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Fourier transform infrared spectroscopy (FTIR) is an experimental test to identify organic/inorganic materials. But for understanding and interpretation of FTIR results, the principles of the analysis should be studied.

Principles of FTIR

To better interpretation of FTIR results, it’s useful to know about the principles of the test.

Generally, by introducing infrared radiation (IR) to a material, FTIR’s detectors detect a sample’s absorbance of infrared light at various wavenumbers to determine the material’s chemical structure. FTIR spectrometer converts the experimental data from the broadband source to the absorbance level at each wavenumber.

This method can be employed for solids, liquids, and gaseous samples. Some times, the weight of samples required for a viable analysis is very low and most analyses can be conducted with little sampling process.

How to interpret FTIR spectra

The X-Axis

The horizontal axis shows the IR spectrum, which represents the intensity of the IR spectra. The absorbance peaks, can be attributed to the various vibrations of the sample’s bonds exposed to the IR energy of the electromagnetic spectrum. Usually, the wavenumber on the IR spectrum is drew between 4000 to 400 cm-1.

The Y-Axis

The vertical axis shows the amount of IR absorbance/transmittance by the under-studied material.

The Absorbance Bands

Generally, absorbance bands can be derived into two groups: Group frequencies and fingerprint frequencies.

Group frequencies are assigned to small groups of bonds such as C-H, O-H, and C=O. These types of bonds are usually observable at 1500-400 cm-1 in the IR spectrum and they are typically unique to a specific bond or functional group in a structure.

For fingerprint frequencies, these are assigned to the chemical structure as a whole. These types of absorbances are usually observable at 400-1500cm-1 in the IR spectrum. Therefore, this region is less reliable for identification, however the absence of a peak is often more indicative than the presence of a peak in this region.

How to analyze FT-IR Spectra

Generally, interpretation of FT-IR spectra starts at the high frequencies end to identify the presence functional groups. The fingerprint regions are then investigated to identify the chemical bonds.

Still Curious About FTIR Analysis?

FTIR is a useful technique for manufacturers and researchers of various industries. If you have more questions about the analysis or are wondering if it may be a fit for your testing needs, contact us for a quote.

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Only 10 $ for interpretation of your FT-IR spectrum
Payment Upon Completion
 Send your spectra...

Only 10 $ for interpretation of your FT-IR spectrum 
Payment Upon Completion

Send your spectra...

1. Determine the X-axis and the Y-axis of the spectrum. The X-axis of an IR spectrum is labeled as “Wavenumber” and ranges in number from 400 on the far right to 4,000 on the far left. The X-axis provides the absorption number. The Y-axis is labeled as “Percent Transmittance” and ranges in number from 0 on the bottom and 100 at the top.

2.Determine the characteristic peaks in the IR spectrum. All IR spectra contain many peaks. However, determine the large peaks on the spectrum because they will provide the data necessary to read the spectrum.

3. Determine the regions of the spectrum in which the characteristic peaks exist. The IR spectrum can be segregated into four regions. The first region ranges from 4,000 to 2,500. The second region ranges from 2,500 to 2,000. The third region ranges from 2,000 to 1,500. The fourth region ranges from 1,500 to 400.

4. Determine the functional groups absorbed in the first region. If the spectrum has a characteristic peak in the range of 4,000 to 2,500, the peak corresponds to absorption caused by N-H, C-H and O-H single bonds.

5. Determine the functional groups absorbed in the second region. If the spectrum has a characteristic peak in the range of 2,500 to 2,000, the peak corresponds to absorption caused by triple bonds.

6. Determine the functional groups absorbed in the third region. If the spectrum has a characteristic peak in the range of 2,000 to 1,500, the peak corresponds to absorption caused by double bonds such as C=O, C=N and C=C.

7. Compare the peaks in the fourth region to the peaks in the fourth region of another IR spectrum. The fourth is known as the fingerprint region of the IR spectrum and contains a large number of absorption peaks that account for a large variety of single bonds. If all the peaks in an IR spectrum, including those in the fourth region, are identical to the peaks of another spectrum, then you can be assured that the two compounds are identical.