Photoionization Detectors

Another different kind of detector for GC is the photoionization detector which utilizes the properties of chemiluminescence spectroscopy. Photoionization detector (PID) is a portable vapor and gas detector that has selective determination of aromatic hydrocarbons, organo-heteroatom, inorganice species and other organic compounds. PID comprise of an ultrviolet lamp to emit photons that are absorbed by the compounds in an ionization chamber exiting from a GC column. Small fraction of the analyte molecules are actually ionized, nondestructive, allowing confirmation analytical results through other detectors. In addition, PIDs are available in portable hand-held models and in a number of lamp configurations. Results are almost immediate. PID is used commonly to detect VOCs in soil, sediment, air and water, which is often used to detect contaminants in ambient air and soil. The disavantage of PID is unable to detect certain hydrocarbon that has low molecular weight, such as methane and ethane.

Instrumentation

Figure 1. Schematic of a photoionization detector

Limitations

1. Not suitable for detecting semi-volatile compounds
2. Only indicates if volatile organic compounds are presents.
3. High concentration so methane are required for higher performance.
4. Frequent calibration are required.
5. Units of parts per million range
6. Enviromental distraction, especially water vapor.
7. Strong electrical fieldsRapid variation in temperature at the detector and naturally occurring compounds may affect instrumental signal.