Fourier transform infrared spectroscopic measurement of carbon monoxide and nitric oxide in sidestream cigarette smoke in real time using a hollow waveguide gas cell and nonimaging optics
Thompson, Bruce Thomas
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The application of a hollow waveguide (HW) was investigated as a gas cell for analytical infrared analysis. The analysis was the measurement of carbon monoxide (CO) and nitric oxide (NO) in sidestream cigarette smoke. An FT-IR analysis system was setup with a 3m multi-pass gas cell and a 55cm by 2mm i.d. Ag/AgI coated HW in tandem with individual CO and NO gas analyzers. The HW demonstrated response times an order of magnitude less than the larger volume multi-pass gas cell and slightly faster than the single analyte gas analyzer. Furthermore, it has been demonstrated that the HW provides up to approx. 60% greater sensitivity on a per meter optical path basis than the multi-pass gas cell of the analytes investigated due to increased optical efficiency maximizing the light concentration within the gaseous sample volume. Simulations in 3-D showed the sensitivity could theoretically improve by more than an order of magnitude if the IR beam was coupled more efficiently into the waveguide. Both FT-IR configurations gave statistically equivalent results for CO to the independent analyzers. With the HW increased temporal resolution, inter-puff measurements comparable to the gas analyzer were achieved at a lower spectral resolution. The HW optical configuration was modeled for ray tracing in MATLAB. Simulations in 2-D and 3-D were accomplished. The simulations show a major drawback to HW optimization is the coupling of the infrared beam into the waveguide. As demonstrated in a 3-D simulation, approximately 97% of the rays are rejected when an off-axis parabolic mirror with 25.4mm focal length is used to focus the IR beam into the 2mm i.d. waveguide. Repeating the simulation with longer focal length mirrors showed improved in IR coupling into the waveguide from 3% to 85%. Simulations applying a compound parabolic concentrator show comparable performance to the traditional design of two OAP mirrors to collect rays from the HW and focus onto the detector, but in a much smaller configuration. The simulation routines can be used to further improve the design of this and other optical sensing systems and enhanced by incorporating a spectral component to the simulation.