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nbsp; The UV spectrometer used for monitoring the eluent from a GC column employs a source that provides light over a wide range of wavelengths and consequently, with the aid of an appropriate optical scanning system, absorption spectra of any substance eluted from a gas chromatograph can be obtained for identification purposes. The actual procedure, however, differs with the type of spectrometer being used.     There are two basic types of UV spectrometer, the dispersion spectrometer and the diode array spectrometer, the latter being the more popular for use in conjunction with the gas chromatograph. Both types require a broad emission light source such as the deuterium or the xenon lamps the use of the deuterium lamp being the most widespread. The two types of spectrometers have important differences. In the dispersive instrument the light is dispersed before it enters the sensor cell and thus virtually monochromatic light passes through the sensor. However, if

fluorescence spectrometer must be fitted with a sensor cell of appropriate dimensions. Such a spectrometer system can be highly complex and versatile and allows excitation spectra to be obtained at any fixed fluorescent wavelength or emission spectra to be obtained for any fixed excitation wavelength. In general fluorescence spectra have very limited use in structure elucidation but can be used for identification purposes providing the necessary reference spectra are available. A diagram of a fluorescence spectrometer is shown in figure 13. It consists of two monochromators, the first that selects the wavelength of the excitation light and the second disperses the fluorescent light and provides a fluorescence spectrum. The spectrometer incorporates two distinctly different light paths and as a result the optical system appears quite complicated. If the different light paths are considered separately, that is firstly, the path of the excitation light and secondly, the path of the

reference compound and, if a match is obtained (the difference is close to zero) the unknown is considered identified. In a similar manner, for certain compounds, UV and fluorescence spectra can also be used to confirm solute identity and the association of the GC with a UV spectrometer or a fluorescence spectrometer has proven to be much easier due to the significantly greater sensitivity of these spectrometers compared with that of the IR spectrometer.   The use of UV spectra and fluorescence spectra, however, are far less useful than mass spectra or IR spectra for solute identification. Except for certain substances (e.g. those containing aromatic rings) the majority of compounds give very similar UV spectra with very little fine structure to allow confident spectra matching. This is due to a multiplicity of adsorption bands merging to produce a broad envelope the cause of which will be discussed later. Similarly fluorescence spectra have much less detail than IR

The Multi Wavelength Fluorescence Detector The multi wavelength fluorescence detector contains two monochromators, one to select the excitation wavelength and the second to select the fluorescence wavelength or produce a fluorescence spectrum A diagram of the multi wavelength fluorescence detector is shown in figure 38. Figure 38.  The Fluorescence Spectrometer Detector The detector comprises a fluorescent spectrometer fitted with suitable absorption cell that is sufficiently small so as not to degrade the resolution of an LC column. There are

3. Acenaphthene 4. Phenanthrene 5. Anthracene 6. Fluoranthracene 7. Pyrene 8. Benzo(a)anthracene 9. Chrysene 10. Benzo(b)fluoranthene 11. Benzo(k)fluoranthene 12. Benzo(k)fluoranthene 13. Dibenz(a,h)anthracene 14.Indeno(1,2,3,cd)pyrene 15. Benzo(ghi)perylene Courtesy of the Perkin Elmer Corporation Figure 37. Separation of the Priority Pollutants Monitored by the Simple Fluorescence Detector There are some compromises between the expensive fluorescence spectrometer detector and the single wavelength excitation fluorescence detector. Some have a single monochromators that select the wavelength of the excitation light, others employ a single monochromator to select the emission wavelength or provide emission spectra

the removal of any  stray scattered incident light.   The fluorescence signal (If) is given by   where (f) is the quantum yields (the ratio of the number of photons emitted and the number of photons absorbed), (Io) is the intensity of the incident light, (c) is the concentration of the solute, (k) is the molar absorbence, (l) is the path length of the cell. Fluorescence detectors can be simple or complex, the simplest consists of a single wavelength excitation source and a sensor that monitors  fluorescent light of all wavelengths. For certain samples, this form of fluorescence detector can be very sensitive and relatively inexpensive. However, employing excitation light of a single wavelength and only a broad emission wavelength, it is not very versatile. Conversely, the fluorescence spectrometer fitted with a small sensor cell is far more