ATR IV: Depth Profiling and Analyzing Filled Polymers

It is finally time to resume our ongoing discussion of the Attenuated Total Reflectance (ATR) sample preparation technique. Recall from my earlier posts that the depth of penetration (DP) in an ATR experiment is a measure of how far the infrared beam penetrates into a sample. The equation that allows us to calculate the DP has a number of variables in it, each of which will be the subject of a separate blog post. The subject of this post will be the refractive index of the ATR crystal, nc. This parameter appears in the denominator of the DP equation, so as nc goes up depth of penetration goes down.

There are a number of materials that can be used as ATR crystals that have different refractive indices. For example, diamond has an nc of 2.4 while germanium (Ge) has an nc of 4.0. Depths of penetration from less than 1 micron to up to 10 microns are possible depending upon the crystal used. This means that if these two crystals are used to measure spectra of the same sample, spectra from different depths in the sample are obtained without having to take the sample apart. This ability of ATR is called “depth profiling”. Many ATR accessories allow the ATR crystal to be changed easily, making taking spectra of the same sample with different crystals straightforward. Now to be clear, the spectra are taken from the outside in; spectra of “slices” or layers internal to the sample by themselves are not obtained. For example, to analyze different layers in a polymer laminate non-destructively, spectra at a shallow DP using a germanium crystal and at a greater DP using a diamond crystal are obtained. The Ge spectrum is then subtracted from the diamond spectrum to reveal the spectrum below the surface of the sample. The depth profiling ability of ATR can be used on any sample where you need to know how composition changes with depth.

An excellent example of how the change of DP with nc can be put to good use is the analysis of filled polymers. A filled polymer consists of an organic resin, such as a rubber, and a filler such as carbon black, silica, or limestone. One of the purposes of the filler is to add bulk to the material and reduce cost; fillers are cheaper than resins. When FTIR spectra are obtained of filled polymers it is generally the spectrum of the resin that is desired. The problem is that many types of filler, particularly carbon black, have broad, intense absorbances that can mask the spectrum of the resin. This is illustrated in the bottom spectrum, which is the spectrum of an O-ring filled with carbon black. Note the strong absorbances, sloping baseline and distorted peak shapes caused by the presence of the carbon black. It is very difficult to identify the resin from such a low quality spectrum.


The top spectrum shows the spectrum of the same O-ring obtained using a Ge ATR crystal. Note that although the spectrum is not perfect the absorbances have been reduced, there is less baseline slope, and the peak shapes are no longer distorted. This spectrum is interpretable, and gives reasonable library matches when searched. The Ge ATR crystal, having a higher refractive index than diamond, has a lower DP. In the diamond spectrum the DP is great enough that the carbon black contributes significantly to the spectrum. With Ge the carbon black contribution is reduced and the spectrum of the resin is easier to see. These results indicate that Ge ATR is the method of choice for obtaining FTIR spectra of filled polymers. This is also a neat illustration of how the dependence of DP on nc allows a normally difficult sample to yield a usable spectrum.