Page:NIOSH Manual of Analytical Methods - 3800.pdf/20

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 20 of 47 "co-added" (averaged) to reduce the system's residual noise area (RSA). A plot of the resulting IR power-vs.-position signal, referred to as an interferogram, is shown in Figure C3. This interferogram is the co-added result of 64 scans with only nitrogen gas (and some low level of water vapor) present in the absorption cell. Nitrogen is one of the few compounds which does not interact with infrared radiation, so this interferogram very closely represents the fundamental FTIR system response in the absence of a sample. Note that the signal is relatively large near the beginning of the interferogram, where the “zero phase difference (ZPD) burst” is located. The ZPD is often used to rapidly obtain an estimate of the IR signal strength during alignment of the optical system.

Figure C2. Sampling Components and Sample Paths

Figure C3. Sampling Components and Sample Paths NIOSH Manual of Analytical Methods, Fourth Edition