1- freon (1,1,2-trichloro-1,2,2-trifluoroethane ) contains no c-h bonds, and thus does not absorb ir radiation in the aforementioned 3100 to 2700 cm1 where hc, oil and grease absorb. 2- compared to standard sample of air with known contaminants. 3- reflectance mode, each reflected band is characteristic for one of the analytes, the reflectance value is proportional to the conc.
Answers
Infrared radiation is absorbed by a molecule when the frequency of the radiation matches that of one of the vibrational modes of the molecule. It is also necessary that the molecule have a vibrational mode for which there is a change in dipole moment. If the energy of a photon does not meet both these criteria, then it will be transmitted rather than absorbed.
A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with N number of atoms, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example H2O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.
Simple diatomic molecules have only one bond and only one vibrational band. If the molecule is symmetrical, e.g. N2, the band is not observed in the IR spectrum.
So N2 (or O2) only has a single vibrational mode (3×2−5=1), no permanent dipole, and can only have a weak induced dipole, so it's absorption in the IR is very small. CO also has only a single vibrational mode but a permanent dipole moment and therefore has the ability to absorb in the IR spectrum.
I've tried to balance "keeping it simple" with giving a meaningful explanation. Don't hesitate to ask for clarifications in the comments.