Is your cold-stream working for you or against you? An in-depth look at temperature and sample motion

It is normally assumed that a commercial gaseous nitrogen cold-stream provides a sample environment near 100 K and that the force of the cold-stream does not induce movement in the sample. As might be expected, the reality is much more complex. Here, an investigation of one cold-stream, starting with the temperature profile, is presented. Using silicon single crystals and flexible mounting loops, an approximate force/vibration profile of the cold-stream is obtained. Results indicate that the center of the temperature profile is offset from the position suggested by the manufacturer-supplied alignment tool and coincides with the area within the cold-stream that has the most consistent force profile. Tests indicate that this region is only about one-third of the width of the cold-stream nozzle opening. To verify that the results were relevant to protein crystallographic data collection, the impact of cold-stream position on the final data quality for lysozyme crystals was analyzed. On the basis of the observations it is recommended that users perform a temperature profile of their cold-streams to ensure proper alignment instead of relying only on the alignment tool for setup. In addition, suggestions are made on what users can look for in data processing to identify problems with loop movement and what users can do to minimize the impact of these problems on their experiments. (C) 2008 International Union of Crystallography Printed in Singapore - all rights reserved