ABSTRACT
Besides fixed-potential anodic measurements, other electrochemical methods have also been coupled to CLC. These modes typically offer detection of other analytes or improved selectivity over the anodic amperometric method. Reductive mode detection for OTLC using an on-column, carbon fiber microelectrode has been described; however, detection limits are approximately 100-fold worse than anodic detection because of the high background currents [18]. Differential pulse detection has been applied for improved selectivity [18]. Integrated, pulsed amperometric detection (IPAD) at Au microwires has also been applied to capillary columns for determination of thiols [19]. Scanning voltammetric detection using carbon fiber microelectrodes has been used to increase the amount of electrochemical information garnered by the on-column detector and to improve resolution by allowing simultaneous chromatographic and voltammetric analysis [20, 21]. A recent innovation, sinusoidal voltammetry, has yet to be used with CLC but has been used with CE and flow injection analysis (FIA) [22, 23]. In this method, the electrode potential is varied in a sinusoidal wave with the resulting current traces analyzed by Fourier transform. (In this method the potential waveform is a sinusoid, with no other DC potential or linear ramps applied.) The background and signal can have different frequency spectra, so analysis in the frequency domains allows significant discrimination against the background, resulting in improved S/N. This method is best used with electrodes that exhibit mainly capacitive background currents. This is because electrodes that generate faradaic currents in their background signal (i.e., surface waves) will have frequency spectra similar to faradaic current from analytes, thus preventing the discrimination offered by frequency analysis. Using this approach with Cu electrodes allowed concentration limits of detection as low as 70 nM for ATP [22]. Because the detector response increases with the number of bases in a nucleotide chain, this technique permits high sensitivity detection for oligonucleotides. For a 9.5 kilobase chain of double-stranded DNA, the concentration limit of detection was 3.2 pM, corresponding to 14 zmol or 8400 molecules (4 nL injection volume) using the sixth harmonic where the signal-to-noise ratio is maximal (Figure 3) [23].
