Radiation Sources

When it comes to Atomic Absorption, there are a couple important factors to note. Absorption lines are extremely narrow, usually only ranging between 0.002 and 0.005 nm. Also, each element has its own unique electronic transition. Picking a source is application driven, the correct source for your specific use must be chosen. These factors must be taken into account when deciding a lamp for analysis.

There are two categories of sources, line and continuum. Line sources emit a narrow band of radiation, which is important because they are highly selective, provide high sensitivity (beam power is in a narrow wavelength band) and reduce spectral interference of other elements, molecules, atoms, or ions that have similar spectral lines.  Continuum sources are typically used for background correction, to eliminate the matrix so only the signal of the analyte is observed.

Hollow-cathode Lamps

             The most common line source used for atomic absorption spectroscopy is the HCL. Structurally, this is an air-tight lamp filled with argon or neon and kept at around 1 to 5 torr. The inert gas is ionized as a high voltage potential difference is created between the tungsten anode and use-specific cylindrical cathode. Cations of the ionized argon or neon gas dislodge metal ions from the cathode. These produce an atomic cloud; where some of the atoms in the cloud are in an excited state and emit an element specific radiation upon returning to ground state. There are many commercially available models of HCLs, some can possess multiple metal cathodes for analysis of several metals.



Electrodeless Discharge Lamps (EDLs)

Electrodeless discharge lamp

Electrodeless discharge lamp

              EDLs are line sources that provide radiant energy that has a greater intensity than HCL. These are sealed quartz tubes filled with inert gas at low pressure, much like HCL. The bulb contains a small quantity of the element of interest. An intense field of  radiation provides ionization and excitation of the metal to produce a spectrum. The line width is typically narrower, but may require more time to analyze because it is powered by a radio-frequency (RF) source and needs time to stabilize the RF coil.  EDLs are generally less reliable than HCLs with the exception of Se, As, Cd, and Sb in which, better detection limits are produced.