I have had FTIR done on my sample of mesoporous silica nanoparticles coated with chitosan. When I received the data I noticed that all the %transmittance values corresponding to the wavenumber (cm-1) are greater than 100. But how can %tranmittance be greater than 100? Logically the maximum value can be 100 whereas the maximum value in my data is around 125. Can someone kindly explain the reason?
Dear Mr. Amin, What are you using as a reference? Is it KBr? Check if your reference is not contaminated.
Unfortunately, instruments may show some drift which means that the reference spectrum should be recorded newly every once in a while. A baseline correction is certainly possible, but the drift is not necessarily constant with regard to wavenumber. What are you actually using as a reference? Note that in infrared spectrophotometry it is generally not a good idea to measure either an empty or a cuvette filled with the pure solvent as reference...
What you have is relative units (125), the maximum transmitance is 100%
Please divide the value that you have to 125 and you will have the transmitance in %.
% transmittance is defined by T/To where To is the reference spectrum. Thus, if the reference sample does not transmit as much as the sample, you can observe more than 100% transmittance. Such is the case if your sample reflect from the surface more than the sample. When you coat a sample with lower refractive index material, such as chitosan, than silica, then the transmission will increase. If you specify how you obtained the spectrum, the answer can be much more specific.
The % transmittance is calculated against your empty cell or a reference. If you reference is not transparent to IR (or your empty cell is not clean) it can give you a lower transmittance than the sample, making your sample signal >100%. In this case, repeat baseline with a clean cell. If you are using diamond cells, stay in the center of the cell (not on the edge) and use only one cell after crushing your sample between two. Remember also to calibrate your instrument with a standard (polystirene?) to adjust attenuation. And be sure to warm-up the instrument for about 15 min befor measurements, otherwise spectra can't be reliable.
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