Sample stacking by acetonitrile-salt mixtures.

A new type of sample stacking for capillary zone electrophoresis (CZE), which occurs in mixtures of acetonitrile and high concentrations of inorganic salts in the sample, is described here. It is different from the stacking obtained in low-ionic-strength buffers in the sample since it is improved by the presence of a high concentration of ions while the latter type deteriorates in the presence of such ions. It is based on the addition of acetonitrile to the sample to eliminate proteins by precipitation. From a practical point of view, the stacking in acetonitrile is more useful since it both removes proteins and counteracts the deleterious effects of high concentrations of inorganic ions present in the sample. Furthermore, it allows a larger volume of the sample to be loaded on the capillary. More than one-third of the capillary can be loaded with the sample by this type of stacking, producing a high sample concentration with good resolution and increased sensitivity of detection. It is well suited for analyzing small molecules in serum, urine, food, tissues, and industrial samples. Furthermore, since proteins are eliminated, the capillary does not need thorough washings between samples, speeding up the analysis and extending the capillary life.