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In addition to being an important indoor air pollutant, formaldehyde is a ubiquitous component of both the remote atmosphere and polluted urban atmospheres. With new regulations coming into force, as well as an improved monitoring network, the demand for gaseous formaldehyde standards at low concentration (from 1 nmol/mol to 10 µmol/mol) is increasing. Dynamic standards as well as static gas mixtures of around 1 µmol/mol are currently being developed at a number of National Metrology Institutes (NMIs), which plan to demonstrate their comparability through the BIPM formaldehyde primary gas standards facility.
The BIPM formaldehyde dynamic facility
The BIPM formaldehyde primary facility has the capability to generate an adjustable level of formaldehyde in nitrogen (between 1 - 10 µmol/mol) by using continuous accurate measurements of mass loss from a permeation tube containing paraformaldehyde coupled with a dilution system. Validation studies of this system have demonstrated that with precise control of the temperature and pressure inside the permeation chamber, as well as accurate calibration of the balance, well characterized and well calibrated flow control and flow measurement devices, the relative uncertainty of the permeation rate can be as low as 0.05 %.
One obvious source of bias in the system is the presence of impurities. Great efforts have been made to work with pure gases, for which purity is assessed using a Fourier Transformed InfraRed (FTIR) spectrometer together with a selected suite of software which is used to perform quantification of impurities. Finally, highly repeatable measurements are obtained using a Cavity Ring Down Spectroscopy (CRDS) analyser, which is used as a comparator between gas concentrations from the permeation facility and the certified gas cylinders.
The BIPM facility for the dynamic generation of formaldehyde in nitrogen standard mixtures.
Towards the international comparison CCQM-K90
The BIPM is currently performing a series of validation studies to demonstrate the suitability of its facility as well as transfer standards which will be used in the proposed international comparison CCQM-K90, which will start in 2013. All stages of this process are regularly discussed during the bi-annual meetings of the Gas Analysis Working Group of the CCQM.
Milestones in the preparation of the international comparison CCQM-K90
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