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The major disadvantage is the need to change the line source whenever the element that is

to be assayed is changed.

Two radiation sources are commonly used in commercial AAS instruments, the
Electrodeless Discharge Lamp (EDL) and the Hollow Cathode Lamp (HCL). Both types of

lamps are operated to provide as much intensity as possible while avoiding line-broadening
problems caused by the collision processes described earlier.

I.3.1.2.1 Electrodeless Discharge Lamp (EDL) or
Gaseous Discharge Lamp or Arc Lamp

All electrodless discharge lamp is prepared by placing a small amount of an element or a
halide of an element into a quartz tube. A few milligrams of the iodide of the element is often

used. The quartz tube is evacuated under an atmosphere of an inert gas to a pressure that is

usually between 0.1 and 5 torr. For most EDLs the optimal pressure is between 0.3 and 3 torr.
After evacuation the tube is sealed and placed into a hollow microwave cavity. The vapor

pressures of the elements that are used in EDL are sufficiently high to permit some gaseous
atoms of the element to be formed in the low pressure environment of the lamp. Application of

microwave field (2450 MHz) causes excitation of some of the gaseous atoms. As the excited

make the transition to the ground state or to other low energy levels, radiation that is
characteristic of the atom is emitted and travels through the transparent end of cavity. A sketch

of EDL is given below.

An EDL can be operated at a power level that corresponds to any of three stages depending upon
the required intensity of emitted radiation.

At a low power the lamp emits a dim glow that is spread throughout the lamp.

At an intermediate power level the lamp exhibits a high intensity glow that is
concentrated in a relatively small volume of the lamp.

At a high power level the high intensity glow fills the entire lamp.

Generally EDL operational lifetimes are greater than 50 h. most EDLs emit radiation that is at
least 10 times as intense as that of corresponding HC lamp. Consequently, EDLs are more often

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