## Ruler vs. Silver behenate: Trust but verify

One of the most important parameters you need when analyzing small-angle scattering patterns is the distance from the sample to the detector, as this defines the scattering wave vector q. This is often determined using the same approach as for wide-angle diffraction: measurement of a standard crystalline sample. Whereas in the past, stretched rat tail collagen was l’objet du jour, these days people prefer the much more sensible silver behenate. This saves you from having to find a suitable rat donor, preparing the tail, and stretching it just so that it would give you the right answer (as the degree of stretching would affect the distance). Silver behenate is easier to apply, easy to get and shows a nice round crystalline peak for all but the smallest angles.

For a while now, though, I and some others have measured the sample-to-detector distance with a ruler when measuring. There are several benefits to this. Firstly, it is dead simple to do (there is no need to recode my diffraction ring-fitting method into Python, for example) and tools are readily available. Secondly, it is easy to calculate that even being off by a centimeter on the meter, the impact on the scattering vector q is minimal compared to the uncertainty in the measured intensity. Lastly, I do this because there is a rumor that silver behenate is in fact unstable, hygroscopic and essentially untrustworthy.

Five silver behenate samples, arranged from fresh (left) to ancient (right).

Now, though, it is time to put it to the test: Is there really so much variety in silver behenate? To test this, Pawel (one of my colleagues) collected all the silver behenate samples from the labs he could find (five, some of which indeed looked rather dubious), and we set about measuring and analyzing them using his standard method (300 seconds, after which the measurement was analyzed using Jan Ilavsky & co.’s excellent brainchild). Without further ado: these are the results from the measurements, the sample-to-detector distance in meters:

Sample A 1.3602
Sample B 1.3598
Sample C 1.3606
Sample D 1.3595
Sample E 1.3578

While this is hardly a comprehensive test, these results seem to indicate that silver behenate is about as accurate as a ruler, with a variance of a few millimeters to the meter. While I am not going to use silver behenate in my methodology just yet (I still haven’t programmed a ring fitting method in my new Python code, you see), I am now less likely to label silver behenate as unreliable.

1. Fred