A Fourier transform infrared spectrometer was built by MIDAC Corporation specifically to identify and quantify impurities in ultra-high purity nitrogen, helium, argon and instrument air. One of the difficulties with optical measurements is that a purge must be maintained on the optical enclosure, which due to its large surface area and material composition tends to off-gas, releasing water and carbon dioxide as well as other contaminants.
The problem of the instrument purge was tackled using an ultra-high vacuum. The vacuum system is integral to the instrument, with a turbo molecular pump fixed beneath the optical enclosure. These, together with a number of other refinements, have resulted in more accurate and precise results than were previously possible, driving detection limits lower.
The detection limits were estimated using the procedure defined in the ASTM Standard Test Method for Determination of Gaseous Compounds by Extractive Direct Interface Fourier Transform Infrared (FTIR) Spectroscopy Appendix 2.