Detectors coulometric

The amperometric detection uses less than 10% of the analyte in the flow cell, unlike the coulometric detector, and can be operated in a pulsed mode (cydic voltammetry, with a gold working electrode) in addition to the constant potential mode. The pulsed mode helps cleaning the working electrode. 

Detection limits (depending on the electroactive properties) are in the picogram range. 

The coulometric detector measures current x time (coulomb), usually with a three-electrode system. It not only utilizes a larger surface area in the working electrode, consuming all the analyte, but also generates more noise. The detection limits are, therefore, not necessarily much better with the coulometric detector, but the stability 

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Oxygen Transport. The most widely used methods for measuring oxygen transport are based upon the Ox-Tran instmment (Modem Controls, Inc.). Several models exist, but they all work on the same principle. The most common apphcation is to measure the permeabihty of a film sample. Typically, oxygen is introduced on one side of the film, and nitrogen gas sweeps the other side of the film to a coulometric detector. The detector measures the rate that oxygen comes through the film. The detector response at steady state can easily be converted to At (eq. 1). Simple... 

Johnson, K. M., Wills, K. D., Butler, D. B. el al. (1993). Coulometric total carbon dioxide analysis for marine studies maximizing the performance of an automated gas extraction system and coulometric detector. Mar. Chem. 44,167-187. 

While the terms amperometric detection and coulometric detection have come into use to describe detectors of less than 100% efficiency and 100% efficiency respectively, these terms are actually misnomers. An amperometric detector is any electrochemical detector where current is plotted as a function of time, regardless of the conversion efficiency. A coulometric detector is any electrochemical detector where charge is plotted as a function of time, again regardless of the conversion efficiency. Preferred terminology should be high efficiency and low efficiency detectors to describe the two situations. 

Further, mention should be made of solid-cell coulometric detectors for halogens and halogen compounds121 and for sulphur compounds122, where for example in the latter instance a solid cell of Pt Ag Agl Ag2S Pt interacts with passing gaseous sulphur compounds. 

One type of ec detector (the coulometric detector) reacts all of the electroactive solute passing through it. This type has never become very popular (there is only one on the market at the moment). Another type (the amperometric detector) reacts a much smaller quantity of the solute, less than 1%. The currents observed with these detectors are very small (nanoamps), but such currents are not too difficult to measure and the detector has a high sensitivity, considerably higher than that of uv/visible absorbance detectors although not as good as fluorescence detectors. Noise equivalent concentrations of about 10, 0g cm-3 have been obtained in favourable cases. Another advantage of these detectors is that they can be made with a very small internal volume. 

Figure 3.8 Amperometric detectors (a) measure the current that flows between the working electrode, usually a glassy carbon electrode, and a reference electrode, at a fixed voltage, usually close to the discharge potential for the compound. Coulometric detectors (b) are less common and are designed with a porous carbon flow cell so that all the analyte reacts in the cell, the amount of current consumed during the process being proportional to the amount of the substance.

In coulometric detectors, the eluent flows through a porous graphite electrode such that, in theory, 100% of any electroactive species will undergo electrolytic conversion. As a result, this significantly increases the detection sensitivity, as the surface area is relatively large. 

An assay for NE, E, L-DOPA, DA, 3-nitrotyrosine, m-,o-, and p-tyrosine compared an amperometric detector with a CoulArray detector. A CoulArray detector has the sensitivity of a coulometric detector applied to eight different electrodes to give an array of applied voltages. A C18 column with a mobile phase consisting of an acetate buffer (pH 4.75) and sodium citrate in methanol was used. The assay was... 

Kissinger, P. T., "Amperometrie and Coulometric Detectors for High-Performance Liquid Chromatography , Anal. Chem., 1977, 49, 447A-456A. 

When the coulometric detector was turned on, both leuco forms were completely oxidized to their nonfluorescing chromatic forms and thus vanished from the fluorescence channel. This disappearance was balanced by the arrearance of their chromatic forms in the diode array channel. The confirmation of malachite green, gentian violet, and tlieir leuco analogs in catfish and trout tissue could be based, therefore, on the correct retention times, the observation of the natural fluorescence of the leuco forms when the coulometric detector was turned off, the absence of the leuco form peaks in the 588 nm channel when the coulometric detector was off, the disappearance of the fluorescence of the leuco forms when the coulometric detector was on, the appearance of peaks of parent drugs formed by oxidation of the leuco forms in tlie. S88-nm diode array channel, and the correct ultraviolet-visible spectra maxima for all four peaks. 

The detector can be used to confirm electronegativity, but there are no good theories linking the structure of practical compounds to electronegativity. In contrast to the ECD, the microcoulomet-ric detector can be made quantitatively specific for chlorine, at the expense of sensitivity. In the food extract example, good procedure would be to rerun the extract on a coulometric detector. 

Oishi et al. (1992) compared the results from classical iodometric PV determinations of edible oils and fats to those using a coulometric detector. Results from each technique expressed as meq active oxygen/kg sample, were consistent with one another. Typical results were sesame oil (4.1), corn oil (8.7), cottonseed oil (14.5), rapeseed oil (33.2), peanut oil (30.5), olive oil (17.0), palm oil (8.9), beef tallow (2.5), and lard (35.0). 

All of the fat-soluble vitamins, including provitamin carotenoids, exhibit some form of electrochemical activity. Both amperometry and coulometry have been applied to electrochemical detection. In amperometric detectors, only a small proportion (usually <20%) of the electroactive solute is reduced or oxidized at the surface of a glassy carbon or similar nonporous electrode in coulometric detectors, the solute is completely reduced or oxidized within the pores of a graphite electrode. The operation of an electrochemical detector requires a semiaqueous or alcoholic mobile phase to support the electrolyte needed to conduct a current. This restricts its use to reverse-phase HPLC (but not NARP) unless the electrolyte is added postcolumn. Electrochemical detection is incompatible with NARP chromatography, because the mobile phase is insufficiently polar to dissolve the electrolyte. A stringent requirement for electrochemical detection is that the solvent delivery system be virtually pulse-free. 

The first combined HPLC-electrochemical measurements of vitamin K used the reductive mode, but this technique suffered from interference from the reduction of oxygen. A redox method was later developed that eliminated this interference, and provided a 10-fold increase in sensitivity over photometric detection and an improved selectivity. The coulometric detector employed in the redox mode is equipped with a dual-electrode cell in which phylloquinone is first reduced upstream at the generator electrode and the hydroquinone is reoxidized downstream at the detector electrode. 

In the case of coulometric detectors (with complete electrolysis), the limiting current is given by Faraday s law ... 

The coulometric detector is a particular type of amperometric detector in which the percentage of electrolyzed analyte is almost 100%. The area of the peak on a current versus time plot (charge flow during electrolysis) is related to the analyte concentration via Faraday s law. This technique cannot be used with very active compounds because the analyte products would pollute the electrode surface if the reaction proceeded to completion (exhausting all the reactant). 

Kissinger, P.T. 1977. Amperometric and coulometric detectors for high-performance liquid chromatography. Anal. Chem. 49 447A-456A. 

After separation with a strongly basic anion-exchange resin, dissolved NOJ and NO2 were separated within nine minutes and were detected by a Cd coulometric detector A rotating Cd disk electrode was also used. 

Taylor, L. R., Johnson, D. C. Deterhiination of antimony using forced-flow liquid-chromatography with a coulometric detector. AnaL Chem. 46, 262 (1974)... 

Coulometric detectors are also used. When placed in a series, such detectors are used to detect and quantify coeluting compounds that differ in their half-wave potentials (the potential at half-signal maximum) by at least 60 mV. These detectors are extremely selective and sensitive, with reasonably wide linear response ranges. They are used in the clinical laboratory for the analysis of metanephrines, vanil-lymandelic acid, homovaniUic acid, and 5-hydroxyindole acetic acid in human urine without extensive sample preparation. 

Y. Takata and G. Muto, Flow coulometric detector for liquid chromatography. Anal. Chem., 45. 1864,1973. 

Eluted peaks were detected by electrochemical oxidation using the ESA 5100A coulometric detector equipped with an ESA 5010 dual electrode detector cell and a guard cell (ESA, Bedford, MA). The guard cell was placed between the pump and injector (19) and set at a potential of 0.75V. The first electrode of the analytical cell was set at a potential of 0.5V and the second electrode at which OA and N-acetyl OA are oxidized was set at 0.7V. 

Through Barrier Materials Using a Coulometric Detector... 

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