For example, EPA test methods 418.1 and 413.2 measure the C-H absorption for either petroleum or total hydrocarbons. Physical Principles Molecular bonds vibrate at various frequencies depending on the elements and the type of bonds. For any given bond, there are several specific frequencies at which it can vibrate. One way to cause the frequency of a molecular vibration to increase is to excite the bond by having it absorb light energy. Sample Preparation Samples for FTIR can be prepared in a number of ways.
Salt is transparent to infrared light.
The drop forms a thin film between the plates. This powder is then compressed into a thin pellet which can be analyzed. Alternatively, solid samples can be dissolved in a solvent such as methylene chloride, and the solution placed onto a single salt plate. The solvent is then evaporated off, leaving a thin film of the original material on the plate. This is called a cast film, and is frequently used for polymer identification.