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It is possible to construct a cathode from more than one element. These are called

“multielement” HCLs, and can be used for the determination of all of the elements in the

cathode. This can be done sequentially but without having to change the lamp, which saves some
time. In general, multielement cathodes do not perform as well for all of the elements in the

cathode as do single HCLs for each element. The multielement cathode may have reduced
intensity for one or more of the elements. All of the elements present will emit their atomic

emission spectrum, resulting in a more complex emission than from a single element lamp. This

may require that a less-sensitive absorption line be chosen to avoid a spectral interference.
The obvious reason to use a multielement lamp is in the hope that more than one element

can be measured simultaneously, making AAS a multielement technique. In fact, there are a few
commercial simultaneous multielement AAS systems available for measuring up to eight

elements or so. Most use a bank of single element lamps all focused on the atomizer rather than
multielement cathodes.

The disadvantage with this approach is that only one set of conditions in the atomizer can

be used, and this set of atomization conditions may not be optimum for each element. HCLs have
a limited lifetime, usually due to loss of filler gas atoms through several processes. Adsorption of

filler gas atoms onto the lamp surfaces causes decreased sputtering and decreased intensity of
emission; eventually the number of filler gas atoms becomes so low that the lamp will not

“light”.

The sputtering process causes atoms to be removed from the cathode; these metal atoms
often re-condense elsewhere inside the lamp, trapping filler gas atoms in the process and

decreasing lamp life. This is particularly a problem for HCLs of volatile metals like Cd and As.
HCLs operated at currents higher than recommended will have shorter lifetimes than those

operated according to the manufacturer’s recommendation. Operating at higher currents results

in more intensity in the lamp output, but also may increase noise, which impacts both precision
and limit of detection.

Since we are measuring the ratio of light absorbed to incident light, there is little to be
gained by increasing the lamp current. Single element HCLs cost between $200 and 400 per

lamp, while multielement lamps cost between $300 and 400 each.

I.3.1.2.3 Temperature-Gradient Lamp (TGL).

A recently developed source of EMR for atomic absorption and atomic fluorescence is
the temperature-gradient lamp (TGL). The TGL uses an electric heater to convert an element

into atomic vapor. A relatively high current (about 0.5 A) which is passed through the vapor
causes excitation of some of the atoms, which subsequently emit radiation at the wavelengths

that are characteristic of the element in the lamp.

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