Electrical Double-Layer Effects and the Origin of Indirect UV Detection in the Liquid Chromatography of a Cationic Sample Using a Cationic Probe.

Two cations, tetra-n-butylammonium (TBA(+)) and (4-nitrobenzyl)trimethylammonium (NBTA(+)), are simultaneously sorbed on the bonded phase sorbent Partisil-10 ODS-3 from water at various ionic strengths. The non-UV-absorber TBA(+) is thought of as the sample ion and the UV-absorber NBTA(+) as the probe ion in an indirect detection system. In the sorbed state, the two ions reside at different charge planes:  NBTA(+) is very close to the ODS-water interface, and TBA(+) is about 11 Å deeper into the ODS phase. Both ions contribute to the electrical potential ψ(I) at the NBTA(+) charge plane, which can be calculated from an electrical triple-layer model. The sorption of TBA(+) influences the amount of NBTA(+) sorbed both by raising ψ(I) and by altering the amount of space available for NBTA(+). The former effect is greater than the latter at ionic strengths up to at least 0.5 mol/L. For the latter effect, the direction and magnitude of the influence of TBA(+) on available space both change with ionic strength (c). At low c, sorbed TBA(+) increases the amount of space available for NBTA(+) by disrupting the conformation of the alkyl chains of the ODS phase. At high c, sorbed TBA(+) decreases the amount of space available by competing with NBTA(+) for space. This dramatically different effect of TBA(+) on available space at low and high c occurs because higher c produces both a weaker interaction of the ODS alkyl chains with water and a reorientation of NBTA(+) from flat to perpendicular in the ODS phase.