ATR V: Depth Profiling Redux

This is the fifth in my intermittent installment series on Attenuated Total Reflectance (ATR), the sampling technique of choice for many FTIR samples. The second post in the series introduced an equation that determines the depth of penetration (DP) in the ATR experiment, a measure of how far the infrared beam penetrates into the sample. The most recent post in this series discussed how changing the refractive index of the ATR crystal can change the DP and allow spectra to be taken at different depths in samples non-destructively, which is called “Depth Profiling”. This blog post is subtitled “Depth Profiling Redux” because altering the angle of incidence of the infrared beam to the sample, called theta, can also alter DP. Examination of the DP equation shows that theta is in the denominator, so as theta goes up DP goes down. If we had some means of varying theta we could take spectra at different depths in a sample non-destructively i.e. perform depth profiling.

Fortunately, varying theta is not difficult. By adjusting the position of the mirror(s) involved in focusing the IR beam onto the ATR crystal, the angle of incidence of the beam at the sample can be adjusted. There exist ATR accessories where changing theta is simply a matter of moving one or more mirrors. The variable angle ATR accessory I use, the VeeMax from PIKE Technologies (details here: http://www.piketech.com/products/atr.html) allows theta to be adjusted by simply moving a knob up or down. This allows you to easily fine tune theta and hence easily fine tune the DP of your spectrum. This is, I feel, superior to adjusting the refractive index to change DP because in this case only certain fixed DPs are available to us depending upon the refractive indices of the ATR crystals mother nature provides us.

The attached figure shows the spectrum of a sample of polyethylene taken using 9 different angles of incidence varying between 42 and 70 degrees. Note how the peaks stack on top of each other; the absorbances are different sizes for the same sample because the DP for each spectrum is different. Adjusting theta to perform depth profiling will be useful for any sample where you would like to know how composition changes with depth. For example, this technique can be used on polymer laminates that consist of layers of different polymers. For example, a low DP scan can measure the spectrum of first layer. A high DP scan can measure the spectrum of the first and second layers. Subtracting this top layer spectrum from this spectrum will yield the spectrum of layer two non-destructively.