Effects of system peaks in a coupled column system for noscapine and its metabolites.

System peaks created in a coupled column system, after transfer of the precolumn mobile phase to the analytical column have been investigated. Co-elution of an analyte with a system peak may affect the chromatographic performance of the analyte. The possibility of using the peak compression effect, in order to increase the detectability of the compounds noscapine, narcotoline and cotarnine, respectively, is studied. Three different mobile phases for the analytical column have been tested, the first contained a co-ion, the second a counter-ion and the last both a co-ion and a counter-ion. A compressed noscapine peak is obtained not only when it is eluted in a positive gradient of the co-ion in the first system, but also in a positive gradient of the counter-ion in the second system. Slightly compressed narcotoline and cotarnine peaks are obtained in the third system as long as they eluted before the deficiency peak of the counter-ion.