Electrokinetic signal

Matson, M. T., Ramstad, T., and Dunn, M. J. (2005). Purity determination of alprostadil by micellar electrokinetic chromatography with signal enhancement involving field-amplified sample stacking and extended path length detection. /. Liq. Chromatogr. Relat. Technol. 28, 3181—3203. 

Figure 26-26 Lower trace Sample injected electrokinetically for 2 s without stacking is limited in volume to prevent band broadening. Upper trace With stacking. 15 times more sample could be injected (for 30 s). so the signal is 15 times stronger with no increase in bandwidth. [From V Zhao and C. f. Lurie, pH-Mediated Field Amplification OmCotumn Preconcentration of Anions in Physiological Samples for Capillary Electrophoresis, Anal. Chem. 1999, 71,3985.]...

The characteristics of electrokinetically controlled fluid flow in microchannel manifolds has been studied in a systematic way by Harrison and coworkers [28, 30]. An illustrative demonstration of the potential of this approach is shown in Fig. 2 for the controlled dilution of a fluorescein solution under voltage control. In parallel with a stepwise decrease of the potential applied to the fluorescein reservoir, a decrease of fluorescence signal downstream after the junction is visible in Fig. 2. As long as the ionic strength and pH in each supply channel is the same (same jieo), mass balance is automatically fulfilled, and the incoming flows at the intersection will be exactly balanced by the outgoing flow of the mixed components (otherwise, mass balance would be enforced by additional hydrodynamic or secondary internal flows). This way of mixing fluids was also... 

Fig. 2.21. Illustration of signal improvement by the usage of a high sensitivity cell in CEC (reproduced from Ref. [55] with permission of the publisher). Column, CEC Hypersil Cl 8, 3 pm, 250 (335) x 0.1 mm mobile phase acetonitrile-50 mM Tris-HCl, pH 8 (90 10 v/v) voltage, 25 kV, injection electrokinetic, pressure, 10 bar both sides, temperature 20°C.

In electrokinetic phenomena such as electroacoustics, theoretical models need to consider the induced movement of charge within the electrical double layer (EDL), the surface current , Is, as well as the interaction of the outer portion of the double layer with the applied signal (acoustic or electric field) and with the liquid medium. Hydrodynamic flows generate surface current as liquid moving relative to the particle... 

Figure 14. Microfluidic realization of capillary electrophoresis analysis on the electrokinetic platform. (Adapted from [123], ( Agilent Technologies, Inc. 2007. Reproduced with permission, courtesy of Agilent Technologies, Inc.) After the sample has been transported to the junction area (a) it is metered by the activated horizontal flow and injected into the separation channel (b). Therein, the sample components are electrophoreticaUy separated (c) and readout by their fluorescence signal (d). The complete microfluidic CE-chip is depicted in the center.

MEKC instrumentation is not different from the apparatus used for capillary zone electrophoresis (chapter 3.3.2). The only deviation is that the run buffer contains micelles. MEKC is sometimes also referred to as micellar electrokinetic capillary chromatography (MECC). The signals are recorded as an electrokinetic chromatogram with signal intensity versus time. 

E.A. Pereira, A.A. Cardoso and M.F.M. Tavares, Determination of low-aliphatic aldehydes indoors by micellar electrokinetic chromatography using sample dissolution manipulation for signal enhancement, Electrophoresis, 24,700-706, 2003. 

Traditionally, electrokinetic injection is the most common method for sample injection and separation into the microchannel utilizing some form of electroosmotic pumping. However, depending on the sample, due to the different mobility in an electric field, a bias to anionic, neutral, or cationic analytes is possible. Careful adjustment of pH and ionic strength can reduce this effect [2]. A broad injection band and sample leakage phenomena are also important defects of the electrokinetic injection method. Both are known to reduce the separation efficiency of a device since they result in a wide sample distribution within the microchannel and an increasing signal baseline as the number of injection runs increases [3-6], respectively. Other injection methods, like pressure injection, do not show this method-dependent effect. 

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