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be emitted by the lamp. If we want to determine Cu in our AAS experiment, the lamp cathode

must be a copper cylinder; if we want to determine gold, the cathode must be a gold cylinder,

and so on. The cathode and an inert anode are sealed in a glass cylinder filled with Ar or Ne at
low pressure (the “filler gas”). A window of quartz or glass is sealed onto the end of the lamp; a

quartz window is used if UV wavelengths must be transmitted. Most HCLs have quartz
windows, because most elements have emission and absorption lines in the UV. Glass can be

used for some elements, such as sodium, where all the strong absorption lines are in the visible

region of the spectrum. The HCL emits narrow, intense lines from the element that forms the
cathode.

Applying a high voltage across the anode and cathode creates this emission spectrum.
Atoms of the filler gas become ionized at the anode and are attracted and accelerated toward the

cathode. The fast-moving ions strike the surface of the cathode and physically dislodge some of
the surface metal atoms (a process called “sputtering”).

The displaced atoms are excited by collision with electrons and emit the characteristic

atomic emission spectrum of the metal used to make the cathode. The process is shown in Fig.
The emitted atomic lines are extremely narrow. Unlike continuum radiation, the narrow emission

lines from the HCL can be absorbed almost completely by unexcited atoms. Using this light

source, atomic absorption is easily detected and measured.
Narrow line sources such as the HCL provide not only high sensitivity, but also

specificity. If only Cu atomic emission lines are produced by the Cu HCL, there are few species
other than Cu atoms that can absorb these lines. Therefore, there are few spectral interferences in

AAS.

The emitted spectrum consists of all the emission lines of the metal cathode, including
many lines that are not resonance absorption lines, but these other lines do not interfere in the

analysis. Each hollow cathode emits the spectrum of metal used in the cathode. For this reason, a
different HCL must be used for each different element to be determined. This is an

inconvenience in practice and is the primary factor that makes AAS a technique for determining

only one element at a time. The handicap is more than offset, however, by the advantage of the
narrowness of the spectral lines and the specificity that results from these narrow lines.

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