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Spectral Basics | Infrared Spectroscopy Guide


What is IR Spectroscopy?

Infrared, or IR, spectroscopy is a chemical analysis technique that takes advantage of the interaction between infrared light and matter. Infrared light is part of the electromagnetic spectrum between visible light and microwaves, with wavelengths ranging from 780 nm to 1 mm.

Due to historical reasons however, we typically discuss IR light in spectroscopy in terms of wavenumbers rather than wavelengths. Wavenumbers tell us the number of wavelengths per unit distance and are given in units of cm-1. Light with shorter wavelengths and higher energy will have a larger wavenumber, while light that has longer wavelengths will have lower energy and wavenumbers.

Mid-IR, near-IR, and far-IR

Infrared light can be further divided into three categories: Near infrared (NIR), mid infrared (MIR), and far infrared (FIR). NIR has the shortest wavelength with higher wavenumbers while FIR has the longest wavelength with lower wavenumbers.

Typically, when discussing IR spectroscopy MIR is the type of IR light that is used. The IR light in this range is useful because it coincides with an important property of chemical compounds: their vibrations.

Figure 1: Positions of infrared radiation in the electromagnetic spectrum. NIR, MIR and FIR are highlighted.

Types of vibrations in IR spectroscopy

The atoms in chemical compounds are constantly moving and vibrating in different ways. Even in a simple molecule like water there are six different ways the molecule can vibrate: the symmetric stretch, the antisymmetric stretch, the deformation or bending vibration, rocking, twisting, and wagging.

Each of these vibrations occurs at a different frequency that is unique to the chemical bond and compound. As mentioned earlier, those frequencies happen to match the frequencies of light in the MIR region of the electromagnetic spectrum.

Figure 2: The different types of vibrations in infrared spectroscopy.

Detecting IR absorbtion

As these frequencies match those of the IR light, chemical compounds can absorb IR light which excites the vibrations in the molecules.

For example, the symmetric and antisymmetric stretches for water occur in the range of 2700 to 3700 cm-1 while the deformation vibration occurs around 1650 cm-1, so water will absorb those energies of IR light.

If we shine IR light through some water, we can use a detector to determine which frequencies of light were absorbed as those frequencies will be "missing" from the original beam of IR light.

Figure 3: Principle of IR absorbtion explained with an example water molecule.

Getting an IR spectrum

After the IR light is detected, we can plot the information obtained from the detector to create the IR spectrum. The spectrum shows which frequencies of light were absorbed by the sample and therefore which vibrations were excited when the IR light passed through the sample.

For the sample of water, the spectrum would show signals at the wavenumbers corresponding to the frequencies of the symmetric stretch and antisymmetric stretch, as well as the deformation vibration.

Figure 4

As each chemical species will have vibrations at different frequencies, the resulting spectrum of each compound will be unique. This means IR spectroscopy creates a "chemical fingerprint" that can be used to identify and quantify almost any chemical species.

Over the years, information on a vast number of chemical species has been complied into spectral libraries making IR spectroscopy extremely straightforward even for those without knowledge of the theory behind it.

What is FT-IR spectroscopy? Where is the difference to IR spectroscopy?

Though IR spectroscopy has become a somewhat general term for the chemical analysis technique where molecular vibrations are detected using IR light, the technique most frequently used is Fourier Transform IR Spectroscopy (FT-IR).

Historically, IR spectroscopy was performed by individually checking each frequency of IR light to see if it was absorbed by the sample. As you can imagine, this was a very time-consuming process!

This technique was superseded by FT-IR, which can check all the wavelengths of IR light at once using an interferometer.

This technique is not only much faster than IR spectroscopy but is also more accurate with a better signal to noise ratio. However, this technique produces a different data set than the traditional IR spectroscopy experiment so a mathematical operation, called a Fourier transform, is used to convert the data into the familiar IR spectrum obtained from IR spectroscopy.

Measurement techniques in infrared spectroscopy

There are three main measurement techniques that can be used during FT-IR spectroscopy: Transmission, Attenuated Total Reflection (ATR), and Reflection. Though all techniques rely on the underlying theory behind FT-IR spectroscopy, each technique uses a slightly different process to analyze the sample and is suitable for different purposes.

Transmission IR spectroscopy

Transmission is the "original" technique used with IR spectroscopy. During transmission detection, IR light passes through the sample where it is absorbed. For transmission to be performed, the IR light needs to completely pass through the sample which often requires the sample to be prepared in a specific way. If the sample is too thick for the IR light to pass through, too much IR light will be absorbed by the sample which is called total absorbance. Total absorbance results in poor spectral quality, with peaks that are not easily distinguished.

Figure 5

To avoid total absorbance, the sample is diluted which allows the IR light to pass through the sample without being absorbed too strongly. The substance that is used to dilute the sample should not absorb IR light in the same range as the sample. Otherwise, the substance used to dilute the sample would also appear on the resulting spectrum.

To analyze a liquid sample, the liquid is diluted with a solvent. The usual choice is carbon tetrachloride (CCl4). To analyze a solid sample, the solid must be ground and mixed with another solid, typically potassium bromide (KBr) which does not absorb light in the MIR range. The resulting mixture is pressed into a pellet to be analyzed. Alternatively, the sample can be very thinly sliced and placed on a KBr window. This sample preparation can only be skipped if the sample is extremely thin (< 15 µm).

The sample preparation process for transmission detection is very time consuming and requires a lot of effort. Additionally, preparing the samples this way destroys the original sample. As a result, transmission detection is only used for specific spectroscopic applications such as examining polymer films, proteins, and samples containing oil in water. However, transmission is widely used in FT-IR microscopy in the field of forensics, as well as while analyzing tissue samples and microplastics.

ATR IR spectroscopy

ATR has largely surpassed transmission and is now the primary measurement technique used as this method involves minimal sample preparation and is non-destructive. To use ATR, the sample is simply placed on top of a crystal which is typically made of diamond, germanium, or zinc selenide. The IR light is directed through the crystal where it is partially absorbed by the sample. The IR light then passes through the crystal again and is detected.

Figure 6

During this technique, the light only interacts with the first few microns in the sample. Since the IR light does not pass completely through the sample like in transmission, little to no sample preparation is required to create an IR spectrum using ATR. This not only results in a technique that is extremely simple to perform, but also one that produces very high-quality spectra no matter what sample is being analyzed.

It is worth knowing that ATR and transmission produce slightly different spectra due to the differences between the two techniques. These spectral differences arise due to the way different wavelengths of IR light interact with the sample when the light is only partially absorbed during ATR. However, the spectra produced by ATR and transmission are still quite similar and the difference can easily be corrected using computer software, allowing for direct comparison of spectra obtained by different measurement techniques.

Reflectance IR spectroscopy

The final measurement technique is reflection. With this technique, the IR light that is reflected off the surface of the sample is detected rather than the IR light that passes through the sample. This makes Reflection IR Spectroscopy useful for examining solid samples that are difficult or impossible to analyze with transmission or ATR. There are many ways to perform Reflection IR spectroscopy depending on the sample being analyzed.

Figure 7
  • Reflection-absorption or "transflectance" shines IR light through a very thin sample onto a reflective substrate. This technique is useful for analyzing thin tissues or coatings.

  • Specular reflection analyzes the IR light that is bounced off a reflective surface, which is useful for examining large samples like polymers, gemstones, or even pieces of art prior to restoration.

  • Diffuse reflection, where light is scattered off a sample surface, is used in a technique called Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRFITS). DRIFTS requires careful sample preparation, so it is more difficult to perform, but it yields excellent quantitative results when analyzing solid samples such as soils, concrete, or catalysts.

Figure 8

Like with ATR, the differences between reflection and other measurement techniques result in different spectra due to the way the IR light interacts with the sample. The different methods of reflection detection impact the spectra differently, but all these differences can be corrected using computer software.

The many applications of (FT-)IR spectroscopy

At its core, IR spectroscopy has two main uses: identification and quantification.

IR spectroscopy has long been used for qualitative analysis as an IR spectrum is like a chemical "fingerprint" of a chemical species. This makes IR spectroscopy a powerful technique for identifying what substances are present in a sample, making it a valuable chemical identification tool not only in a laboratory setting, but in virtually any industry. FT-IR can be used for identification purposes in forensics, plastic recycling, failure analysis, quality control, and much more.

FT-IR can also be used to quantify components in a sample made of more than one substance. Individual components can absorb IR light more strongly if the concentration of that component is higher. Using computer software, the concentration of each component can be easily determined by analyzing the amount the IR light that was absorbed. FT-IR can provide extremely precise quantitative results, depending on the nature of the sample. This makes FT-IR useful for quantitative applications in areas such as the pharmaceutical industry, soil science, and biological research on proteins.

Figure 9: Polymers are a typical example where IR spectroscopy is used for both, identification and quantification.