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suffer from the lack of specificity, due to a range of potential interferences from other gases. In addition temperature and humidity can affect the results. Electrochemical detectors are another type of detectors that are not specific, like MOX and PID but can also suffer from a range of interferences. Gas chromatography, because of the intrinsic capability to separate each single component, is the best solution to selectively quantify BTEX. In fact, this approach allows for detection and quantification of BTEX compounds in a sub- ppb concentration.
Figure 2: PID Signal for BTEX analyses.
To ensure the optimal performance, a GC requires the instrument to be operated by a trained and qualified technician. Most commercial GC instrumentation requires a reasonable amount of fixed footprint in a laboratory environment. It also requires a carrier gas, which will be derived from a generator or a gas cylinder, as a consequence traditional GC’s are not the best solution for large network monitoring area.
Figure 3: Pre-concentrator operation.
US EPA released Methods 325A and 325B especially designed for the refinery sector rules. These methods describe the use of passive absorbent tubes. In Europe the monitoring of benzene is also mandatory according to European Air Quality Directive - EN14662-3:2015.
The EN14662-3:2015 defines the standard method for the measurement of benzene concentrations. Part 3 of this legislation, in particular describes the use of automated pumped sampling with in situ gas chromatography. This standard specifies the performance characteristics and sets the relevant minimum criteria required to select an appropriate automated gas chromatograph (GC) by means of type approval tests. It also includes the evaluation of the suitability of an analyser for use in a specific fixed site so as to meet the data quality requirements as specified in
Annex I of Directive 2008/50/EC [1] and requirements during sampling, calibration and quality assurance for use. The method is applicable to the determination of the mass concentration of benzene present in ambient air in the range up to 50 µg/ m3 benzene. This concentration range represents the certification range for the type approval test.
At the moment, there are several technologies and sensors available for the detection of BTEX: each of them has advantages and disadvantages. PID (Photoionisation detection) is very sensitive to Benzene but not selective, therefore Benzene, Toluene and Xylenes cannot be distinguished – in fact the PID lamp usually responds to molecules with carbon double bonds and aromatics. Metal oxide sensors (MOX), are similarly not specific to individual organic compounds, and
Equipment developments since the early 1990’s have provided on-site analytical instruments fine-tuned to the user’s specific requirements (e.g. Pollution Analytical Equipment (PAE)). The PyxisGC BTEX by PAE for BTEX monitoring provides a miniaturised GC on chip that can work as a stand-alone gas chromatograph for real time environmental monitoring of aromatic hydrocarbons and other volatile compounds (VOCs). There is also a High-sensitivity PID - Photo Ionisation Detector (10.6 eV). The small size makes it more applicable than larger fixed instrumentation for use in remote location (i.e. industrial area) or urban installations. The system integrates miniaturisation with ease of use combining Micro-Electro-Mechanical-System (MEMS) microfluidics for selective pre-concentration and GC separation using a micro column on a chip.
The miniaturised GC, specifically the micro column on a chip, allows for substantially reduced gas consumption, and it also allows for continuous automonous operation over long periods of time. When the miniaturised GC-PID is switched on, the module operates in sampling mode, i.e. the sampling pump provides a constant flow of sample through the pre-concentrator, while at the same time the GC pump cleans the separation column using ambient air as carrier gas.
After approximately 10 minutes, the preconcentration column is connected to the GC pump and heated to 100°C: the
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