Fourier Transform Infrared Spectrometry
Fourier Transform Infrared Spectrometry ASTM E168, ASTM E1252
FTIR Spectroscopy: What It Is and How It's Used
FTIR, or Fourier Transform Infrared Spectroscopy, is a common analytical technique used to identify and characterize materials based on their absorption of infrared light. It is widely used in the fields of chemistry, materials science, and engineering for various purposes, such as identifying unknown materials, assessing material quality, and detecting contamination.
The basic principle of FTIR is straightforward: when a material absorbs infrared light, it produces a unique "spectral fingerprint" based on the frequencies at which it absorbs the light and the intensity of those absorptions. By comparing the spectral fingerprint of an unknown material to the spectral fingerprints of known materials stored in a computerized library, one can identify the unknown material.
The mid-infrared region of the light spectrum, which ranges from 400 to 4000 wavenumbers (2.5 to 25 microns), is typically used in FTIR analysis. The resulting spectral scan, which can be either absorbance or transmittance, is usually specific to a general class of material. For example, the spectral scan of polycarbonate looks different from that of nylon, but all nylon scans have unique similarities.
To perform an FTIR analysis, a sample is placed in the spectrometer, and the instrument generates a spectral scan. The unknown spectral scan is then compared to a database of known materials, which can be done manually or with the help of a computerized program. The computerized program can quickly compare the unknown spectrum to a large number of spectra located in multiple databases in a short period of time. Computer programs are very helpful for comparing unknown spectral scans to those of known materials, but a skilled FTIR analyst is needed to examine the computer-selected spectral matches to ensure that sample identifications are both accurate and complete.
The sample size required for FTIR analysis can vary depending on the type of material being tested. Reflective FTIR can scan samples the size of a single resin pellet, and samples that can be easily tested by reflective FTIR include polymer pellets, parts, opaque samples, fibers, powders, wire coatings, and liquids. However, materials with large quantities of carbon, such as carbon black or carbon fiber, are difficult to obtain a usable spectral scan from because carbon strongly absorbs infrared light in a broad range of frequencies, resulting in an FTIR spectrum without the minute details necessary to identify the unknown material.
FTIR analysis is often used in polymer identification, where it is the first logical step in identifying an unknown polymer. It can easily identify classes of polymers, such as nylons, polyesters, polypropylenes, polycarbonates, acetals, or polyethylenes, based on the infrared peaks of the material. However, FTIR spectral scan alone should not be expected to identify the type of nylon or polyester, identify a polypropylene or acetal as a homopolymer or copolymer, or determine whether the polyethylene is a high density or low-density material. Further identification may be aided by other techniques such as differential scanning calorimetry (DSC) or an ash test.
FTIR is also used for quality control of materials. A reference material's spectral scan can be generated and stored in a spectral library database, which allows all future material scans to be compared back to the same earlier scan. Differences noted in a newly generated spectral scan could indicate a change in processing or a possible contamination problem.
FTIR spectral subtractions are used to look for internal contamination in polymers. A computer program is used to subtract the peaks associated with the base polymer from the spectral scan, and then an analysis of the remaining spectral scan is performed. The amount of contamination that can be detected depends on the spectral scans of the base polymer and the