Thanks Alexandra.

I usually measure background every 2 samples, but this spactrum was taken just after background measurment. I thought the background spectrum should have been automatically subtracted by the software.

Is there any way to minimize the effect of the background? I was looking for a peak at 788 cm-1 which is overlapped by this negative absorption. Does it mean that the peak does not exist or I just can't see it?

Do you have any chance to obtain the raw background and sample spectra? Actually, the background R0 is most probably not directly subtracted, but what you got is a kind of "Reflectance Absorbance" given by -lg(R/R0) which is, anyway, mostly far off the limits where it makes sense...

Many ATR-FTIR equipment do not subtract the background and appear negative values. You must subtract the background.

Thank you all for your suggestions.

I do have the background spectrum, but if I subtract it from the spectrum of my sample it doesn't produce any sensible result (you can find attached the screenshot from the software).

Is there anything else I might try to correct the distored region of this spectrum?

You could do a spectrum with a KBr pellets. Too you could use a calibrated sample to check the FT-IR.

Alexandara, thanks so much for your explanation!

I guess I don't really have means of calculating the ratio of probe/backround signal. I've asked for the spectrometer to be realigned, so hopefully the results will be better now. If not, I'll use the KBr pastil, although it's a bit labourious.

Many thanks once again!

The cause of negative peaks in the spectra are due to something in the background not in the sample spectrum. This can be a contaminant on the crystal, atmospheric water/co2 or residual solvent in the atmosphere immediately after cleaning the crystal. As well as your 750 band there is an inbalance band at 1100-100. As this occurs intermittently it strongly suggests  atmospheric solvent by running the background too soon after cleaning. A chlorinated solvent is suggested by the bands, which solvent do you use ?

Hi,

I have asked the lab technician to realigned and recalibrate the appratus and now it works fine. Thank you all for your valuable feedback.

@Geoffrey, I have been using methanol or isopropanol to clean the cristal. Would you suggest a diffrent solvent? I will be more careful now and will wait longer after cleaning the crystal to take the background sygnal.

 I would prefer isopropanol. Methanol can be aggressive, not on the crystal but on any adhesive holding it in. Some grades also contain trace water.

the important thing is to only use solvents from glass bottles without plastic stoppers. Both plastic wash bottles and tops contain plasticisers which leach into the solvent and can then be deposited on to the  "clean" crystal.

Geoffrey,

We have a problem with negative absorbance in the > 3000 cm-1 range.  We use 91% isopropanol to clean the ATR diamond crystal.  It can be purchased in a pharmacy, and is contained in a plastic bottle.  Do you recommend we use "dry" isopropanol in glass bottles to clean ATR crystals?

If it is 91% what is the other 9% ? Water ?. If so this takes longer to evaporate than the alcohol so you would have to leave the crystal "longer" to dry. However the main problem with solvents in plastic bottles is that the solvent leaches out the plasticisers. These are often phthalate S which have bans ain the 3000 and 1700 cm-1 region. This is why plastic wash bottles containing solvents harden and eventually crack. Try to clean a crystal with dry isopropanol , run that as your background. Let some 91% solvent dry onto the crystal then run a spectrum to see if an peaks are showing. If there are the spectrum will tell you what the additive is in there.

The negative absorption at ~750. The probable reason may be using of any halogenated solvent near by. Purge the system.

perhaps the reason is that the device exclusively was not read as in the radiation  counters which is called dead time 

I too have a similar query, I have done the FTIR of fly ash and spherule (file attached) but the absorbance is in negative. Kindly suggest?

It looks like you have a strong negative Absorbance due to a contaminated background. If this is ATR run the "clean" crystal to check for bands. If it is clean it could be you are seeing Christiansen effects on highly scattering samples

Check the contact quality with the ATR crystal; check other well known samples to see if the effect persists; and of course, all above comments.. Good luck!

I had a similar problem, but this time a strong peak in >3000cm-1. However, I was doing ftir with pallets of kbr, so the cleaning contamination problem wouldn't apply for this case. Also, I reproduced the experiement concerned I may had contaminated the pallet before, but the peak persists. Is exactly the hydroxyl peak but on the contrary.

Any idea?

so its due to contamination ?