Electrokinetic injection (EKI) is usually considered as one of the useful approaches to improve sensitivity of CZE analysis. In the present study, we explored the relationship between electrode position and sample amount injected during EKI process by using 2D computer simulation (CFD-ACE+) and real experiments, aiming to obtain higher detection sensitivity. Two different models of electrode configuration, a capillary inserted in a hollow electrode and a capillary surrounded by a cylindrical electrode on the reservoir wall, were simulated to evaluate the efficiency of EKI. It was found that analytes, occurring only in an effective potential field, could be introduced into the capillary while the other analytes remain outside of the field because of slow diffusion. Consequently, the longer distance between the electrode and the end of capillary, the higher efficiency of EKI was found by the simulation. This finding was verified by the real CZE analysis of dilute rare-earth metal ions in a chloride solution (pH almost neutral). In fact, when the distance of Pt electrode and the capillary end in a CE apparatus (an Otsuka CAPI-3100) was default (ca. 1 mm), LOD of Er was 0.27 microg/L. When the distance was increased to 19.5 mm, the LOD was improved over ten times down to 0.02 microg/L. The LOD achieved is 50-fold better than that of inductively coupled plasma atomic emission spectrometry (1-2 microg/L for Er).

Electrokinetic supercharging (EKS) is defined as a technique that combines electrokinetic sample injection with transient ITP. Quantitative repeatability of EKS-CZE and the other CE methods using electrokinetic sample injection process is usually inferior in comparison with the CE methods using hydrodynamic or hydrostatic injection. This is due to some effects, such as the temperature change and the convection of the sample solution in the reservoir, as well as the change of the distance between an electrode and a capillary end (D(ec)). In particular, we have found that the D(ec) change might most seriously affect the repeatability, especially when the electrode is a thin Pt wire that could be unintentionally bent during sampling. By using a Teflon spacer to fix D(ec) to 1.1 mm, the RSD of peak area (n=5) was decreased from 20 to 3.4% in EKS-CZE for several metal cations. This D(ec) dependence of the sample amount injected was supported by computer simulation using CFD-ACE+ software. The improved repeatability (down to 5.1% at n=5, averaged RSD for Co(2+), Li(+), Ni(2+), Zn(2+) and Pb(2+)) was also experimentally attained by increasing the D(ec) to ca. 20 mm, which was also effective to obtain high sensitivity. Since the temperature and the convection effects on the repeatability are comparatively small in a proper laboratory environment, these effects were estimated from the EKS-CZE experiments using conditions such as warming and agitating the sample solution during EKS process. Finally, EKS-CZE was applied to the detection of ions from atmospheric electrolytes in high-purity water exposed to ambient air for 2 h. The microgram per liter levels of anions (chloride, sulfate, nitrate, formate, acetate and lactate) and cations (ammonium, calcium, sodium and magnesium) could be detected using conventional UV detector.