ATR-FTIR spectroscopy is used worldwide in industry and academia. It is a versatile and ubiquitous technique, adopted in a variety of scientific fields. This article aims to explore the different types as well as some of the uses of ATR-FTIR spectroscopy.
What is ATR-FTIR spectroscopy?
ATR stands for Attenuated Total Reflectance. It is a form of FTIR (Fourier Transform Infrared) sampling which uses a small amount of light, called an evanescent wave, to interrogate the shallow surface of a sample. The evanescent wave is generated where light undergoes total internal reflection within a material.
Although the underlying physics is fairly complex, ATR is an extremely simple technique from the user point of view. By placing the sample material firmly in contact with the reflecting surface, the evanescent wave can be used to generate a spectrum of the sample.
The resulting spectrum provides information about the molecular structures of the sample, making it a valuable method for identifying and characterising materials.
One of the key advantages of ATR-FTIR spectroscopy is that it is a non-destructive and non-invasive technique. This makes it a convenient and efficient method for analysing a variety of samples in a wide range of applications.
What are the types of ATR-FTIR spectroscopy?
Single reflection ATR
The commonest form of ATR uses a small reflecting prism, or ‘crystal’, made of infrared transparent materials like diamond, zinc selenide, or germanium. Inside the crystal, the infrared beam is reflected just once and has one localised point of contact with the sample.
Multiple reflection horizontal ATR
Another form of ATR uses a long trapezoidal prism, usually made of zinc selenide. The infrared beam can bounce evenly several times along the top and bottom surfaces of the crystal. This means that more of the sample can be in contact with the IR beam and the sensitivity of the measurement will be increased. It also averages out spatial variations in the composition of the sample, giving a better representation of inhomogeneous samples.
Silicon wafer ATR
A recent innovation in ATR spectroscopy is one that uses a thin wafer of silicon material with shallow grooves on one surface to couple with the infrared beam. Multiple reflections take place inside the crystal giving improved sensitivity. However, the primary advantage of silicon wafer ATR is the low cost and consumable nature of the silicon. This makes it ideal for analyses of samples that may be difficult to clean or those that can cause damage to equipment. It’s also good for preparing batches of samples for analysis.
Microscope ATR objectives
Single reflection ATR can be applied to IR microscopy as a “point-and-shoot” analysis technique. The localised nature of the interaction with the sample makes it possible to spatially analyse samples under magnification and, unlike transmission methods, does not require the sample to be sliced to a fine thickness beforehand.
What are the applications of ATR-FTIR spectroscopy?
The characteristics of ATR spectroscopy make it the most commonly applied form of FTIR analysis. These include its inherent ease-of-use as well as its robustness to both chemical and physical damage when using a diamond ATR accessory. Equally, its specificity to just the surface layers of a sample can be useful in some circumstances.
Below are just a few examples of where ATR spectroscopy is useful.
Checking and confirming the identity of substances
Routine checking of incoming materials and quality testing of final products is a common application of ATR-FTIR, used in all major chemical manufacturing industries, from pharmaceuticals to polymers and plastics. Incoming materials are checked against a reference spectrum to ensure that they are correct.
In a similar vein, pharmacopoeia often require an infrared spectrum for acceptance of new molecules [1] and identification of substances from a chemical fingerprint has been used in the search for microplastics and identification of drugs and so-called “legal highs”.
Single reflection ATR is usually employed for this kind of analysis.
Monitoring cure times for adhesives
Many simple reactions are suitable for monitoring via ATR-FTIR spectroscopy. It involves monitoring spectra over time for depletion of peaks associated with unreacted components or growth of peaks associated with newly created chemical bonds. A simple example is monitoring the following reaction of isocyanate in cold adhesives for the food packaging industry.
Polyol (-OH) + Isocyanate (-N=C=O) –> Polyurethane (-(H)NC(=O)O-)
Chemical classification and machine learning
Recent years have seen an explosion of interest in using machine learning and classification algorithms such as principal components analysis (PCA) and partial least squares regression (PLS). These methods may be used to create calibration plots for quantitation of more complex samples, or to classify samples based on spectral similarities.
In some of the most exciting recent applications, machine learning has been used to identify brain cancers from spectral data on blood and serum samples. But it has other uses as well, such as quantifying polypropylene and polyethylene blends in plastics recycling feedstocks, distinguishing real and fake food stuffs such as wine, honey, tea, and olive oils; indeed, this field promises to be one of the most active in years to come.
Analysis of biomolecules, organisms, and other macromolecules with horizontal ATR
Large molecules such as proteins, vesicles, and even whole cells in solution can be difficult due to their low spatial coverage and tendency to aggregate. Equally, water-based solutions exhibit strong water bands which require careful subtraction.
Horizontal ATR can be used to average out differences in distribution between measurements and increase sensitivity to the objects of interest. It has been used to look for signs of aggregation in proteins associated with Alzheimer’s and also to obtain more representative spectra of foodstuffs such as olive oils, cheese, and meat products.
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