Abstract

A theoretical model has been developed for the comparison of the performance of a detector-noise-limited, flame infrared emission (FIRE) spectrometer with that of a room-temperature, nondispersive, infrared absorption (NDIR) spectrophotometer. The ratio of the signal-to-noise ratio (SNR) in emission to that in absorption is found to depend on the product of five terms: (1) the ratio of the noise in absorption to that in emission; (2) the ratio of the solid angles in emission and absorption; (3) the ratio of the pathlengths in emission and absorption; (4) the ratio of the number densities in emission and absorption; and (5) the ratio of the Boltzmann factor for the flame to the Bose-Einstein factor for the blackbody source. Implications for gas chromatographic detection are considered. Under conditions in which chemical conversion of the sample into an infrared-active species is not required and both measurements employ an uncooled PbSe detector, a sample cell volume of 30 μL, and a filter bandwidth of 0.30 μm at a wavelength 4.35 μm (the antisymmetric stretching vibration of CO₂), the major factor which determines the ratio of the two SNRs will be the ratio of the solid angle × pathlength product, which has a constant value of 100, regardless of the sample cell pathlength chosen. If a hydrogen/air flame is used in emission (2272 K) and a Nernst glower is used in absorption (1800 K), the FIRE spectrometer is predicted to produce an SNR that is at least 19 times better than that produced by the corresponding NDIR absorption photometer, regardless of the pathlength of the absorption cell.

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