Photoionization Detector - PerkinElmer Photovac 2020 Manual

2020 Software Manual
another, but displays a number indicating the total concentration of
all photoionizable compounds in the sample.
A standard of isobutylene at a known concentration may be used for
setting the sensitivity. If 2020 is calibrated with isobutylene, it
displays concentrations in units equivalent to ppm of isobutylene. If
isobutylene were the only photoionizable chemical in the sample,
then 2020 would display its concentration directly.
2020 responds more or less readily to other chemicals than it does to
isobutylene. Because it has a medium sensitivity to isobutylene, this
gas has been chosen as a reliable means of reporting an average
concentration of total ionizables present.
For special applications, gases other than isobutylene can be used to
calibrate 2020.

Photoionization Detector

2020's PID is shown in Figure 30. The PID measures the
concentration of photoionizable chemicals in the gas stream from the
sample inlet and produces an electrical signal for the
microprocessor.
A UV lamp generates photons which ionize specific molecules in the
gas stream. The permanent air gases (argon, carbon dioxide,
nitrogen, oxygen, water vapor etc.) require a relatively high energy
for ionization, and are not ionized by the UV photons. Many of the
chemicals considered pollutants, including most hydrocarbons, are
ionized.
The gas stream is directed into the PID through a small port at the
center of the UV lamp window and through a series of larger ports
around the perimeter of the lamp window. This arrangement permits
a high sample flow rate and short response.
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