Fourier Transform Infrared (FTIR) spectroscopy is an analytical method used to identify the chemical makeup of solids, liquids, and gases by examining how its molecules interact with infrared light. The technique provides a distinct profile of a sample, creating a “molecular fingerprint” that helps scientists determine its components with a high degree of specificity.
How FTIR Technology Works
The core of FTIR analysis involves shining a beam of infrared light on a sample. This beam contains a wide range of infrared frequencies simultaneously. The light passes through a device called an interferometer, which is central to the “Fourier Transform” part of the name and spectrally encodes the light before it reaches the material.
Molecules are not static; their chemical bonds are in constant motion, vibrating by stretching and bending at specific frequencies. These vibrational frequencies are unique to the types of atoms and the bonds connecting them, much like different tuning forks vibrate at distinct pitches.
When the infrared light passes through the sample, the molecules absorb energy from the frequencies that match their own natural vibrational frequencies. The rest of the light passes through and is measured by a detector. The detector records the pattern of absorbed energy, producing a signal known as an interferogram.
This raw data is not directly interpretable. A computer applies a mathematical process called a Fourier transform to this signal. This calculation converts the interferogram into a recognizable spectrum, which plots the intensity of absorption against the frequency of the light. The resulting graph provides the detailed molecular fingerprint used for analysis.
What an FTIR Scan Reveals
The output of an FTIR scan is a spectrum, a graph that plots the amount of infrared light absorbed by the sample against the frequency of the light. The vertical axis shows absorbance, while the horizontal axis represents frequency in wavenumbers (cm⁻¹). Each peak on this graph corresponds to a specific molecular vibration, indicating the presence of a particular chemical bond.
The positions of these peaks are informative. For example, a strong absorption band around 3300 cm⁻¹ signifies the presence of an oxygen-hydrogen (O-H) bond, found in alcohols and water. A sharp peak near 1700 cm⁻¹ indicates a carbon-oxygen double bond (C=O), a feature of compounds like ketones and esters. The intensity of each peak relates to the concentration of that specific bond within the sample.
A useful part of the spectrum is the “fingerprint region,” which typically spans from 1500 cm⁻¹ down to 400 cm⁻¹. This area of the graph is dense with many overlapping peaks that represent complex vibrations involving the entire molecular structure. The intricate pattern in the fingerprint region is unique to each specific molecule.
To identify an unknown substance, an analyst compares the measured spectrum against a digital library containing reference spectra from known compounds. Software searches for a match between the unknown sample’s fingerprint and those in the database. A successful match provides a confident identification of the material’s chemical composition.
Common Uses for FTIR Scans
In industrial quality control, FTIR is used to verify the identity and purity of substances. For example, pharmaceutical companies use it to confirm the correct active ingredient is present in a medication and to screen for potential contaminants. Recycling facilities can use it to rapidly sort plastics by identifying their polymer type.
Forensic science uses FTIR to analyze trace evidence found at crime scenes. A single textile fiber or paint chip can be analyzed and its spectrum compared to a sample from a suspect, helping to link individuals or objects to a location. The technique is also used to identify unknown powders or liquids, such as seized drugs, providing confirmation of their chemical identity.
In art conservation, the non-destructive nature of some FTIR methods is beneficial. Analysts can identify the specific pigments used in a painting to authenticate the work or understand an artist’s methods without damaging the art. It can also be used to analyze residues on ancient artifacts, like pottery, to determine what they once held.
In food science, FTIR helps ensure product safety and authenticity. It can detect fraudulent ingredients or contaminants, such as the illegal addition of melamine to milk powder. The analysis can also verify the composition of products like cooking oils, identifying cases where a cheaper oil has been substituted for a more expensive one.
Analyzing Different Types of Samples
FTIR spectroscopy is a versatile technique capable of analyzing samples in solid, liquid, or gaseous states. The instrument is a desktop-sized unit containing the light source, interferometer, sample compartment, and detector. Analysis involves placing a small amount of the substance into the machine using a method suited to the sample’s form.
A common technique is Attenuated Total Reflectance (ATR). This method is ideal for solids and liquids and requires minimal sample preparation. In an ATR setup, the sample is pressed firmly against a durable crystal, such as diamond or germanium, and the infrared beam is directed into it to reflect internally.
At the point of reflection against the sample, a phenomenon called an evanescent wave extends a very short distance beyond the crystal surface and into the material. If the sample contains bonds that vibrate at the frequencies present in the light, it will absorb energy from this evanescent wave. This attenuation of the light is measured, producing a spectrum about the sample’s surface. The ease of pressing a sample against the crystal makes ATR a popular choice for rapid analysis.