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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Jwelsh142 on November 08, 2021, 08:55:37 AM

Title: Help in identifying mystery FTIR peak
Post by: Jwelsh142 on November 08, 2021, 08:55:37 AM
Hi everyone, I wonder if somebody could help me identify a mystery peak that appears in my FTIR data? The problem has been on-going for several years and I've finally decided to get to the bottom of it!

We're generating a low-temperature plasma in air which creates a host of gas phase chemical species that we flush through ~ 30 cm of pipe into our FTIR gas cell. The cell uses KBr windows and has a 10 cm path length. Plasma produces many reactive species but these typically react quickly to form long-lived species such as O3, N2O, and NO2.
When assessing this 'exhaust' gas with FTIR we see the expected peaks in their expected positions. However, we also see an extremely strong peak at 1356 cm-1 which does not look like anything we would expect. It is this peak I am interested in identifying...

Some clarifying information:
- We always take a background prior to plasma generation, the peak does not appear in this background spectra.

- The peak first appears during plasma generation; critically, it remains even after we have flushed the cell.

- The peak must be related to the KBr windows, other window materials show no sign of it.

- We have tried with brand new windows, straight from the sealed pack, the peak immediately appears once plasma species have been in the cell.

- Our only option to remove the peak is to take a second background, after the cell has been flushed following plasma generation.

- A collaborator in a different country reports a similar issue using a similar plasma system, their FTIR system has a different manufacturer than ours, but again they use KBr windows.

I've tried to attached a plot showing the mystery peak, hope it has worked!

If anyone could help shed some light on this annoying issue I'd be most grateful...


Title: Re: Help in identifying mystery FTIR peak
Post by: Corribus on November 08, 2021, 10:24:39 AM
I'm assuming the peak is not observed if you do the experiment without the plasma? Does the peak go away eventually, gradually or instantly?

To hazard a vague guess, your plasma is probably generating a lot of charged species that could easily adsorb on or react with the KBr surface.
Have you tried using a window made with a different window material, e.g., CaF2, to see if that helps?
Title: Re: Help in identifying mystery FTIR peak
Post by: mjc123 on November 08, 2021, 10:47:18 AM
I remember seeing similar peaks (can't remember if it was in exactly the same place) after using gas mixtures (not plasma) containing NOx. Didn't have windows of alternative materials available. I assume a component of the gas is reacting with the window. The peak seems in about the right place for KNO3, dunno if that's what it is.
Title: Re: Help in identifying mystery FTIR peak
Post by: Corribus on November 09, 2021, 11:09:47 AM
You should take care when trying to identify surface adsorbed/bound species based on literature values for isolated molecules. The match of your peak to KNO3 could be coincidental. Depending on how a species is interacting with a surface, vibrational energies can change quite a bit. Not saying it isn't KNO3 - just that for true identification you would probably want some other piece of evidence.

I threw out CaF2 as just a random example. There are a number of different options for FTIR windows. Your ozone peak comes out at what... somewhere around 1000-1100 wavenumber? BaF2 windows cut off around 830 wavenumber, so that could be a workable option.

Title: Re: Help in identifying mystery FTIR peak
Post by: marquis on November 30, 2021, 09:14:52 PM
Did the mystery peak occur all at once or gradually?  If it was gradually, have all consumables( filters, etc. ) been changed?  A sharp peak in that area is interesting.  But nothing makes sense.