A coulostatic electrochemical detector was developed for use In flowing streams. The detector Is capable of scanning the applied potential at rates of up to 3 V/s and recording multiple channel chromatograms, where each channel corresponds to a separate electrode potential. The faradaic current Is measured between coulostatic pulse applications, at which time no current flows In the bulk solution passing through the detector. The detector was able to resolve and quantitate liquid chromatographic peaks which were completely overlapped In the time domain, provided that the half-wave potentials of the components differed by more than 0.25 V. The Instrument exhibited good precision and linearity for acetaminophen samples ranging from 6.85 ng to 1 pg In mass. The relative standard deviation for three determinations at the 6.85 ng level was 3.8%.Electrochemical liquid chromatographic detectors have become very popular (1) due to their high sensitivity and the increased selectivity afforded by a judicious choice of the applied potential. However, for an unknown sample, one must select the proper applied potential through a trial-and-error approach, or use another technique, (e.g., cyclic voltammetry) to determine the optimum value. Further, while it is often possible to select a potential at which an unwanted peak is eliminated, it is impossible to resolve overlapping peaks and simultaneously quantitate both components. This problem can be alleviated to some degree through the use of multiple electrode cells (1); however, in general, each set of overlapping peaks would require two separate working electrodes with a different control potential at each. A more suitable solution to this problem is to rapidly scan the potential repeatedly during the elution and concurrently record chromatograms at several different potentials. In this way, the voltage axis can be used to resolve peaks which are overlapped in the time domain, and all resolved peaks can be accurately quantitated.
A coulometer which is based on the coulostatic prlnclple has been developed and shown to be useful For analyses in low electrolyte (0.1 PA KNOB) solution. The instrument injects dlscrete charge pulses rind measures the cell potentlal between pulses when the current flowing through the bulk solution is negllglble. Thus, the cell potentlal can be measured precisely, free from any effects of the ohmic potential drop. As a result, the caulomettric performance of the Instrument is unaffected by the relatlve positioning of the reference and worklng electrodes. Lead determlnations were performed in 0.5 M or 0.1 M KNO, scalutlon, and the reference electrode was positioned off the axis of minimum separatlon between the working and auxlllary electrodes, in order to test the instrument's performance under adverse conditions. The preclslon and accuracy ranged from 0.09 % to 0.05 % (relative error) for lead samples Iin the range 1-8.5 mg, respecllvely (3.5 mL cell volume).Coulometry is a technique characterized by high current, and therefore a large ohmic potential drop exists in the solution during the analysis. The ohmic potential drop can, and generally does, lead to a nonuniform potential distribution across the surface of the working electrode. In unfavorable cases, the potentials measured at various locations on the surface of the working electrode may differ by more than 100 mV (1). A potential variation of this magnitude could easily lead to an erroneous coulometric result due to competing side reactions. One consequence of this problem is that cell ge. ometry (i.e., the relative orientation of the three electrodes) can critically affect the performance of the coulometer (2). The best behavior is usually obtained for cells whose working and auxiliary electrodes are axially symmetric and separated by a distance roughly equal to the radius of the working electrode. The reference electrode should be placed on the line of minimum separation between the other electrodes. situated as close as possible to the working electrode.The coulostatic electrochemical technique is essentially immune (3-6) from the effects of the ohmic drop which exists in the solution. The immunity derives from the fact that the potential is measured at times when no current flows in the bulk solution. This is accomplished by applying the current in the form of discrete charge pulses which are applied intermittently to the cell. Within the time period between pulse applications, it is possible to measure the potential across the electrical double layer without any error due to ohmic contributions. A coulometer, which is based on the coulostatic principle, has been developed and shown to be useful in eliminating many of the problems associated with the ohmic potential drop. DESCRIPTION OF T H E COULOMETERA block diagram of the instrument is shown in Figure 1. A precision pulse generator can apply a precise increment of charge (via the auxiliary electrode (A)) to the electrical double layer which exists a t the interface between the working electrode (W...
Voltammetric electrochemical detection has been employed by various workers (1-5) because of the increased selectivity and additional qualitative information provided by the technique. Most electrochemical detection (including voltammetric) has been performed in aqueous media because of the need for supporting electrolyte and the widespread interest
Two high-current analog switches are described. One is based on a JFET (junction field effect transistor) and the other on a VMOS (vertical metal oxide semiconductor) field effect transistor. The JFET switch has an input voltage and current range of ±10 V and ±350 mA, respectively, with a channel resistance of 3 Ω. The switch exhibited turn-on and turn-off times of 250 and 150 ns, respectively, for a positive 9 V signal (180 mA load current). The switch off-state leakage current was less than 1 nA. The VMOS switch has an input voltage and current range of ±10 V and ±750 mA, respectively, with a channel resistance of 5.5 Ω. The turn-on and turn-off times for the VMOS switch were 600 and 200 ns, respectively, for an input voltage of 9 V (360 mA load current).
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