Voltammetric detectors selectivity

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

The use of FIA is not limited to optical sensors. It has been previously used to test ion selective electrodes [12, 68], and most recently to testing in the development of pneumatic gas voltammetric detectors [1050, 1051]. Also the surface treatment of glassy carbon electrodes has been evaluated by means of their voltammetric response observed in the FIA mode [853]. 

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

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). 

Potentiometric, voltammetric, or polarographic electrochemical detectors are useful for the quantitation of species that can be oxidized or reduced at a working electrode. These detectors are selective, sensitive, and reliable, but require conducting mobile phases free of dissolved oxygen and reducible metal ions. A pulseless pump must be used, and care must be taken to ensure that the pH, ionic strength, and temperature of the mobile phase remain constant. Working electrodes are prone to contamination by reaction products with consequent variable responses. 

Another detector, which has found considerable application, is based on the changes in the refractive index of the solvent that is caused by analyte molecules. In contrast to most of the other detectors listed in Table 32-1, the refractive index detector is general rather than selective and responds to the presence of all solutes. The disadvantage of this detector is its somewhat limited sensitivity. Several electrochemical detectors that are based on potentiometric, conductometric, and voltammetric measurements have also been introduced. An example of an amperometric detector is shown in Figure 32-9. 

Furthermore, the voltammetric data established a background for the quantitative determination of catecholamines using a selective am-perometric detector in combination with liquid chromatography (LCEC technique) 254). The tendency of this class of compounds to absorb at electrode surfaces (such as platinum) allowed the determination of... 

We also developed an electrochemical flow system for on-line monitoring of trace TNT in marine environments, based on a square-wave voltammetric operation at a carbon-fibre based detector [9]. The flow system offers selective measurements of sub-ppm concentrations of TNT in untreated natural water matrices (with a detection limit of 25 ppb). It responds rapidly to sudden changes in the TNT concentration with no observable carry-over. About 600 runs can be made every hour with high reproducibility (e.g., RSD=2.3%, n=40) and stability. The system lends itself... 

Greater selectivity and better information for qualitative identification can be obtained with the detector electrode operated in the voltammetric mode, where its potential is scanned over a given potential window during elution. However, the detection limits... 

When analyzing complex samples by HPLC, the selection of detection system is important. If chromatographic separation is incomplete or analytic concentration very low, then the more universal and insensitive ultraviolet absorbance (UV) detectors are not satisfactory, and other detection systems must be used. Fluorescense (FL) detection is a sensitive and selective alternative for those compounds that fluoresce. Electrochemical (EC) detection differs from UV and FL in that it is based on a chemical reaction rather than a physical phenomenon, and is the best choice for the many electroactive compounds. In this system analytes are either oxidized or reduced on the electrode surface. More precisely, this technique may be called ampero-metric or voltammetric detection, though in practice it is commonly referred to as EC. 

Pervaporators are amenable to coupling to any type of detector via an appropriate interface such as a transport tube, a microcolumn packed with adsorptive or ion-exchange material, or a gas liquid separator. The acceptor stream can be either liquid or gaseous depending on the characteristics of the detector. The detectors most frequently used are the spectroscopic - atomic or molecular, electroanalyti-cal (potentiometric, voltammetric), electron capture, and flame ionization types. The low selectivity of some of these detection techniques is overcome by that of the pervaporation step, endowing the overall analytical process with the selectivity required for the analysis of complex matrices. The potential use of the pervaporation technique for sample insertion into water-unfriendly detectors such as mass spectrometers or devices such as those based on microwave-induced plasma remains unexplored. 

Unnikrishnan et al. [100] reported the fabrication of a selective voltammetric sensor for the determination of chlorpromazine (an antipsychotic drug) using GCE modified with MWCNT-PEI, in the presence of uric acid (UA), Do, and acetaminophen. The GCE/MWCNT-PEI platform was also employed for the quantification in batch of flavonoid compounds in onion samples [135] and peanut hull samples [136], and as a detector in capillary electrophoresis, for the simultaneous detection of eight polyphenols (t-resveratrol, (-i-)-catechin, quercetin and /)-coumaric, caffeic, sinapic, ferulic, and gallic acids) in Spanish white wines [137]. 

The versatility of the flow system enables the analyst to perform voltammetric analysis with the same detector or even using similar apparatus. Suteerapataranon etal. [116] have proposed the use of components of the SIA system similar to the FIA system with exception of the syringe pump and the 10-port selection valve for the simultaneous quantification of cadmium, copper, lead, and zinc. The proposed FIA and SIA systems associated with stripping voltammetry were easily operated and required little amount of sample (30 pi provided reproducible current signals) with high degree of automation. 

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