16 Agilent InfinityLab LC Series 1260 Infinity II FLD User Manual
1
Introduction to the Fluorescence Detector
How the Detector Operates
When a more complex molecule transforms from its ground energy state into
an excited state, the absorbed energy is distributed into various vibrational
and rotational sub-levels. When this same molecule returns to the ground
state, this vibrational and rotational energy is first lost by relaxation without
any radiation. Then the molecule transforms from this energy level to one of
the vibrational and rotational sub-levels of its ground state, emitting light, see
Figure 4 on page 16. The characteristic maxima of absorption for a substance
is its λ
EX
, and for emission its λ
EM
.
Figure 4 Relationship of Excitation and Emission Wavelengths
Photoluminescence is the collective name for two phenomena, fluorescence
and phosphorescence, which differ from each other in one characteristic way
— the delay of emission after excitation. If a molecule emits light 10
-9
to 10
-5
seconds after it was illuminated then the process was fluorescence. If a
molecule emits light longer than 10
-3
seconds after illumination then the
process was phosphorescence.
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