Ion interaction chromatography of organic anions in the presence of tetraalkylammonium salts

Abstract

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

Simultaneous determination of free mycophenolic acid and its glucuronide in serum of patients under mycophenolate mophetil therapy by ion-pair reversed-phase liquid chromatography with diode array UV detection

“…The retention behaviour of some organic anions as quaternary ammonium ion pairs on hydrophobic alkyl modified silica [22] or non polar poly(styrene-divinylbenzene) adsorbent [23] has been already described. When an analyte ion, X − , is introduced in the chromatographic system above specified, it is distributed to the stationary phase as the ion pair QX, which competes with QC (where C − is the co-anion of TBA + ) and QZ (where Z − indicate a competing anion, e.g.…”

“…H 2 PO 4 − , in the buffer) for the available adsorption sites, A s . Under certain assumptions (negligible adsorption of X − in the acidic form, capacity factor independent of X − concentration) it can be shown [23,24] that retention can be expressed by Eq. (1):…”

Dye-coated stationary-phases: A retention model for anions in ion-interaction chromatography

“…where 9 is the phase ratio; the constant term A is given by the product KadsKp (Eqs (4) and (8) [4]. From our results (see the following discussion) it seems that ion-pair partition plays a minor role in the retention process compared with ion-exchange; in these circumstances Eq.…”

“…The column was coated by pumping mobile phase containing the dyestuff and the monobasic potassium phthalate at the same concentrations as used for the elution, until breakthrough [4] occurred. The system was controlled by a computer.…”

Ion Pair Chromatography

“…Analyte retention has been suggested to follow a double-layer model in which the organic pairing ion occupies a primary layer on the stationary phase and the other ions in the system complete for the secondary layer [1]. Analyte retention has been suggested to follow a double-layer model in which the organic pairing ion occupies a primary layer on the stationary phase and the other ions in the system complete for the secondary layer [1].…”