Jf. Alder et Jg. Baker, QUANTITATIVE MILLIMETER WAVELENGTH SPECTROMETRY - PART IV - RESPONSE CURVES FOR OXYGEN IN CARBON-DIOXIDE AND NITROGEN AT 60 GHZ, Analytica chimica acta, 367(1-3), 1998, pp. 245-253
A millimetre wavelength (MMW) Fabry-Perot cavity spectrometer describe
d in earlier work has been applied to the measurement of oxygen absorp
tion at the 60 306.05 GHz transition, in gas matrices of air, nitrogen
acid carbon dioxide. The cavity was enclosed in a magnetic field impe
rmeable Mumetal case to minimise the splitting of the spectral line by
the magnetic field of the earth. Two depths of frequency modulation (
FM), 240 and 600 kHz frequency deviation, at a rate of 1 kHz were appl
ied to the microwave source. At the working pressure of 100 mTorr, the
240 kHz FM deviation gave the most intense absorption signal, yieldin
g a limit of detection (signal: background = 3) of around 1% pressure
fraction of oxygen in either of the matrices. There was no matrix effe
ct observed in passing from nitrogen to carbon dioxide as diluent, The
response curves of absorption signal vs. fraction of oxygen in the mi
xtures was essentially linear at low concentrations, curving towards t
he abscissa at higher concentrations. This behaviour is not completely
understood, but fitting the curves to response functions that reflect
ed the characteristics of the absorption process, the cavity and elect
ronic transfer functions and the effect of line broadening by matrix m
olecules proved to be a reasonable approach to modelling the instrumen
t response. The pressure broadening coefficient of the oxygen line was
measured in the range up to 1000 mTorr and found to be about 2.4 MHz
mTorr(-1), somewhat larger than the 1.89 MHz mTorr(-1) reported in the
literature for oxygen in the range 1-20 mTorr. An explanation for the
discrepancy may involve some modulation broadening of the spectral li
ne and possibly incomplete screening of the earth's magnetic field in
the cavity. The MMW technique showed itself to be a useful approach to
the monitoring of oxygen in gas mixtures of practical importance. (C)
1998 Elsevier Science B.V. All rights reserved.