Hollow Cathode Lamps
The most common source for atomic absorption measurements
is the hollow cathode lamp, such as the one
shown in Figure 9-11.10 This type of lamp consists of a
tungsten anode and a cylindrical cathode sealed in a
glass tube that is filled with neOIil or argon at a pressUl;e
of 1 to 5 torr. The cathode is constructed of the metal
whose spectrum is desired or serves to support a layer
of that metaL
Ionization of the inert gas occurs when a potential
on the order, of 300 V is applied across the electrodes,
which genetates a current of about 5 to 15 rnA as ions
and electrons migrate to the electrodes. If the potential
is sufficiently large, the gaseous cations acquire enough
kinetic energy to dislodge some of the metal atoms from
the cathode surface and produce an atomic cloud in a
process called sputtering. A portion of the sputtered
metal atoms are in excited states and thus emit their
characteristic radiation as they return to the ground
state. Eventually, the metal atoms diffuse back to the
cathode surface or to the glass walls of the tube and are
redeposited.