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Ionization potential is typically specified in electron volts (eV) and refers to the energy required to remove a single electron from a single atom or molecule to produce an ion. The I.P. of an individual element or compound (found in instrument manufacturers’ charts) may be used as an indicator of its detectability using a PID.
A PID lamp generates a specific voltage, the most common being 10.6eV. If the I.P. of the compound is less than the lamp voltage then that compound will be ‘seen’ by the PID. Most volatile organic compounds have an I.P. of less than 10.6 eV, so will be measured.
For those compounds with an I.P. greater than 10.6eV, a higher voltage 11.7eV lamp is available. This is frequently used to detect and measure compounds that will not be seen by the standard lamp. Any compound with an I.P. greater than 11.7eV is not detectable by PID
Unfortunately the operating costs for this higher voltage lamp are significantly greater, and the lamp lifetime much shorter. This lamp is therefore only used when absolutely necessary.
A lower voltage, 10/9.8eV lamp is also available. This can be used as an alternative to the standard lamp in order to discriminate between compounds. If two compounds are known to be present and the 10.6eV lamp sees them both it is sometimes possible to substitute the low voltage lamp in order to only see one of the compounds.
This lower voltage lamp may also be used to effectively ‘screen out’ a large number of compounds that are not of interest. This is used, for example, in Benzene specific instruments widely used in the petrochemical industry.