Investigation of the effects of halide ions on indirect UV detection in capillary zone electrophoresis

The use of halide salts in combination with sulfonated calixarenes has been recently demonstrated as a novel and effective approach for the indirect detection of a number of significant analytes, including amino acids, biogenic amines, and inorganic cations and anions in capillary zone electrophoresis (CZE). In this method, a sulfonated calixarene, such as p-sulfonic calix[6]arene (SCX6), was employed as the selectivity modifier as well as the UV-absorbing background electrolyte for the indirect detection of the sample ions, while the halide salt was supposed to serve as the 'ionic strength adjuster' in the run buffer. However, although halide ions are not commonly employed as UV-active chromophores in CZE for the indirect detection of various analytes, certain halide ions, especially Br^- and I^-, are capable of absorbing appreciable amounts of UV photons near 200 nm. Therefore, in a run buffer system containing a calixarene compound and a halide salt, the overall background absorbance can arise from both of these background electrolytes. In the present work, new insights into the influence of halide ions on the detector response patterns (both peak intensity and direction) were provided by examining the effects of the presence of SCX6 and/or halide ions in the run buffer on the indirect detection of various model analytes. The results indicated that halide ions played a significant role in governing detection sensitivity, while the cations of the halide salts were also found to influence the peak intensity, as well as the peak direction for the various analytes studied. These response patterns were briefly discussed in terms of mathematical treatments based on the Kohlrausch regulation function.