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)trimeth ylammonium (NBTA(+)), are simultaneously sorbed on the bonded phase so rbent 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-absorbe r NBTA(+) as the probe ion in an indirect detection system. in the sor bed state, the two ions reside at different charge planes: NBTA(+) is very close to the ODS-water interface, and TBA(+) is about 11 Angstrom deeper into the ODS phase. Both ions contribute to the electrical pot ential psi(I) at the NBTA(+) charge plane, which can be calculated fro m an electrical triple-layer model. The sorption of TBA(+) influences the amount of NBTA(+) sorbed both by raising psi(I) and by altering th e 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 l atter effect, the direction and magnitude of the influence of TBA(+) o n available space both change with ionic strength (c). At low c, sorbe d TBA(+) increases the amount of space available for NBTA(+) by disrup ting the conformation of the alkyl chains of the ODS phase, At high c, sorbed TBA(+) decreases the amount of space available by competing wi th NBTA(+) for space. This dramatically different effect of TBAf on av ailable space at low and high c occurs because higher c produces both a weaker interaction of the ODS alkyl chains with water and a reorient ation of NBTA(+) from flat to perpendicular in the ODS phase.