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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

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

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

lamp. This simple type of fluorescence detector was the first to be developed, it is relatively inexpensive and for certain compounds can be extremely sensitive. Typical specifications for a fluorescence detector are as follows:- Typical Specifications for a Fluorescence Detector        Sensitivity (Anthracene) 1x 10-9 g/ml Linear Dynamic Range 1 x 10-9 to 5 x 10-6 g/ml Response Index 0.96 - 1.04 A more elaborate form of fluorescence detector uses a monochromator to select the excitation wavelength and a second monochromator to select the wavelength of the fluorescent light. This instrument gives the maximum versatility and allows the maximum sensitivity to be realized for any type of solute. The system can also provide a fluorescence spectra by arresting the flow of mobile phase when the solute resides in the detecting cell and scanning the fluorescent light. The Refractive Index Detector 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 Fluorescence Detector When light is emitted by molecules that are excited by electromagnetic radiation, the phenomenon is termed photoluminescence. If the release of electro-magnetic energy is immediate, or stops on the removal of the excitation radiation, the substance is said to be fluorescent. If, however, the release of energy is delayed, or persists after the removal of the exciting radiation, then the substance is said to be phosphorescent. Fluorescence has been shown to be extremely