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ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 24 of 47 C6.

Figure C6. Double Beam Absorbance Spectrum of Water

The absorbance spectrum of a sample gas is determined from single beam spectra of an infrared-transparent gas (the background spectrum) and the sample gas (see Section 5). The fundamental relationship connecting the absorbance spectrum of a sample gas to the concentrations of the compounds comprising the sample is referred to as Beer’s Law. This relationship can be written as

$$A_1=\sum_{j=1}^M{L_S}{a_{ij}}{c_j}$$ (Equation C1) where

The procedures described in this method are all related to the parameters which comprise Beer’s Law. The following list describes these procedures and relationships in a general context:

1) Record a reference absorbance spectrum for each interesting compound of a nitrogen-diluted sample of the pure compound, at a known concentration and pathlength, then divide the absorbance at each point in that spectrum by the product of the concentration and pathlength. This process yields the absorptivity spectrum ($$a_{ij}$$), or reference spectrum, for each compound. NIOSH Manual of Analytical Methods, Fourth Edition