INFICON 2020ComboPRO Portable Photoionization Detector - Manual (Page 42)

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In TVOC mode, the 2020ComboPRO does not distinguish one type of
compound from another, but displays a number indicating the total
concentration of all photoionizable compounds in the sample.
In TVOC mode, a standard of isobutylene at a known concentration may be used for
setting the sensitivity. If the 2020ComboPRO is calibrated with isobutylene, it
displays concentrations in units equivalent to ppm of isobutylene.
In TVOC mode, the 2020ComboPRO 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 ionizable compounds present.
Gases other than isobutylene can be used to calibrate the 2020ComboPRO.
However, all response factors in TVOC mode are based on an isobutylene
calibration.
For benzene measurements in GAS mode, a pre-filter tube holder and benzene pre-
filter tube are installed, and calibration with a known concentration of benzene is
advised.
Photoionization Detector
The 2020ComboPRO’s PID is shown in Figure 3. 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.
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 time.