Voltammetric detectors sensitivity

Amperometric detection is a very sensitive technique. In principle, voltammetric detectors can be used for all compounds which have functional groups which are easily reduced or oxidized. Apart from a few cations (Fe , Co ), it is chiefly anions such as cyanide, sulfide and nitrite which can be determined in the ion analysis sector. The most important applications lie however in the analysis of sugars by anion chromatography and in clinical analysis using a form of amperometric detection know as Pulsed Amperometric Detection (PAD). 

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. 

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... 

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

Figure 25-17c. These configurations permit rrptimiza-tion of detector sensitivity under a variety of experimental conditions. Working electrcxle blocks and electrode materials are described in Section 25B-1. This type of application of voltammetry (or amperometry) has detection limits as low as lO to 10 M. We discuss voltammetric detection for liquid chromatography in more detail in Section 28C-6.

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). 

The multichannel coulometric detection system serves as a highly sensitive tool for the characterization of antioxidant phenolic compounds because they are electroactive substances that usually oxidize at low potential. The coulometric efficiency of each element of the array allows a complete voltammetric resolution of analytes as a function of their oxidation potential. Some of the peaks may be resolved by the detector even if they coelute (Floridi and others 2003). 

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. 

Kalvoda and Benidakova and Kalvoda have recently reported detection limits of 1 ppb for prometryne and 0.18 ppb for ametryne, repectively, using the adsorptive stripping technique this approach is sensitive but must be combined with a separation step for real applications. The detection limits found by these authors for ametryne were about 10 times higher (cf. Table 7). A swept-potential electrochemical detector, operating in the square wave voltammetric mode is used to detect mixtures of simazine, atrazine, cyanazine, propazine and anilazine after separation on a reverse-phase resin column. The cell used was a jet ceU with a... 

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. 

We have developed a new voltammetric method using a quinone reagent [5]. Based on the voltammetry of quinone and acids, we assembled an electrochemical detector for measuring acid concentration at a given potential. The flow injection analysis with electrochemical detection (FIA-ECD) method we developed is preferable, as it is a simpler, more sensitive, and rapid method for acid determination. This chapter assesses this method for determining the acid content in foods and beverages. 

The main advantage of electrochemical techniques over others such as chromatography, spectrophotometry, etc. is that they require less expensive equipment, less solvent use, are quieker and show, in some cases, a greater sensitivity. Besides, voltammetric techniques also offer the possibility of developing electrochemical detectors for coupling to flow systems when becomes necessary to implement a pre-separation step in complex samples in the presenee of several analytes. 

Fig. 80. Specular reflection spectrophotometer a) Light source ib) Monochromator (c) Polarizer d) Reflecting metal (e) Voltammetric circuit with modulator (/) Photo cathode or photomultiplier (g) Phase sensitive detector.

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