Operating Principle - Thermo Scientific Dionex UltiMate 3000 Series Operating Instructions Manual

UltiMate 3000 Series:
Fluorescence Detectors FLD-3100 and FLD-3400RS

2.2 Operating Principle

Fluorescence detectors are optical detectors. In a fluorescence detector, the sample is
exposed to light at a defined wavelength (excitation). The thus excited sample substance
emits light at a higher wavelength (emission). The photomultiplier is positioned at an angle
of 90° to the light source and detects the light that was emitted from the fluorescing
substances. In contrast to UV/Vis detectors, a fluorescence detector measures a very weak
light signal rather than the difference between light intensities (absorbance).
Fig. 2: Simplified presentation of the light emission in a molecule (fluorescence)
Fluorescence is used, for example, in highlighters or in whitening agents (optical
brighteners). Fluorescent paint used in highlighters reacts to the blue and near, non-visible
ultraviolet range of the daylight and gives off light at a longer wavelength (typically blue-
green, yellow and red).
As shown in Fig. 3 (→ page 17), the light beam from the xenon lamp (no. 1) is focused by the
lamp optics (no. 2) through the entrance of the excitation monochromator (no. 3). The
excitation monochromator transmits only light of the user-selected wavelength to the sample
inside the flow cell (no. 4). Most of the light penetrates the sample. A reference detector
(no. 5) behind the flow cell measures the intensity of this light. The excitation light stimulates
the sample to emit fluorescence light.
Page 16
Fig. 1: Simplified presentation of absorption in a molecule
Absorption of light
Emission of
light of a longer
wavelength
Operating Instructions