Study on mechanism of stacking of zwitterion in highly saline biologic sample by transient moving reaction boundary created by formic buffer and conjugate base in capillary electrophoresis

The reason why a moving reaction boundary (MRB) can stack analyte in highly saline sample in capillary electrophoresis [C.X. Cao, Y.Z. He, M. Li, Y.T. Qian, S.L. Zhou, L. Yang, Q.S. Qu, Anal. Chem. 74 (2002) 4167] is still unclear. To illuminate the mechanism of such stacking, three MRBs formed by formic acid-NaOH buffer and sodium formate as well as 40, 80 and 120 mmol/L sodium chloride in matrixes were studied. The computation with MRB theory shows that sodium chloride in matrix has weak effect on the stacking efficiency, whether the concentration of sodium chloride is set at 40, or 80, or 120 mmol/L. The conclusion has been highly manifested by numerous experiments. Furthermore, the computer simulation and theoretical analyses depict that this kind of stacking is induced by the mechanism of MRB, rather than that of electrostacking or isotachophoresis (ITP) under the given electrolytic system. Finally, the application of the sample condensation was achieved for the stacking of analyte(s) in highly saline biological sample of skeletonema costarum’ culture with up to 527 mmol/L total salt and health human urine with 150–320 mmol/L inorganic ions (Cl−, Na+, K+, PO43−, etc.). The results herein have a clear significance to the design on stacking of analyte in highly saline biological sample.