changes in lamp intensity and detector sensitivity are facilitated but those for flame background are not.
Sharp-line Sources
Ideally, the emission line used should have a half-width less than that of the corresponding absorption
line otherwise equation (8.4) will be invalidated. The most suitable and widely used source which
fulfils this requirement is the hollow-cathode lamp, although interest has also been shown in
microwave-excited electrodeless discharge tubes. Both sources produce emission lines whose half-
widths are considerably less than absorption lines observed in flames because Doppler broadening in
the former is less and there is negligible collisional broadening.
Hollow-cathode Lamps
A diagram of a hollow-cathode lamp is shown in Figure 8.28. It consists of a sealed glass envelope with
a quartz end-window, and containing a hollowed-out cylindrical cathode of some 2 mm internal
diameter together with a tungsten wire anode. The cathode is fabricated from the element to be
determined. By reducing the pressure inside the envelope to about 1.3 kN m–^2 and passing a current of
5 – 50 mA at an applied potential of about 300 V, a low-pressure glow-discharge confined to the inside
of the cathode and characteristic of the cathode material is produced. The action of the gas used to fill
the envelope is to bombard the cathode thereby vaporizing atoms from the surface, a process known as
'sputtering'. The resulting emission spectrum contains lines from both the cathode material and the
filling gas. The choice of gas is restricted to those whose emission lines do not coincide with lines from
the cathode itself and which will not ionize the sputtered atoms. In practice, neon or argon are preferred.
The emission spectrum of the cathode material includes a number of intense, sharp lines arising from
transitions between excited states and the ground state, so-called resonance radiation. Generally, only a
few resonance lines per element are suitable for quantitative work and there will be variation in the
ranges of concentration over which absorbance measurements
Figure 8.28
Diagram of hollow-cathode lamp.