Light sources for atomic absorption

It is well known that an atom in its ground state absorbs light at discrete wavelengths (E = h*c/λ). In order to measure this narrow light absorption with maximum sensitivity, it is necessary to use a line source, which emits the specific wavelengths which can be absorbed by the atom. Narrow line sources not only provide high sensitivity, but also make atomic absorption a very specific analytical technique.

The two most common line sources used in atomic absorption are the “hollow cathode lamp, HCL” and the “electrodeless discharge lamp, EDL”.

The Hollow Cathode Lamp HCL

The common source of light is a “hollow cathode lamp”. This contains a tungsten anode and a cylindrical hollow cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas – e.g. argon or neon – at a pressure between 1Nm^-2 – 5Nm^-2. 

When an electrical potential is applied between the anode and cathode, some of the inert gas atoms are ionized. These gaseous ions bombard the cathode and eject metal atoms from the cathode in a process called sputtering (Fig. 1). Some sputtered atoms are in excited states and emit radiation characteristic of the metal as they fall back to the ground state. The shape of the cathode concentrates the radiation into a beam which passes through a quartz window, and the shape of the lamp is such that most of the sputtered atoms are redeposited on the cathode.

Hollow cathode lamps have a finite lifetime. With extended use, the sputtering process removes some of the metal atoms from the cathode and these are deposited elsewhere.

Fig 1.: Hollow cathode lamp emission process

 

The Electrodeless Discharge Lamp  (EDL)

For most elements, the hollow cathode lamp is a completely satisfactory source for atomic absorption. In a few cases, however  such as the more volatile elements -, the quality of the analysis is impaired by limitations of the hollow cathode lamp.

In cases like the above the electrodeless discharge lamp (EDL) is used. A small amount of the metal or salt of the element for which the source is to be used is sealed inside a quartz bulb. The bulb is placed inside a small, self-contained RF generator. When power is applied to the RF generator, an RF field is created. The energy will vaporize and excite the atoms, inside the bulb, causing them to emit their characteristic spectrum.

Electrodeless discharge lamps are typically much more intense and in some cases, more sensitive than comparable hollow cathode lamps. They therefore offer a better precision and lower detection limits.