Liquid Chromatography Detectors - The Fluorescence Detector > Page 56

The optical system of most fluorescent detectors is arranged such that the fluorescent light is viewed at an angle to the exciting incident light beam. This minimizes the amount of incident light that can interfere with the fluorescent signal. Under such circumstances, the fluorescent signal is viewed against a an almost black background and thus, furnishes the maximum signal to noise. A filter can be used to reduce the background light still further by 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 complex but with both selectable excitation wavelengths and emission wavelengths is extremely versatile. In addition, excitation and emission spectra can be obtained as required.