Page:NIOSH Manual of Analytical Methods - 3800.pdf/27

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 27 of 47 employs such corrections, he or she should also calculate the residual (Equation C6) using the reference spectrum which minimizes the squared error (Equation C5). FTIR analysts and manufacturers have devised other correction procedures to improve the sample analysis accuracy in such circumstances, and these are included in some commercially available software packages.



Note: Intense infrared radiation, such as that produced by some lasers, can induce non-equilibrium populations of a molecule’s rotation-vibration energy states. However, the thermal infrared sources used in commercially available FTIR spectrometers are much weaker than such lasers. In gases at atmospheric pressure, the commercially available thermal sources induce transition rates between quantum energy states which are small compared to those of the competing collisional relaxation processes, and cannot induce non-equilibrium energy state populations. In addition, the induced transition rates related to absorption and emission at mid-infrared frequencies are much larger than the corresponding spontaneous (natural) emission rate. As a result, all accurate FTIR measurements at mid-infrared frequencies obey Beer’s Law to within the uncertainty related only to the S/N ratio of the measured spectra. NIOSH Manual of Analytical Methods, Fourth Edition