An Infrared Radiation spectrum chart, or IR spectrum chart, is the product of a Fourier Transform Infrared Spectroscopy (or FTIR) material analysis test. FTIR tests are one of the most commonly conducted tests for characterizing molecules within a material largely because IR spectrum charts are so easy to produce, clear, and reliable.
This is what you should know about an IR spectrum chart, including how FTIR tests produce them, what they represent in detail, and even how you can start reading them to understand what they have to tell you about the molecules and molecular bonds within a sample.
What is an IR spectrum chart?
When molecules are irradiated with infrared light, the bonds between them respond by vibrating faster. Depending on the spectrum of infrared light and the type of bond absorbing the light, these vibrations occur at different speeds.
The bonds between different functional groups of molecules, such as alkanes, alkenes, and alcohols, always vibrate at different characteristic speeds. By bombarding a sample with infrared radiation and watching for bonds to begin vibrating at these characteristic speeds, analysts can identify the functional groups of the molecules within a sample.
An IR spectrum chart makes this identification possible. These charts depict and map the speeds at which all of the molecular bonds within the sample vibrate in response to absorbing and transmitting different spectra of infrared radiation onto a readable graph with an X and Y axis.
By reading this graph, analysts can understand how the material’s molecular bonds vibrate in response to radiation and therefore which functional groups of molecules the sample contains. By comparing the most complex reactions of the molecule’s bonds at specific spectra to known reference material, analysts can even begin to identify what’s happening within the molecules they find.
How is an IR spectrum chart created?
FTIR testing produces a great deal of raw data about how a physical sample absorbs, transmits, and vibrates in response to different wavelengths and spectra of infrared radiation by bombarding the sample with all infrared spectrums in very quick succession. This data is known as an “interferogram.”
After generating the interferogram, the FTIR testing device’s computer applies a mathematical algorithm known as the “Fourier transform” algorithm to it. The Fourier transform converts the interferogram’s data on how the substance absorbed each spectrum of infrared radiation into an IR spectrum chart.
This makes it possible for analysts to understand how the substance absorbed each spectrum over time, allowing them to see how the different spectra affected the vibrations of the molecules inside the sample and therefore which molecules the sample contains.
What is an IR spectrum chart useful for?
It’s quite difficult to understand the complete molecular structure of most materials by interpreting their IR spectrum chart alone. Most organic molecules have dozens of different bond stretching and bending motions, all of which absorb IR light in different ways.
This means even though all molecules belonging to the same function groups all vibrate at characteristic speeds in response to specific spectra, the individual molecules within those functional groups all vibrate at slightly different speeds.
These differences can be very minute and difficult to distinguish on a conventional IR spectrum chart, and so analysts usually can’t rely solely on IR spectroscopy to characterize and analyze large, complex biomolecules that contain a large number of individual molecules and complex molecular bonds within the same functional groups.
In other words, an IR spectrum chart can show analysts which functional groups of molecules a sample contains and even provide some information about the molecules within those groups, but it often can’t help them definitively tell those molecules apart.
Fortunately, an IR spectrum chart is still a very useful way to derive structural information about a physical sample despite these limitations. In fact, IR spectrum charts partially rely on the complexity of individual molecule’s bond motions to produce the distinctive molecular “fingerprint” that helps analysts characterize the molecules in question.
Different functional groups generate “absorbance bands” when they absorb the infrared spectrum that makes their molecules vibrate faster. These broad absorbance bands are how analysts determine which functional groups the sample contains.
Absorbance bands will also have slightly different peaks with the range of the spectrum they react to. These small, differing peaks are known as molecular fingerprint frequencies. These frequencies help analysts begin to understand what’s going on within the molecule they’ve identified, and can sometimes even help analysts tell different molecular bonds within the same functional group apart.
How can I read an IR spectrum chart?
An IR spectrum has an X and Y axis that provides information about a single line that should wave up and down across both. The IR spectrum is interpreted from the left to right along the x-axis. It allows analysts to determine which spectra of infrared radiation molecules within the sample react – which, in turn, allows them to determine their functional groups.
The X-axis of the IR spectrum chart plots the different intensities of the infrared spectra the sample was exposed to in wavenumbers, or cm-1. This shows analysts which spectra of infrared light is producing the response they’re looking at along the y-axis.
Meanwhile, the Y-axis of the IR spectrum chart measures and plots the amount of infrared light absorbed by the sample material at each of these frequencies as a percentage of that absorbance. When the line plotted along these axes bends “up” along the Y-axis, it means the molecules inside of the sample are absorbing the infrared spectrum and vibrating in response. These are called the absorbance bands.
By studying where the absorbance bands occur and cross checking them with reference material, analysts can determine which functional groups the sample contains. For example, molecules in the Alkane functional group (C-H) will always demonstrate an elevated absorbance within 2850 to 2960 cm-1. That means, if the IR spectrum chart shows an absorbance band at this frequency along the x-axis, the sample contains Alkane molecules.
After identifying the major functional groups within the molecule using the IR spectrum chart, analysts will typically take a closer look at the absorbance bands to identify the molecular fingerprint frequencies within them. These look like the small peaks and dips at the top of an absorbance band.
These frequencies provide additional information about what’s happening within the molecules of each functional group. Analysts can occasionally even use this information to more accurately characterize the individual types of molecules and bonds within each functional group.
How can I get an IR spectrum chart of my substance?
Innovatech’s professional analysts are experts at conducting FTIR tests to produce and interpret IR spectrum charts. If you’d like to produce an IR spectrum chart of the molecules inside a physical substance, just get in touch right away for a quote and we can get started.
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