Isotachophoresis in open systems : Problems in quantitative analysis

So far, isotachophoretic (ITP) analyses have been carried out in commercially available or laboratory-made ITP instruments, generally in closed systems. At present several instruments are commercially available, originally designed for capillary zone electrophoresis with open capillaries, and these instruments can also be used for ITP. If ITP experiments are carried out using such apparatus, however, an electroosmotic flow (EOF) will act on the ITP system. The velocity of the EOF strongly varies with the choice of the leading electrolyte and terminating electrolyte and also the composition of the sample. Hence the reproducibility in quantitative analyses is a serious problem. For quantitative experiments at least an internal standard must be used to correct for undesirable fluctuations in the EOF and irreproducible injections. Better results can be obtained by effectively suppressing the EOF by using additives such as methylhydroxyethylcellulose. Results of quantitative experiments using the Beckman P/ACE System 2000 HPCE are presented, showing some of the problems in quantitative analyses with ITP in open capillaries.