Isotachophoretically Assisted On-Line Complexation of Trace Metal Ions in a Highly Saline Matrix for Capillary Electrophoresis

Trace metal ions such as Cd2+, Ni2+, and Zn2+ in a highly saline sample were subjected to on-line complexation with 4-(2-thiazolylazo) resorcinol (TAR) dissolved in a background electrolyte (BGE) under transient isotachophoresis (TITP) conditions. A long plug of the saline sample, containing the trace metal ions but devoid of TAR, was injected into a coated capillary filled with a BGE composed of 150 mM 2-(cyclohexylamino) ethanesulfonic acid (CHES) and 110 mM triethylamine (TEA) at pH 9.7. Since the electrophoretic mobility of TAR fell between the mobilities of the anionic leading electrolyte (Cl- in the sample) and the anionic terminating background electrolyte (CHES-), a highly concentrated zone of TAR from the BGE was formed at the rear of the sample matrix and then the metal cations toward the cathode were swept by isotachophoretically assisted on-line complexation (IAOC) between the metal ions and the isotachophoretically stacked TAR. As a result, anionic metal-TAR complexes were formed efficiently, which satisfy the TITP conditions between Cl- and CHES-. The enrichment factors of metal ions including Cd2+ were up to 780-fold compared to a conventional CZE mode using absorbance detection. The detection limits were 17 nM, 15 nM, and 27 nM for Ni2+, Zn2+, and Cd2+ in a 250 mM NaCl matrix, respectively. Our method was successfully applied to the analysis of urine samples without desalting.