It depends on the hardness of the sample. ATR FTIR is frequently used with relatively "soft" polymers, where pressing to the ATR crystal smooths out the surface roughness. Even relatively hard polymeric materials can be studied with diamond anvil ATR. If your sample is really hard you may try to use diffuse reflection.

It depends on the hardness of the sample. ATR FTIR is frequently used with relatively "soft" polymers, where pressing to the ATR crystal smooths out the surface roughness. Even relatively hard polymeric materials can be studied with diamond anvil ATR. If your sample is really hard you may try to use diffuse reflection.

Dear Nicholas:

You asked a question which is very important for those using FTIR-ATR-techniques In studies of rough high-moduli (hard) materials. Prof. Gyorgy Banhegyi  gave you excellent recommendations and I fully agree with him. Here I am going to add just a few words concerning the ATR measurements of such materials. Namely, since the relative intensity of ATR-spectrum strongly depends on penetration depth  i.e., on amount of material which comes in contact with IR-radiation  In the case where the contact of the rough material with the internal reflection element occurs only in several points - the resulting spectra will be inevitably distorted. At higher frequencies (due to the dependence of the absorbance intensity on the penetration depth) the spectral information can be completely lost and in the spectrum will be seen only a few very weak bands in the frequency range below 800 cm-1.

Leonid

Indeed, those losses of spectral information appear to have been observed for a chemical image generated by FTIR microscopy.  It uses a Ge ATR crystal, which has a penetration depth of 0.66 µm at 45 deg.  There were several spots that yielded spectra with structure similar to the background, which suggests very weak contact in those areas.  Also, the specimen was HDPE, which can be easily deformed.  However, we were analyzing for contamination, which could have had very weak absorbances and very high moduli.  These two factors may have coupled to produce spectra marred by steep baselines and %T > 100.  

Many thanks for the detailed answers!  

The sample should be measured at different locations (where you can specify the FTIR instrument to run several spectra in one run, e.g. 32 scans, which are automatically averaged into one spectrum), but since you’re doing ATR-FTIR you should not make an average of the spectra taken at different locations with different contact variations between the sample and the ATR crystal. Relatively hard surfaces complicate accurate quantitative measurements (height of absorbance peaks) due to varying contact with the ATR crystal for the different samples. Air between sample and ATR crystal results in a weaker absorbance signal. Thus, unless other conditions indicate otherwise (e.g. inhomogenities, impurities, etc.), the FTIR spectrum with the largest absorbance peaks represents the most correct measurements on one and the same sample, and hence this spectrum should be chosen as it is assumed to be the most correct one. Qualitative measurements (location of absorbance peaks at wave numbers) do not represent a problem as long as the contact area is large enough to ensure a sufficient strong measurement signal. When interpreting ATR-FTIR spectra where quantitative results are important, note that non-corrected ATR spectra have much stronger absorbance bands at longer wavelengths (smaller wave numbers) than at shorter wavelengths (larger wave numbers) compared to normal FTIR transmittance spectra. The reason being that the penetration depth is dependent on (in addition to refractive indices of sample and ATR crystal, and angle of incident radiation) the radiation wavelength, and increases with increasing wavelength (decreasing wave number). Various details about the ATR-FTIR technique which may be helpful may be found in the following articles (may be requested through Research Gate): (a) B. P. Jelle, T.-N. Nilsen, P. J. Hovde and A. Gustavsen, ”Accelerated Climate Aging of Building Materials and their Characterization by Fourier Transform Infrared Radiation Analysis”, Journal of Building Physics, 36, 99-112, 2012, and (b) B. P. Jelle and T.-N. Nilsen, ”Comparison of Accelerated Climate Ageing Methods of Polymer Building Materials by Attenuated Total Reflectance Fourier Transform Infrared Radiation Spectroscopy”, Construction and Building Materials, 25, 2122-2132, 2011, where the latter article is applying polypropylene (PP) and high density polyethylene (HDPE) as samples, i.e. as I noticed in your answer that your specimen was in fact HDPE.