Some common difficulties encountered when taking FTIR spectra are listed below. Please contact the service manager for further help.
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1. No signal:
Problem:
Cannot see peaks at all relative to baseline noise.
Causes:
Sample not in contact with surface OR beam misaligned.
Solutions:
- Verify sample is face DOWN.
- Go to “Check Signal” tab in the Data Collection window, and verify that signal is consistent with listed values. If not, contact NBTC staff for assistance.
- Put the sample in firm contact with the surface, if using ATR—make sure the sample is clamped finger‐tight.
- Very small particles can impede signal—try
- cleaning ATR crystal with IPA and a cotton swab or lens paper (no Kimwipes!) and
- putting a piece of lens paper (not a Kimwipe) between the sample and ATR and pulling the lens paper out.
2. Low signal:
Problem:
Peaks very small compared to baseline noise.
Causes:
Insufficient signal
Solutions:
- Use liquid nitrogen cooled MCT detector.
- Increase number of runs per measurement (shouldn’t need more than 256 even for monolayers).
- Increase sample thickness, if possible.
- Use larger sample mask (for VeeMAX)
- Very small particles can impede signal—try
- cleaning ATR crystal with IPA and a cotton swab or lens paper (no Kimwipes!), then
- putting a piece of lens paper (not a Kimwipe) between the sample and ATR and pulling the lens paper out.
ALSO READ: GC Troubleshooting Tips
3. Noisy baseline:
Problem:
Sharp peaks around 4000‐3500 and 2000‐1300 cm‐1, as can be seen in the light blue trace below (Dark blue is of the same blank sample):
Other notes: Primarily a problem for samples with low signal—monolayers etc. Same sample will change over time In the example below (absorbance): Blue trace is immediately after pumping down, pink after several minutes, and brown is several minutes after that. Note that the peaks around 1400‐1600cm‐1 are obscuring important spectroscopic data that can be seen in the pink trace.
Cause:
Atmospheric water—either more or less of it compared to background.
Solutions:
- Pump down the chamber for about the same amount of time as was done for the background spectrum.
- Take multiple spectra over several minutes and select the one where the atmospheric water peaks are smallest.
- To minimize the amount of water in the system, pump down, vent without using the air bypass, and pump down again (this will purge the chamber with nitrogen).
- If you’re still having difficulties, retaking the background spectrum may be helpful.
ALSO READ: Troubleshooting Guide – HPLC
4. Triangular Peak
Problem:
Large triangle‐shaped peak around 2200cm‐1, when using diamond ATR setup.
Other notes: May not occur on all samples. See spectra below for an example:
Causes:
Refractive index mismatch (Silicon on diamond ATR).
Solutions:
Use germanium ATR crystal with VeeMAX sample holder—contact NBTC staff for assistance.
5. Errant Peak
Problem:
Doublet peak around 2350 cm‐1, as can be seen in the light blue trace below (Dark blue is of the same blank sample):
Other notes: More frequently seen at atmospheric pressure. Same sample will change over time.
Causes:
Atmospheric carbon dioxide
Solutions:
- If working with a nonvolatile sample, pumping down the chamber for the background and sample spectra should remove this peak.
- If working with liquids or other vacuum‐sensitive samples, let the sample sit in the covered chamber for several minutes before taking a measurement, as the chamber is nitrogen‐purged.
- Take multiple spectra over several minutes and select the one where the doublet peaks are smallest.
6. Ringing:
Problem:
Spectra that look like the one below, with continuous wavy features that obscure sample peaks.
Other notes: The ringing may not occur on all samples. It may happen on your background sample if you re‐run it. The exact wavelength at which the peaks appear may vary. You may see sample peaks transposed over the ringing section (as seen here at around 1650 cm‐1).
Causes:
Most frequently caused by the IR beam reflecting off the underside of the VeeMAX sample holder.
Solutions:
- Prepare a larger sample to use with the larger sample mask (5/8” diameter recommended).
- Use the VMax at a non‐grazing angle (<60°).
- Recoating the underside of the mask—contact NBTC staff for assistance.
- Adjust aperture size—contact NBTC staff for assistance.
Tags
Chromatography
