The development and application of a method to quantify the quality of cryoprotectant solutions using standard area-detector X-ray images

An X-ray based method for determining initial cryoprotectant concentrations necessary to protect solutions from crystalline ice formation on flash cooling was developed. X-ray images from a charge-coupled device (CCD) area detector were integrated as powder patterns and quantified by determining the standard deviation of the slope of the normalized intensity curve in the resolution range where ice rings are known to occur. The method was tested by determining the concentrations of glycerol, PEG400, ethylene glycol and 1,2-propanediol necessary to form an amorphous glass at 100 K with each of the 98 crystallization solutions of Crystal Screens I and II (Hampton Research, Laguna Hills, California, USA). For conditions that required glycerol concentrations of 35% or above, cryoprotectant conditions using (2R,3R)-(-)-2,3-butanediol were determined. The method proved to be remarkably reliable. The results build on previous work [Garman & Mitchell (1996). J. Appl. Cryst. 29, 584-587] and extend the number of suitable starting conditions to alternative cryoprotectants.