Environmental Engineering Reference
In-Depth Information
of VOCs can be detected and measured. Major disadvantages are the rela-
tively small portion (1 ml) of the 1 to 10 L sample that is analyzed, and
potential contamination of the sample from pumps, tubes, and fittings used
with the technique.
Both sorbent tubes and samples collected in evacuated canisters are
analyzed using gas chromatography, often in conjunction with a mass spec-
trometer. The mass spectrometer allows for relatively reliable identification
of individual VOC species. Because of low concentrations (ppbv,
), it
is desirable to thermally desorb VOCs collected on sorbent tubes before
analysis. Thermal desorption has several advantages over solvent (CS
µ
g/m
3
) des-
orption. These include fewer analytical operations, shorter operating time,
recovery of the entire undiluted sample, and increased sensitivity to low
VOC levels. In the evacuated canister method, a collected sample is injected
directly into a gas chromatograph for analysis.
Total volatile organic compounds are usually quantified by using a gas
chromatograph with a flame ionization detector (GC-FID), with toluene or
hexane as a standard for determining concentrations. Concentrations of
TVOCs, using a mixture of toluene, hexane, or cyclohexane and helium as
a calibration gas, can be calculated based on measured FID response and the
following equations:
2
f = c
/V
/C
(9.2)
s
s
s
where f
= response factor (counts/
µ
g)
c
= FID count for a known volume of standard
s
V
= volume of standard (m
)
3
s
C
= concentration of the cyclohexane/helium standard (
µ
g/m
)
3
s
The TVOC concentration is then calculated as
T = c/f/V
(9.3)
where c = FID count for air sample
V = volume of air sampled (m
)
3
T = TVOC concentration, (
µ
g/m
)
3
The determination of TVOC concentration takes less effort than identi-
fying and quantifying individual VOCs, with a shorter turnaround time for
air sampling results. Total VOC concentration can also be compared to guide-
line values that have been proposed for use in assessing whether elevated
VOC levels may be responsible for health complaints in a building.
The determination of TVOC concentrations alone may be less informa-
tive than a full quantitative analysis of individual VOCs. Such a full analysis
is useful as a “fingerprinting” tool in identifying potential VOC sources such
as photocopiers, etc.
