Effect of a predetection open segment in the column on speed and selectivity in capillary electrochromatography

Columns in capillary electrochromatography (CEC) most commonly have the detection window located immediately after the retaining frit of the packed segment. Here, the properties of "duplex" columns having a predetection open segment between the frit and the detector window are examined with particular regard to the effect of the relative lengths of the packed and open segments on the separation of mixtures containing neutral and charged components. This configuration allows the use of columns with short packed segments in contemporary instruments for rapid separations. It is shown that, by varying the length of the packed segment, the balance of chromatographic and electrophoretic forces can be shifted, and the selectivity can be adjusted if the separation involves the interplay of both mechanisms. Expressions are presented for estimating the retention time in a duplex column if the chromatographic and electrophoretic properties of the sample components are known. The results are expected to facilitate CEC method development in selection of the respective column segment lengths for optimum separation.