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I started doing IR spectroscopy a few months ago and have run into the following problem: Here's the IR background that everyone knows and loves:Typical Background

CO2 peaks in the middle, water lines on either side. I get that. But what about those lines in the CH-stretch region? They are obviously very normal for an FT-IR background- everyone sees them, they're all over the internet. But WHAT ARE THEY???? No one knows- not even the IR spectroscopy veterans I've talked to. They're some kind of hydrocarbon, I guess, and they're not a gas because they are unaffected by purging. This is a spectrometer looking at no sample, with no windows, nothing, and fwiw this is using an MCT/A detector. I feel like I should know this, but maybe I'm not using the right resources?

Thanks for any help!

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    $\begingroup$ Would this be an artifact of the FT? The "beat" frequency difference between the large peaks at about 1500 and and 3600? $\endgroup$ – MaxW Nov 7 '15 at 3:18
  • $\begingroup$ @MaxW- Interesting idea! I think though, that if it were the beat frequency of the water lines, then it would be affected by purging the air inside the spectrometer. What's more, the spectrometer can easily resolve all those little rotational states- so I think the difference frequencies should also show up as a comb of lines? The mystery lines are always broad and have no smaller scale structure. $\endgroup$ – a yam Nov 7 '15 at 21:37
  • $\begingroup$ The only other idea that I had was the beam splitter. Does it have two pieces of glass glued together? Some sort of organic goo would probably be used between the two pieces. $\endgroup$ – MaxW Nov 7 '15 at 22:05
  • $\begingroup$ This Shimadzu page has separate IR spectra for H2O and CO2, neither of which shows that 2800-3000cm-1 feature. That is in the C-H stretch region so hydrocarbons sound like a fair guess. Possibly a film of oily contamination on the interferometer or detector? If it appears in every spectrum recorded on that instrument, then contamination is likely. $\endgroup$ – AndyW Oct 31 '17 at 15:05

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