non-specific detection of hydrocarbons
and may be used to measure these at
concentrations in the ppm range. The
photoionization detector (PID) can be
used to detect various airborne analytes
that can be ionized by ultraviolet (UV)
energy, typically at 10. 6 eV. Oxygen,
nitrogen, water, methane and carbon
dioxide are not ionized by relatively
low-energy UV photons and are not detectable using a PID. Both FIDs and PIDs
create ions, and the resulting ion current
produces a signal with intensity proportional to the analyte concentration.
Methane exists at low ppm concentrations in ambient air, and thus the stand-alone FID isn’t useful for detection of
hydrocarbon analytes below these levels.
In 1974, after researchers reported a
link between exposure to vinyl chloride
monomer (VCM) and cases of angiosarcoma of the liver,
4 OSHA lowered the
relevant occupational exposure limit
from 500 ppm to 1 ppm as an 8-hour
time-weighted average. The ability to
quantify airborne VCM below the quantitative range of the FID has been credited
with the rise of the PID as a stand-alone
detector.
The identities of target analytes must
be known beforehand to use either the
FID or PID quantitatively. A response
factor for a specific analyte is entered for
microprocessor calculation of selected
analyte concentration readout. The broad
selectivity of these detectors presents
problems for airborne mixtures, as individual components are likely to have
varied response factors. However, these
detection tools remain useful for deter-
mining relative exposure magnitude and
temporal variability, even when complex
airborne exposures exist. For example,
datalogging PID or FID instruments can
show concentration spikes over time; the
associated activities can be investigated
and controls applied as appropriate. The
FID is not particularly affected by high
humidity, whereas attenuation of PID
response has been noted when high airborne concentrations of compounds that
are not detected by the PID are present
(for example, water vapor5 or methane6).
Handheld detectors with high selectivity also exist. The flame photometric detector (FPD) is a highly sensitive detector
for compounds containing either sulfur
or phosphorous. When compounds that
contain these elements are burned in a
hydrogen flame, the intensity of light
emissions at a frequency specific for
each of these elements is proportional to
analyte concentration. A handheld FPD
is fielded by some military organizations for sensitive detection of chemical
warfare agent compounds that contain
either or both sulfur and phosphorous
atoms. The FPD will not respond to analytes that do not contain either of these
elements, which greatly reduces false
positive responses.
A drawback to handheld instruments
that require a flame is the need to ensure
intrinsic safety, as well as the need to
have compressed hydrogen and clean air
available to produce the flame.
Man-portable Detection Systems
Person-portable instruments derived
from complex laboratory instruments
populate Level 3 of Figure 1. These
include fourier transform infrared analyzers for solid, liquid and gas phase
analytes; gas chromatographs; and mass
spectrometers. A number of person-portable gas chromatography (GC) instruments have been available since the
1970s and have undergone steady improvements for field portability.
In gas/liquid chromatography, organic
analytes partition between a carrier gas
and a liquid film in a column. Because
different analytes have different vapor
pressures and solubilities in a liquid film
stationary phase, those that partition
more to the stationary phase will spend
more time there as a carrier gas flows
through the column and, thus, will elute
from the column later than analytes that
remain in the gas phase longer. If col-
umn temperature conditions and carrier
gas flow are reproducible, the retention
time provides a means of identifying
the analytes, although it is possible that
a specific retention time may not be
unique to a single analyte.
