Unlimited-volume stacking of ions in capillary electrophoresis. Part 1: Stationary isotachophoretic stacking of anions

An online technique for stacking based on the generation of a stationary isotachophoretic (sITP) boundary is presented. By balancing the anodic migration of an ITP boundary with a cathodic EOF, a stationary boundary is formed that can be used to indefinitely concentrate analytes according to ITP principles during electrokinetic injection. The ITP boundary is created by using an electrolyte containing a leading ion (chloride) and a suitable terminating ion added to the sample (2-morpholinoethanesulphonic acid, MES). Destacking and separation are achieved simply by replacement of the sample vial with electrolyte. The formation and stabilisation of the sITP boundary were evaluated through computer simulation which revealed that the pH had little impact upon the formation of the sITP boundary, but did govern the position at which it becomes stationary. Simulations also demonstrated that similar results were obtained when the capillary was initially fined with sample/terminator or leader/electrolyte, which was also supported by experimental results. Using 100 mM Cl-, 200 mM Tris, pH 8.05 as the leader/electrolyte and adding 100 mM MES, 200 mM Tris, pH 8.05 to the sample, the sITP boundary was established after 5 min at -20 kV and was stable for at least 60 min. This provided detection limits for NO2-, NO3- and SCN- of 0.05-0.66 ppb, which are 10000 times lower than hydrodynamic injection and 10-50 times lower than other stacking approaches used for these inorganic ions.