Unexpected surface chemistry in capillaries for electrophoresis

Good and reproducible capillary quality is needed to develop robust methods and to facilitate method transfer in CE. Physical surface defects no longe r play a major role in variability of fused-silica capillaries. Nevertheles s, problems are frequently being reported when buffers in the pH range betw een 4 and 7 are used. Thus the surface chemistry has been studied by X-ray photoelectron spectroscopy. Silicon-carbon bindings have been found on inne r capillary surfaces for electrophoresis. This binding type is not complete ly removed by pre-conditioning with 1 M NaOH for 30 min. This corresponds t o the result, that capillaries provide more stable migration times, especia lly in the pH range 4-7, when they are pre-conditioned for longer than 1 h. The origin of this Si-C bond is still not quite clear. They could be cause d by graphite which is used during the fabrication of the raw cylinders pri or to capillary drawing. Further investigations are intended in order to un derstand if there are any differences in surface carbon content from batch to batch and if this can influence experimental results in CE. A better und erstanding of the surface chemistry should not only improve robustness in C E, but also help to facilitate and accelerate capillary pre-conditioning an d rinsing procedures to remove strongly adsorbed analytes or matrices. (C) 2000 Elsevier Science B.V. All rights reserved.