The major equilibria that contribute to the retention of an organic analyte anion on PRP-1 (a nonpolar poly(styrenedlvlnylberizene) adsorbent) In the presence of a tetraalkylammonlum cation (R4N+), a coanlon, and OH' are identified and used to derive an equation describing the analyte retention which was experimentally verified. The significant parameters are structure and concentration of R4N+, mobile phase solvent composition and pH, analyte concentration, and type and concentration of coanlon. The ability of a R,,N+ salt to enhance analyte retention changes significantly as the coanlon Is changed. The analyte retention, referred to as Ion Interaction, Is suggested to follow a double layer model where the R4N+ salt occupies a primary layer at the stationary phase while the analyte anion and other anions in the system compete for the secondary layer. A major equilibrium is the selectivity of one anion over another. Several applications are Illustrated.Adding: hydrophobic counterions to a mobile phase to enhance retention and resolution has been widely used in the liquid chromatographic (LC) separation of charged organic species on alkyl-modified silica (1-3). Other studies have focused on identifying the interactions that occur in the presence of the counterions, and several retention mechanisms have been proposed (1-12).The ion pair mechanism is one where it is suggested that ion pairs form between analyte ions and the hydrophobic counterions prior to sorption on a hydrophobic alkyl-modified silica stationary phase. Experimental evidence supporting this are isotherm measurements and correlation of retention with counterion concentration. Other workers suggested an ion exchange mechanism, where the counterions are first sorbed and these charge sites serve as exchange sites for the analyte ions. Retention of counterion on the stationary phase and correlation of this to analyte retention has been the major supporting evidence for an ion exchange mechanism. It has also been suggested that both occur and the extent to which one is more significant than the other is a function of the conditions employed in the LC separation. If very longchained, bulky, hydrophobic counterions are used, micelle formation and molecular size factors are significant parameters that influence retention.Because neither the ion pair nor ion exchange mechanism individually explains all the experimental results, retention models that are broader in scope and which attempt to account for all equilibria have been suggested (2, 4, 5,10-14). Thus, Biddlingmeyer et al. (2,5) proposed a model that does not require ion pair formation in either phase and is not based on classical ion exchange. This model assumes dynamic equilibrium which is affected by electrostatic, eluophilic, eluphobic, adsorbophilic, and adsorbophobic forces. Although this study was done on a C-18 bonded phase the model is very similar to the double layer model proposed by Cantwell and

2427in the present study lends support to this suggestion. The role of C10, in the present study is that of a so-called "pairing-ion" reagent which produces on the adsorbent surface a charge density that depends on its bulk solution concentration and on the overalU ionic strength. Thus, adsorbed C10, transforms the adsorbent into a "dynamic cation exchanger" and, at the same time, imparts to the surface an electrical potential which varies as a result of changes in both surface charge density and bulk solution ionic strength. Studies in the field of so-called "ion-pair" chromatography should not focus solely on the "dynamic ion exchange" contribution to retention (13,32-34) but, should also evaluate the contribution of potential-dependent adsorption. In1 some cases other phenomena, such as ion pairing in the bulk solution, may also make some contribution to the overall retention of an ion. However, the retention of NBS-on QXAD apparently does not involve ion pairing,

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Journal of Liquid ChromatographyPublication details, including instructions for authors and subscription information:ABSTRACT A1 kyl-modified s i l i c a (RSi) and polystyrenedivinyl benzene (PRP-1) stationary phases a r e compared f o r the chromatographic separation of inorganic analyte anions and cations using hydrophobic ions of opposite charge as mobile phase additives. alkylammonium s a l t s were used f o r anion separations and alkyl sulfonate s a l t s f o r cation separations. Two major e q u i l i b r i a influence the retention of analyte ions on PRP-1. These are: retention of the hydrophobic ion on PRP-1 and an ion exchange s e l e c t i v i t y between the hydrophobic counterion and the analyte ion. a t residual silanol groups, which a c t a s weak cation exchange s i t e s . analyte retention are identified. favorable eluting conditions f o r the separation of inorganic ionic analytes. Of particular i n t e r e s t i s the potential use of PRP-1 and RSi columns f o r the separation of inorganic cations; condit i o n s f o r the separation of a l k a l i metals and a l k a l i n e earths a r e discussed.