Liquid Chromatography Detectors - The Fluorescence Detector > The Multi Wavelength Fluorescence Detector > Page 59

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 paths are depicted separately, the excitation light in dark blue and the emitted light in light blue.

The broad band excitation source (usually a deuterium lamp) is placed at the focal point of an ellipsoidal mirror (shown at the top left hand corner of the diagram). The resulting parallel light beam falls on a toroidal mirror that focuses it onto the grating on the left-hand side of the diagram. This grating selects the wavelength of the excitation light. Light of the selected wavelength passes to a spherical mirror and then to a ellipsoidal mirror (shown at the base of the diagram) which focuses it onto the sample. The excitation light path is in dark blue and is situated on the left-hand side of the diagram.

A beam splitter is situated between the spherical mirror and the ellipsoidal mirror (in the center of the diagram) which reflects a portion of the incident light onto another toroidal mirror which focuses it onto the reference photo cell. The path of the fluorescent light is in light blue and is largely on the right hand side of the diagram. Fluorescent light from the cell is focused by an ellipsoidal mirror on to a spherical mirror which then focuses the light onto a grating situated (seen at about center right of the figure). The grating selects the specific wavelength of the fluorescent light to be monitored. Light of the selected wavelength passes to a photoelectric cell which monitors its intensity.