one of my synthesised compound is showing only 5% transmittance. What it signifies?
Scattering (Rayleigh or Mie) might be considered if your sample is powder.
Hi,
I assume that you are referring to a transmittance at a certain wavelength. Using FTIR, one usually gets whole spectrum - transmittaance at different wavelengths.
Transmittance 5% menas that only 5% of the incoming light is detected on the other side of the sample. It can be due to either high absorption or scattering of the sample. Both phenomena affect the transmitted beam. In addition, thicket the sample, less light is coming through, according to Beer's law: I = I0*exp(-mu*d).
Hope this helps.
Infrared frequencies correspond to vibrational modes in specific chemical bonds. The stronger the bond, the higher the frequency (shorter the wavelength, higher the energy)the atoms vibrate at. IR absorption occurs when the incoming light frequency matches the vibrational frequency of the bond. This gives chemists a powerful tool, enabling us to identify specific functional groups in molecules in the sample.
Light can be detected either through transmission - the amount of light that passes through the sample - or by reflection - the amount of light that bounces off the sample. Each detection mode has its benefits.
High transmittance at a frequency means there are few bonds to absorb that "color" light in the sample, low transmittance means there is a high population of bonds which have vibrational energies corresponding to the incident light.
There are lots of textbooks and on-line sources you can look for that will explain the details in as much depth as you want.
You may want to revise the experimental protocol, bearing in mind its adequacy for the analysed substance. For instance, the capillary thin film technique would be suitable for a methyl salicylate (wintergreen oil) sample, but not the KBr disk method ─ a related discussion can be found elsewhere at this forum:
https://www.researchgate.net/post/Can-we-take-FTIR-Fourier-Transform-Infrared-Spectroscopy-spectrum-of-wintergreen-oil-methyl-salicylate-a-liquid-using-KBr-disk-method
It's quite difficult to understand what's going on. But if your higher transmittance peak shows only 5%, maybe you should check how the sample is being prepared.
Depending on the substance and the method of spectra acquisition, maybe you should try to use a thinner film of the substance to higher the amout of light passing through the sample and reaching the detector.
I agree with Steven and thank him for his short and comprehensive explanation. It may be useful to say that by using of absorbance scale instead of transmittance, the interpretation of the results will be different. In other words, the higher absorption number would indicate the presence of more specific bond numbers in a sample at a specific wavelength.
I agree with what Steven has told. IR is a very powerful technique for the identification of functional groups in a chemical compound. It can be used for both qualitative and quantitative analysis. For qualitative analysis, normally IR spectrum is recorded in the transmission mode while for quantitative analysis, it is generally recorded in the absorbance mode.
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