5 research outputs found
The interference of tetrachloromethane in the measurement of benzene in the air by a gas chromatography–photoionisation detector (GC-PID)
The European Union requires that benzene in the air is continuously measured due
to its toxicity and widespread presence in the population nuclei, mainly
motivated by vehicle emissions. The reference measuring technique is gas
chromatography (GC). Automatic chromatographs used in monitoring stations
must verify the operating conditions established in Standard EN 14662 part 3,
which includes a type approval section with a number of tests that analysers
must pass. Among these tests, the potential interference of a number of
compounds is evaluated. The 2005 version of the mentioned standard requires
the evaluation of the potential interference of tetrachloromethane (TCM). The
2015 version eliminates TCM as a potential interferent. Although most
consumer uses of TCM have been banned, recent studies have measured
significant concentrations of TCM in the air. In this paper, the potential
interference of TCM in benzene measurements obtained with gas chromatography
coupled to a photoionisation detector (GC-PID) has been investigated. Our study
shows that the simultaneous presence of benzene and TCM causes a significant
decrease in benzene readings. For TCM concentrations of 0.7 µg m−3
(typical of urban areas) and 4.5 µg m−3 (detected in the vicinity
of landfills), the relative errors in benzene measurements were 34 % and 70 %, respectively, which are far too high compared to the maximum overall
uncertainty allowed for benzene measurements (25 %). Possible mechanisms
to qualitatively and quantitatively explain the behaviour of the PID when
measuring benzene with and without TCM have been proposed.</p
Influence of sample temperature and environmental humidity on measurements of benzene in ambient air by transportable GC-PID
Calibration of in situ analysers of air pollutants is usually done
with dry standards. In this paper, the influence of sample temperature and
environmental humidity on benzene measurements by gas chromatography coupled
with a photoionisation detector (GC-PID) is studied. Two reference gas
mixtures (40 and 5 µg m−3 nominal concentration
benzene in air) were subjected to two temperature cycles (20/5/20 °C and
20/35/20 °C) and measured with two identical GC-PIDs. The change in sample temperature did not produce any significant change in readings. Regarding ambient humidity, the chromatographs were calibrated for benzene with dry gases and subjected to measure reference standards with humidity (20 and 80 % at 20 °C). When measuring a concentration of 0.5 µg m−3
benzene in air, the levels of humidity tested did not produce any significant
interference in measurements taken with any of the analysers. However, when
measuring a concentration of 40 µg m−3, biases in measurements of
18 and 21 % for each respective analyser were obtained when the
relative humidity of the sample was 80 % at 20 °C. Further
tests were carried out to study the nature of this interference. Results show
that humidity interference depends on both the amount fractions of water
vapour and benzene. If benzene concentrations in an area are close to its
annual limit value (5 µg m−3), biases of 2.2 % can be expected
when the absolute humidity is 8.6 g cm−3 – corresponding to a relative humidity of 50 % at 20 °C. This can be accounted for in
the uncertainty budget of measurements with no need for corrections. If
benzene concentrations are above the annual limit value, biases become
higher. Thus, in these cases, actions should be taken to reduce the humidity
interference, as an underestimation of benzene concentrations may cause a
mismanagement of air quality in these situations
Abstracts of the 6th FECS Conference 1998 Lectures
International audienc