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Spectroscopy Basics

Guide to Infrared Spectroscopy

We briefly explain the fundamentals of infrared (IR) spectroscopy and focus on the basic questions about FT-IR, ATR, transmission and reflection.

FT-IR Spectroscopy Basics

Getting started

What is infrared and FT-IR spectroscopy? Is there a difference?

About infrared (IR) 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 is characteristic of the nature of the chemical bonds present in a sample.

With a spectrometer, this absorption is measured as a function of wavelength (as wave numbers, typically from 4000 - 600 cm-1). The result is an IR spectrum that serves as a characteristic "molecular fingerprint" whichcan be used to identify organic and inorganic samples.

About FT-IR spectroscopy

In the past, the samples were analyzed step by step, whereby the sample was irradiated with different single wavelengths (dispersive). FT-IR on the other hand, collects the spectral data of all wavelengths in one pass.

Here, a continuous source generates IR light over a wide range of infrared wavelengths. The infrared light then passes through an interferometer and is then directed at the sample.

In contrast to dispersive measurements, we first obtain an interferogram, that needs to be converted to an IR spectra.

The difference between IR and FT-IR

This interferogram (a raw signal), represents the light intensity not as a function of wavelength, but as a function of the position of a mirror inside the interferometer.

As a result, the signal must first be Fourier-transformed (FT) to produce the more familiar IR representation of intensity as a function of wavenumber. Hence the name "FT-IR" or FTIR.

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, have much higher wavelength accuracy.

Getting into details

How do you measure an IR spectrum?

This figure shows the most basic setup to measure IR spectra in transmission or reflection.

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.

If a sample is thin enough (<15 µm), as for example a polymeric film, a coating on a metallic surface or a biological tissue section, enough IR light can pass the sample to be analyzed directly without dilution in KBr or a solvent.

Another technique would be reflection. Here the IR light only interacts with the surface of a material to collect chemical information. Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) is a special reflection sampling technique that allows to collect great quality spectra of solid samples that are very difficult to analyze in transmission, like soil or concrete.

However, by now, ATR FT-IR spectroscopy as succeeded many other sampling techniques, since it is mostly non-destructive, very easy to apply and suited to analyze solids and liquids in their present states.

Getting into details

What is ATR or Attenuated Total Reflectance?

This figure shows how the IR beam passes through the ATR crystal and is reflected at the interface between crystal and sample.

As the title states, 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. Be advised, that good contact between sample and crystal is very important!

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.

But this does not mean that ATR spectra are more difficult to interpret, on the contrary. ATR and transmission spectra can be easily converted into each other. This is especially useful, if you want to compare recently acquired ATR data with older spectra contained in a spectral reference library.

Getting into details

What is IR reflection? What is DRIFTS?

The three different types of reflection used for IR spectroscopy.

Reflection FT-IR measurements have a special place in infrared spectroscopy. In Principle, with the exception of DRIFTS, reflection measurements are non-destructive and are used, for example, in the analysis of valuable art objects and paintings.

The strength of the reflection is determined by the refractive index, so wherever there is an absorption band, the extent of the reflection also changes. Therefore, a (specular) reflection spectrum looks very different to transmission.

The reason for this is found in the absorbance properties, let's look at an isolated IR signal. For an isolated absorption band, the refractive index shows a maximum towards higher wavelengths, and a minimum towards lower wavelengths. So similiarly, the spectra will exhibit the same pattern - that of a first derivative.

FT-IR Videos & Tutorials

Getting into details

FT-IR basics tutorial videos

We explain how infrared (IR) light is used to create a spectrum.
We explain the most common IR technique: ATR.
We explain how IR reflection can be used to create spectra.
Getting practical

Application of FT-IR spectroscopy

FT-IR Basics - Chemical Identification.
FT-IR Basics - Substance Verification.
Analysis of smoking hot bitumen / asphalt by FT-IR.

FT-IR FAQ

The last straw

Frequently asked questions about FT-IR

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.