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ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 22 of 47 definitions—see Reference 15, Chapter 1, Section IV) and nominal spectral resolution parameters, the MIL is a parameter which is readily measured to the accuracy required for practical applications of FTIR spectrometry. It can be measured using the water absorption bands present in low-pressure works pace air samples (see Appendix B, Section 3).

C4.

The mathematical result of the FFT (applied to an apodized IR interferogram) is a called a single beam spectrum. Single beam spectra represent the infrared power transmitted through the FTIR spectrometer as a function of the infrared "wavenumber" $$w$$, which is usually expressed in the units of reciprocal centimeters (cm$-1$). The wavenumber is actually a measure of the frequency, rather than the wavelength, of the infrared radiation. In a vacuum, the wavelength and frequency are related through the equation $$\textstyle{\nu = s / \lambda}$$, where λ is the wavelength (cm), ν is the frequency (sec$-1$, or Hz), and s(cm/sec) is the speed of light, equal to 2.99792954×10$10$ in a vacuum. In these units, the wavenumber in cm$-1$ is given by the equation $$\textstyle{w = 1 / \lambda = \nu / s}$$. Figure C4 presents the single beam spectra for two samples consisting mainly of nitrogen gas (≥99%) but with different concentrations (≤ 1%) of water vapor. The vertical scales of the two spectra are nearly identical, but they have been offset for clarity. The detected infrared power in certain wavenumber regions is clearly lower in the high moisture sample, indicating both the qualitative nature and strength of water's absorption of infrared radiation.

C5.

Combinations of pairs of single beam, such as the pair of spectra S and B shown in Figure C4, are referred to as double beam spectra; they provide the quantitative basis for FTIR spectroscopy. One type of double

Figure C4. Single Beam Spectra of N2 at Different Moisture Levels. NIOSH Manual of Analytical Methods, Fourth Edition