Detector in liquid chromatography

Figure 1 Ultraviolet spectra for benzaldehyde and benzoic acid solvent, methanol reference, methanol cell, 1.0 cm. (From Pfeiffer, C. D., Larson, J. R., and Ryder, J. F., Linearity testing of ultraviolet detectors in liquid chromatography, Anal. Chem., 54, 1622, 1983. Copyright American Chemical Society Publishers. With permission.)...

Stewart, J. E., Spectral-bandwidth effects of variable-wavelength absorption detectors in liquid chromatography, ]. Chromatogr., 174, 283, 1979. 

Esquivel, J. B., Wavelength accuracy testing of UV-visible detectors in liquid chromatography, Chromatogmphia, 26, 321, 1988. 

Munk, M., Refractive index detectors, in Liquid Chromatography Detectors, Vol. 23, Vickrey, T. M., Ed., Marcel Dekker, New York, 1983, chap. 5. 

A number of lines of evidence suggested early on that 8-oxo-7,8-dihydrogua-nine (8G) is more readily oxidized than guanine [42, 86-88]. The most direct evidence in this regard was the ability to detect 8G selectively using electrochemical detectors in liquid chromatography [89]. We therefore suspected that 8G would react with metal complexes with potentials lower than... 

Figure 11.13 —Schematic am optical path showing the principle and simplified view of a diode array spectrophotometer. The shutter is the only mobile piece in the assembly, allowing subtraction of the background signal (dark current) without any light intensity striking the photodiodes. This inverted optical design allows the sample to be exposed to the exterior light. These instruments are widely used as detectors in liquid chromatography (cf. 3.7).

Two approaches to the venting of the solvent prior to the detector have been presented in detail [137], Packed GC columns coupled to capillary columns have been used for the total transfer of effluent from the LC [138]. The current status of LC-GC has been reviewed [139]. The use and performance of the ELCD, NPD, and FPD GC detectors in liquid chromatography has also been reviewed [140]. Even though the majority of applications are not directly related to the analysis of pharmaceuticals, they may nevertheless be useful [141-146]. 

A general discussion regarding the instrumentation which can be used for the measurement of CD spectroscopy has been provided by Donald Bobbitt, while the more specialized requirements associated with vibrational optical activity have been discussed by Laurence Nafie. This latter chapter deals with both the methods of vibrational CD spectroscopy as well as raman vibrational optical activity. The use of CD as a detector in liquid chromatography represents an important area of heightened activity, and this has been covered by Andras Gergely. 

A rather broad applicability of FTIR as a detector in liquid chromatography can be achieved when the mobile phase is removed from the sample prior to detection. In this case the sample fractions are measured in a pure state without interference from solvents. Experimental interfaces to eliminate volatile mobile phases fromHPLC effluents have been tried with some success [133-135] but the breakthrough towards a powerful FTIR detector has only been achieved by Gagel and Biemann, who formed an aerosol from the effluent and sprayed it on a rotating aluminum mirror. The mirror was then deposited in a FTIR spectrometer and spectra were recorded at each position in the reflexion mode [136-138]. 

Another means of identification is a two-detector system. One detector should be nonspecific, such as the thermal conductivity cell in gas chromatography, or the refractive-index detector in liquid chromatography, while the other should have... 

Boehme, W. Wagner, G. Oehme, U. Spectrophotometric and polarimetric detectors in liquid chromatography for the determination of enantiomer ratios in complex mixtures. Anal. Chem. 1982, 54, 709-711. 

Byrne, S.H., Jr. Detectors in liquid chromatography. In Modem Practice of Liquid Chromatography Kirkland, J.J., Ed. Wiley New York, 1971 95-124. 

The last two popular detectors in liquid chromatography are nuclear magnetic resonance and mass spectrometers. The mass spectrometer as a detector was described in the section in this chapter on gas chromatography. On the other hand, the principles of the nuclear magnetic resonance are described in chapter 5 of this book because of its important in asphaltene chemistry. 

Describe some commonly used detectors in liquid chromatography and then-bases of operation. 

An important application of flow cells is their use as detectors in liquid chromatography (LC), capillary zone electrophoresis (CZE), and flow injection (FI) methods (46-50). Such cells may be coulometric ones, where all of the material flowing into the cell is electrolyzed, but more frequently they are amperometric or voltammetric cells, sometimes utilizing UMEs as described in Section 9.7. 

Jewett, K.L. Brlnckman, F.E. In "Detectors in Liquid Chromatography" Vickrey, T.M., Ed, Marcel Dekker New York, 1983 Chap.6. Schwedt, G. "Chromatographic Methods in Inorganic Analysis" ... 

The Kissinger type twin electrode thin layer cell is a widely used tool in the everyday analytical practice especially in the field of flow injection techniques and as an amperometric detector in liquid chromatography. Less attention is paid to the possibilities offered by this cell 2LS a microanalytical tool when it is filled with a quiescent solution sample. In this way the determination of electroactive components in a volume of about 50-100 pi can be carried out by applying a proper excitation potential program. 

For a characterization within a series of compounds often polarimetry supersedes CD spectroscopy because the chosen concentration for its measurement is not restricted by a too high absorption. The broad acceptance of the polarimetric standardization of chiral compounds always has been a motivation for improvements of polarimeters. Especially for the daily routine work there is a requirement for instruments with high convenience (see section on Polarimetry and CD detectors in liquid chromatography). 

Polarimetry and Circular Dichroism Detectors in Liquid Chromatography... 

A polarimetric equipment is an important detector in liquid chromatography. The combination of an [ajJmax itieasurement combined with a detection of the UV absorption allows to determine the optical purity even in the case of an incomplete chromatographic enantiomeric separation. The use of CD as a chromatographic detector is limited in liquid... 

Since the appearance of this work, many different kinds of electrocatalytic reactions of organic compounds with CMEs have been carried out with various mediators and surface modification methods. Additional applications of mediator functionalized CMEs have been their use as amperometric detectors in liquid chromatography and as biosensors. 

The choice of selective detectors in liquid chromatography is still more limited. Electrochemical detectors and the UV-spectrophotomet-ric detector can provide clues, but many types of compounds resist oxidation or reduction in an electrochemical cell, and the paucity of clues provided by a UV spectrum has been a source of disappointment to many chromatographers. 

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