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The first traceable spectroscopic standard for Hydrogen Chloride worldwide has been developed and metrologically validated at PTB
Hydrogen Chloride (HCl) is a corrosive, acidic gas which can be released in a vast range of concentrations as an airborne molecular contaminant from the generation of biomethane, the burning of materials, such as coal or biomass, or from processes in microelectronics manufacturing. HCl is harmful to human health, animals, or crops, but also deteriorates e.g., the product quality of semiconductors. Because of this, levels of HCI are carefully monitored in semiconductor clean rooms, chimney stacks and any other areas from which it could be released.
Pollution monitors must be calibrated with metrologically validated reference gases to demonstrate the SI-traceability of the measurements, but this has been problematic for HCl. Hydrogen Chloride is a highly reactive gas and can stick to gas vessel walls, altering the concentration and drastically limiting the range of static standards. The few available standards are also only available as HCl-N2 mixtures - leading to systematic calibration errors for instruments measuring HCl in gas matrices other than nitrogen.
New spectrometer using optical gas standards
Building on the work of the EMRP project Metrology for airborne molecular contamination in manufacturing environments (IND63, MetAMC) during the EMPIR project Metrology for Airborne Molecular Contaminants II (17IND09, MetAMCII) PTB, the National Measurement Institute (NMI) of Germany, developed a new instrument to detect HCl amount fractions, which is directly traceable to the International System of Units, the SI.
This ‘optical gas standard’ spectrometer (OGS) at PTB is based on direct tuneable diode laser absorption spectroscopy (dTDLAS). Using a 1st-principles data evaluation approach the dTDLAS instrument operates without the need for calibration and instead relies on SI-traceable HCl spectral data and on traceable gas pressure and temperature measurements to detect HCl in the range 1 nmol/mol to 10 μmol/mol. Moreover, the HCl OGS instrument can also be used for field calibration or direct measurements of HCl. In the above projects PTB expanded the approach by fast multiplexing of dTDLAS with wavelength modulation spectroscopy (WMS), which allows a higher sensitivity.
As well as the optical gas standard spectrometer, the project also upgraded or developed other instrumentation, including:
- Project partner GASERA Ltd developed a prototype HCl photoacoustic trace gas analyser sensor using a 3375 nm Interband Cascade Laser as the light source.
- VSL, the NMI of the Netherlands, upgraded its OPO-based commercial cavity ring-down spectrometer system used to determine HCl at trace levels as observed in clean rooms.
- A noise immune cavity-enhanced optical heterodyne molecular spectroscopy instrument developed in the EMRP project was adapted by the National Physical Laboratory (NPL), the NMI of the UK, to target both HCl detection (at 1742 nm) and water vapour (at 1854 nm) within the same instrument.
The project has published a report on the potentials and capabilities of the new spectroscopic systems developed. More information on the HCl-OGS can be found on PTB’s website.
Knowledge gained in the project has also allowed PTB to participate in the key comparison CCQM K175 which aims to ensure the equivalency in the measurement capability of NMI’s to measure 30 µmol mol-1 HCl in nitrogen. This is necessary to support industry at this relevant HCl concentration and is expected to be completed soon.
The new instrumentation, like the optical gas standard spectrometer, will help industry detect harmful airborne contaminants, such as HCl, reducing the cost of wastage in the microelectronics industry, and help support monitoring and mitigation steps of this toxic gas in the environment.
Geoffrey Barwood (NPL) who coordinated this successful project said about the work:
“This is a great example of the ongoing impact of our EMPIR project. PTB have continued to improve traceability in the detection of trace levels of HCl via a traceable spectroscopic instrument allowing participation in the key comparison CCQM K175”.
This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States. The EMRP joint research project was part of EURAMET’s European Metrology Research Programme. The EMRP was jointly funded by the EMRP participating countries within EURAMET and the European Union.
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