System eigenmobilities in zone electrophoresis: A general moving-boundary approach

System zones in capillary zone electrophoresis represent an important topic, very interesting from the theoretical point but also important for practice. This paper is aimed at contributing to the understanding of system zones as one of the very fundamental properties of electrophoretic systems, by developing an alternative approach to the so far used vector-matrix model of calculation of system mobilities (system eigenmobilities). The presented model is based on the solution of the differential form of the moving-boundary equation. The result for acid-base systems is a single algebraic equation valid universally for a zone comprising any number of constituents (mono- or polyhydric strong or weak acids or bases and/or amphoteric compounds). The value of the described solution against previous models consists in its explicit form, expressing the system eigenmobility of a homogeneous zone of given composition as a function of only known quantities. The obtained equation is shown to be the common source of various simplified equations obtained in the past for particular simple systems. The applicability of the simplified equations is discussed in terms of completeness of the results (number of output system eigenmobilities). For non-buffered systems, the occurrence of a previously unreported non-zero value of system eigenmobility is discussed that is equal to the arithmetic average of mobilities of the solvent ions.