Infrared Spectroscopy is important to scientific researchers and food analysts across the globe, due to its usefulness in determining and identifying structures of compounds. Understanding Infrared Spectroscopy and how to use Fourier Transform Infrared Spectroscopy (FTIR) to identify and characterize organic compounds is vital to your success as a food analyst or executive. Although primarily used in the packaging industry, it’s important for everyone to have a baseline understanding of how Infrared Spectroscopy works, and how it can help your company succeed.
What is Infrared Spectroscopy?
Infrared Spectroscopy, also known as IR Spectroscopy, is an analytical technique for chemical compound identification. This specific branch of science is based on the fact that different chemical functional groups absorb infrared light at different wavelengths, dependent on the nature of the particular chemical function group. Infrared Spectroscopy can reveal the structures of covalently bonded chemical compounds, like organic compounds.
What is a Fourier Transform?
A Fourier Transform is a mathematical conversion that allows for the split of the entire infrared light spectrum, then converts the scanning results into spectra that assess wavelength versus absorbance.
A Fourier Transform combined with Infrared Spectroscopy creates Fourier Transform Infrared Spectroscopy, or FTIR. FTIR can be used to identify and characterize organic compounds. The simplicity of FTIR methods allows for widely used analysis of different materials. FTIR methods are often used in the packaging industry, specifically to analyze monomeric materials for purity, as well as identify polymers (polyethylene, polyester, nylon, etc.) and their compositions.
How is Infrared Spectroscopy Used?
FTIR is routinely used for forensic analysis. For example, by matching the spectra of the material in question with the spectra of known compounds, FTIR methods can identify foreign materials in food or beverage products.
Infrared Spectroscopy can be used to investigate solids in powder, film or block form, pure liquids and liquid solutions, and gases. It is particularly useful for the identification of pills and the component makeup of multi-layer coatings. When used for compound identification purposes, IR Spectroscopy is typically operated in the Mid-IR range between 4000-400 cm-1 (2.5-25 μm). IR in either the Far-IR range, between 400-10 cm-1 (25-1000 μm), and the Near-IR 14,000-4,000 cm-1 (0.71-.5 μm), is typically carried out for special purposes.
Near-IR applies to methods such as the routine, rapid quantitation of macro components of complex mixtures, such as protein and moisture in grains and flour. Modern IR instruments can be operated in transmission or reflectance mode.
In the past, analyses were often limited by the thickness of the samples, the sample shape, and the sample’s bulk quantity. These limitations have been largely overcome by the use of adapters that change the presentation of the samples to the IR beam.
Attenuated Total Reflectance (ATR) and Diffuse Reflectance accessories allow for analyzing the surfaces of coatings and compare the bulk properties with the surface properties of a material.
How Does IR Analysis Work?
There are three basic procedures, or modes, used to obtain infrared spectra of samples. Each has its own unique advantages for optimizing the quality of the spectrum obtained.
The transmission sampling technique involves passing infrared energy through the sample and detecting the portion of the beam that is transmitted, or not absorbed. The infrared beam permeates the sample, then the energy passing through the sample is measured against the respective wavelength to generate a spectrum.
Transmission Using IR Sampling Cards
IR sampling cards are accessories frequently used to conduct transmission analyses, which utilize a chemically inert or resistant, non-hydroscopic microporous sampling substrate. When applied as a solution, the microporous sample substrate permits rapid solvent evaporation.
The two most popular types of cards are polytetrafluoroethylene cards, which provide an absorption-free background from 4000-400 cm-1 and polyethylene substrates, which provide an absorption-free background for the entire spectrum, except in the region of 3000-2800 cm-1. Using these two cards in combination allows full transmission scanning of the spectral range from 4000-400 cm-1. Because they are disposable, use of the IR cards typically shortens analysis time, eliminates cross-contamination concerns, and reduces solvent clean-up waste.
IR cards allow the analysis of a wide range of samples, including—but not limited to—inorganic and organic liquids of low volatility, solids that can be solubilized in volatile solvents, insoluble solids, semi-solids, pastes, lubricants, paints, and more.
Pressing an alkali halide pellet—most often a potassium bromide pellet—is another widely used technique when investigating solid samples in the transmission mode. The method consists of grinding the sample together with pure, dry spectroscopic-grade KBr to a fine powder, then transferring the mixture to a compression die. The mixture is then placed under high pressure until the mixture forms a pellet that is transparent to infrared light.
Diffuse and Specular Reflectance
Reflectance IR analysis is an optically simpler technique than transmission IR, especially for solids. It involves reflecting the infrared light off of the sample. When infrared radiation is directed onto the surface of a solid sample, the result is one of two types of energy: Specular or Diffuse Reflectance.
Diffuse reflectance is the radiation that penetrates the surface of the sample (usually to a short depth), and then re-emerges after reflecting off internal portions of the sample. External reflection techniques provide a non-destructive method for measuring surfaces and coatings without sample preparation.
Si-Carb paper is a small disk of silicon carbide used for diffuse reflectance analyses. This technique allows us to abrade the surface and transfer a small amount of sample—hard polymer, paints, and coatings—to the disk. An infrared spectrum is obtained from the material that clings to the surface of the Si-Carb disk after abrading. If abrading extremely hard samples, consider trading Si-Carb paper for diamond paper.
Attenuated Total Reflectance
The primary function of Attenuated Total Reflectance spectroscopy—otherwise known as ATR—is to measure the changes that occur when a totally, internally reflected infrared beam comes into contact with the sample. Of the available reflection methods, companies are increasingly using ATR due to its ease of use and the limited amount of sample preparation needed.
In the ATR mode of analysis, the sample is pressed into place, making physical contact with a special ATR crystal. The sample and crystal are then placed in the IR beam in such a position that the IR beam enters the crystal at an angle and is then reflected along the length of the sample. IR energy exiting the opposite end of the crystal is measured against the respective wavelength.
This technique is ideal for rapid quantitative and qualitative analyses because no sample preparation is required. Single and multiple internal reflection ATR accessories are well-suited for highly infrared-absorbent materials such as rubbers and polymers. In addition, ATR is equally well-suited for providing information about the surface makeup or surface conditions of a material. Single reflection ATR technology is also used in the ATR Microscopy objectives.
FTIR microscopy is a technique that uses a modified light microscope in combination with an FTIR spectroscope. The design goal of a quality microscope modified for infrared use is twofold. The first is to collect infrared spectra free of spectral contributions of the surrounding matrix. Secondly, to focus on the fine detail of the sample—for example, looking at an individual layer of a multilayered film structure. Applying IR microscopy capabilities is for solving problems such as:
- Micro-contaminants in plastics, paper, pharmaceutical products and packaging materials
- Surface analysis of rubbers, paints and coatings
- Forensic science (paint chips, crystals, powders, fibers)
FTIR Library Matching
The powerful computing capabilities of the FTIR spectrometer can also be used for comparing spectra of unknown compounds against spectra of known compounds. Typically, FTIR equipment suppliers provide the instrument user with a library of spectra, including spectra of different polymers and organic and inorganic materials.
Take the Next Step
As you now know, Infrared Spectroscopy is used by chemists and analysts alike to determine functional groups in molecules. IR Spectroscopy measures the vibrations of atoms and uses the extracted data to determine these functional groups. Most often, Infrared Spectroscopy analysis is used in the packaging industry to determine package characterization. Contact us to see if this analysis is right for you and your product matrix.