Fourier Transform Infrared (FTIR) Spectroscopy bases its functionality on the principle that almost all molecules absorb infrared light. Only the monatomic (He, Ne, Ar, etc) and homopolar diatomic (H2, N2, O2, etc) molecules do not absorb infrared light. Molecules only absorb infrared light at those frequencies where the infrared light affects the dipolar moment of the molecule. In a molecule, the differences of charges in the electronic fields of its atoms produce the dipolar moment of the molecule. Molecules with a dipolar moment allow infrared photons to interact with the molecule causing excitation to higher vibrational states. The homopolar diatomic molecules do not have a dipolar moment since the electronic fields of its atoms are equal. Monatomic molecules do not have a dipolar moment since they only have one atom. Therefore, homopolar diatomic molecules and monatomic do not absorb infrared lighti. But all other molecules do!
Most FTIR spectroscopy uses a Michelson interferometer to spread a sample with the infrared light spectrum and measure the intensity of the infrared light spectrum not absorbed by the sample. FTIR spectroscopy is a multiplexing technique, where all optical frequencies from the
source are observed simultaneously over a period of time known as scan time.
The spectrometer measures the intensity of a specially-encoded infrared beam after it has
passed through a sample. The resulting signal, which is a time domain digital signal, is called an Interferogram (Figure 1) and contains intensity information about all frequencies present in the infrared beam. This information can be extracted by switching this signal from a time domain
digital signal to a frequency domain digital signal, which is accomplished by applying a Fourier transform over the interferogram and producing what is called a Single Beam Spectrum. Another characteristic of this signal is that it is a statistically stationary signal, so the higher the number of scans of which this signal is composed, the better the estimate that can be extracted from the data.
As mentioned earlier, almost all molecules absorb infrared light, and each type of molecule only absorbs infrared light at certain frequencies. This property provides a unique characteristic for each molecule. It provides a way to identify the molecule type (Qualitative analysis) and the amount or quantity of this molecule in the sample (Quantitative analysis). Since each type of molecule only absorbs at certain frequencies, it provides a unique absorption spectral pattern or fingerprint through the entire infrared light spectrum. In this way, the more molecules of the same type in the sample, the more infrared light is absorbed at those specific frequencies at which those molecules absorb infrared light.
As an example, Figure 2 and Figure 3 show two of the moisture fingerprints with absorption bands centered at 1596 and 3756 cm-1. Figure 2 presents a moisture concentration of 10 ppm/v, and Figure 3 presents a moisture concentration of 100 ppm/v. We can observe, by looking at both
figures, that they present almost identical patterns, but the sample with the most moisture has higher peaks (observe higher Absorbance values on the ordinate scale).
The height of the peaks are defined by the Beer-Lambert relationship (or Beer’s law). Beer’s law states that the concentration C is directly proportional to the absorbance A.
where a is the absorptivity of the molecule and b is the pathlength or distance that the light travels through the sample gas.
This relationship can be used to determine the concentration of a molecule in a gas by comparing the height of a maximum absorption peak of the molecule in a reference gas spectrum to the height of the corresponding peak in a sample spectra.
An absorption spectrum is calculated by performing the logarithmic ratio between a sample single beam spectrum ‘sample’ (Figure 5) and a background single beam spectrum ‘background’ (Figure 4). The sample measures the intensity of the infrared light reaching the detector with a sample placed in the path of the encoded infrared light. The background is a single beam spectrum that measures the intensity of the infrared light reaching the detector without any sample. The frequencies of infrared light radiation that are absorbed and the strength of these absorption bands are determined by the sample’s chemical makeup.
It is important to note that the single beam background spectrum is used as a point of reference to determine the absorption of a sample inside the sample compartment. This background reflects the conditions inside the spectrometer at the moment it was taken, so it is important to maintain a stable environment inside the spectrometer. This means that the conditions inside the spectrometer must have minimal fluctuations, through all background and sample collections.