Fourier transform infrared spectroscopy (FTIR) is an incredibly versatile materials analysis technique, helping identify organic and some inorganic materials that could be the source of product contamination or cause a malfunction. As a result, we often recommend FTIR to our clients as the first step in the testing process.
But when it comes to understanding and analyzing FTIR results, how we interpret FTIR spectra is a common question.
Your materials testing lab should be providing you with clear, detailed, and actionable insights and recommendations in an easy to understand format—something Innovatech Labs is dedicated to.
With that said, we’ve developed this helpful guide to interpreting FTIR spectra to help our customers—and anyone else who’s interested—know how to analyze FTIR data.
In order to understand FTIR results, it’s helpful to know a little bit about how FTIR works.
Essentially, by applying infrared radiation (IR) to samples of materials, FTIR analysis measures a sample’s absorbance of infrared light at various wavelengths to determine the material’s molecular composition and structure. The Fourier transform spectrometer works to convert the raw data from the broad-band light source to actually obtain the absorbance level at each wavelength.
FTIR can be used on solid, liquid, and gaseous samples. Usually, the amount of material required for a viable analysis is very small and most analyses can be done relatively quickly with little sample preparation.
How to Read FTIR Results Graphs
The X-Axis: The Infrared Spectrum
The x-axis—or horizontal axis—represents the infrared spectrum, which plots the intensity of infrared spectra. The peaks, which are also called absorbance bands, correspond with the various vibrations of the sample’s atoms when it’s exposed to the infrared region of the electromagnetic spectrum. For mid-range IR, the wave number on the infrared spectrum is plotted between 4,000 to 400 cm-1.
The Y-Axis: Absorbance or Frequency
The y-axis—or vertical axis—represents the amount of infrared light absorbed or transmitted by the material being analyzed.
The Absorbance Bands
Typically, absorbance bands are grouped within two types: Group frequencies and fingerprint frequencies.
Group frequencies are characteristic of small groups of atoms or functional groups such as CH₂, OH, and C=O. These types of bands are typically seen above 1,500cm-1 in the infrared spectrum (See top spectrum in the graph below) and they’re usually unique to a specific functional group, making them a reliable means of identifying functional groups in a molecule.
As for fingerprint frequencies, these are highly characteristic of the molecule as a whole; they tell what is going on within the molecule. These types of absorbances are typically seen below 1500cm-1 in the infrared spectrum (See bottom graph of figure below); however, some functional groups will absorb in this region as well. As a result, this region of the spectrum is less reliable for identification, but the absence of a band is often more indicative than the presence of a band in this region.
How to Interpret FTIR Spectra
Once the initial testing and spectrum collection is complete, interpretation of FTIR spectra comes next.
Typically, interpreting FTIR spectra starts at the high frequency end to identify the functional groups present. The fingerprint regions are then studied to positively identify the compound. Thankfully, there are vast libraries of infrared spectra available, allowing our team to compare unknown materials to ensure quick and accurate identification.
Still Curious About FTIR Analysis?
Fourier transform infrared spectroscopy is an incredibly helpful and versatile quality control and troubleshooting tool for manufacturers and researchers across industries. If you have more questions about the technique or are wondering if it may be a fit for your testing needs, contact us for a quote. Our team is ready to help.