Page 36 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Figure 2.2
The hollow-cathode lamp.
with the metal of interest. The lamp is contained within a glass envelope filled with an inert gas
(usually Ne or Ar) at 1-5 Torr. A potential of about 500 V is applied between the electrodes and, at the
pressures used, the discharge concentrates into the hollow cathode. Typically, currents of 2-30 mA are
used. The filler gas becomes charged at the anode, and the ions produced are attracted to the cathode
and accelerated by the field. The bombardment of these ions on the inner surface of the cathode causes
metal atoms to sputter out of the cathode cup. Further collisions excite these metal atoms, and a simple,
intense characteristic spectrum of the metal is produced. Marcus, and Kirkbright and Sargent (see
Appendix C) describe this action and hollow-cathode lamps in more detail.

The insulation helps to confine the discharge within the hollow cathode, thus reducing the possibility
of self-absorption and the appearance of ion lines. Both of these effects can cause bending of calibration
curves towards the concentration axis. A glass envelope is preferred for ease of construction, but a silica
window must be used for ultraviolet light transmission. A graded seal between the window and
envelope ensures excellent gas tightness and shelf-life. A moulded plastic base is used. The choice of
filler gas depends on whether the emission lines of the gas lie close to useful resonance lines and on
the relative ionization potentials of the filler gas and cathode materials. The ionization potential of
neon is higher than that of argon, and the neon spectrum is also less rich in lines. Therefore, neon is
more commonly used.

Modern hollow-cathode lamps require only a very short warm-up period. Lifetimes are measured in
ampere hours (usually they are in excess of 5 A h). A starting voltage of 500 V is useful, but operating
voltages are in the range 150-300 V. In many instruments, the current supplied to the lamp is
modulated. Hollow-cathode lamps may also be pulsed or