I have been working on a common reaction for the conversion of mPEG into mPEG-COOH. I used for the characterization ATR-FTIR that requires small amount of the powder sample (ThermoScientific Nicolet iS10). I looked for the appearance of the strong peak of the terminal carboxylic group that should be added by the reaction.
I couldn't find the peak and so I tried several different conditions for the reaction, but it was all useless although this reaction is commonly reported. When I tried to measure FTIR absorbance using another device that requires compressing the sample into KBr Cell (Shimadzu), I found that all samples showed the terminal COOH peak.
I tried several other polymeric substances with terminal functional (attached is the spectrum for PLGA-COOH) group on several devices and found the same observation: terminal group does not usually appear on ATR-FTIR devices and it appears clearly on devices using KBr cells. Does any have an explanation for this observation?
I don't know what your level of expertise is, but just to be sure, here are some basic things to take care of:
To make a thorough comparison between your measurements (KBr pellet and ATR), make sure the IR machine is thoroughly purged with dry nitrogen or argon, so that you do not see water and/or carbon dioxide present in air (and do not breathe into the measurement chamber). Make sure the KBr powder is thoroughly dry.
Another possibility is that the ATR crystal you're using is not transparent in the region you want to observed the desired vibrational mode.
More in-depth: Did you expect that the broad signal in the first spectrum is the OH-stretch mode of the terminal COOH group? In general, that mode appears around 3000 /cm. Did you do a quick google on the IR spectra of, say, water versus acetic acid?
Hope this helps...
The difference you are seeing is due to the unique nature of the ATR experiment. While the transmission spectrum using a KBr pellet does not have this effect, the depth probed by the ATR experiment is a function of the frequency. The higher the frequency, the shallower the sampling depth. Thus, the spectrum in the 4000-3000 represents roughly speaking 1/10 of the material probed in the 1500-500 cm-1 range. Since you are using the strong peaks in the range of 1500-500 cm-1 as a standard for plotting, the bands in the range of 4000-3000 is shrunk (relatively speaking). Therefore, it looks like the band OH stretching band around 3600 cm-1 looks missing. However, it does exist in the spectrum, albeit weak appearance. If you want to convert the ATR spectrum into spectrum similar to the transmission spectrum without the frequency effect, you need to perform analysis using the exact optical theory, which is extremely theoretically demanding. The methodology is however published in the literature. Simply look for papers using keywords such as ATR, optical theory and others.
Nicely explained above by Hatsuo. Simply, ATR is much less sensitive compared to transmission experiment through KBr. Measurement with ATR is highly dependent on the material (i.e., germanium, diamond, silicon etc.) of the crystal.
Whilst the above answers are correct they don't address a more fundamental difference in your spectra. The ATR spectrum is sharp and has several bands typical of esters. The transmission spectrum is broad. The position of the OH bands and carbonyl at 1759cm-1 are not typical of carboxylic acids. They also appear far too strong for end groups. I think a more likely explanation is that in the KBr holder you have pressure melted the sample changing it from a crystalline to a more liquid like spectrum. I have seen this effect previously with low mol wt polymers.
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