Peak deformation in cationic analysis caused by system zones

In electrophoretic processes, often zones migrate through the separation compartment, with a composition different from that of the background electrolyte (BGE) but which do not contain, however, any component of the sample mixture. These zones migrate with a mobility mainly determined by the composition of the BGE and are called system zones (SZs). If these SZs are visible in electropherograms they are called system peaks (SPs). If sample components have a mobility close to that of a SZ, the separation process can be disturbed and the sample peak shapes are deformed. SZs can appear applying BGEs containing more co-ionic species or if BGEs are used at high or low pH. Recently, the existence of SZs has been described applying BGEs containing weak multivalent anionic species. In this paper, the diverse kinds of system zones, are discussed for cationic systems and the effect of invisible SZs on separations is shown. As an example of a weak multivalent cation, the behavior of the divalent cation histamine is studied, which can be used as co-ion in BGEs for the separation of cations in the indirect UV mode. Applying BGEs containing histamine, SZs are visible in the electropherograms and there existence could also be established theoretically by the use of SystCharts. A mathematical model for the calculation of the mobility of SZs is verified and it has been shown that an unsafe region with a mobility window of msp +/- 10% can be indicated, for the separation of fully ionized sample components.