Optimizing the Analysis of Volatile Organic Compounds

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PID Operation
The photoionization detector, PID, is a selective, non-destructive detector most commonly
used for characterizing aromatic compounds (Figure 19). It has excellent sensitivity (low pg
detection) and provides a linear dynamic range of 3 orders of magnitude.
In a PID, a krypton lamp emits photons in the form of light energy at a wavelength of
116.6nm and 123.6nm. The photons excite compounds having an ionization potential of less
than 10.2eV. Charged particles produced in this manner pass through a reaction cell with an
electrical potential of 50 to 200 volts, producing an electrical charge that is measured as a
signal. Sensitivity is a function of the chemical structure of the analyte, including the number
of carbon atoms, the nature and position of functional groups, and the position of double
or conjugated double bonds. 10 For suitable analytes, a PID is 10-times more sensitive than an
FID. Compounds such as benzene (9.3eV ionization potential) have ionization efficiencies of
less than 0.1%, allowing the majority of the sample to pass through the detector unaffected.
Even with this minute portion of the sample ionized, sensitivity for aromatic compounds is
measured at the pg level.
PID Maintenance and Troubleshooting: It is very common for silicone from column bleed
to collect on the PID window and reduce transmission from the lamp. Reduced sensitivity
for all components is evidence of this condition. We recommend cleaning the window on a
regular basis, using a mild abrasive material such as iron oxide. Alumina powder is more
abrasive than iron oxide and can scratch the lens. This will reduce sensitivity. Create a slurry
with the powder, scrub the window, then rinse with acetone or methanol. Avoid touching the
clean window. If sensitivity is not restored, replace the lamp. Dead volume within a PID will
produce broad peaks, and might cause peaks to tail. PIDs designed for packed column systems
have a cell volume intended for high flow rates. When using these detectors with capillary
columns, 20-30mL/min. of make-up gas is required to minimize dead volume and, in
turn, reduce peak tailing. Newer PID designs have smaller cell volumes that are compatible
with capillary columns.
Flush
Gas
Collector Electrode
Capillary
Column
Capillary
Column
Adapter
Make Up
Gas
Courtesy of
Thermo Finnigan
Exhaust
Capillary
Column
Figure 19.
Diagram of a typical PID.
Lamp
Polarizer Electrode
Quartz Reaction Cell
Photoionization Lamps
Description eV Rating Base qty. cat. #
PID Lamp, Model 103 C 10.2 1.375" ea. 20676
PID Lamp, Model 108 10.0/10.6 0.781" ea. 20675
PID Lamp, Model 108-βTEX 10.0/10.6 0.781" ea. 23020
PID Lamp, Model 108 9.6 0.781" ea. 23021
PID Lamp, Model 107 8.4 0.781" ea. 23022
PID Lamp, Model 109 11.8 0.781" ea. 23023
PID Lamp Polishing Kit kit 20674
10. Jessie Crockett Butler, “Tandem Detector Technology for Gas Chromatography.” Environmental
Analysis Technical Report No. 9110, p3. Thermo Finnigan, GC and GC/MS Division, Austin, TX.