Page:NIOSH Manual of Analytical Methods - 3800.pdf/19

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 19 of 47 APPENDIX C. General Description of FTIR Spectrometry.

C1. Figure C1. FTIR Spectrometer Components and Beam Path

Figure C1 illustrates the basic FTIR spectrometer configuration required for gas phase analyses. The infrared radiation emitted by the infrared (IR) source contains energy at all wavelengths between 2.0 and 20 microns; this is the portion of the electromagnetic spectrum usually referred to as the "mid-infrared" (MIR). In the units typically employed in FTIR spectroscopy (wavenumbers, or cm$-1$), this is the wavenumber range 5000 to 500 cm$-1$. The IR radiation passes through an interferometer, where the motion of an optical element-usually a mirror-optically modulates the infrared beam. The modulated IR beam then enters an absorption cell through a window (typically made of KBr or ZnSe) and interacts with the gases of interest. The physical length over which this interaction takes place is the "absorption pathlength." In "multi-pass" (or "White") absorption cells, mirrors within the cell direct the IR beam through the sample gas many times; in such cells, the absorption path length can be from 4 to 50 (or more) times the cell's physical length. (A larger absorption pathlength generally leads to greater sensitivity.) The IR beam then exits the sample cell via a second window and is re-focused onto an IR detector. Because this extractive technique requires the transport of gas samples through the FTIR absorption cell, the design and integrity of the sampling system is of great importance. It is also important that the sampling system allow the operator to perform all the necessary calibration and sampling procedures without compromising the speed and flexibility of the analytical system. Figure C2 illustrates a sampling system con figuration that meets these requirements, though other configurations are possible. A computer controls the actions of the interferometer and records the voltage output of the IR detector throughout the motion of the interferometer's optical element. Ideally, the detector voltage is proportional to the total power in the IR beam. The computer must accurately record the detector voltage as a function of the position of the moving element in the interferometer, so a secondary, laser-based optical system is usually used to measure the moving element's position very precisely. In most circumstances, the motion of the mirror or other optical element is repeated many times, and the resulting individual "scans" are NIOSH Manual of Analytical Methods, Fourth Edition