Fourier transform infrared (or FTIR) is a method of infrared spectroscopy (Bradley, n.d.). Figure one below shows the components in an FTIR machine (Bradley n.d.).
Figure 1: Diagram of interacting components of an FTIR machine (Bradley, n.d.)
The overall concept for FTIR is that when a sample is exposed to radiation, some of the radiation will be absorbed and some will be transmitted (Bradley, n.d.). The transmitted radiation creates the “fingerprint” of the sample (Bradley, n.d.). Thus, it provides very specific information that may not be available using other methods (Bradley, n.d.).
An FTIR machine produces data through recording an interferogram and then utilizing Fourier transform to create the spectra (Birkner and Wang, 2016). In order to create the interferogram, a beam and mirrors are used (Newport, n.d.). The source will generate radiation, which travels through an interferometer and then to the sample (Birkner and Wang, 2016). In the interferometer, the beam splitter splits the beam into two components, one of which travels to a moving mirror and the other travels to a stationary mirror (Bradley, n.d.). Then, the two beams are recombined, travel through the sample, and then to the detector (Bradley, n.d.). The bonds in the sample will absorb specific frequencies from the radiation, and the rest is transmitted to the detector (Bradley, n.d.). This transmission is what creates the spectra, as different bonds have different frequencies that they can absorb (Bradley, n.d.). An example of a spectra that compares different amino acids can be seen in figure two below (Wolpert and Hellwig, 2006). As seen in the figure, since different amino acids can absorb different frequencies, each one produces a different spectrum (Bradley, n.d. and Wolpert and Hellwig, 2006).
Figure 2: Comparative FT-IR spectrum of different amino acids (Wolpert and Hellwig, 2006).
FTIR has several advantages (Eaton, 2001). First, FTIR is very fast (Eaton, 2001). This method also has a good signal to noise ratio (Eaton, 2001). The lack of slits on the machine create “greater optical throughput” (Eaton, 2001). Finally, since this method is widely used, the accurate frequency of different bonds is known, and thus can be used as a reference (Eaton, 2001).
The disadvantages of FTIR include that the method itself creates an interferogram, not a spectrum, thus Fourier transform is needed (Eaton, 2001). This is a disadvantage because it creates more steps in the process (Eaton, 2001). Finally, since a single beam is emitted from the source, all of the regions are measured together (Eaton, 2001). Thus, if there is some noise or a change in the environmental conditions, it will affect the entire spectra (Eaton, 2001).
There are several different ways that the samples may be handled in FTIR (Bradley, n.d.).
The first method is the transmission method (Bradley, n.d.). This method includes placing a sample into the infrared beam (Bradley, n.d.). This method may require some sample preparation (Bradley, n.d.). The strengths of this sampling method are that it is inexpensive, often used, and gives plenty of information (Bradley, n.d.).
The second method is attenuated total reflection (Bradley, n.d.). This method measures changes in the “internally reflected infrared beam” when it encounters a sample (Bradley, n.d.). This method is useful for thick samples (Bradley n.d.). The benefits of utilizing attenuated total reflection are that it requires little sample preparation, it is easy to clean, and analyzes thick samples (Bradley, n.d.).
The third method of FTIR is diffuse reflectance (Bradley, n.d.). In this method, the sample is ground and mixed with KBr in a cup (Bradley, n.d.). Then, the infrared beam interacts with the sample and is reflected off of the KBr (Bradley, n.d.). This method is utilized for soft powder samples (Bradley, n.d.). The strengths of this sample handling method are that it requires little sample preparation and is easy to clean (Bradley, n.d.).
The final method of sample handing for FTIR is spectral reflectance (Bradley, n.d.). This method measures the energy reflecting from the sample (Bradley, n.d.). This method is useful for large flat surfaces, requires little preparation, and does not destroy the sample (Bradley, n.d.).
There are also disadvantages to these sample handling techniques (Bradley, n.d.). Scientists cannot use thick samples unless utilizing the attenuated total reflectance method (Bradley, n.d.). The particles also have to be small in many of the methods (Bradley, n.d.). For the diffuse reflectance, the sample and the KBr have to be mixed in the correct ratio for the experiment to work (Bradley, n.d.)
FTIR can be applied to differentiation between chronic and aggressive periodontitis (Ozek et al., 2016). Periodontitis is a gum infection that damages the tissues and bones in a patient (“Periodontitis (Advanced Gum Disease)”, n.d. and “Periodontitis”, 2017). Chronic periodontitis shows no well-defined pattern of bone loss (Slim, n.d.). However, aggressive periodontitis progresses faster and appears in younger patients who show more bone loss and gingival inflammation (Slim, n.d.). The only method of differentiating between chronic and aggressive periodontitis is through family history and x-rays (“Periodontitis (Advanced Gum Disease)”, n.d. and “Periodontitis”, 2017).
The aim of this study was to determine if there is a way to efficiently differentiate between aggressive and chronic periodontitis (Ozek et al., 2016). The authors collected saliva from patients and analyzed the biomolecules present in the samples (Ozek et al., 2016). The participants also self-reported their smoking status, as smoking affects the biomolecules in the body, which affects the FTIR spectra (Ozek et al., 2016).
The findings of this study showed that FTIR can be utilized to differentiate between chronic and aggressive periodontitis (Ozek et al., 2016). The spectra that measured thiocyanate differentiated smokers and non-smokers (Ozek et al., 2016). Thus, that measurement can be utilized to confirm that participants are accurately self-reporting their smoking status (Ozek et al, 2016). The lipid spectra showed differentiation between the non-smoking group with chronic periodontitis and the non-smoking group with aggressive periodontitis (Ozek et al., 2016). Finally, since smoking affects so many different biomolecules, the only way to differentiate between the smoking groups was to utilize a cluster analysis of all the data (Ozek et al., 2016). The groups of smokers with chronic versus aggressive periodontitis were differentiated in the 1800 cm-1 to 950 cm-1 regions of the spectra (Ozek et al., 2016).
In conclusion, this study confirmed that FTIR has a practical application to disease, and can be used in the future to quickly differentiate between two forms of periodontitis (Ozek et al., 2016).