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The fluorescent signal (IF) is given by

where is the quantum yield (the ratio of the number of photons

(IF) emitted to the number of photons absorbed),
(I0) is the intensity of the incident light,
(c) is the concentration of the solute,
(k) is the molar absorbance,
and (1) is the path length of the cell.

There are two basic types of fluorescent spectra that can be taken:

1. The fluorescence intensity taken at a fixed wavelength while programming the excitation wavelength.

2. The fluorescence intensity taken over a range of wavelengths while employing a fixed excitation
wavelength.

This flexibility can produce a very large number of fluorescence spectra from a given compound either
monitored at a range of fixed emission wavelengths or over a range of fixed excitation wavelengths, all
of which will be significantly different. Examples of a pair of such spectra are given in Figure 2.5. The
spectrum on the left was obtained by monitoring the fluorescence at 405 nm and programming the
excitation light from 230 nm to 410 nm which provides an excitation spectrum. The spectrum on the
right is obtained by fixing the excitation light at 292 nm and monitoring the fluorescent light from 360
nm to 480 nm which provides an emission spectrum. Fluorescence spectra can be used for identifying
substances by comparing them with reference spectra. However, they have very limited use for the
structure elucidation of a completely unknown substance.

Figure 2.6 shows the basic fluorescence spectrometer detector. 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.