Nonstoichiometric Theories

Modern retention theories recognize that the layer of the bonded phase is so thin that the surface area to bonded phase volume ratio is extremely high. As a consequence, the surface science view on adsorption of both the analyte and the ion pair reagent to the stationary phase can be applied. In these theories, the electrostatic interaction between the ion pair reagent and the analyte ion leads to a non-stoichiometric retention theory. The non-stoichiometry is a consequence of the fact that the electrostatic interaction is long ranged and therefore gives rise to a multibody interaction (Figure 1).

The common basis of modern retention theories is that a charged surface is created when the ion pair reagent adsorbs at the interface between the polar mobile phase and the hydrophobic stationary phase. The inorganic counterions to the ion pair reagent are territorially bound to the charged surface in a diffuse layer and a diffuse double layer is formed. Physically this results in a difference in electrostatic potential, AY0, between the bulk of the mobile phase and the surface (AY0 has the same physical origin as the more

Figure 1 Schematic illustration of the long-range nature of electrostatic forces between ions in typical ion pair chromatographic systems. Open and full arrows represent electrostatic repulsion and attraction forces, respectively. (Reproduced with permission from Bartha A and Stahlberg J (1994) JournalofChromatographyA 668: 255.)

Figure 1 Schematic illustration of the long-range nature of electrostatic forces between ions in typical ion pair chromatographic systems. Open and full arrows represent electrostatic repulsion and attraction forces, respectively. (Reproduced with permission from Bartha A and Stahlberg J (1994) JournalofChromatographyA 668: 255.)

frequently encountered zeta-potential, although they may differ in their numerical value). For a positively charged ion pair reagent the numerical value for AY0 is positive and for a negative reagent it is negative. Qualitatively, the charging of the stationary phase surface infers that the retention of analyte ions of opposite charge to the pairing ion increases due to electrostatic attraction. It also infers that the retention of analyte ions of similar charge to the

Figure 2 A schematic picture of the electrical double layer in reversed-phase ion pair chromatography. (Reproduced with permission from Bartha A and Stahlberg J (1994) Journal of ChromatographyA 668: 255.)

pairing ion decreases due to electrostatic repulsion (Figure 2).

Several non-stoichiometric theories have been proposed, which all use the difference in electrostatic potential between the mobile and stationary phase as a parameter that influences the retention of an ionic analyte. However, there are important differences between the theories in the physical description and also in the role that the electrostatic surface potential plays in retention. For example, in the theory proposed by Cantwell and co-workers, retention is considered to be due to a mixed ion exchange and electrostatic effect. The disagreement today among the proponents of non-stoichiometric theories seems to be on how retention is best described under conditions of high surface loadings of the ion pair reagent, i.e. when the electrostatic surface potential is higher than + 60 mV. The disagreement is a consequence of the fact that under these conditions a number of different, not fully understood, physical phenomena may occur in the diffuse double layer.

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