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A few weeks ago I collected an IR spectrum of iodoform in KBr as part of a computational chemistry course. For some reason, there is consistently a trough right before every peak, and I am unsure of this mechanism. I have spoken to a spectroscopist who said he has never seen the phenomenon in anything other than in KBr and iodoform. Is there a mechanism for this that anyone is aware of? More specifically, the iodoform was ground into KBr using a mortar and pestle in a 1:20 m/m ratio and the spectrum was collected using a PerkinElmer Spectrum 100 FT-IR spectrometer.

IR Spectrum Iodoform

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  • $\begingroup$ Looks like a phasing issue which is common in raw NMR data, but I've never seen it for IR. $\endgroup$ Commented Oct 28, 2022 at 10:33
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    $\begingroup$ These 'derivative ' type features do seem to occur at the three most intense iodoform transitions, so it might be possible that the at these particular wavelengths no light is being transmitted by the sample and the fourier transform performed by the instrument is, for some unknown reason, responding as shown. You could try diluting the sample by, say, 10 times. $\endgroup$
    – porphyrin
    Commented Oct 28, 2022 at 12:50

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Prior to using an instrument, I always recommend benchmarking it. It would have been appropriate for the instructor to test this. A polystyrene film should be placed in the sample compartment and its FTIR spectrum should be collected. Does it appear to be a well-behaved spectrum? This test sheet is included with most FTIRs. If so, then the instrument is functioning properly. If the spectrum is not well-behaved, it is time for a servive. However, a good polystyrene spectrum this does not guarantee that your sample has been prepared properly.

Please keep your cell phone away from the instrument. Electromagnetic interference can lead to the formation of these fake derivative shapes. While the Michelson interferometer collects data while phones ring and chats take place, the infrared spectrum may exhibit these derivative-like spikes. The majority of instruments are shielded from electromagnetic interference, but it is possible for these things to occur.

People who perform microwave spectroscopy, for example, know that their detectors pick up the Wi-Fi signal in the room. Wi-Fi peaks contaminate the spectrum.

Briefly, the spectrum should be re-done to ensure that everything, including the KBr/sample, is bone dry. Handling spectroscopic instruments requires a great deal of respect. They demand a high-level of attention before their output can be trusted. Your overall transmittance is pretty high, so I do not think it is a concentration problem.

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    $\begingroup$ All other spectra are well-behaved. Every spectrum that has been collected by this instrument works as expected. This exclusively appears with Iodoform in KBr. $\endgroup$ Commented Oct 29, 2022 at 0:22
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    $\begingroup$ Acquire the spectrum again with a fresh sample with new background. Even try another bottle of iodoform. Collect at least 32 scans. There is nothing unusual with iodoform. Its FTIR spectra are available and they look perfectly normal. chemicalbook.com/SpectrumEN_75-47-8_IR1.htm $\endgroup$
    – ACR
    Commented Oct 29, 2022 at 0:36
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Molecular absorbances change the bulk refractive index of a material in that derivative-like shape. You have reflection and transmission going on in that KBR pellet and the absorbance peak is changing the refractive index as well as absorbing light. both of which changes how much radiation goes through. That's a great spectrum for physics, not what a chemist wants to see.

And yes, my email says PerkinElmer.com

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  • $\begingroup$ Thanks for the answer! Are there any textbooks that you know of or could recommend I look in to for more info on these phenomena? $\endgroup$ Commented May 1, 2023 at 2:03

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