• protein crystallography;
  • sample mounting loops;
  • cold-streams;
  • loop motion

The purpose of this study was to determine the relative stiffness of the more common sample mounting loops used in protein crystallography experiments and to see if they were moving under the influence of the nitrogen cold-stream gas. The `stress test' involved mounting a silicon single crystal onto a loop and seeing how reproducibly a single reflection could be measured at low temperature using a photodiode detector. Once a general ranking of loop stiffness was obtained, crystals of tetragonal lysozyme were mounted in these loops to investigate if data quality was being degraded as a result of cold-stream gas-induced loop motion. Sample motion was assessed using a differential measurement based on data sets taken at two different κ orientations on the same sample. Four of the eight sample mounting loops tested showed evidence of motion in at least one lysozyme data set using typical sample sizes and normal data collection conditions. These results suggest that loop thickness is key to increased stiffness, and factors such as loop design and frozen solvent can also play an important role.