Page 155 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

Page 140

6.3 Sources

A source for AFS must be very intense to give low detection limits. In contrast with AAS, continuum
sources may be used but, given the distribution of wavelengths from a black-body radiator, few give
high intensity in the vital ultraviolet region. The most suitable continuum radiation source is the 150-
500 W Xe high-pressure arc lamp, used in a continuous mode. Xenon arcs have been used for AFS,
but scatter problems are encountered. The actual integrated intensity over the absorption half-width is
still relatively low, whereas for a line source the majority of the emission is concentrated in a few lines.
Hence the preference is for line sources.

Where vapour discharge lamp sources exist (for volatile elements such as Hg, Na, Cd, Ga, In, Tl and
Zn) they can be used. Hollow-cathode lamps are insufficiently intense, unless operated in a pulsed
mode. Microwave-excited electrodeless discharge lamps are very intense (typically 200-2000 times
more intense than hollow-cathode lamps) and have been widely used. They are inexpensive and simple
to make and operate. Stability has always been a problem with this type of source, although
improvements can be made by operating the lamps in microwave cavities thermostated by warm air
currents. A typical electrodeless discharge lamp is shown in Fig. 6.3.

Figure 6.3
A microwave-excited electrodeless discharge lamp.