For many applications in ambient air monitoring and industrial safety monitoring the instrument will be provided with a Photo Ionisation Detector, 10.6 eV, with a measuring cell of 50 ml.
This detector will achieve a high resolution and allow the measurement of very low concentrations: for aromatic hydrocarbons, alkadienes and sulfur compounds down to 150 ppt, i.e. 0,4 mg/m3.
Principle : Photo ionisation is the process by which a photo excited electron absorbs enough radiant energy to be ejected from the atom or molecule. Photo ionisation is initiated by the absorption of a 10.6 eV photon by the molecule. If the molecule has an ionisation potential equal to or less than 10.6 eV, the following process occur.
R + hv -----> R+ + e-where hv is a photon with approximately 10.6 eV and R is the species of interest. Molecules with ionisation potentials greater than 10.6 eV will yield a lesser response.
The detector source is a low pressure discharge lamp filled with noble gas. It provides a stable monochromatic source of high energy photons suitable for ionisation of the various components in gas chromatography.
The UV source is contained in a vacuum-tight envelope with a UV grade magnesium fluoride window maintaining isolation of the UV source from the gas chromatograph's carrier gas. The lamp’s output is approximately 100 microwatts of 10.6 eV photons contained in a beam 6 degrees wide.