Anomalous behaviour of sodium in isotachophoresis

Using a cationic operational system in capillary isotachophoresis, millimolar concentrations of sodium were determined in the absence and presence of serum proteins. With several internal standards, the reproducibility of the sodium determination was significantly poorer in the presence of proteins, even after partial thermal degradation. The results are explained by protein-sodium interaction in the injection port, where the electric field strength was sixteen times lower than in the separation capillary.