When an atom absorbs energy, for example from a laser beam, it pushes the system of electrons in the atom into a higher energy level. This could be a higher-energy vibrational state, or, if there is enough energy, a higher electronic state.
In many cases, this excitation is short lived - any electrons which move into higher energy levels soon drop back down again, releasing the excess energy as they do so. Because the energy levels in a particular atom are discrete and precisely defined, the energy is released as photons with characteristic wavelengths, allowing identification of the specific substance under examination.
Two types of measurement can be taken using this phenomenon. The excitation spectrum is measured at a single emitted wavelength, whilst varying the wavelength of the excitation laser beam. An emission spectrum works the other way around, measuring the emitted intensity across a range of wavelengths whilst exciting the analyte with a fixed wavelength beam.
Atomic fluorescence spectroscopy is ideal for measuring the concentration of an element in a particular sample. The good signal-to-noise ratio of the fluorescence signal compared with spectroscopic methods based on absorption means it is also more suited to measuring trace element compositions. It is is particularly effective for analyzing the metal content of water, oils, and biological samples.
Components of an Atomic Fluorescence Spectrometer
Fluorescence spectrometers consist of at least three basic components - a light source, a sample container and a detector. These components also need to meet certain criteria to make the spectrometer useful as an analytical tool. The light source must be able to produce a range of excitation wavelengths - this could be achieved using a tunable laser, but is more often simply a broadband source with a relatively even continuous output across a range of wavelengths, such as a Xenon arc lamp. diffraction grating monochromators can then be used to select the required excitation or emission wavelength.
Fluorescence instruments typically use detectors based on photomultiplier tubes. The detectable spectral range of a single photomultiplier tube is dependent on the material used for the photocathode, so two or more tubes are often used to cover the whole UV and visible light spectrum.
For atomic fluorescence spectroscopy, the analyte is almost always in the gas phase. This is achieved using an atomizer or nebulizer, or by producing hydrides with a strong reducing agent like sodium borohydride. For mercury analysis, a cold vapour can be produced by reaction with tin salts.
Types of Interference in Atomic Fluorescence Spectroscopy
Species with spectra where the lines overlap or are very close together are rare, and are well known and characterized, so they do not adversely affect the accuracy of the analysis. Most of the problems which can occur with AF spectra are from the atomization process, such as matrix effects, chemical reactions which create unexpected species in the analyte, or light source drift. With the exception of the chemical interference, these can be taken care of with background correction.
Typical Applications for Atomic Fluorescence Spectroscopy
- Environmental health/disease control
- Food safety
Sources and Further Reading