takes place due to the effect of the background co-ion in the eluant and/or sample solution on the distribution of the ions between the bulk water and the water incorporated in the packing material, which acts as the stationary phase.…”

“…[24][25][26][27][28][29][30][31] Furthermore, we have succeeded in accumulating analyte anions having distribution coefficients larger than that of the component anion of the added electrolyte in a very narrow zone at the trailing edge of the elution band of the electrolyte using ion-exclusion effect exerted by fixed anionic charges on a hydrophilic polymer packing material as well as partition chromatographic ion stacking. 23 It was shown that the enrichment factor as high as several hundred for some inorganic anions could be achieved with a hydrophilic polymer column of 300 mm long. This ion-enrichment method is very simple, cost effective, and versatile since only pure water and a simple inorganic salt suitable for the enrichment of the analyte ions are needed, and the operation can be performed in a single run.…”

Characterization of the Interfacial Liquid Layer Formed on Hydrophobic Packing Material Surfaces by Liquid Chromatographic Analysis of the Distribution of Ions and Molecules

“…It has so far been reported that not only inorganic ions but also some small hydrophilic compounds such as urea show retention volumes obviously smaller than that of deuterium oxide on RPLC columns in some water–organic solvent and pure water eluent systems. 27 − 33 These phenomena have been interpreted with ion exclusion or size-exclusion mechanism assuming that hydrophilic molecules would have effective sizes much larger than their own by strong hydration. 29 − 32 We have shown that this specifically weak retention of small hydrophilic molecules should be attributed to their sensing of the interfacial solvated liquid layer.…”