CAPILLARY ZONE ELECTROPHORESIS WITH INDIRECT UV DETECTION APPLYING A UV-ABSORBING COUNTER ION

The method of choice for the determination of non-UV-absorbing ions in capillary zone electrophoresis (CZE) usually consists in applying indirect UV detection with a non-UV-absorbing counter ion and a UV-absorbing co-ion (chromophore). Nevertheless, the use of chromogenic species with a sign of charge opposite to that of the analyte has recently been mentioned with regard to its principle but has not yet given rise to practical development. In this paper, the indirect UV detection of cations using anionic chromophores (benzoate, anisate) and cationic buffers (Tris, ethanolamine) is described. Based on the use of Kohlrausch's regulation function, a new equation for the response factor (k(i)) has been derived. Depending on the absolute mobility of the analyte ion relative to that of the co-ion, both negative and positive peaks can be observed. The separation of some alkali metal ions and quaternary ammonium ions was used to confirm the dependence of k(i) on the absolute mobilities of the three components of the system (analyte ion, co-ion and counter ion). In addition, the peak shape of a weak analyte (ammonium ion) is also presented as a function of pH. From these results, it can be shown that using the proper pH, a higher efficiency can be reached by matching the effective mobility of the UV-transparent co-ion more closely to those of the analyte ions.