Electrochemical cells pulsed detection

Time-resolved microwave conductivity measurements with electrodes in electrochemical cells can conveniently be made with pulsed lasers (e.g., an Nd-YAG laser) using either normal or frequency-doubled radiation. Instead of a lock-in amplifier, a transient recorder is used to detect the pulse-induced microwave reflection. While transient microwave experiments with semiconducting crystals or powders have been performed... 

Pulse voltammetry techniques are characterized by a succession of potential steps. During the sequential potential steps, the rates of current decay of the capacitive (If.) and the faradaic currents (7p) are essentially different (specifically, while 4 in Equation 2.1 decays exponentially with time. Ip decreases as a function of t 2, characteristic of a diffusion-controlled electrochemical reaction). In this way, the rate of decay of If. is significantly faster than that of Ip, and thus I. is negligible at a time of 57 uQ after the potential step is imposed (where 7 uQ is the time constant, Tj-gy, for the electrochemical cell having values from microseconds to milliseconds, and R is the uncompensated resistance between reference and working electrodes). Consequently, Ip is the main contribution to the measured current I when its value is measured at the end of a potential step. The detection limits of these techniques therefore fall around 10 M making them suitable for quantitative analysis. 

Pulse polarographic studies have been described using a microcell of 0.5 mL capacity, which analyzed two 1,4-benzodiazepines, with the lowest detection limit reported to date being 10-20 ng/mL of blood [199]. Detailed construction of the cell and electrode assembly was also described (shown in Fig. 26.16). Further miniaturization of this type of three-electrode cell is not practical hence further increases in sensitivity will have to rely on electrochemical detector flow cells of microliter capacity such as those used in conjunction with liquid chromatography (see Chap. 27). 

Commercial electrochemical detectors whose cells are directly connectable to the postcolumn efflux of the HPLC apparatus are available from a number of manufacturers. This method of detection is comparable in sensitivity to detection by absorption spectroscopy when the amperometric circuit is operated as a DC pulse polarograph. 

The most common electrode material used in LC-EC is carbon, either as solid glassy carbon disks in thin-layer cells, or as a high-surface-area porous matrix through which the mobile phase can flow. Gold electrodes are useful to support a mercury film and these are primarily used to determine thiols and disulfides, and also for carbohydrates using pulsed electrochemical detection... 

To overcome the problem of detection in CE, many workers have used inductively coupled plasma-mass spectrometry (ICP-MS) as the method of detection. " Electrochemical detection in CE includes conductivity, amperometry, and potentiometry detection. The detection limit of amperometric detectors has been reported to be up to 10 M. A special design of the conductivity cell has been described by many workers. The pulsed-amperometric and cyclic voltametry waveforms, as well as multi step waveforms, have been used as detection systems for various pollutants. Potentiometric detection in CE was first introduced in 1991 and was further developed by various workers.8-Hydroxyquino-line-5-sulfonic acid and lumogallion exhibit fluorescent properties and, hence, have been used for metal ion detection in CE by fluorescence detectors.Over-... 

M. E. Herrera, Analytical and Neurochemical Studies with Flow Injection Analysis and Electrochemical Detection. Part I. Pulsed Flow Electrolysis with the Reticulated Vitreous Carbon Electrode. Part II. On-Line Determination of Secreted Catecholamines from Cultured Adrenal Cells. Diss. Abstr. Int. B, 46 (1986) 1894. 

A mixed self-assembled monolayer was used for the aptamer immobilization on the gold electrode. The aptamer-modified electrodes were then incubated for 1 h at 37°C with thrombin (18 ag/mL). Electrochemical measurements were recorded in the thin-layer cell configured to contain a total volume of 20 [xL. Thrombin chromogenic substrate (/3-Ala-Gly-Arg-p-nitroaniline) was injected into the cell and differential pulse voltammetry (DPV) measurements between -0.2 and -1V with a pulse height of—0.05 V and pulse duration of 70 ms were carried out. The DPV measurements showed that /3-Ala-Gly-Arg-p-nitroaniline substrate and the p-nitroaniline product have different redox potentials. Moreover, the DPV experiments showed a current peak at -0.45 V in the presence of the thrombin substrate. After 5 min, the peak at —0.45 V decreased and a new peak was detected at -0.70 V, indicating the formation of p-nitroaniline. The same measurements carried out on a control electrode in order to test the specificity of the assay in this experiment bovine serum albumin (BSA) substituted thrombin and in this case only the peak at 0.45 V was measured. 

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