Quantitative millimetre wavelength spectrometry at pressures approaching atmospheric II. Determination of oxygen at atmospheric pressure

摘要

A millimetre wavelength (MMW) Fabry-Perot cavity spectrometer described in earlier work has been applied to the measurement of oxygen absorption at 60 GHz and atmospheric pressure in a gas matrix of nitrogen. The spectrometer has also been modified such that the MMW source is stabilised by a sub-harmonic microwave signal transmitted by an infrared carrier on a single model telecommunications fibre optic. This is a step towards developing an instrument comprising minimal electronic components that can perform MMW spectrometry remotely. Oxygen determinations wee achieved by monitoring the change in the quality factor (Q) of a resonant Fabry-Perot cavity due to the presence of an absoring sample. The MMW absorption of the sample was determined by incrementing the frequency modulation (FM) deviation of the source frequency scanning the cavity resonance profile. The response curve of absorption signal versus fraction oxygen in nitrogen was found to be linear throughout the working range of 1-100% O_2(v/v) in N_2 with a slope of (1.407 (+-) 0.007) X 10~(-4) m~(-1) (% O_2)~(-1). The detection limit (3 X standard deviation of the background) was found to be approx 0.8% (v/v). The MMW technique employed is advantageous since, unlike common MMW techniques, there is no vacuum requirement. Application of this method, to the monitoring of oxygen in gas mixtures of practical importance, is proposed. Values of the oxygen spectral absorption coefficients of lines between 55 and 60 GHz were measured at reduced pressure and found to be within (+-) 2% of previous literature values. A pressure correction coefficient for O_2 absorption of 60 GHz in the 45-121 kPa range was obtained and found to be (1.354 (+-) 0.041) X 10~(-4) m~(-1) kPa~(-1).