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 two distinctly different light paths one for the excitation light and one for the emitted light. The different light

The fluorescence detector is one of the most sensitive LC detectors and for this reason is often used for trace analysis. Unfortunately, although the detector is very sensitive, its response is only linear over a relatively limited concentration range. In fact, the response of the detector can only be assumed to be linear over a concentration range of two orders of magnitude. Unfortunately, the majority of substances do not naturally fluoresce which is a serious disadvantage to this type of detector. It follows, that in many instances fluorescent derivatives must be synthesized to render the substances of interest detectable. There are a number of regents that have been developed specifically for this purpose but derivatizing procedures will be discussed in detail in a later Book. A diagram of the Fluorescence Detector is shown in figure 23. Figure 23. The Fluorescence Detector

The Single Wavelength Excitation Fluorescence Detector The single wavelength excitation fluorescence detector is probably the most sensitive LC detector that is available, but is achieved by forfeiting versatility. A diagram of a simple form of the fluorescence detector is shown in figure 36. The excitation light is normally provided by a low pressure mercury lamp which is comparatively inexpensive and provides relatively high intensity UV light at 253.7 nm. Many substances that fluoresce will be excited by light of this wavelength.     Figure 36. The Single Wavelength Excitation Fluorescent Detector The

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

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 useful as a detection process and detectors based on fluorescent measurement have provided some of the highest sensitivities available in LC. When a molecule adsorbs light, a transition to a higher electronic state takes place and this absorption is highly specific for the molecules concerned; radiation of a specific wavelength or energy is only absorbed by a particular molecular structure. If electrons are raised to an upper excited single state

nbsp; It is seen that the anthracene is clearly picked out from the mixture of aromatics by the fluorescence detector and the chloride ion, not shown at all by the UV adsorption or fluorescence detectors, is monitored by the electrical conductivity detector. The simultaneous use of all detector functions make this detector very useful but, the real advantage of the trifunctional detector is that it allows the analyst a choice of the three most useful detector function in one detecting system. In addition, any of the three functions can be chosen at the touch of a switch and without any changes in hardware. An example of the use of the three individual detector function in