Capillary electrokinetic separation techniques offer high efficiency and peak capacity, and can be very useful for the analysis of samples containing a large variety of (unknown) compounds. Such samples are frequently met in impurity profiling of drugs (detection of potential impurities in a pharmaceutical substance or product) and in general sample profiling (determination of differences or similarities between samples). In this paper, the potential, merits, and limitations of electrokinetic separation techniques for profiling purposes are evaluated using examples from literature. A distinction is made between impurity profiling, forensic profiling and profiling of natural products, and the application of capillary zone electrophoresis, micellar electrokinetic chromatography, and capillary electrochromatography in these fields is discussed. Attention is devoted to important aspects such as selectivity, resolution enhancement, applicability, detection, and compound confirmation and quantification. The specific properties of the various electrokinetic techniques are discussed and compared with more conventional techniques as liquid chromatography.
The potential of on-column preconcentration in capillary electrochromatography (CEC) to improve the detection limitwas investigatd. Two test mixtures containing a pharmaceutically relevant steroid (Desogestrel or Tibolon) together with several structurally related compounds were used for evaluation. For both test mixtures, the mobile phase composition was optimised resulting in a baseline separation of all components and plate numbers of up to 1.2.10 s plates m -1 within 15 min. An equation was derived which describes the obtainable gain in injection time as function of the analyte retention factor in the mobile phase and in the injection solvent. The proposed model accounts for the focussing of the analytes due to both the retention during injection and the step-gradient during elution. For the experimental study, the least hydrophobic component of the Desogestrel mixture was used. When the mobile phase was used as injection solvent, a considerable decrease in plate numberwas observed when the injection time exceeded 5 s. By dissolving the analyte in a less-eluting solvent, injection times could be increased up to 60s without causing significant extra band broadening. Two mixtures containing a relatively high amount of Desogestrel or Tibolon, and the related components at the 0.1% level were analysed to study the potential of the system for impurif,/profiling. With the mobile phase as injection solvent, the low level components could hardly be detected. By applying large volume injection from a less-eluting injection solvent, a gain in sensitivily of a factor of 7 -9 was achieved.
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