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Why stick to reflection geometry in powder diffraction?

4 September 2018 No Comment

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Since I made my first steps in the powder diffraction community, I have been amazed by the many possibilities for collecting diffractograms. In this blog, I’ll describe how I learned about the advantages of transmission geometry over reflection geometry for specific samples.

Chocolate-Milk_Dark-300x270In almost every powder diffractometer, the reflection geometry is being used: the detector is at the same side of the sample surface.

This geometry works very well for inorganic materials that can be prepared as flat samples with randomly oriented crystallites.

One of the disadvantages of reflection geometry is that it is very difficult to measure reflections below 5 degrees 2Theta accurately:

  1. Special care is needed to keep the background to a reasonable level
  2. The peak position is very sensitive to the sample preparation method

The first point can be dealt with, but the second point is inherent to the reflection geometry using slits, and it cannot be overcome. In transmission geometry, however, measuring reflections at low angles is very easy and straightforward. I learned about this years ago when I started to work with this method, and one of my contacts, Prof. Henk Schenk from the University of Amsterdam, suggested that I use chocolate as a test sample. This works best with thin chocolate slabs of a few mm thickness. They immediately fit in a standard sample holder after biting off the edges (!). The results were surprising to me.

Why stick to reflection geometry

As you can see in the picture, the transmission geometry gave much more intense peaks, and a clear visibility of reflections at 2.3 and 2.5 degrees 2Theta, respectively. I used a 1/8 degrees divergence slit in the reflection geometry, knowing that I would get better low angle data by going to smaller divergence slit sizes. This would, however, negatively impact the intensity difference even further, over 10x!

Of course, the transmission geometry only works if the samples are sufficiently transparent to X-rays. If this is not the case, one can easily switch to reflection geometry and back with PreFIX. The range of samples one can analyze in transmission geometry is increasing: one can now analyze even cell phone batteries by using Ag radiation. Organic materials, like for instance pharmaceutical substances, can very well be done with the more common Cu radiation. Our pharma PXRD customers using transmission geometry praise the massive improvement in data quality over reflection geometry and use transmission in their drug discovery and scale-up processes.

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