Page 37 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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run continuously. Hollow-cathode lamps are comparatively free from self-absorption, if run at low
current.
Normally, a different lamp is used for each element. Multi-element lamps (e.g. Ca-Mg, Fe-Mn or Fe-
Ni-Cr) are available, but are less satisfactory owing to the differing volatilities of the metals.
Demountable (water-cooled) hollow-cathode lamps have also been marketed, but are not widely used.
2.2.1.2 Electrodeless Discharge Lamps
Electrodeless discharge lamps were first developed for use in AFS. These lamps are microwave excited
and are far more intense than hollow-cathode lamps, but more difficult to operate with equivalent
stability. Radiofrequency-excited electrodeless discharge lamps (the radiofrequency region extends
from 100 kHz to 100 MHz, whereas the microwave region lies around 100 MHz) are typically less
intense (only 5-100 times more intense than hollow-cathode lamps), but more reproducible.
Commercially available radiofrequency lamps have a built-in starter (the starter provides a high-voltage
spark to ionize some of the filler gas for initiation of the discharge), are run at 27 MHz from a simple
power supply (capable of supplying 0-39 W), pre-tuned and enclosed to stabilize the temperature and
hence the signal.
A diagram of such a lamp is shown in Fig. 2.3 [taken from Barnett et al., At. Absorpt. Newsl. 15, 33
(1976). This paper gives a good account of the analytical performance of electrodeless discharge
lamps].
High intensity is not a source requirement in AAS and therefore electrodeless discharge lamps will not
replace hollow-cathode lamps. However, for those elements that produce poor hollow-cathode lamps
(notably arsenic
Figure 2.3
Cutaway diagram of an RF-excited electrodeless discharge lamp.
