1. Introduction

Infrared (IR) spectroscopy, a type of vibrational spectroscopy, is a common analytical technique that characterizes molecules based on how they absorb, emit, or reflect IR light. [1] With this technique, functional groups of molecules are identified by measuring the vibration of atoms within the molecule. The instrument used is called an IR spectrometer. When IR light interacts with a molecule, specific wavelengths are absorbed which causes the bonds between atoms to vibrate at unique, characteristic frequencies. These vibrations correspond to different functional groups, thus identifying the functional groups and overall molecular structure of the molecule. The infrared range of the wavelength spectrum spans from 12,800 to 10 cm-1 and can be divided into three regions: near-infrared (NIR) (12,800 ~ 4,000 cm-1), mid-infrared (MIR) (4,000 ~ 200 cm-1). and far-infrared (FIR) (50 ~ 1,000 cm-1) (Figure 1). [2] IR spectroscopy is a qualitative and quantitative technique, as it can identify the sample and measure the concentration of a specific component in a mixture. IR spectroscopy encompasses several techniques, including NIR spectroscopy, MIR spectroscopy, FIR spectroscopy, Raman spectroscopy, and Fourier-Transform Infrared Spectroscopy (FTIR). Advantages of IR spectroscopy are that it can measure a variety of samples (solids, liquids, gases, semi-solids, powders, polymers, organics, inorganics, biological materials), it is a relatively fast, easy, and inexpensive process that produces information-rich spectra. [3] Disadvantages are that there are several materials that are not measurable due to the lack of vibrations, and also mixtures may produce complex spectra, making it difficult to identify specific peaks. [3]

Figure 1. The infrared (IR) range of the wavelength spectrum. [4]

FTIR spectroscopy is a type of MIR spectroscopy that is measured using a Fourier-Transform Infrared (FTIR) spectrometer. Developed in the 1980s, the FTIR spectrometer was a major breakthrough in IR spectroscopy by making the process faster and more accurate. Previous dispersive IR spectrometers measured absorption at each wavelength sequentially, resulting in longer measurement times. [4] In contrast, FTIR spectrometers measure all wavelengths simultaneously, making the process significantly faster. The scan time of all frequencies is approximately one second. [5] In addition, FTIR spectrometers have improved signal-to-noise ratios, wavenumber accuracy, wide scanning range, and extremely high resolution. [4, 5] This innovation relies on the Michelson interferometer, a component within the spectrometer that produces an interferogram, which contains raw information on how IR light is absorbed across wavelengths as a function of time. This raw information must be converted into a readable IR spectrum using a mathematical model called a Fourier transform. Due to these numerous advantages, FTIR spectrometers have replaced dispersive IR spectrometers. [5]

[6]

2. FTIR Sampling Techniques

FTIR spectroscopy has four main sampling techniques: transmission, attenuated total reflection (ATR), specular reflection, and diffuse reflectance (DRIFTS). [7] These techniques differ in how IR light interacts with the sample, and the sample type required.

2.1. Transmission

Transmission FTIR spectroscopy is a non-contact technique and is considered the most traditional method to analyze liquid, solid, or gas samples. [8]  In this process, incident IR light passes through the sample, the amount of light that transmits through is measured, which produces an FTIR spectrum in %Transmission (Figure 2). With transmission FTIR, sample preparation is necessary. Solid samples may need to be dispersed in potassium bromide (KBr) and pressed into pellets, and liquid samples must be placed into IR transparent windows such as calcium fluoride (CaF2). [8] The required sample preparation for solids and liquids makes transmission FTIR unfavorable, thus making ATR-FTIR the standard sample technique. Transmission FTIR is still necessary for gas measurements.

Figure 2. Schematic of FTIR transmission spectroscopy. [8]

2.2. Attenuated Total Reflection (ATR)

Attenuated total reflection (ATR) FTIR spectroscopy is an internal reflection technique that requires little to no sample preparation for solids and liquids. In ATR-FTIR, the IR light beam is directed towards an Internal Reflective Element (IRE) (Figure 3). The IRE must have a higher refractive index than the sample to ensure that the light does not pass through the sample. The IRE material is chosen based on application area, some examples are zinc selenide (ZnSe), germanium, and diamond. When the light enters the IRE and is reflected, an evanescent wave is created that projects into the sample. [8] The evanescent wave interacts with the sample, causing a certain amount of energy to be absorbed, and thus the evanescent wave is attenuated (weakened). The attenuated light is then reflected out of the IRE and to the detector. [8] With ATR-FTIR, the sample must be in contact with the IRE, making it crucial that the equipment is properly cleaned after each use. ATR-FTIR is the most common FTIR technique for polymers.

Figure 3. Schematic of FTIR-ATR spectroscopy. [8]

2.3. Specular Reflectance

Specular reflectance spectroscopy is a non-contact external reflectance technique where the angle in which the angle of incidence (angle at which light hits the surface) equals the angle of reflection (angle at which light is reflected) (Figure 4). [8] This technique is the most suitable for samples that have smooth, solid surfaces, such as thin (polymer) films or materials with a reflective metal substrate.

Figure 4. Schematic of specular reflectance FTIR spectroscopy. [8]

2.4. Diffuse Reflectance (DRIFTS)

Lastly, diffuse reflectance (DRIFTS) spectroscopy is another non-contact external reflectance technique suitable for samples with rough surfaces such as powder. [8] The IR incident beam enters the sample and is scattered in all directions (Figure 5).

Figure 5. Schematic of DRIFTS FTIR spectroscopy. [8]

3. FTIR Data Analysis

3.1. Qualitative Analysis

The y-axis of the resulting spectra varies by sampling technique, with percent transmittance for transmittance FTIR, absorbance for ATR-FTIR, and reflectance for specular reflectance and DRIFTS. The y-axis is generally presented as absorbance, as transmittance and reflectance can be converted to absorbance. The x-axis is the wavenumber throughout the spectrum, and is the same across all techniques.

Figure 6 shows example FTIR spectra of cotton, silk, polyester, and polyamide. [9] Each fiber has its own unique characteristic spectrum, or ‘fingerprint’, based on its molecular structure. For example, cotton has a broad peak at ~3,380 cm-1, which is attributed to extensive hydrogen bonding in the cellulose structure. [8] Another broad peak occurs at ~1,085 cm-1 due to C-C, C-OH, C-O-C stretches, which are characteristic of the bonding in cellulose. [9]

Figure 6. FTIR spectra of cotton, silk, polyester, and polyamide. [9]

Online resources such as the Infrared Spectroscopy Absorption Table by LibreText provide information on the absorption frequencies corresponding to various functional groups. To quickly identify an unknown sample, FTIR software can automatically compare the spectrum of the sample to a library database containing FTIR spectra of a wide range of materials. An example is the KnowItAll Spectral IR Spectral Database by Wiley Science Solutions.

3.2. Quantitative Analysis

FTIR is also a quantitative technique as it can measure the intensity of a sample’s IR absorption, which is directly related to the concentration of a sample based on the Beer-Lambert Law. The Beer-Lambert Law is the linear relationship between the absorbance and concentration of a sample:

Figure 7. Beer-Lambert Law formula. [10]

This law allows the concentration (c) of a sample to be calculated based on the absorbance (A) produced from the FTIR spectra. The molar absorption coefficient (ε) is the measure of how strong an absorber the sample is at a particular wavelength, and is sample-dependent. [10] The optical path length (l), is the path length of the IR beam through the sample. A calibration curve is created by plotting the absorbance values of samples with known concentrations against their corresponding concentrations. This curve establishes a linear relationship, which can then be used to interpolate the concentrations of unknown samples based on their absorbance values. An example of the absorption spectra of Rhodamine B solutions at different concentrations and its respective calibration curve is shown in Figure 8.

Figure 8. (a) absorption spectra of Rhodamine B solutions at different concentrations and its (b) respective calibration curve. [10]

4. FTIR Applications

FTIR is used in the textiles and material science fields to examine the composition and properties of polymers, coatings, and other materials, allowing researchers to understand its behavior, performance, and potential applications. [11] Other applications that use FTIR include pharmaceuticals, food and beverage analysis, forensics, and environmental monitoring. In the pharmaceutical industry, FTIR is critical to check the proper ingredients, amount, and purity are being used to ensure the consumers wellbeing. [12] In the food and beverage industry, ATR-FTIR is being used to quickly determine the trans-fat content of manufactured food products, ensuring compliance with food labeling requirements. [12] For forensics, FTIR is used to analyze evidence in criminal investigations. For environmental monitoring, FTIR is used to identify pollutants and contaminants in samples used for air and water quality assessments.

5. Conclusion

Fourier-Transform Infrared (FTIR) spectroscopy is a type of infrared (IR) spectroscopy widely-used to characterize materials in a qualitative and quantitative manner. The four main FTIR sampling techniques – transmission, attenuated total reflection (ATR), specular reflection, and diffuse reflectance (DRIFTS) – differ in how IR light interacts with the sample, and the sample type needed. FTIR allows researchers to identify molecular structures, functional groups, and concentrations with high accuracy, making it invaluable in various fields such as textiles, material science, pharmaceutical, and the food and beverage industry.

6. Peer-Reviewed Paper

The article I selected was “FTIR imaging, a useful method for studying the compatibility of silk fibroin-based polymer blends”. [13] Published in 2013, the authors used FTIR innovatively as an imaging technique. FTIR imaging combines the principles of FTIR spectroscopy with spatial imaging to create a 2D (sometimes 3D) chemical composition map of a sample. By detecting variations in the chemical composition within this spatial image, the phase behavior and phase separation in polymer blends can be analyzed. In comparison to traditional techniques used to analyze phase behavior (scanning electron microscopy and differential scanning calorimetry), FTIR imaging shows the chemical distribution across blends, providing a more direct understanding of their phase behavior. The authors used FTIR imaging to study the phase behavior of three silk fibroin-based polymer blends: silk fibroin/chitosan (SF/CS) blend, silk fibroin/sodium alginate (SF/SA) blend, and silk fibroin/polyvinyl alcohol (SF/PVA) blend.

First, the FTIR spectra of the pure silk fibroin (SF) film, chitosan (CS) film, sodium alginate (SA) film, polyvinyl alcohol (PVA) film, and the three polymer blend films were collected to analyze their characteristic peaks. The samples were prepared as as-cast films, and additional samples were treated with a 70% ethanol aqueous solution before measurement.

Afterwards, FTIR images of the three polymer blend as-cast films were taken.

FTIR images of the three polymer blend films after treatment with 70% aqueous ethanol solution were also taken.

Comparing the FTIR images of the polymer blend sample prepared as as-cast films or treated with 70% aqueous ethanol solution, the authors concluded that SF/CS blend was compatible, the SF/SA blend was partially compatible, and the SF/PVA blend was incompatible. The increase in differing colors of the SF/PVA blend represents the phase behavior of SF and PVA, indicating poor compatibility between the two polymers.

This paper relates to the chapter as it demonstrates a real-world application of FTIR to identify the polymer characteristics of samples. FTIR spectra were measured for the pure polymer films and the three polymer blend films. FTIR imaging was used as an innovative method to qualitatively analyze the phase behavior and blend compatibility of the different polymer blends.

7. Interactive Elements

• Interactive website that shows the IR spectra of chemical structures you draw.

8. References

[1] Infrared Spectroscopy. (n.d.). Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy

[2] Klein, C. (2019, December 12). Infrared Radiation and Infrared Spectroscopy. ThermoFisher Scientific. https://www.thermofisher.com/blog/materials/all-about-infrared-radiation-and-spectroscopy/

[3] Smith, B. C. (2011). Fundamentals of Fourier Transform Infrared Spectroscopy. CRC Press. https://doi.org/10.1201/b10777

[4] What is FTIR Spectroscopy? (2023, March 1). Edinburgh Instruments. https://www.edinst.com/blog/what-is-ftir-spectroscopy/

[5] How an FTIR Spectrometer Operates. (2019). Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy/How_an_FTIR_Spectrometer_Operates

[6] What is FTIR Spectroscopy? – Technology Introduction – METTLER TOLEDO – EN. (2022, April 1). YouTube. Retrieved December 10, 2024, from https://www.youtube.com/watch?v=0e_xBwQ7znI

[7] FTIR Spectroscopy Academy. (n.d.). ThermoFisher Scientific. https://www.thermofisher.com/us/en/home/industrial/spectroscopy-elemental-isotope-analysis/molecular-spectroscopy/fourier-transform-infrared-spectroscopy/resources/ftir-spectroscopy-academy.html

[8] Common Sampling Techniques of FTIR Spectroscopy. (2023, March 1). Edinburgh Instruments. https://www.edinst.com/blog/common-sampling-techniques-of-ftir-spectroscopy/

[9] Forensic Analysis of Fibers Using FTIR Microscopy. (n.d.). Quantum Analytics. https://www.lqa.com/resource/forensic-analysis-of-fibers-using-ftir-microscopy/

[10] The Beer-Lambert Law. (2021, January 15). Edinburgh Instruments. https://www.edinst.com/blog/the-beer-lambert-law/

[11] 7 Applications of FTIR Analysis. (2023, August 21). Richmond Scientific. https://www.richmondscientific.com/what-are-the-applications-of-ftir?srsltid=AfmBOop8EaSbq9YmLPT18IvcfFKXaIRSqWC6n8DDVJzzgHBo7c_cnW-F

[12] FTIR Applications. (n.d.). ThermoFisher Scientific. https://www.thermofisher.com/us/en/home/industrial/spectroscopy-elemental-isotope-analysis/molecular-spectroscopy/fourier-transform-infrared-spectroscopy/applications.html

[13] Ling, S., Qi, Z., Knight, D. P., Shao, Z. & Chen, X. (2013, May 28). FTIR imaging, a useful method for studying the compatibility of silk fibroin-based polymer blends. Polymer Chemistry, 4, 5401. https://doi.org/10.1039/C3PY00508A