HPLC instrumentation electrochemical detector

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

CE methods will develop further. By now, only UV detectors have been used after ascorbic acid separation in capillary, but there is strong evidence from the field of HPLC that electrochemical detection will be adopted also in CE. The instrumentation of CE will improve further to become more convenient for the user, and especially the capacity will increase with modern autosamplers that allow also the increased capacity of automatic washes between samples. 

HPLC, when compared to other instrumental methods, presents significant advantages for the simultaneous analysis of creatinine and purine derivatives. The variety of instrumental and experimental conditions (columns, buffers, organic modifiers, detectors, etc.) of these methods reported in the literature offers versatility and flexibility. Chromatographic conditions for these analytes are not complicated when reversed-phase columns are employed. New stationary phases with high separation power provide short analysis times. The mobile phases used are also very simple ones (organic-water mixmres with controlled pH) both isocratic elution and gradient elution are recommended. Different sensitivity detectors (UV, electrochemical, fluorescence, and combined techniques such as HPLC-MS) are very valuable for the... 

Very often baseline problems are related to detector problems. Many detectors are available for HPLC systems. The most common are fixed and variable wavelength ultraviolet spectrophotometers, refractive index, and conductivity detectors. Electrochemical and fluorescence detectors are less frequently used, as they are more selective. Detector problems fall into two categories electrical and mechanical/optical. The instrument manufacturer should correct electrical problems. Mechanical or optical problems can usually be traced to the flow cell however, improvements in detector cell technology have made them more durable and easier to use. Detector-related problems include leaks, air bubbles, and cell contamination. These usually produce spikes or baseline noise on the chromatograms or decreased sensitivity. Some cells, especially those used in refractive index detectors, are sensitive to flow and pressure variations. Flow rates or backpressures that exceed the manufacturer s recommendation will break the cell window. Old or defective source lamps, as well as incorrect detector rise time, gain, or attenuation settings will reduce sensitivity and peak height. Faulty or reversed cable connections can also be the source of problems. 

Photo-acoustic spectroscopy has been used for ultratrace levels of Hg in air and snow (de Mora etal. 1993). X-ray fluorescence is nondestructive, rapid, requires minimal sample preparation, and was, for example, used successfully to determine the maximal level of mercury in maternal hair to assess fetal exposure (Toribora et al. 1982). However, the procedure is less sensitive compared to AAS and INAA if no pre-concentration is used. Electrochemical methods have been replaced as detectors in chromatography by other instrumental techniques because of poorer detection limits. High-performance liquid chromatography (HPLC) with reductive amperometric electrochemical reduction, however, was shown to be capable of speciating Hg(II), methyl- ethyl- and phenylmercury, with detection limits <2pgL (Evans and McKee 1987). 

Despite the numerous advantages the instrumental demands of microcolumn LC are considerable, and these demands are further accentuated as the requirements vary from one column type to another. A consequence of the reduced flow rates is that the detector flow-cell volume should be reduced to <10nl for OTCs, 0.1 pi for packed microcapillaries and 1 pi for microbore columns. An additional demand of the detector is that it should have a rapid response, <0.5 s. Development of suitable detectors is paramount if the potential of micro-LC is to be realised. Study of detector systems has focused in two areas firstly, the miniaturisation of ultraviolet, fluorescence and electrochemical systems, using in the former two systems LASERS as excitation sources and ultraviolet fibre optic and on-line cells to reduce band broadening and increase sensitivity [123,124] secondly, the direct interfacing with systems which previously required transport and/or concentration of the eluant. Interfacing of HPLC with mass spectroscopy has been undertaken by Barefoot et al. [125] and Lisek et al. [126] and flame systems (FPD and TSD) have been reviewed by Kientz et al. [127]. Jinno has reviewed the interfacing of micro-LC with ICP [128]. 

The detection and identification of phenohc compounds, including phenolic acids, have also been simplified using mass spectrometry (MS) techniques online, coupled to the HPLC equipment. The electrospray ionization (ESI) and atmospheric pressure chemical irmization (APCI) interfaces dominate the analysis of phenohcs in herbs, fruits, vegetables, peels, seeds, and other plants. In some cases, HPLC, with different sensitivity detectors (UV, electrochemical, fluorescence), and HPLC/MS are simultaneously used for the identification and determination of phenolic acids in natural plants and related food products. " In some papers, other spectroscopic instrumental techniques (IR, H NMR, and C NMR) have also been applied for the identification of isolated phenolic compounds. 

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