Photoionisation lamps are compact long life devices that produce ultra violet light which can ionise many volatile organic molecules. They are primarily used in fixed or portable environmental hazard detectors and can provide ionising energies ranging from 4.9 to 11.8 electron volts (eV).
Photoionisation lamps are available in two fundamental types dc voltage excited and RF excited. However since the electron voltage available is a function of the gas in the lamp and the window material the output data is relevant for both types of lamps.
The majority of photoionisation lamps have windows made from lithium fluoride, magnesium fluoride, sapphire or spectrosil, the available ionisation potential being limited by their spectral cut off.
The most commonly used gases in photoionisation lamps are xenon, krypton, hydrogen, argon, and mercury vapour. The combination of the fundamental gas lines created by striking a discharge in the gas with the window materials defines the chemical ionisation potential available for each type of lamp.
Whilst the various ionisation potentials can theoretically be used to provide added selectivity to the analysis, care must be taken when interpreting established data.
Due to the technical difficulties of measuring both wavelength and intensity accurately within this spectral region, modern analytical tools available to Cathodeon have helped clarify the true situation.
Earlier analytical data quoting a 10.2 eV ionising potential from Krypton filled lamps is erroneous being derived from early generation VUV vacuum monochromators that were unable to resolve the 10.0 and 10.6 eV lines. Similarly results based on individual 10.0 or10.6 eV ionising potentials are erroneous as both ionising potentials are available in all krypton filled photoionisation lamps. u
ENQUIRY No 84
Cathodeon Ltd is based in Cambridge, UK. www.cathodeon.com
