Electrochemical cells array detectors

Voltammetric and amperometric measurements in BIA can be performed with the same electrochemical cells and detectors utilized in FIA, including wall-jet arrangement with injection of sample volume of a minimum 10 pi and the use of microelectrode arrays [104, 105]. Special attention has been attributed to the monitoring in agriculture due to the possibility of exploring portable instruments for in situ analysis. It occurs mainly through association between BIA and stripping voltammetric techniques. 

Electrochemical detection is sensitive and selective, and it gives useful information about polyphenolic compounds in addition to spectra obtained by photodiode array detectors. Differences in electrochemically active substituents on analogous structures can lead to characteristic differences in their voltammetric behavior. Because the response profile across several cell potentials is representative of the voltammetric properties of a compound, useful qualitative information can be obtained using electrochemical detection (Aaby and others 2004). 

A schematic diagram of a working FIA system is shown in Fignre 9.9. FIA can be nsed with many types of detector incinding electrochemical, e.g. pH probes, ISEs and condnc-tivity, and spectroscopic, e.g. UV-Vis, infrared and flnorescence. Diode array detectors allow the monitoring of many components simnltaneonsly, and conpled with chemometrics can be a very fast and information-rich technique. The detector itself should have a small cell volume to avoid undue dispersion. 

The most widely used detection methods in HPLC analysis of anilines and phenols are UV (especially diode array) and electrochemical detection (ED). UV detectors provide very good signal stability and in the case of diode-array detectors they can be used for analyte tentative confirmation purposes using UV spectra libraries. Electrochemical detectors are more sensitive than the UV detectors however, their performance is highly dependent on the type of samples analyzed. Components from dirty samples are deposited on the electrochemical cell and the detector sensitivity is rapidly decreased. ° ° The heterocyclic amines can be measured with UV, electrochemical, and fluorescence detectors. 

Often, low levels of carotenoids in biological samples provide significant challenges in quantification by HPLC-PDA alone. Electrochemical detection (ECD) has been successful in quantifying low concentrations of carotenoids (MacCrehan and Schonberger, 1987 Finckh et ah, 1995 Yamashita and Yamamoto, 1997). More information about ECD can be found in Chapter 2. ECD has also been successful in quantifying carotenoid isomers in foods, plasma, prostate tissue, cervical tissue, and buccal mucosal cells (Ferruzzi et ah, 1998,2001 Allen et ah, 2003 Unlu et ah, 2007). Electrochemical array detection for all-irans - 3-carotene has been reported to be 10 fmol on column, which is approximately 100-1000 times more sensitive than UVA is detectors (Ferruzzi et ah, 1998). 

The use of multielectrode BCD in combination with SPE using Ci8 material and HPLC separation was described for the identification of 27 phenolic compoimds in water samples [64]. The multielectrode consisted of four coulometric array cells, each containing four electrochemical detector cells. These employed porous graphite-working sensors with palladium as reference and coimter electrodes were arranged in series after the analytical column. Tap water and mineral water were analyzed the authors reported very low detection limits for the phenols. 

Commercially available cells with rate constant of 500 s and a cell volume of about 5 pi assure coulometric efficiency for typical HPLC flow rates with minimal extra-column band broadening. Each electrochemical unit has a central porous carbon electrode, on either side of which is situated a reference electrode and an auxiliary electrode. The characteristics of porous graphitic carbon electrode facilitate the construction of electrode arrays, lypical commercial systems include two units placed in series but arrays of up to 16 units are commercially available (Thermo Scientific, formerly ESA/Dionex). These cells have some degree of resistance to flow and with use can develop a significant back pressure. To minimize such back pressure changes, they need to be protected from particulate materials. Their intrinsic back pressure should also be borne in mind when connecting other types of HPLC detector cell in series. 

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