Quantitative study on selective stacking of zwitterions in large-volume sample matrix by moving reaction boundary in capillary electrophoresis

The paper advanced the theoretical procedures for quantitative design on selective stacking of zwitterions in full capillary sample matrix by a cathodic-direction moving reaction boundary (MRB) in capillary electrophoresis (CE) under control of electroosmotic flow (EOF). With the procedures, we conducted the theoretical computations on the selective stacking of two test analytes of L-histidine (His) and L-tryptophan (Trp) by the MRB created with 30 mM pH 3.0 formic acid-NaOH buffer and 2-80 mM sodium formate. The results revealed the following three predictions. At first, the MRB cannot stack His and Trp plugs if less than 12.5 mM sodium formate is used to form the MRB and prepare the sample matrix. Second, the MRB can stack His and/or Trp sample plugs completely if higher than 50 mM sodium formate is chosen to form the MRB. Third, the MRB can only focus His plug completely, but stack Trp plug partially if 20-50 mM sodium formate is used; this implied the complete MRB-induced selective stacking to His rather than Trp. All the three predictions were quantitatively proved by the experiments. With great dilution of sample matrix and control of EOF, controllable, simultaneous and MRB-induced selective stacking and separation of zwitterions were achieved. The theoretical results hold evident significances to the quantitative design of selective stacking conditions and the increase of detection sensitivity of zwitterions in CE. In addition, the control of EOF by cetyltrimethylammonium bromide (CTAB) can evidently improve the stacking efficiency to both His and Trp.