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Molecular fluorescence is the optical emission from molecules that have been excited to higher energy levels by absorption of electromagnetic radiation. The main advantage of fluorescence detection compared to absorption measurements is the greater sensitivity achievable because the fluorescence signal has in principle a zero background. Analytical applications include quantitative measurements of molecules in solution and fluorescence detection in liquid chromatography. The theory of quantitative fluorescence measurements is given in a separate document.
Transitions between molecular electronic energy levels:
A typical fluorimeter contains an excitation source, sample cell, fluorescence detector. Molecules in solution are usually excited by uv light and the excitation source is usually a deuterium or xenon lamp. Broad-band excitation light from a lamp passes through a monochromator, which passes only a selected wavelength. The fluorescence is dispersed by another monochromator and detected by a photomultiplier tube. Scanning the excitation monochromator gives the excitation spectrum and scanning the fluorescence monochromator gives the fluorescence spectrum. Simple instruments sometimes use only a bandpass filter to select the excitation wavelength.
Because of the differences in the nature of the energy-level structure and dynamics, discussion of atomic-fluorescence spectroscopy (AFS) and high-resolution laser-induced molecular fluorescence are in separate documents.
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