Atomic spectroscopy as a potential primary method

Project Description

Project objectives:

• to develop a methodology whereby optical spectroscopy (ultraviolet and infrared) may be used for absolute calibrations of amount of substance measurements, particularly for reactive gases difficult to calibrate by other means.
• to use accurate gravimetrically-preparde stable gas mixtures to validate the spectroscopic method and to determine its scope and accuracy (candidate gases CO and CO2).
• to compare, by means of trnsfer standards, the accuracy with which participating laboratories can calibrate reactive gas mixtures (candidate gases HCl, HF, NH3 and NO2)

Project rationale :

It is difficult to prepare reacitve gas mixtures gravimetrrically with known stable concentrations in gas cylinders. For these mixtures, therefore, traceability to the kilogram may not be practical. However, the frequency dependent spectral absorption of any molecule in a gas matrix in any given volume is given by the product of its molecular linestrength, the number density of the molecule in the matrix, and the optical path through the volume. The molecular linestrength is an intrinsic and unique property of the given molecule and may be measured by carefully designed experiments. The amount of absorption of the molecule over a known optical pathlength can also be measured. This therefore enables the number density (i.e. the concentration of the gas in the mixture) to be calculated. The potential of this spectroscopic method will be investigated by means of stable gas mixtures where the traceability of their concentrations has been established by accepted ( usually gravimetric) means. The additional difficulties associated with reactive gas mixtures will also be investigated through cooperative research and through intercomparisons using stable transfer standards.

Final Report 2002-01-31

During the course of the project one DFM staff member has visited NPL. DFM has performed some work in the near infrared spectral region, focusing on the molecule CO [1] and on general aspects of quantitative spectroscopy [2].
In September 2000 NPL decided to discontinue their efforts, while DFM is continuing. One expressions of interest from Lithuania has been assessed, but it is judged that there is not sufficient overlap of the specificresearch activities that synergy can be hoped for. Consequently, the project is hereby closed.

[1] J. Henningsen, H.Simonsen, T.Møgelberg, E.Trudsø: The 0→3 Overtone Band of CO: Precise
Linestrengths and Broadening Parameters, J. Mol.Spectrosc. 193, 354-362 (1999)
[2] J. Henningsen, H. Simonsen: Quantitative wavelength modulation spectroscopy without certified gas
mixtures, Appl. Phys. B 70, 627-633 (2000)