Fourier Transform Infrared Spectroscopy

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Overview

FTIR basic schematic.gif

Technique: FT-IR
Formal name: Fourier Transform Infrared Spectroscopy
Basic schematic of this technique: Michelson interferometer (see image at right)
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.

Most common uses in conservation

Examples of chemical substance(s) or physical feature(s) identified: alcohols (C OH), aldehydes (H-C=O), carbonyls (C=O), silanes (Si-O-Si), methyl groups (-CH3)
Phases it can be used to examine: Solids, liquids, and gases
Is the technique quantitative? Generally, no, but can be made to be semi-quantitative
Is this technique destructive or non-destructive: Can be both depending on the application used (see below) and how the sample is prepared
Size of sample necessary to use this technique:

Applications

Examples of how this technique is used in the field of art conservation: To identify materials used in paintings
Risks associated with using this technique: One shouldn’t look into the laser of the instrument; if preparing solutions, follow safety protocols associated with solvent
Cost to purchase this instrument: ~$100,000 (USD), with costs increasing as you add accessories/applications
Annual costs to maintain this instrument: dry air feed; liquid nitrogen
Sample analytical costs:
Complementary Techniques:

  • FT-Raman

Variations of this technique:

Links to external resources/databases:


Data:

FTIR sample spectrum.gif

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

Case Study


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