Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: poco on January 30, 2016, 01:31:28 AM
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Hello everyone!
I am just curious if this had happened to anyone else and if they know exactly what had caused this issue. I was analyzing compounds using an FTIR and comparing them with the standards. All of the compounds exhibit peaks at the same wavelengths of the standard. However in each compound despite them being different I kept getting a reading at 704cm-1 that doesn't exist in either compound's standard. It also isn't actually a peak its more of a massive spike that seems to go as high as 150-500% transmission. My blank also exhibited broader peaks as well as a few peaks that usually aren't apparent. I had purged the system with Nitrogen and my problem was solved. My blank was normal and whatever peak or interference was now gone and had not appeared on a single sample's spectrum. I'm assuming this was interference of some sort. I just assumed interference would just appear as little fringes and not so much a very large sharp spike. Two days ago the system was running perfectly. Can anyone give me a little more insight as to why this would occur?
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I've encountered some FTIR weirdness in my day too, so I can understand the urge for a quick answer. But this is a learning forum, so we'll start with hints.
Because other atoms cause shifts, 704 cm-1 isn't what you should look for. And the intensity of the peak is , while unnaturally high, can be based on sample size and the like. Instead, you should say that you have found a strong, sharp peak in the 700 cm-1 area.
You can find many tables that identify bands and peak types for FTIR. Here's one from Wikipedia:https://en.wikipedia.org/wiki/Infrared_spectroscopy_correlation_table Do you see something associated with a strong, sharp peak in the vicinity of your peak? And can you exclude a possible contamination of your sample by that functional group for some reason?
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It's always helpful to provide an image. Troubleshooting spectra is hard just from words.
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Corribus,
I attached one spectrum. This one is Polyethylene Glycol, the spike I'm talking about occurs in every single sample I had run. I had also run a certified reference standard that was not supposed to exhibit a peak around 704cm-1 and of course it was still there. So I'm assuming the contamination comes from within the instrument. I'm just not entirely sure what exactly happened in there to cause this.
Arkcon,
Alright, if this is something introduced in the sample the peak does correspond with a mono-substituted benzene or a cis alkene but if I had continued to recontaminate the sample and reference standard wouldn't more peaks be unaccounted for. It seems unlikely that the reference standard would not correspond to a previous one from the same bottle made a couple years ago.
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That looks like nothing more to me than noise in the system: no peak in your spectrum is remotely like that one. No "sharp peak in FTIR" looks like that. The C=O stretch in hexanoic acid is a strong, sharp peak.
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Almost certainly an artifact. It's possible your detector has a bad pixel/spot or something. If so, if your instrument is under warranty they may replace it. Even if not under warranty you should still contact the manufacturer and they may have some suggestions.