At NPL the grating spectrophotometers used up to the present for determining the Mid-Infrared standards and for the associated measurement services have been kept in use^[1] and well maintained, in spite of the use elsewhere of Fourier transform (FT) spectrometers^[2]. This is because of the large number of categories of systematic ordinate error that can arise with the FT instruments, and the difficulties in distinguishing and quantifying them^{[3, 4]}. There is a long-standing programme of R &  D to assess the problem^[5], to find out how to determine standards with known uncertainties of measurement and to transfer the measurement services to suitable FT spectrometers.

One measurement service based on a Fourier transform instrument has now been validated: the calibration of polystyrene wavenumber standards. This uses a Bomem MB100 spectrometer.

Ongoing research uses a specially modified Perkin-Elmer Spectrum GX spectrophotometer for investigation of new methods of determining the basic standards. This instrument has an alternative Jacquinot Stop located between the interferometer and the sample area to inhibit interreflection errors. It is not ideal for future use in the measurement services, because it cannot cover in a single range the essential spectral range of 4000 cm^-1 to 200 cm^-1 needed for the important thermally related applications. However, to cover the measurement service need there is also a specially modified Bruker Equinox 55 instrument. This is fitted with a special interferometer, which has a special interferometer with a thin CsI beamsplitter that only requires one transmission through it for each of the interfering beams. This can give useful response to below 180 cm^-1.

References

1. F J J Clarke "The absolute calibration of mid-infrared transmittance standards" Anal. Chim. Acta, 380, 127 - 141 (1999)
2. F J J Clarke, "Ordinate Scale Standards for Mid-Infrared Spectrometry", in Handbook of Vibrational Spectroscopy, J.M. Chalmers and P.R. Griffiths (Eds), John Wiley & Sons, Ltd, Volume 1, pp. 891 - 898 (2002).
3. J R Birch and F J J Clarke "Fifty categories of ordinate error in Fourier transform spectroscopy" Spectroscopy Europe 7(4), 16-22, (1995)
4. J R Birch and F J J Clarke "Interreflection errors in Fourier Transform Spectroscopy: a preliminary appraisal" Anal. Chim. Acta, 380, 369 - 378 (1999)
5. J R Birch, C Chunnilall and F J J Clarke "Validation of the ordinate scale in FTIR" Twelfth International Conference on FTS, 191 - 192 (Waseda Univ. Press, Japan, 1999)