Photoionization Detector (PID)
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.
See a Flash-based
If the energy of an incoming photon is high enough (and the molecule is
quantum mechanically "allowed" to absorb the photon) photo-excitation can
occur to such an extent that an electron is completely removed from its
molecular orbital, i.e. ionization.
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.
Selective detection using a PID
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. Ta da! selective detection of one compound
in the presence of another.
Since only a small (very reproducible but basically unknown) fraction of
the analyte molecules are actually ionized in the PID chamber, this is
considered to be a nondestructive GC detector. Therefore, the exhaust port
of the PID can be connected to another detector in series with the
PID. In this way data from two different detectors can be taken simultaneously,
and selective detection of PID responsive compounds augmented by response
from, say, an FID or ECD. The major
challenge here is to make the design of the ionization chamber and the
downstream connections to the second detector as low volume as possible
(read small diameter) so that peaks that have been separated by the GC
column do not broaden out before detection.
Schematic of a gas chromatographic photoionization detector
These notes were
written by Dr. Thomas G. Chasteen; Department
of Chemistry, Sam Houston State University, Huntsville, Texas 77341.
© 1995, 2007, 2009.
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