Page:NIOSH Manual of Analytical Methods - 3800.pdf/23

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 23 of 47 beam spectrum is the transmittance. The percent transmittance of a sample gas possessing the single-beam spectrum S—with respect to the background single-beam spectrum B—is defined as T(%)=100 * S/B; a transmittance value is defined for each wavenumber value of the two spectra. If the background spectrum B closely represents the response of the FTIR system to a transparent sample, then the percent transmittance T closely approximates the percentage of the infrared radiation transmitted by the sample (represented by the spectrum S). Because water is the only absorbing compound present in the single-beam spectrum B, the spectrum T (shown in Figure C5) closely approximates the percent transmittance spectrum of water.

The same pair of spectra define the (double beam) absorbance A of the sample through the equation A = -log$10$(S/B). The absorbance spectrum of water, as approximated by the two single-beam spectra S and B, is shown in Figure C6. The absorbance is the desired quantity because it appears in the general linear absorption model known as Beer's Law (see below).

Figure C5. Double Beam Transmittance Spectrum of Water NIOSH Manual of Analytical Methods, Fourth Edition