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IR spectra for un unknown substance?

IR spectra for un unknown substance? Topic: Sample on how to write a report
June 16, 2019 / By Jaye
Question: I have received a few IR spectra of un unkown compound, which is a solid probe.The IRs were taken on both surfaces: glossy and opaque. The peaks of each face of the probe are almost the same for each type of absorbtion, just the amount of energy absorbed is different(for ex.: at 1238cm-1 for one face I have 85%(glossy face) of absorbance and for the other 80%(opaque face)). What could cause this difference of absorbance for the same functional group? The two spectra, often, are overlaped, the shape of the peak is the same, but also I have a difference of 5-10% in absorbance. Around 2350cm-1 on one spectra I have a weak (double peak) absorbtion and on another spectra I have a peak with the same shape but seen in a mirror; instead to have a peak downward, I have upward. Please help me.... For more information, I could send you a pdf copy of the spectra.
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Best Answers: IR spectra for un unknown substance?

Gair Gair | 10 days ago
There will be a bit more scattering with the opaque sample with respect to the glossy sample. I suspect that your spectra are written in transmission mode, since you report the values as percentage values, which would make sense if the opaque sample is scattering more light (i.e., less light is making it back to the spectrometer.) Absorbence is slightly different, and typically not reported for IR spectra. That peak at 2350 wavenumbers is carbon dioxide. It's usually present in most IR spectra, and it can appear positive or negative depending on how the air in the room has changed since you took the blank. Most programs have a collection mode to cancel out the CO2 signal so you don't have to bother with it later.
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