Effect of pH and ions in the sample on stacking in capillary electrophoresis

Concentrating the sample on the capillary called ''stacking'' is a simple technique that overcomes the poor detection limits of capillary electrophoresis (CE). Understanding the factors that affect the stacking mechanism is vital. Two general stacking methods are used in CE: (1) low ionic strength buffer in the sample (LISS) and (2) stacking by inclusion of acetonitrile (AS) in the sample. As the sample volume, especially in the latter method, is greatly increased we show that the pH, buffer type and ionic strength of the sample affect greatly the plate number, resolution and migration time. Surprisingly, we find inclusion of a pH, molarity or ion type in the sample, different from that of the separation buffer, can greatly improve the resolution and the stacking for some components of the sample, especially for the acetonitrile induced stacking. (C) 1997 Elsevier Science B.V.