Fourier Transform Infrared Spectroscopy

Contributors: Jennifer Herrmann, Catherine H. Stephens

Overview

Technique: FT-IR
Formal name: Fourier Transform Infrared Spectroscopy
Basic schematic of the signal generator for this technique: Michelson interferometer (see image at right)
Basic image of data generated using this technique: See FTIR spectrum of vanillin (below)


x-axis: wavenumbers [cm-1] = 1/(wavelength of light that was absorbed by the functional group)
y-axis: either Absorbance (with values 0 – 1.5) or Transmission (with values 0-100); both give the same information, the only difference is how the peaks are oriented. Transmission is used in the example spectrum of vanillin given above.

Details

What this techniques measures: Fourier-Transform Infrared Spectroscopy measures the intensity of the absorbed or transmitted infrared radiation when the latter passes through a solid, liquid or gaseous sample. The resulting spectra represent the molecular absorption or transmission of the sample or samples tested.

In particular, when infrared radiation passes through a compound part of it is absorbed by the constituent molecules which vibrate or rotate at certain frequencies. The rest of the infrared radiation, i.e. the part that is not absorbed by the sample, is transmitted according to Beer's Law and is picked up by a detector. The resulting signal is processed by the Central Processing Unit of the PC attached to the FT-IR spectrometer, using complex mathematical operations known as Fourier Transformations, and a unique spectrum is produced.It should be stressed that FT-IR spectra are unique to each compound since no two molecular compounds produce the same infrared spectra.

Several different sample analysis techniques can be used during FTIR analysis, including attenuated total reflectance, fiber optic reflectance, microscopy, transmission, diffuse reflectance, salt pellets, etc. The type of sample to be analyzed may influence which specific sample analysis technique is most effective.

Limitations of this technique: Mixtures can be difficult to analyze…. Sampling can be problematic in conservation applications which by necessity require very small samples, so care must be taken that the sample is representative of the whole or else several small samples may be required to achieve an overall identification. For example, in painting analysis, if only a tiny grain of a certain mixtures is analyzed, complete identification of the chemical composition may not be possible if the specific sample did not contain all the components of the paint mixture.

Can/how can this technique be made quantitative? The technique is typically not quantitative but can be made semi-quantitative where comparisons can be made between bulk properties and trace analysis. For example, during the analysis of plastics, information can be obtained both on the general polymer composition, but also on plasticizers that may be present in trace amounts. Similarly, lignin and paper additives can be found when analyzing paper or photographs.

Samples

Phases it can be used to examine: Solids, liquids, and gases
Is this technique non-destructive? Depending upon the specific analysis method, FTIR can be either destructive, requiring various sample sizes, or non-destructive allowing in situ analysis. Non-destructive sampling is the best for conservation applications. Attenuated total reflectance and fiber optic probes/handheld devices can be used non-destructively.
How invasive is this technique? Depending upon the specific analysis method, FTIR can be non-invasive to very invasive requiring many samples. Fiber optic probe or handheld device analysis methods tend to be the least invasive, since attenuated total reflectance, while non-destructive, may leave a submillimeter indentation on the surface analyzed. Microscopic FTIR techniques often will not allow in situ analysis due to the constraints of the optical lenses, and therefore require invasive sampling.
Minimum size of sample necessary to use this technique? Depending upon the specific analysis method, various sample sizes are required. Sampling is likely not necessary with fiber optic probe analysis accessories or handheld devices. Microscopic FTIR techniques require the smallest samples to be taken. Attenuated total reflectance devices may not require sampling or may be used with various sample sizes including as small as a grain of sand or a fiber. Other sampling techniques tend to require larger samples which are not as relevant to most conservation applications, though could be useful if quality assurance analysis is required for conservation and housing supplies, such as adhesives and polyester sleeves.
Time to run one experiment: Analysis times can be as short as under a minute to as long as many minutes. Typical times are a few minutes, though data analysis with peak identification and library searches will often take much longer than the actual run time of the experiment. Certain techniques requiring sample preparation will also take longer than analysis methods requiring no sample preparation.
Sample preparation methods: Depending upon the specific analysis method, little to no sample preparation is necessary. Fiber optic probe, handheld devices, or attenuated total reflectance analyses require no sample preparation, while other analysis methods may require more, for example typical microscopic sample preparation or powder grinding and pellet formation.

Applications

Examples of how this technique is used in the field? Common analytes include pigments, synthetics/plastics, adhesives, residues, pesticides, and many more.
Risks associated with using this technique? Depending upon the specific analysis method, no potential damage (non-destructive analysis methods) to slight marking that may disappear after analysis (attenuated total reflectance) to physical changes due to sampling (destructive sampling methods).

Budgetary Considerations

Approximate cost to purchase equipment for this technique? ~$100,000 (USD) , with costs increasing as accessories/applications are added
Annual cost to maintain this equipment? A service contract will ensure the instrument remains in the best working order (estimate ~ $6ooo [USD, 2021]), however, the instrument is robust and often requires very little actual maintenance. Some instruments rely on dry air feed or liquid nitrogen for the detectors, which adds to the annual cost and upkeep.
Sample analysis costs? Little to none
Time it may take to get results from a contract laboratory? This depends on a contract laboratory, but data collection itself can take as little as 1 minute.

Case Studies

• Rosina Herrera Garrido, Suzan de Groot & Tom Callewaert-Dore (2020) The Coated Salted Paper Prints from the Eduard Isaac Asser Collection at the Rijksmuseum: FTIR and OCT Identification and Characterization, Journal of the American Institute for Conservation, 59:3-4, 246-261, DOI: 10.1080/01971360.2020.1774725
  
• Arthur McClelland, Elena Bulat, Brenda Bernier & Erin L. Murphy (2020) Specular Reflection FTIR: A Non-Contact Method for Analyzing Coatings on Photographs and Other Cultural Materials, Journal of the American Institute for Conservation, 59:2, 123-136, DOI: 10.1080/01971360.2019.1660546
  
• Silvia A. Centeno, Marcelo I. Guzman, Akiko Yamazakikleps & Carlos O. Della Védova (2004) Characterization by Ftir of the Effect of Lead White on Some Properties of Proteinaceous Binding Media, Journal of the American Institute for Conservation, 43:2, 139-150, DOI: 10.1179/019713604806082528
  
• Corina E. Rogge & Bradford A. Epley (2017) Behind the Bocour Label: Identification of Pigments and Binders in Historic Bocour Oil and Acrylic Paints, Journal of the American Institute for Conservation, 56:1, 15-42, DOI: 10.1080/01971360.2016.1270634
  
• Eric F. Hansen, Michele R. Derrick, Michael R. Schilling & Raphael Garcia (1991) The Effects of Solution Application on Some Mechanical and Physical Properties of Thermoplastic Amorphous Polymers Used in Conservation: Poly(vinyl acetate)s, Journal of the American Institute for Conservation, 30:2, 203-213, DOI: 10.1179/019713691806066764
  

Additional Information

Complementary Techniques: FT-Raman, x-ray fluorescence (XRF) Variations of this technique: There are myriad accessories to use WITHIN the instrument to collect data. Techniques can include attenuated total reflectance (ATR), which looks at surfaces; Diffuse reflectance infrared fourier transmission (DRIFTS), which is for analyzing powders, or photoacoustic (PA) FTIR, which can be used to depth profile a sample

References Resources, databases, publications
Links to external resources/databases:

• Infrared & Raman Users Group (IRUG)
• Further reading from Michigan State University
  

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