Gas Chromatography - Tandem Techniques - The Fluorescence Spectrometer > Page 22


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 absorbance,
(l) is the path length of the cell.


For use with gas chromatography, the 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 fluorescent light, the diagram is easier to understand.


The excitation source that emits UV light over a wide range of wavelengths (usually a deuterium lamp) is situated at the focal point of an ellipsoidal mirror shown at the top left hand corner of the diagram. The parallel beam of light is arranged to fall on a toroidal mirror that focuses it onto the grating on the left-hand side of the diagram.


This grating allows the wavelength of the excitation light to be selected or the whole excitation spectrum scanned providing a complete range of excitation wavelengths. The selected wavelength then passes to a spherical mirror and then to a ellipsoidal mirror at the base of the diagram which then focuses it onto the sample. The excitation light path is situated on the left-hand side of the diagram. Between the spherical mirror and the ellipsoidal mirror, in the center of the diagram, is a beam splitter that reflects a portion of the incident light onto another toroidal mirror.




Figure 13. The Fluorescence Spectrometer Detector