SYSTEM PEAKS IN CAPILLARY-ZONE-ELECTROPHORESIS .3. PRACTICAL RULES FOR PREDICTING THE EXISTENCE OF SYSTEM PEAKS IN CAPILLARY-ZONE-ELECTROPHORESIS OF ANIONS USING INDIRECT SPECTROPHOTOMETRIC DETECTION

A theoretical and experimental study of the existence and evolution of system peaks in capillary zone electrophoresis (CZE) with indirect sp ectrophotometric detection is presented with respect to the effect of the number of colons present in the background electrolyte (BGE). It i s shown that in BGEs having only one colon (i.e., the UV-absorbing pro be anion), the sample produces only negative peaks due to each analyte anion and no system peaks, with the number of sample peaks correspond ing to the number of analytes present in the sample injected. In BGEs containing two colons, a sample with one analyte anion produces one ne gative indirect detection peak and one system peak. The transition bet ween BGEs having one colon and those with two colons has also been stu died and it has been shown that an addition of ca. 5% of the second co lon to a single colon BGE causes the resulting BGE to behave macroscop ically as a regular two-colon BGE. A descriptive model is proposed, ba sed on transient isotachophoresis (transient stacking) of the sample s pecies and of the colon from the BGE which has the closest mobility to the sample ion. This model explains qualitatively the formation and e volution of the sample peak (containing the sample species and being d etected by indirect detection due to displacement of the UV-absorbing probe in its zone) and the system peak (containing no sample species a nd being a vacancy in the continuum of colons of the BGE). It is shown that the system peak may be positive or negative as it corresponds to the situation where the vacancy of one component of the BGE results i n an enhanced concentration of the other component. It has been demons trated that the system peak is created by a vacancy of that component of the BGE which has the greatest difference in mobility relative to t hat of the sample species. On indirect detection in BGEs containing tw o colons the sample displaces predominantly the BGE colon which has a mobility closest to that of the analyte anion. In systems with BGEs co ntaining two colons, a sample having n analytes produces n sample peak s and one system peak, the sign and magnitude of which are dependent o n the sum of the UV absorbances of the analytes involved. The effect o f bicarbonate from atmospheric CO2 has also been studied and it has be en shown that weakly alkaline BGEs with a single anionic UV-absorbing colon, such as those currently used for anionic analyses with indirect detection, may suffer from the presence of system peaks due to bicarb onate.