Introductory Chapter: Infrared Spectroscopy - Principles and Applications

1. Spectroscopy

Spectroscopy is a term used for the study of the spectra generated as a result of interaction of electromagnetic radiation with matter. It is used for the detection and/or identification of atoms, molecules, functional groups, and nuclei based on the produced spectra following the interaction of matter with the radiation. When light strikes matters, it can physically be reflected, refracted, scattered, or transmitted. During transmittance through matter, light with specific energy may interact with molecules in different ways, depending on the energy (E, Joule) of lights, which is directly proportional to its frequency (υ, with the unit sec⁻¹ or Hz) and inversely proportional to wavelength ($\mathrm{\lambda }$) as according to the equations below-mentioned:

$$\text{Energy},\mathrm{E}=\mathrm{h}.\mathrm{\upsilon }$$E1

where h is Planck’s constant (6.6260755 × 10⁻³⁴ J·s)

$$\text{Frequency},\mathrm{\upsilon }=\mathrm{C}/\lambda$$E2

where c is the speed of light (2.998 × 10⁸ m/s in vacuum).

From Eqs. (1) and (2),

$$\mathrm{E}=\mathrm{h}.\mathrm{C}/\lambda$$E3

Expressing 1/$\mathrm{\lambda }$ as wavenumber (ῡ), Eq. (3) can be described as below:

$$\mathrm{E}=\mathrm{h}.\mathrm{C}.\mathrm{ῡ}$$E4

Based on the above relationships, the energy of light can be scaled on a sector of spectrum by the unit of wavelength (300–700 nm, in UV–visible spectrophotometry) or in wavenumbers (400–4000 cm⁻¹, for the mid-IR (MIR) region). Schematic description showing other electromagnetic regions of the spectrum, along with molecular processes rising from the interactions in each region, is illustrated in Figure 1. Matter may interact with light of microwave region causing the rotational movement of entire molecule. Light of particular energy in the IR region may cause the vibrational movement of bonds in the molecules, including stretching, bending, scissoring, rocking, wagging, and twisting, giving different patterns for different molecules and allowing the identification of different functional groups. UV–visible light may cause electron excitation. Ionization and bond breaking can happen in the X-ray region. The patterns of the UV and IR spectra extended on the x-axis work for the identification of different substances/molecules, while the intensities of the spectrum’s peaks on the scale of y-axis correspond proportionally to the concentration of measured molecules, allowing the quantitative determination of molecules.

Figure 1.

Schematic description of electromagnetic radiation regions, showing representative molecular processes that occur in each region.

2. Infrared spectroscopy (IR)

Appearing at a longer wavelength compared to the visible region, as shown in Figure 1, the invisible IR radiation extends from the red edge of the visible region at 780 nm to 10⁶ nm reaching the edge of microwave region. As shown in Figure 1, the IR region is divided into three main zones:

1. The far-IR (FIR, 400–10 cm⁻¹, 25–300 μm)

2. The mid-IR (MIR, 4000–400 cm⁻¹, 2.5–25 μm)

3. The near-IR (NIR, 14,000–4000 cm⁻¹, 0.7–2.5 μm)

Most bond stretches and other vibrations of the functional groups of the organic compounds occur at the MIR zone.

2.1 Assigning peaks in an IR spectrum

The MIR range can also be divided into two regions. The region below 1400 cm⁻¹ is called “fingerprint region” giving peaks that are relatively difficult to assign, yet unique and specific for particular compounds. The region above 1400 cm⁻¹ is called “diagnostic or functional group region,” as it is often easier to pull specific details about the functional groups from this region. More specifically, regions can be divided into: 1) 2500–4000 cm⁻¹ for Z-H single bond stretches (C▬H, N▬H, C▬H); 2) 2000–2500 cm⁻¹ for triple bond stretches (C☰C, C☰N); 3) 1500–2000 cm⁻¹ for double bond stretches (C〓C, C〓O); 4) 1000–1500 cm⁻¹ for single bond stretches (C▬O); and 5) 1000 cm⁻¹ for mostly bending vibrations. Table 1, below, summarizes the locations and nature of some functional group peaks in organic compounds.

Table 1.

Wavenumber and nature of peaks for different functional groups.

2.2 Factors affecting peak location

In order to rationalize the different locations of bond stretches frequency in IR spectrum, we can think about bonds connecting two atoms, as springs connecting two objects. The factors affecting the stretches frequency of spring were described, in physics, by Hooke’s law (Eq. (5)),

$$\mathrm{\upsilon }=\frac{1}{2\mathit{\pi c}}{\left[\frac{f(m_1+m_2}{m_1{m}_2}\right]}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.},$$E5

where f = force constant or bond strength and m = mass of atoms.

Based on this analogy, we can see that the locations of different stretches band are determined by two factors:

1. Bond strength: Stronger bond will need higher energy (higher wavenumber) to stretch (C〓O > C▬O & C☰C > C〓C > C▬C).

2. Mass of bonded atoms: If we are comparing stretching of same number of bonds but between two different atoms, the lighter the atoms connected, the greater the stretch frequency (H▬Cl > H▬Br > H▬I).

2.3 Identification of organic compounds

IR spectroscopy was used to qualitatively analyze the organic compounds, such as identifying the polysaccharide type based on the presence of specific polysaccharides in a complex system [1]. Most of the absorption bands of the polysaccharides are present in the region between 1800 and 800 cm⁻¹. Canteri et al. (2019) revealed that the presence of several absorption bands in plant walls at 1075, 1440–1450, 1616, and 1740 cm⁻¹ were distinguishing bands for pectin, and the absorption bands at 895, 1035–1041, and 1160–1163 cm⁻¹ were used for cellulose [2]. Moreover, IR can also be used to identify substitution groups in the organic components, such as the presence of sulfate groups in the sulfated polysaccharides, which is confirmed by the presence of three characteristic bands at 1200–1270 cm⁻¹, 1010–1060 cm⁻¹, and 900–800 cm⁻¹, conforming to symmetric, asymmetric stretching of S〓O, and stretching of C▬O▬S, respectively [3]. In addition, it can identify the anomeric carbon configurations by the presence of specific absorption bands in the anomeric region. The band at about 890 cm⁻¹ shows the existence of β-configuration, while the absorption bands at around 840 cm⁻¹ or 920 cm⁻¹ reveal the presence of α-configuration [4, 5]. On the other hand, IR spectroscopy was used to determine the presence of n-alkane and long-chain alcohol in plant species. Ferreira et al. found that in the presence of intense peaks in the range 1190–1485 cm⁻¹, many elongations such as (C▬C), (C▬O) coupled to (CH) and (OH) in-plane deformations co-occur, which are associated with n-alkanes and alcohols in the studied samples [6].

AFM-IR (photothermal-induced resonance) was developed and used to characterize four industrially important polymers: a polypropylene-based reactor thermoplastic polyolefin (rTPO), linear low-density polyethylene (LLDPE) for molding, and two recycled post-consumer polypropylene/polyethylene blends. Using nanoscale spatial resolution IR imaging, we could picture each polymer’s key components, including the mineral fillers (talc and calcium carbonate) in each mixture and the morphology acquired using AFM. The obtained results show the possibility of individually specifying the polymer and CaCO₃ with the peak at 1427 cm⁻¹ and the peak at 1459 cm⁻¹, which is related to the CH₂ group vibration and 1376 cm⁻¹ absorption band, which corresponds to the CH₃ symmetric vibration band [7].

2.4 Analysis of the inorganic compounds

Simple inorganic compounds, such as NaCl, do not generate vibrations in the MIR range, despite their lattice vibrations in the FIR area; because of this, the IR windows are manufactured of simple inorganic compounds such as NaCl, KBr, and ZnSe. Several variables influence the inorganic compound IR spectra. The compound’s crystal structure, like the crystalline lattice bands that appear in the FIR range and changes in the crystalline structure, could be detected from the IR spectra. The IR technique is a nondestructive sampling approach and is preferred for such samples. When analyzing spectra, the degree of hydration of an inorganic substance is also considered. Due to O▬H stretching and bending absorption bands, water molecules in a crystalline compound’s lattice framework form characteristic sharp bands in the 3800–3200 and 1700–1600 cm^₋1 regions [8]. IR spectroscopy may directly analyze nanoparticles and the functional groups on their surfaces. Furthermore, various ligands attached to nanoparticles may be quickly, precisely, and nondestructively identified using their vibrational fingerprints. In addition to directly analyzing such materials, specific nanostructures alter the local optical field, amplifying IR signals and enabling better-IR nanoparticle imaging processes [9]. According to various investigations, IR spectra indicate vibrational fingerprints of synthesized nanoparticles. On the surface of persimmon peel biochar, the IR spectra of the Si▬O vibration can be detected, and the absorption band emerges at 1029 cm^₋1 [10]. Also, silicate (Si▬O▬Si) deformation vibrations appear on the surface of olive tree leaves biochar in the range 1120–950 cm^₋1 [11], and tetrahedral complexes (Mn²⁺▬O²⁻) can occur at 700–500 cm^₋1 [12]. Fe▬O bond vibration can be shown in the IR spectrum of magnetite nanoparticles, appearing in the range 564–570 cm^₋1 [13, 14]. Finally, bending vibration modes of Ti▬OH and Ti▬O stretching modes could appear at 1630 and 1383 cm^₋1, respectively [15].

3. Applications of IR spectroscopy

FT-IR spectroscopy has found many appreciations in the field of medicine, especially as a diagnostic tool, as well as environmental research including protein and nucleic acid analysis [16, 17, 18], environmental analysis, forensics, drug syntheses, and pharmaceutical fields. IR spectroscopy has been extensively used for deep analysis of organic and inorganic compounds based on observations of specific absorption bands. The typical applications of IR spectroscopy are compound-type detection, substitution groups, anomeric carbon structures, and crystal allomorphs. However, because some compounds’ structures are so similar, the qualitative characterization of these compounds using IR spectroscopy is complex. Furthermore, the overlap of bands in spectra may obscure certain important information, adding to the difficulty of this study. As a result, qualitatively analyzing different compounds using only one characteristic IR peak may be purely empirical and inaccurate. The applications of IR spectroscopy in different science branches are as follows:

3.1 Application to environmental and biological samples

Food products may be assessed qualitatively and quantitatively using MIR and NIR spectra. As significant components, foods are complex mixtures of water, proteins, fats, and carbohydrates. Fats and oils have long been studied using IR spectroscopy. Some vegetable oils, such as canola (CaO), maize (CO), soybean (SO), and walnut (WO), have a similar color to cod liver oil (CLO), making their detection with the human eye very difficult [19]. As a result, FT-IR spectroscopy, in combination with chemometrics, was used to detect vegetable oils. The IR spectra of all these samples show the modest changes in band intensities and the specific frequencies at which maximal absorbance is obtained in each oil due to variances in fatty acid content, chain length, degree, and location of double bonds in triglyceride [19]. These spectrum variations may be found in frequency ranges of 3007 (cis-vinylic stretching vibrations), 2922, and 2852 cm^₋1 due to asymmetrical and symmetrical ▬CH₂▬ stretching vibrations and 1237, 1117, and 1098 cm^₋1 related to C▬O stretching vibrations [20]. As a result, these frequency regions were used to differentiate between CLO and other vegetable oils. Due to the consumption of hazardous chemicals, fabricated eggs can cause various human health concerns. As a result, developing technology for identifying fabricated eggs is a top concern for food safety. FT-IR spectroscopy is commonly employed because of its ability to analyze such objects using their optical features. The albumen results show that the wavenumber region between 1650 and 1550 cm^₋1 provides excellent separation between synthetic and actual albumen. Even though absorbance values of natural and created albumen exhibit a peak around 3300 cm^₋1, those peaks linked to water content were eliminated since the water contents in the synthetic egg are easily modified by the recipe [21].

Evaluating the overall soil quality necessitates estimation of the soil’s physical, chemical, and biological characteristics. MIR spectroscopy is a rapid, low-cost, environmentally friendly, nonhazardous, and reproducible approach and can be used in different applications as shown in Figure 2. The C and N content of the soil was thoroughly investigated using MIR spectroscopy. MIR spectroscopy may also illustrate and assess clay minerals, soil water (volumetric and gravimetric water content), and soil microorganisms [22]. NIR spectroscopy has emerged as a popular method for measuring heavy metals and other pollutants in soils, sediments, and water [23]. Multi-molecular IR (MM-IR) spectroscopy was used to identify and quantify the amount of fluorescent whitening agent in the wheat flour to check the quality of the flour [24]. MM-IR integrated with two-trace 2D correlation spectroscopy was used to analyze deoxynivalenol (DON). DON is considered a potent pollutant in wheat flour, even in low quantities, and causes a range of adverse health impacts on people and cattle [25].

Figure 2.

Applications of MIR spectroscopy.

3.2 Applications to biomedical and bioanalytical investigations

IR spectroscopy has become one of the most powerful tools in modern bioscience. Because of its high molecular specificity, applicability to a wide range of samples, rapid measurement, and noninvasiveness, IR can provide important qualitative and quantitative information for various types of biological material. IR became a well-established bioanalytical approach with several applications [26, 27]. It can be used as an efficient tool for predicting protein secondary structure. One of the most significant obstacles to estimating protein structure using FT-IR spectroscopy is interference from H₂O and the protein IR amide I and II bands. Earlier research used D₂O solution for protein and peptide spectroscopy since there are no D₂O absorption spectra in the area where the amide I and II bands are visible. Later, using a drying apparatus and a tiny sample cell (<10 μm) enabled the quantification of H₂O. Correlations were established between various protein secondary structures in H₂O solution and their FT-IR spectroscopy bands [28]. IR spectroscopy was also used to characterize various systems, including structure elucidation and identification, crystalline and amorphous forms, and quantitative and remote sensing applications [29]. Hence, it can be used to determine roxithromycin in its formulations using conventional KBr to acquire the FT-IR spectra of standards and samples. Based on Beer’s law, a calibration curve with (R² = 0.9992) was created using the FT-IR carbonyl region (C〓O) range 1765–1705 cm⁻¹ [30]. In addition, the early detection of breast cancer was done using FT-IR spectroscopy. The ranges 900–1425 cm^₋1 and 1475–1710 cm⁻¹ displayed better classification performance. The lipids, proteins, sugars, and nucleic acids were represented by the region 900–1425 cm^₋1, while the proteins were shown by the region 1475–1710 cm^₋1. The biological compounds in the other bands also added some unique potential to the categorization, resulting in the best classification accuracy in the entire band [31]. In addition, research showed that integrating imaging technique IR spectroscopy can be promising apparatus for the early diagnosis of diseases such as cancer [32, 33, 34, 35] and for monitoring brain stroke damages after treatment [36, 37, 38].