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The reason to use more than one kind of detector for gas chromatography is to achieve selective and/or highly sensitive detection of specific compounds encountered in particular chromatographic analyses. The selective determination of aromatic hydrocarbons or organo-heteroatom species is the job of the photoionization detector (PID). This device uses ultraviolet light as a means of ionizing an analyte exiting from a GC column. The ions produced by this process are collected by electrodes. The current generated is therefore a measure of the analyte concentration.
A Photoionization Reaction
If the amount of ionization is reproducible for a given compound, pressure, and light source then the current collected at the PID's reaction cell electrodes is reproducibly proportional to the amount of that compound entering the cell. The reason why the compounds that are routinely analyzed are either aromatic hydrocarbons or heteroatom containing compounds (like organosulfur or organophosphorus species) is because these species have ionization potentials (IP) that are within reach of commercially available UV lamps. The available lamp energies range from 8.3 to 11.7 ev, that is, lambda max ranging from 150 nm to 106 nm. Although most PIDs have only one lamp, lamps in the PID are exchanged depending on the compound selectivity required in the analysis.
Here is an example of selective PID detection: Benzene's boiling point is 80.1 degrees C and its IP is 9.24 ev. (Check the CRC Handbook 56th ed. page E-74 for IPs of common molecules.) This compound would respond in a PID with a UV lamp of 9.5 ev (commercially available) because this energy is higher than benzene's IP (9.24). Isopropyl alcohol has a similar boiling point (82.5 degrees C) and these two compounds might elute relatively close together in normal temperature programmed gas chromatography, especially if a fast temperature ramp were used. However, since isopropyl alcohol's IP is 10.15 ev this compound would be invisible or show very poor response in that PID, and therefore the detector would respond to one compound but not the other.
Schematic of a gas chromatographic photoionization detector
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