Page 104 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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incorporating a crystal oscillating at a fixed frequency. When the power load changes, owing to sample
introduction into the plasma, for example, the generator retunes by means of vacuum or airgap
capacitors to maintain a fixed frequency. Free-running generators do away with the necessity for a
separate auto-tuning circuit. In this design changes in the power loading are compensated for by slight
shifts in the frequency of the oscillation circuit to bring the whole circuit back into resonance. The
majority of RF generators operate at 27.12 MHz, although there has been a move towards higher
frequencies, particularly 40.68 MHz, which is thought to yield lower continuum background emission
and greater stability. Originally RF generators were based on vacuum tube technology, although recent
instruments have solid-state RF generators which are more compact.

Q. Explain the difference between a free-running and a crystal-controlled radiofrequency generator

4.4.3 Sample Introduction

The commonest form of sample introduction is by means of an aerosol generated using a pneumatic
nebulizer. Several types of nebulizer can be used. All-glass concentric nebulizers (Fig. 4.7a) operate in
a similar manner to those used for FAAS. Cross-flow nebulizers (Fig. 4.7b) operate by directing a high-
velocity stream of gas across the mouth of a capillary

Figure 4.7
Nebulizers used for ICP-AES: (a) concentric; (b) cross-flow;
(c) glass frit (reproduced with permission from Caruso et al.
, Spectrochim. Acta, 1985, 40B , 3); (d) Hildebrand grid (expanded
view) (reproduced with permission from Caruso et al., J. Anal.
At. Spectrom., 1987, 2, 389); (e) V-groove or 'Babington type'.