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Understanding Infrared (FTIR) Spectroscopy for Chemical Analysis: How Infrared Light Reveals Chemical Fingerprints

Home » Blog » Understanding Infrared (FTIR) Spectroscopy for Chemical Analysis: How Infrared Light Reveals Chemical Fingerprints

Introduction

Direct chemical analysis of unknown substances without further chemical manipulation is rare. Even more uncommon is performing this analysis without breaking down or destroying the sample. However, when chemical testing is rapid, reliable, and repeatable, it becomes a powerful technique.

Fourier Transform Infrared (FTIR) Spectroscopy is one such technique. By using infrared light (heat), the instrument interacts with the sample through molecular vibrations. Much like playing a musical note produces a unique tone, infrared waves can be used to make different chemical bonds to vibrate. When these bonds vibrate, the energy to a detector decreases. Therefore, when playing a series of notes, or a broadband spectrum of continuous wavelengths, then recording the ‘notes’ where energy decreases, unique chemical fingerprints are generated.

How Does Infrared (FTIR) Spectroscopy Work?

FTIR spectroscopy is rooted in quantum mechanics, which explains how matter and energy interact at the atomic and subatomic levels. Once classical physics failed to explain the interactions of light and matter, alternate explanations were sought. The result of that inquiry over many years is the foundation of the modern electronics age brought about by harnessing the nature of matter at the atomic level. 

The instrument uses a mathematical technique called the Fourier Transform (FT) to convert raw data from the time domain into a readable frequency spectrum.  Electrons are the negative particles that complement the positive protons and the neutral neutrons.  These three particles are the simplest common reduction of matter to common components to uniquely identify the basis of all matter, the elements.  These elements as documented by the periodic table provide the basis through which modern chemistry is possible. 

The Role of Light in Chemical Analysis

Light is a spectrum of frequencies.  Visible light impacts daily life for it is the range through which our eyes view the world.  This idea is illustrated well by the rainbow which is the separation of white light into different frequencies, we recognize as color from the interaction of white light with water vapor.  UV, Infrared, X-rays, radio waves all are light of different frequencies, or energies.

Thanks to quantum mechanics, we understand that light only interacts with matter when the energy “note” matches a molecular vibration. This principle makes infrared spectroscopy incredibly powerful for identifying compounds based on how their bonds absorb IR energy.

What is FTR Spectroscopy?

The infrared experiment is called infrared or FTIR spectroscopy.  The ‘FT’ is a mathematical transformation that allows the spectrum to be collected in the time domain, but the data is transformed to the reciprocal of time, which is frequency.  Light energy is the combination of frequency and a constant named after Planck.  The speed of light constant is the product of the light frequency and the wavelength. In the infrared region, this corresponds to the frequency that bonds vibrate.  If atoms are considered spheres, and bonds are considered springs, then it is straightforward to imagine that atoms of different masses with different strength bonds will vibrate at different frequencies. There is a subset of carbon-based compounds that can be grouped by unique sets of atoms vibrating in unison.  These are described as functional groups that allow the trained eye to quickly identify the unique fingerprint of the type of molecules. 

Practical Applications and Real World Impact

Since a complex molecule will have unique ‘chords’, every material has a unique fingerprint. With the advancements of mathematical matching against a library of known compounds, in a matter of seconds, one can learn the most probable type of material that is being analyzed. Certain crime shows often show these results, and this test does work on TV time, as it often takes longer to prepare the sample than run the test. At Sparx Engineering, we use FTIR spectroscopy for a variety of real-world applications, including:

  • Identifying unknown substances
  • Detecting water in plastic materials
  • Monitoring chemical reactions
  • Characterizing polymers and plastic types

With advanced spectral libraries and fast data matching algorithms, we can determine a material’s identity in seconds, often faster than the time it takes to prepare the sample.

Example 1

Spectroscopy testing comparing acetaminophen vs ibuprofen.

For context, the power for the identifications of two unknown chemical substances is straightforward.  In this case, household pain relievers acetaminophen and ibuprofen were analyzed, then matched versus the spectral library owned by Sparx. First it is easy to distinguish the two.  In cases like this, unknowns have been identified for projects at Sparx when a customer part failed and an unknown substance was observed as part of the failure mode.  In one example, a battery failure was identified, instead of the customers part, by analyzing substances on the outside of the battery.

Example 2

FTIR Diagnostic ID of water from improperly dried plastic. Image shows graph of FTIR results of authentic polycarbonate with an overlayed result of water in polycarbonate. It can show where the water was affecting the results.

In this example, product injection molded parts were failing.  The molder blamed the plastic, the customer questioned the choice of plastic.  In a matter of minutes, Sparx was able to show that both were not correct.  The molder was not drying the plastic according to vendor specifications.  The result was water damaging the polymer.  From an engineering point of view, the water damage resulted in plastic with weaker structural properties, and the plastic was cracking during field use.  By correcting the drying problem, Sparx was able to get parts prepared according to specification that met teh customer requirements.

Example 3

Bag placed of spectroscopy tester

This picture shows the ATR accessory. For solids like plastic, the sample can be simply clamped to the sample platform. With this accessory, the infrared light is manipulated with mirrors to the diamond sample platform. This accessory makes the instrument accessible to untrained engineers to evaluate plastic. The preparation of chemical samples by transmission requires training to produce quality samples. For this use case, as customer specified high density polyethylene. The the sample was compared with the authentic libary compound, it was evident that the molder had infused a polyester to cut cost. This resulted in a plastic that did not meet engineering requirements for mechanical strength.

Image of spectroscopy testing results to ID plastic. Plastic shows similar wavenumbers to standard polyethylene.

Conclusion

FTIR is a powerful nondestructive technique for the evaluation of samples usually within minutes. At times expert review of the data is needed to draw conclusions of value to the customer. After a high level summary of the method, examples were shown for the ID of substances with common household pain relievers. Next it was shown that a molding failure was a result of improper drying of the polymer before it was injected into the mold. Finally, a part advertised to be comprised with pure high density polyethylene was instead shown to be contaminated with polyester.

FTIR is a power chemical analysis technique. This is one of the tools Sparx uses to support customers, aid engineering and troubleshoot failure modes when they occur in the field or as part of the development process. Whether you’re analyzing plastics, coatings, or contamination, Sparx Engineering’s FTIR capabilities can help you get fast answers and smarter solutions.

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