Electrodeless Discharge Lamps
Electrodeless discharge lamps (EDLs) are useful sources of atomic line spectra and provide radiant intensities that are usually one to two orders of magnitude greater than hollow cathode lamps. II A typical lamp is constructed from a sealed quartz tube containing a few torr of an inert gas such as argon and a small quantity of the metal (or its salt) whose spectrum is of interest. The lamp contains no electrode but instead is energized by an intense field of radio-frequency or microwave radiation.
Ionization of the argon occurs to give ions that are accelerated by the high-frequency component of the field until they gain sufficient energy to excite the atoms of the metal whose spectrum is sought.
Electrodeless discharge lamps are available commercially for 15 or more elements. Their performance does not appear to be as reliable as that of the hollow
cathode lamp. Figure 9-12 is a schematic of a commercial electrodeless discharge lamp, which is powered by a 27-MHz radio-frequency source.
Source Modulation
In the typical atomic absorption instrument, it is necessary to eliminate interferences caused by emission of radiation by the flame. Much of this emitted radiation
is, of course, removed by the monochromator. Nevertheless, emitted radiation correspondrhg in wavelength to the monochromator setting is inevitably present in
the flame due to excitation and emission by analyte atoms. In order to eliminate the effects of flame emission, it is necessary to modulate the output of the source
so that its intensity fluctuates at a constant frequency. The detector then receives two types of signal; an alternating one from the source and a continuous one from