FT-IR Spectroscopy Basics

What is infrared spectroscopy?

Infrared spectroscopy (IR) relies on the fact that most molecules absorb light in the infrared region of the electromagnetic spectrum, converting it to molecular vibration. This absorption corresponds specifically to the bonds present in the molecule. With a spectrometer, this absorption of infrared radiation by the sample material is measured versus wavelength (as wavenumbers, typically from 4000 – 600 cm^-1).

The result is a spectrum that gives a distinctive "molecular fingerprint" which can be used to screen, scan and identify organic and inorganic samples.

Example: Infrared spectrum of polycarbonate measured with ATR.

What is Fourier-Transformed Infrared Spectroscopy (FT-IR) Spectroscopy?

Instead of sequentially irradiating the sample with varying single wavelengths (dispersive), FT-IR allows to collect all spectral data in one go. For this, you need a continuum source of light to produce light over a broad range of infrared wavelengths. The infrared light then passes through an interferometer and is afterwards directed at the sample.

This yields an interferogram, a somewhat raw signal, that represents the intensity of light as a function of the position of a mirror. This signal first needs to be Fourier-transformed (FT) to produce the more familiar IR plot of intensity vs. wavenumber. Hence the name “FT-IR” or FTIR.

Why use FT-IR?

Not only is the acquisition of FT-IR spectra much faster than by conventional dispersive Instruments. Additionally, these spectra show a significantly higher signal-to-noise ratio and, since the wavelength scale is calibrated with a very precise laser, spectra obtained have much higher wavelength accuracy.

How do you Measure FT-IR?

This depends on the sample that needs to be analyzed. Classically, an otherwise solid sample is either ground with IR transparent potassium bromide (KBr) and pressed into a pellet, or it is thinly sliced and placed onto a KBr window, while liquids are directly measured or diluted with an IR transparent solvent, e.g. CCl₄.

By now, this laborious preparation is in many cases succeeded by ATR FT-IR spectroscopy, a technique that is mostly non-destructive and suited to analyze solids and liquids in their present states.

What is ATR?

ATR stands for attenuated total reflection and has become the standard technique for the measurement of FT-IR spectra. The infrared light passes through a crystal of a certain material (diamond, ZnSe or germanium) and interacts with the sample, which is pressed onto the crystal.

From this a spectrum is obtained, that shows all substance specific characteristics, while the intensity ratio of the observed absorption bands might differ from a traditional transmission spectrum due to physical effects.

To learn more about FT-IR analysis, you can either download our interactive IR tutorial or watch our series of educational videos. In these we try to give a step-by-step tutorial on how to measure FT-IR spectra and evaluate the obtained data.

What is infrared light?

Infrared (IR) light, or more precisely infrared radiation, is an electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye but can be perceived in the form of thermal radiation. Fun fact: more than half of the energy radiated by the sun reaches the earth in the form of infrared.

How does infrared light interact with materials?

When infrared radiation is directed at matter, it can stimulate the movement of molecules and atomic bonds. This movement can take various forms, such as rotation or vibration. Depending on how the molecule is excited, we can obtain information about the structure and identity of the irradiated material.

Can infrared light analyze all materials?

In general, yes, because organic and inorganic substances can be examined equally well with infrared radiation. The basic requirement for analysis with infrared is that the material absorbs infrared radiation. Certain substances, however, including metals and monatomic gases (e.g. noble gases) cannot be examined directly.

Which materials are commonly analyzed?

Especially for organic substances IR spectroscopy is a frequently used tool to obtain a lot of information. This includes the identification of polymers, drugs, pharmaceuticals, or industrial chemicals as well as the determination of contents like water in oil. IR spectroscopy is very flexible, and its applications are so numerous that you can find IR users in all industry and research areas.

What kind of analysis is possible?

With IR it is possible to find out what sample is made of, but also how much of a certain ingredient or component is present. Qualitative analysis is the most common application of IR spectroscopy and is mainly used in quality control of raw materials, failure analysis, and in scientific research. Quantitative analysis is widely used in industrial processes to evaluate production parameters.

Do I need to be an expert to use IR spectroscopy?

Definitely not. IR spectrometers are easier to use today than ever before. Most of the time there are simple software solutions (e.g. touch operation) that allow non-experts to perform IR analyses in an uncomplicated way. Even the analysis can be automated, so anyone can become a spectroscopist!

How long does IR analysis take?

This depends strongly on the analytical question posed. But a simple verification of the identity of a chemical substance hardly takes more than a minute.

What is attenuated total reflectance (ATR)?

ATR is a special sampling technique to obtain IR information. The IR light is directed at a crystal made from IR transparent material (e.g. diamond). The IR radiation will then interact with samples and materials that are in close contact with the diamond. Watch our video on ATR basics to learn more!

Where do I use ATR?

Almost everywhere, as ATR is a truly universal approach. Whether solid or liquid, organic or inorganic - you simply take your sample and place it on top of the crystal. There is no need to cut, dilute or prepare your sample. In the last decades, ATR has become the standard technique in IR spectroscopy.

What is transmission?

Different to ATR, this method requires light to penetrate the whole sample. This means that the sample must be either very thin or diluted. For dilution, samples are often mixed with potassium bromide (KBr) and pressed into a pellet. Very thin samples, on the other hand, are produced with a microtome and then placed on a KBr window. These preparations require a lot of time and effort.

When do I use transmission?

Today transmission is only necessary for very specific analytical questions. These include the quantification of lowly concentrated components in liquids or the application IR microscopy. In certain industrial sectors there are also standardized procedures that require transmission measurements (e.g. Pharma).

What is reflectance?

Reflection is the third main technique in IR spectroscopy. It is based on the reflection of IR light and allows conclusions about the surface of materials. If the surface in question cannot be examined directly, dilution with KBr is often necessary. It is also possible to place very thin samples on metallic mirrors (transflectance).

Where is reflectance used?

Due to the special requirements of reflectance measurements, it is used for very specific analytical objectives. It is possible, for example, to examine valuable works of art completely non-destructively and carefully to enable their restoration.