Electrochemical Detector ECD

A conductivity detector measures the electrical conductivity of the HPLC eluent stream and is amenable to ppm-ppb levels analysis of ions, organic acids, and surfactants. It is the primary detection mode for ion chromatography.30 

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Electrochemical detectors (ECD) gas chromatography, 4 615 liquid chromatography, 4 622 6 387, 449 supercritical fluid chromatography, 4 631... 

GC = gas chromatography EC = electrochemical (detector ECD = electron capture (detector HCD = Hall conductivity detector HFBA = heptafluorobutyric anhydride HPLC = high performance liquid chromatography NCI-MS = mass spectrometry in the negative chemical ionization mode NPD = nitrogen-phosphorus detector ppb = parts per billion UV = ultraviolet absorption SPE = solid phase extraction wt wt = weight weight... 

HPLC-based electrochemical detection (HPLC-ECD) is very sensitive for those compounds that can be oxidized or reduced at low voltage potentials. Spectrophotometric-based HPLC techniques (UV absorption, fluorescence) measure a physical property of the molecule. Electrochemical detection, however, measures a compound by actually changing it chemically. The electrochemical detector (ECD) is becoming increasingly important for the determination of very small amounts of phenolics, for it provides enhanced sensitivity and selectivity. It has been applied in the detection of phenolic compounds in beer (28-30), wine (31), beverages (32), and olive oils (33). This procedure involves the separation of sample constituents by liquid chromatography prior to their oxidation at a glassy carbon electrode in a thin-layer electrochemical cell. 

Some phenols are readily oxidized or reduced, which allows detection by electrochemical detectors (ECD). However, when using single-electrode ECD, the peak identification is only based on retention times (RT). To enhance the selectivity of this technique, it is essential to use dual electrode or electrochemical array detectors, both of which would give more information based on the shape of the voltammograms of the eluting compounds. 

This method of analysis relies on the same chemical principles as the determination by TLC, except that the efficiency (and the cost) of the technique has increased greatly. Instead of the R value, the retention time of the drug is measured and related to P by equations similar to equation (2.5) for TLC. The retention time, as its name suggests, is the time taken for the sample to elute from the HPLC column. The major drawback with using this technique to determine P is detecting the drug if it does not possess a chromophore, when a UV detector cannot be used. In cases like this, use must be made of an HPLC system connected to a refractive index (RI) detector or an electrochemical detector (ECD). 

Another important detector for ion chromatography is the electrochemical detector (ECD). This makes it possible to detect anions such as cyanide, nitrite, sulphide, bromide, iodide and sulphite with maximum sensitivity. Thanks to its specific detection capabilities it is also possible to identify these ions without interference alongside high concentrations of other ions which may also display similar retention values (Fig. 75). 

In the introductory Section 2.1.3, it was discussed that an important aspect of optimization can be to improve a method for its applicability in trace analysis. The nature of the mode of detection is very relevant in this case whether the applied detector is concentration proportional like the very common UV detector or mass proportional hke nebulizer-based detectors, for example, evaporative light scattering detector (ELSD) or charged aerosol detector (CAD). This textbook contains dedicated chapters on nebulizer-based or aerosol detectors (Chapter 10 on trends in detection), as well as for the coupling of LC with mass spectrometry (Chapter 1). Here, the focus is on concentration proportional detectors UV detectors (VWD, DAD), fluorescence detectors (FLD), electrochemical detectors (ECD), and refractive index (RI) detectors. 

The electrochemical detector (ECD), which is still used for anodic oxidation of phenols and amines offers interesting perspectives. With the aid of a commercial detector we were recently able to detect diethylstil-bestrol in plasma in the low pg-region by an oxidation reaction (see Figure 13). Diethylstilbestrol represents the main metabolite and one of the active principle of fosfestrol, which has been used for treating metastasizing prostate carcinomas for about 30 years. 

FIGURE 2.10 Schematic diagram of the SIA system for the evaluation of total antioxidant capacity with an in-house flow-through electrochemical detector (ECD) (1.9 cm width X 5.1 cm length x 2.3 cm height). CE, counter electrode WE, working electrode RE, reference electrode. (Reprinted with permission from Chan-Eam, S. et al. 2011. Talanta 84 1350-1354.)... 

Detection of metabolites derived from Trp-P-1 during primary culture was performed by the method described by Minamoto and Kanazawa (i 7) with some modifications. Briefly, intracellular metabolites and Trp-P-1 were extracted with ethyl acetate firom these cells. The metabolites were detected using a HPLC (Hitachi L-7100) equipped with a UV detector (Hitachi L-7420) and electrochemical detector (ECD IRJCA E 875) connected in series. An Inertsil column, ODS (i.d. 4.6 x 150 mm), was maintained at 35 C. The mobile phase... 

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