Vacuum-assisted electrokinetic supercharging in flow-gated capillary electrophoresis for rapid analysis of high-salt cerebrospinal fluid samples

On-line sample preconcentration is often required to achieve sensitive detection of specific analytes in capillary electrophoresis. However, available on-line techniques for concentrating high-salt samples such as cerebrospinal fluid (CSF) are mainly limited to sweeping and dynamic pH-junctions, etc. Electrokinetic supercharging (EKS) for sample preconcentration combines the techniques of field-amplified sample injection and transient isotachophoresis that may generate tremendous signal enhancement while maintaining high separation efficiency. This paper reports a novel EKS method for the on-line preconcentration of high-salt CSF samples in flow-gated capillary electrophoresis. The basic procedures include three major steps. First, a sample plug was hydrodynamically injected under vacuum; second, a voltage with the reversed polarity was applied to perform EKS with the assistance of a vacuum during the sample plug push-out; and finally a normal-polarity voltage was applied for separation. Specifically, amino acids were fluorogenically derivatized with naphthalene-2,3-dicarboxaldehyde (NDA) in the presence of cyanide. The optimal final sample medium included 20% CSF (or aCSF in volume), 50% acetonitrile, 12.0 mM borate, 5.0 mM KCN, 5.0 mM NDA, and 2.0 mM EDTA; and the separation buffer was composed of 40.0 tetraborate pH 9.2, 50.0 mM SDS, and 3.5 mM hydroxypropyl-β-cyclodextrin. The enhancement factors for a series of amines were in the range of 30-100 fold, and the detection limits were estimated to be below 0.10 nM. The developed method was coupled with alternate injections for the one-point standard addition method to determine γ-aminobutyric acid (GABA) in CSF from rat brain striatum. The results demonstrated detection sensitivity and accuracy for the rapid analysis of high-salt CSF samples. Vacuum-assisted electrokinetic supercharging was used to preconcentrate high-salt cerebrospinal fluid samples. A shows the procedure; B shows a typical voltage configuration; and C demonstrates the imaging evidence of the sample concentration.