Detection modes coulometric

Microcoulometric titration is used as the detection mode in some commercial sulfur-specific analysers. Sulfur in PP and waxes (range from 0.6 to 6 ppm S) were determined by means of an oxidative coulometric procedure [537]. The coulometric electrochemical array detector was used for determining a variety of synthetic phenolic antioxidants (PG, THBP, TBHQ, NDGA, BHA, OG, Ionox 100, BHT, DG) in food and oils [538],... 

Many published articles on HPLC-ECD refer to the use of one of three voltammetric detectors (amperometric, coulometric, or polarographic). More detailed information on principles and techniques of various electrochemical detection modes can be obtained from the recent book, Coulometric Electrode Array Detectors for HPLC (34). There are also two electrode array detectors, the coulometric electrode array system and the CoulArray detector, currently available. Both detectors offer the qualitative data of PDA and the extreme sensitivity of ECD (34). The... 

Among others, this approach has been used to calculate concentration profiles and liquid junction potentials at controlled electrolyte flow, current-induced concentration changes of mobile ions in liquid-membrane electrodes, as well as for the computer simulation of membranes exposed to different sample ions. It was also applied to calculate non-steady-state responses of membranes under current polarization. Also, the response behavior of ISMs operated in a thin-layer coulometric detection mode has been simulated with this model. - ... 

An ECD measures the current generated by electroactive analytes in the HPLC eluent between electrodes in the flow cell. It offers sensitive detection (pg levels) of catecholamines, neurotransmitters, sugars, glycoproteins, and compounds containing phenolic, hydroxyl, amino, diazo, or nitro functional groups. The detector can be the amperometric, pulsed-amperometric, or coulometric type, with the electrodes made from vitreous or glassy carbon, silver, gold, or platinum, operated in the oxidative or reductive mode. Manufacturers include BSA, ESA, and Shimadzu. 

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. 

Fats, oils, and dairy products digest sample with lipase, extract with hexane. Purify hexane extracts obtained from either extraction technique by silica solid-phase extraction in the sample cleanup mode. The internal standard is phylloquinone 2,3-epoxide (unlabeled for UV detection and tritium labeled for coulometric detection). 

Fig. 14 Analytical HPLC of the phylloquinone fraction from an extracted sample of brown rice isolated by semipreparative HPLC. Column, Spherisorb C8 (octyl) mobile phase, methanol/50 mM acetate buffer pH 3.0 (97 3) containing 0.1 mM EDTA, dual-electrode coulometric detection (redox mode), porous graphite electrodes, — 1.5 V (generator electrode), +0.05 V (detector electrode). The arrows signify the fraction containing tritiated phylloquinone 2,3-epoxide (internal standard) and phylloquinone (analyte) that is collected for quantitation by radioisotopic dilution. (Courtesy of M. J. Shearer.)...

MeOH/50 mM acetate buffer pH 3.0, 97 3 containing 0.1 mM EDTA Dual-electrode coulometric detection (redox mode), porous graphite electrodes,... 

Dual-electrode coulometric detection (redox mode), porous graphite electrodes ... 

The three most common modes of operation of electrochemical detection are amperometric, coulometric, and potentiometric. An amperometric detector is an electrochemical cell that produces a signal proportional to the analyte concentration usually the percentage of the analyte that undergoes the redox reaction is very low, about 5%. 

The selectivity of electrochemical detection can be improved by the use of two electrodes (dual-mode detection). Basically, two different combinations are used two amperometric cells and the combination of a coulometric cell with an amperometric cell. The difference between these two cell types is that in amperometric cells only a fraction of the eluting analytes react, whereas in coulometric cells analytes (and all other eluting compounds ) may be quantitatively converted depending on the working potential. Using these two combinations a variety of different experimental setups are possible [74,271]. 

Hordenine was detected with a dual-electrode coulometric cell. The potential of the first electrode was set to -I-0.50V [vs. solid palladium reference electrode (Pd)], which was at the base of hordenine s hydro-dynamic voltammogram. This electrode cleaned the sample by removing easily oxidizable impurities. Hordenine and its precursors were subsequently detected by a second electrode at a potential of -I-0.75V. This method of detection is called the screen mode . In addition, the mobile phase was purified by connecting a coulometric cell between the pump and injector as a guard cell. This additional cell operated at +0.80 V. The detection limit of hordenine was at 1.1 ng, which was 25 times better than detection by UV at 275 nm. 

Based on these electrochemical studies we developed a method for the quantitation of ajmalicine and catharanthine in cell cultures. These alkaloids were extracted from freeze-dried cells and purified by the solid-phase procedure described by Morris et al. (1985), except that ethanol was used as the extracting solvent instead of methanol. A dual-electrode coulometric cell was used in the screen mode. The potential of the first electode was set at +0.2 V (vs. Pd), which was at the base of catharanthine s voltammogram. The alkaloids were detected by the second electrode at +0.8 V, as this offered the best S/N ratio. Higher potentials led to lower S/N ratio, since the background current and noise started to increase exponentially above +0.85 V, due to the oxidation of water. The mobile phase was purified by a guard cell between the pump and injector. The guard cell operated at +0.8V. 

Figure 5 Cell arrangement for coulometric titration in the constant current mode. The counter-electrode is isolated from the sample solution by a liquid junction formed, for example, with a glass frit. The indicator electrodes may belong to a potentiomet-ric, amperometric, or conductometric detection system.

In a coulometric detection, all the co-eluting compounds with an oxidation potential lower than that of the compounds of interest will be oxidized at the screen electrode. This use of the screen mode is a powerful tool that enhances selectivity (Flanagan et al. 2005). The advantage of our HPLC method with coulometric detection is that is offers a possibility to quantify vitamins Be and B12 in food samples without prior transformation of cobalamins to cyanocobalamin. 

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. 

As phenols are electrochemically reactive on carbon electrodes, LC coupled with electrochemical detection (LC-ECD) can provide a more selective and sensitive analysis [50-53]. A further increase in sensitivity can be obtained by using a preconcentration technique like SPE [54-56]. Several different modes of ECD have been used, with amperometric detection [50,52,57-62] being the one most frequently employed. Coulometric detection [63,64] has also been used (for a summary of different ECD of phenols in water, see Table 16.2). 

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