I am quite new to FTIR and getting a little confused, I have some basic questions and would like to have some simple explanations for them ...
To begin with I did FTIR for the same sample twice, but the spectra is not exactly the same for both. From my little knowledge of FTIR understanding the peak position is what really matters and that is the same for both spectra, however, there is a huge difference in the transmittance(pics attached) what does this indicate? And, what could have caused the same sample to show different transmittance in two runs?
On a separate note, I had a sample for which I did FTIR and noticed a peak at 3300 indicating the presence of hydroxyl groups, the sample was then chemically treated and was again analysed by FTIR and it still showed a peak at around 3300, however the peak was much broader than before, what does this change in sharpness of peak indicate?
Thank you..
I assume your spectra have been recorded in transmission mode. What kind of material are you using? The uploaded spectra show a significant amount of water in your sample. Using the "wrong" window material (e.g. NaCl or KBr) in the transmission experiment would explain the difference between the two runs as the window material is affected.
The shape of the OH band depends on the hydrogen bonding network. A broad band indicates that there a many different bonding states of OH groups (e.g. in liquid water), a narrow peak can be found e.g. in the vapor phase where no hydrogen bonds are present.
The problem is probably due to wrong sample preparation. Is there any hydroxyl group in compound?
If you are using KBr pellete techinique, than you must take care of ratio of sample and KBr (1:200). you must dry enough your sample before recording.
http://www.niu.edu/analyticallab/ftir/samplepreparation.shtml
Can you please tell us, what sort of sample you used?
& how you took the spectra? i mean is it involve any sample preparation ?
It will help us to understand the problem
Fatima, try acquiring FTIR in absorbance mode rather transmittance. If peak position remains constant, the area under the peak gives you the qualitative estimation of the amount of that particular moiety. Please be aware that you shouldn't be using KBr window as KBr is hygroscopic and will affect the measurements. Try using NaCl window rather or collect in reflectance mode. Looking to your spectra, it appears that there is no apparent change in peak positions but the upward shift in the curve that happens when your material's quantity decrease as a function of time in the area of the scan.
On your other question, regime 3300 cm-1 is a combination of lots of functionalities associated with water molecules through hydrogen bonding. If you have carbonyl, amine or hydroxyl stretching in your compound, the shape of this region will change with the amount of different species linked to water. I recommend collecting data in absorbance mode for more infromation.
yes it is very First you must the functional group posyton may from book or from google searcher
Thanks for the replies everyone.
So the sample was a thin film of a natural polymer. Now, the FTIR technician did not make any sort of pellet he insisted that a pellet is only necessary if I have a very small amount of sample and that wasnt the case so he directly placed the film under the probe, but from all the answers it seems the pellet is necessary....
Your spectrum is quite close to a cellulose polymer. At the link below you could find the ATR/IR spectrum of linen fiber (compare peaks position, not intensity that in the ATR mode can be quite different from transmission IR), a natural fibre made of cellulose. Maybe after your chemical treatment you induce a breakdown of cellulose chains inducing a decrease in the crystallinity of the material. Hope it could help!
http://tera.chem.ut.ee/IR_spectra/index.php?option=com_content&view=article&id=131&Itemid=100
Mr. Ahmad,
There have no any difference between both the spectra, they have been measured at different concentration. Please find as attachment an overlapped figure.
it is difficult to answer to you without having more details about the FT-IR technique (maybe ATR?) and the acquisition parameters you used. If you used the same film of polymer and you recorded both spectra in transmission mode under the same condition, the difference cannot be explaned.
Please add more details and we will try to help you.
Thanks for the answers everyone, I did TGA for the sample to further investigate the samples https://www.researchgate.net/post/How_to_interpret_a_TGA_graph
For anyone who is trying to understand FTIR spectrum I recommend this video series from Khan Academy (https://www.khanacademy.org/science/organic-chemistry/spectroscopy-jay)
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