Following up on the crystals from my last post I sent them along on the last Diamond trip we had to beamline I04 but regrettably didn’t get any usable datasets. Perhaps fortunately there is a possible explanation for this that isn’t just ‘the crystals were a bit too rubbish’. We send our crystals to the Diamond Light Source and because of the high number of crystals sent on each trip by the department (124 on the last trip) not everyone goes along on every trip and we quite often use something called ‘auto-centering’ that uses a computer algorithm to find the edge of the loop and to align the loop and your crystal in the beam. Centering is necessary but also quite tricky, because to collect a data set you need ‘slices’ of data taken with the crystal at different angles so the loop the crystals are mounted in, is rotated by a robotic arm so that the crystal also rotates around inside the x-ray beam. Thus if your crystal isn’t aligned properly, as the loop rotates it moves you crystal out of the beam and thus you get no data at that point.
Usually the automatic alignment is really good, however this time around it didn’t work so well and quite often either my crystal wasn’t in the beam at all (it mistook the loop stem for the crystal) or it started in the beam and then rotated out of it (which is what I think happened in the case shown below where you can see two ‘periods’ of data with dips in between. This is, of course, suboptimal and leads to a data set that you can’t really use.
I’m resending the crystals to the beamline this Friday and hopefully going with them myself. The silver lining to this is that I know that some of them definitely diffract which is encouraging. Hopefully they’ll behave well when fully in the beam.
Fig. 1 – Beam aligned at very edge of loop showing cyclic diffraction suggesting the crystal was moving in and out of the beam rather than degrading in the beam.
You can read more about this over on Zenodo.