[Baileys]

I have two FTIR spectra of the same sample. However, my wavenumbers are slightly different for the peak; 3315 cm-1 on one and 3325 cm-1 on the other.

Does anyone know what might cause a slight shift of the peak?

Could it be because a different instrument was used for collecting each spectrum?

[marquis]

Please take a look at the spectrum.  In this range, the peaks are usually very broad (often water or OH stretches).  If this is the case, then the instruments can have trouble identifying the exact peak.

Normally, a daily calibration is done on the FTIR instruments.  This is typically a polystyrene spectrum.  See if you can get ahold of the polystyrene standards for both days.  Any drift should show up there (the polystyrene peaks should show peak drift, as well).

Good luck.   

[Furanone]

As mentioned by Marquis, this peak is typically very broad and in hygroscopic samples will indicate the bound water due to the (-OH) stretch. If your sample was hygroscopic and sitting out in the atmosphere longer than the first sample, it could have absorbed some moisture and this could change the peak. Likewise, if your sample has a high water activity, the longer your sample was left out, it may have lost moisture to the atmosphere thus changing this peak.

[dipesh747]

This is an interesting piece, very relevant http://www.americanlaboratory.com/913-Technical-Articles/1244-Positional-Fluctuation-of-IR-Absorption-Peaks-Frequency-Shift-of-a-Single-Band-or-Relative-Intensity-Changes-of-Overlapped-Bands/