Solvent and stationary phase selectivity in capillary electrochromatography method development: Comparison of C18, C8, and phenyl-bonded phases for the separation of a series of substituted barbiturates

The capillary electrochromatographic separation of six substituted barbiturates in the ion-suppressed mode was investigated on commercially available C18, C8, and, phenyl-bonded packing materials. The effect on the retention behavior of mobile phase composition, buffer pH, buffer ionic strength, and temperature was studied. Successful chromatography of the barbiturates was achieved with all three packing materials over a pH range from 2.3 to 7.8. It was notable that a significant electroosmotic flow (EOF) was generated with a pH 2.3 mobile phase for all the packing materials, suggesting that capillary electrochromatography (CEC) in the ion suppression mode can be applied to a wide range of weakly acidic compounds of pharmaceutical interest. The C18, C8, and phenylbonded packing materials gave similar selectivities and retention times, and approximately linear relationships between percentage organic modifier in the mobile phase or temperature versus In k were observed. Increasing the buffer ionic strength reduced the EOF but did not result in changes in selectivity. The use of methanol compared with acetonitrile had no effect on selectivity but resulted in a large increase in retention of the analytes. The experimental investigations have shown that the CEC separation of a series of weakly acidic compounds of pharmaceutical relevance can be achieved readily using a range of commercially available packed capillaries. (C) 1999 John Wiley & Sons, Inc.