Another Approach Toward over 100 000-Fold Sensitivity Increase in Capillary Electrophoresis: Electrokinetic Supercharging with Optimized Sample Injection

Abstract

Electrokinetic supercharging (EKS) is a powerful and practical method for multifold in-line concentration of various analytes prior to capillary electrophoresis (CE) analysis. However, a problem of insufficient sensitivity has always existed when trace analyte quantification by EKS-CE is a target, especially when coupled with conventional detectors. Normally this requires a greatly increased amount of analyte injected without separation degradation. In this contribution, we have shown that it is possible to substantially improve analyte loading and hence CE method detectability by modifying sample introduction configuration. The volume of sample vial was increased (from typical 500 μL to 17 mL), the common wire electrode was replaced by a ring electrode, and the sample solution was stirred. With these alterations, more analyte ions are accumulated within the effective electric field during electrokinetic injection and then maintained as focused zones due to transient isotachophoresis. The versatility of the customized EKS-CE approach for sample concentration was demonstrated for a mixture of seven rare-earth metal ions with an enrichment factor of 500 000 giving detection limits at or below 1 ng/L. These detection limits are over 100 000 times better than can be achieved by normal hydrodynamic injection, 1000 times better than the sensitivity thresholds of inductively coupled plasma atomic emission spectrometry (ICP-AES), and even close to those of inductively coupled plasma mass spectrometry (ICPMS).

Stacking in a continuous sample flow interface in capillary electrophoresis

“…To examine this idea, a continuous sample flow interface was developed using a commercially available Tee connector with a cylindrical electrode at the sample outlet. This electrode design was inspired by the cylindrical electrode that allowed near quantitative injection of all sample ions in work published by Hirokawa et al [37].…”

“…Therefore long injection times may be required to inject a large fraction of ions from the total sample volume. This led them to develop circular electrodes placed at the very top of the sample such that a 3D electric field is created that covers almost the entire sample volume [37]. This greatly improved the transport of ions to the capillary tip, providing near quantitative injection of all the sample ions into the capillary.…”

Two‐step stacking by sweeping and micelle to solvent stacking using a long‐chain cationic ionic liquid surfactant

“…With insight into the CE theory, special concentration procedure could be designed for target analytes. By optimizing sample injection, electrokinetic supercharging attained EFs of 500 000 for seven rare-earth metal ions [18]. Using reversed electrode polarity stacking mode, sensitivity enhancement factor of gold nanoparticle was improved 50-500-fold [19].…”

Simultaneous Determination of Pyridine-Triphenylborane Anti-Fouling Agent and Its Degradation Products in Paint-Waste Samples Using Capillary Zone Electrophoresis with Field-Amplified Sample Injection

“…However, CE has a low concentration sensitivity from minute sample volumes (at the nL level), and a short optical path length (typically 25 -100 μm) available with UV detection. 1 Different types of in-line sample concentration techniques have been proposed to enhance the CE sensitivity: large volume sample stacking, 2 large-volume sample stacking with an electroosmotic flow (EOF) pump (LVSEP), 3,4 sweeping, [5][6][7] dynamic pH junction, 8,9 field-amplified sample injection (FASI), [10][11][12][13][14][15] two-end field amplified sample injection (TE-FASI), 16 acetonitrile (ACN)-mediated stacking, 17 isotachophoresis (ITP) and transient ITP, [18][19][20] electrokinetic supercharging (EKS), [21][22][23] counter-flow electrokinetic supercharging (CF-EKS), 24 pressure-assisted injection techniques, 25,26 and simultaneous electrokinetic and hydrodynamic injection (SEHI). 27,28 These procedures are useful, but some were somewhat complicated in their application to real samples.…”