Atomic Fluorescence Spectrometry

Atomic fluorescence is an extremely sensitive technique for determination of elements in samples. We should reiterate that in atomic fluorescence an external light source is used to excite the analyte atoms. An ideal light source for AFS must be much more intense than a hollow cathode lamp to achieve improvements in sensitivity. As a result, pulsed hollow cathode lamps and lasers are frequently used in AFS measurements. Excitation with a light source such as a hollow cathode lamp, which only emits radiation specific for the element of interest, makes AFS virtually completely free from spectral interferences. In addition, AFS is like AES in that a multi-element analysis can be achieved by putting several light sources around the atom cell, as discussed below.

A. Theoretical Background

This discussion assumes that the spectral line width of the light source is narrow relative to the absorption profile of the analyte atoms, as illustrated in Fig. 12A. The atoms absorb light from the source, and some of the energy is re-emitted as fluorescence, while various collisional processes in the atom cell deplete the remainder of energy. The ratio of the amount of light absorbed to that emitted is called the quantum efficiency (Y) and is ideally equal to one. In its simplest form, the equation for fluorescence radiant power (O f) resembles general expressions in atomic absorption (because for optically unsaturated systems the fluorescence signal is a function of the initial source radiant power, O0):

In the above equation, k is the absorption coefficient and l is the optical path length for the atom cell. Therefore, in AFS the signal size is directly proportional to both the light-source intensity and the atom concentration. Calibration curves for AFS with HCL excitation are linear with a slope of 1 (on a logarithmic plot) at low concentrations and bend back towards the concentration axis with a limiting slope of -0.5 at high concentrations. The curvature at high concentration is related to self-absorption in the atom cell.

1. Light Source

The multi-element capability of AFS is realized by use of several light sources. The HCLs used for AFS are special high-intensity versions of the ones used for atomic absorption and up to 12 of them can be arranged in a practical experimental arrangement. Lasers are also frequently used

FIGURE 14 Laser atomic fluorescence in flame atomizers. The photomultiplier tube (PMT) is equipped with a band-pass filter.

for AFS measurements because of their inherently large radiant intensity. The general diagram for a flame atomic fluorescence instrument with laser excitation is shown in Fig. 14.

2. Detection System

Generally, monochromators are not used in AFS measurements. For hollow cathode excitation, each HCL has a paired dedicated photomultiplier tube detector. In front of each photomultiplier tube is a filter, which allows a range of wavelengths to pass through it including the atomic-fluorescence wavelength of the element excited by the HCL. The filter provides discrimination against the background emission from the atom cell. A high-resolution monochromator is not used for AFS, because the resolution is provided by the specificity of the light source for the element of interest. Although a low-resolution monochromator can be, and often is, used for AFS, filters pass more total light onto the photomultiplier tube and provide adequate resolution (in the range 2-10 nm) to discriminate against much of the background from the plasma. The light sources used for AFS measurement are always modulated to allow discrimination against background emission.

3. Spectral Interferences

Because of the selectivity of the excitation source, the spectral interferences are almost completely absent. Nonetheless, some spectral interference has been reported. This interference is caused by scatter of the incident source radiation off large droplets in the flame and, without back-

ground correction, could be mistaken for fluorescence. When an ICP is used as the atom cell, the scattering interference is not observed. The ICP is so efficient at breaking down droplets and particles that very few scatter signals have been detected in an HCL-ICP instrument.

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