Abrasive Sampling: Spectra of Intractable Samples

There is a sampling technique used on powders and solids called Diffuse Reflectance Infrared Fourier Transform Spectroscopy, or DRIFTS for short. I think some graduate student spent several months thinking up that acronym. At any rate, the technique involves using a special sampling accessory that fits into your FTIR sample compartment to bounce the light off the sample. The light is then collected and sent to the IR detector (more on DRIFTS in a later blog post). Abrasive sampling is an interesting application of DRIFTS. Some FTIR accessory manufacturers make flat metal posts that fit into their DRIFTS accessories and SiC disks with an adhesive backing. The SiC disk is adhered to the metal post, and then the disk is rubbed against the sample to abrade off sample particles. The SiC disk with particles is then placed at the focal point of a DRIFTS accessory, the light is reflected off of the sample particles, and is collected and sent to the detector to obtain the sample spectrum. The background spectrum is run on a clean SiC disk.

The attached spectrum is of the white paint on a light fixture obtained by abrasive sampling using an Alpha spectrometer from Bruker Optics (http://www.brukeroptics.com/). This spectrum would have been difficult to obtain any other way. The light fixture was firmly attached to the ceiling and so could not be taken down. In theory, one could scrape a lot of the paint off and make a KBr pellet or cast a film of the paint. However, this would damage the light fixture, involve time consuming trial and error, and may still not work. With abrasive sampling, only a small hidden part of the light fixture was scratched, and the entire measurement process took about 2 minutes. Abrasive sampling is useful for spectra of large, intractable objects such as furniture, large pieces of plastic, or anything that is simply too big to be analyzed by normal FTIR sampling techniques. The beauty of abrasive sampling is that it is fast and easy. However, the SiC scatters the IR beam a lot, so abrasive sampling spectra can be noisy. This can sometimes be dealt with by increasing the number of scans, perhaps to as many as 256.

A note on the SiC disks. I have seen labs try to save a little money by going to the hardware store, buying SiC paper, and then using a cork borer to punch out SiC disks of the proper size. As long as these disks fit into the sample cup that came with your DRIFTS accessory this should at least in theory allow you to obtain abrasive sampling spectra.

Use your imagination…what type of applications might abrasive sampling have at your company?

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.