CAPILLARY ZONE ELECTROPHORESIS OF COMPLEX IONIC MIXTURES WITH ONLINE ISOTACHOPHORETIC SAMPLE PRETREATMENT

Separation modes provided by the column-coupling configuration of the separation unit in an on-line combination of capillary isotachophoresi s (ITP) with capillary zone electrophoresis (CZE) were studied from th e point of view of their potential in the (trace) determination of ion s present in complex ionic matrices. Urine was arbitrarily chosen as s uch a matrix while sulphanilate and 3,5-dinitrosalicylate (currently n ot present in urine) served as model analytes. In one of these modes, ITP was employed to remove only the most abundant sample constituent ( chloride) and concentrate the rest of those migrating between the lead ing and terminating zones for injection into the ZE stage. In the othe r mode, ITP was employed for maximum sample clean-up. Here, only the a nalyte(s) with a minimum of the matrix constituents was transferred fo r a final separation in the ZE stage. The fraction to be transferred w as defined via a pair of discrete spacers added to the sample. Althoug h a highly efficient sample clean-up was typical in this instance, the use of identical migration regimes in both stages (the separations ac cording to ionic mobilities) did not prove the resolution of one of th e analytes (sulphanilate) from the matrix constituent(s) in the ZE sta ge. A considerable improvement in this respect was achieved easily whe n the ITP clean-up was based on the separation according to pK values while the constituents present in the transferred fraction were finall y separated via differences in their ionic mobilities. This two-dimens ional approach provided a way to achieve a 150 ppb (10(-6) mol l-1) co ncentration detection limit for sulphanilate in a 1-mul volume of urin e taken for the electrophoretic run.