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 = -log10(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).
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/NIOSH_NMAM_3800_-_23.png/600px-NIOSH_NMAM_3800_-_23.png)
Figure C5. Double Beam Transmittance Spectrum of Water
NIOSH Manual of Analytical Methods, Fourth Edition