Steady-state current and absorbance are characterized for a thin-layer spectroelectrochemlcal flow detector with the working electrode on one wall, using complementary Nernst diffuslon-layer-approxlmatlon and finite dlfference shnulatlon models. Both models rigorously treat the Important effects on the measured absorbance of the electrochemically Induced sample Inhomogeneity across the optical beam, requlrlng calculation of absorbance by krtegratlng local transmittances rather than absorbances over the appropriate distance coordinates, except for weakly absorbtng samples. Predicted flow rate exponents of current and absorbance are approxlmately equal and of opposite stgn: +'I3 for current and -'I3 for absorbance under fully developed laminar flow or +'I2 for current and .-'I2 for absorbance under developing lamlnar flow. Optimum optical path geometries are predlcted, havlng long optical paths parallel to the electrode surface and eRher perpendlcular or parallel to flow and lllumlnatlng the entire electrode length If shot noise Is domlnant.Detector selectivity is an important problem for determination of trace species in complex mixtures using flow techniques such as high-performance liquid chromatography (HPLC) or flow injection analysis (FIA). Liquid chromatographic identification and quantitation using a single detector of specific trace species in complex matrices may be subject to interferences by other species which have the same chromatographic retention characteristics and similar detector response. Correlation of two independent detector signals, such as electrochemical current and optical absorbance (spectroelectrochemistry) can be used to improve selectivity by ensuring that a sample component is recognized only when both detection signals yield appropriate responses simultaneously ( I ) .Previous spectroelectrochemical cell (SEC) designs include thin-layer cells with illumination perpendicular to optically transparent electrodes (2-4), planar thin-layer cells with illumination parallel to the electrode and coupled to a spectrometer beam either indirectly by fiber optics (5,6) or directly (7), a cell with the optical path inside a narrow hole drilled through a carbon electrode (8), and planar bulk cells, with illumination from a continuum source (9, 10) or a laser (11,22) passed through a thin layer parallel to the electrode surface. These designs are for static solutions, whose spectroelectrochemical responses are described by semiinfinite (9-22), thin-layer planar (2-7), or concentric cylindrical (8) diffusion. A thin-layer spectroelectrochemical flow detector reported by Heineman and co-workers used a minigrid opt i d y transparent electrode (13). Recently, an electrochemical detector for HPLC and FIA was reported that used photoexcitation to enhance sensitivity (14).This paper characterizes the steady-state flow-rate dependence of absorbance of a species either generated or consumed
Spectroelectrochemical experiments were performed In rectangular thln-layer flow cells wlth long optical paths, for the oxldatlon of ferrocyanlde to ferrlcyanlde In phosphate buffer. The optlcal beam was coupled to the cell wlth fiber optlcs or thln-slab waveguldes as the cell sldewalls. Steady-state amperometrlc and absorptometrlc responses both depended critically on cell deslgn, agreeing well wlth theory for fully developed lamlnar flow when the cell Inlet and outlet were several mlllhneters upstream and downstream of the working electrode, but agreed better wlth theory for developlng laminar flow when the electrode completely covered one channel wall. The slgnal to noise ratio was considerably poorer for absorbance measurements than for current but considerably better than for more conventlonal thln-layer spectroelectrochemlcal cell deslgns wlth short optical paths. Flow InJectlon callbratlon curves demonstrated much greater absorbance sensttlvlty when the reactant rather than the product was the prlmary absorber. Experimental results support the potentlal use of the cells as correlatlon detectors.Thin-layer (TL) spectroelectrochemical flow cell (SEFC) detectors offer intriguing possibilities for enhanced analytical selectivity as well as assessment of flow hydrodynamics and reaction mechanisms in applications such as high-performance liquid chromatography (HPLC) or flow injection analysis (FIA). Although numerous stationary spectroelectrochemical cells (SEC) have been reported (I-@, with both short ( I , 2, 6) and long (3-5) optical paths, we are aware of only one flow application of a TL-SEFC having a short optical path length of low sensitivity in an HPLC stopped-flow coulometry configuration (6).Theory describing the current and absorbance response and optimum optical geometries for a rectangular TL-SEFC is reported elsewhere (7), demonstrating the utility of configurations with long optical paths. The present work experimentally evaluates the steady-state flow response of TL-SEFC detectors with long optical paths parallel to the electrode surface and coupled to the light beam by fiber optics or slab waveguides serving as cell side windows. Experimental data agree well with theoretical predictions. The absorptometric measurements provide a powerful means of confirming observations on the nature of channel flow hydrodynamics derived from amperometric measurements. The results show that a slight change in geometry can shift response and flow rate exponents (8) between fully developed laminar flow behavior and a behavior consistent with developing or stagnating laminar flow. The feasibility of correlation detection based on coupled amperometric and absorptometric measurements is also evaluated. EXPERIMENTAL SECTIONThe thin-layer, long optical path SEFC cell designed for this study is shown in Figure 1. The working electrode substrate consisted of either glass or a silicon wafer with a thin, thermally generated oxide insulating layer, coated with Liquid Bright Gold (Engelhard Industries), and fired in a muffl...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.