4D data collection strategies: Helical Scans, Grid Scans & Serial Crystallography
Dedicated to modern MX crystallography beamlines, the MD3 offers many advanced data collection strategies:
The Ω-axis is mounted on an alignment table with three linear and orthogonal axes, the so-called xyz-alignment table. Two linear tables, perpendicular to itself and the Ω-axis, are mounted on top of the Ω-axis. This is the so-called xy-centering stage. Finally, the sample holder or any other goniometer head, e.g. the Mini-Kappa MK3, is mounted onto the xy-centering stage. All of these different motorized axes can be moved simultaneously and are controllable in real-time. This is exactly what is necessary for 4D data collection strategies. Combined with high accuracy and dynamic precision for all of these motion elements the MD3 becomes the ideal high precision goniometer for modern MX-diffraction techniques like helical scans, grid scans and serial crystallography.
These advanced control features and scan-types are integrated and fully supported by the MD3 control software.
A helical scan consists in an oscillatory movement with the Ω-axis while scanning the sample with a series of linear movements. These combined movements result in a helical sample trajectory. The aim is to entirely scan the crystal with a limited impact due to radiation damage. This method is especially adapted to elongated or needle like crystals, (Gabadinho, 2010), (Flot, 2010).
The grid scan method consists in scanning a crystal according to a virtual grid: each cell of the grid has a dimension close to the x-ray beam size. Each cell is irradiated with a sufficiently low x-ray dose and diffraction images are recorded. Then, the diffraction power is mapped for the entire grid.
The main application is to find the area of a crystal which is best suited for data collection. The diffraction power map allows selecting the best diffracting crystal area. This method is also well-suited to locate small diffracting crystals on a sample support with many crystals. This second application is not restricted to special sample supports and can be used with standard loops or crystallization plates (in situ). (Song, 2007), (Stepanov, 2011)
Serial crystallography is a method that combines the advantages of grid and helical scans: thousands of small sized crystals (0.2 to 5 µm) randomly distributed on a single sample support are scanned in only one operation. Then, the collected diffraction data from all of these crystals is merged in a single data set (Gati, 2014). Serial crystallography is a very powerful diffraction technique which overcomes the difficulties of x-ray data collection from micro-crystals.