Some applications require samples to be frozen during preparation in order to preserve sample integrity or halt chemical / biochemical reactions which would otherwise affect the analyte. Homogenizing cryogenically frozen samples can be difficult, however, as homogenization often adds a significant amount of heat and not all homogenizers can effectively process frozen samples.
The best way to homogenize frozen samples is often with a good, old-fashioned mortar and pestle style homogenizer. They add minimal heat to the sample during operation, and many can be pre-cooled to prevent undesirable levels of heat transfer from the mortar and pestle to the sample. Additionally, some cup-style mortars with hard pestles can be used with liquid nitrogen or dry ice, providing cryogenic cooling during homogenization.
Bead mills can be used in certain circumstances. Due to the relatively short processing times of most high-powered units, you can often achieve good results if the sample is kept frozen prior to homogenization then returned to cryogenic conditions immediately after. Refrigerated units are available which maintain a temperature of about 4°C, and these are highly recommended for cryogenic applications to prevent the sample from thawing during processing.
Rotor-stators are generally incapable of homogenizing cryogenic samples, however there are some exceptions. For instance, the PRO rotor-stators have a number of probes specifically for grinding cryogenic samples, however they are only suitable for large volumes.
High-pressure homogenizers are not suitable for cryogenic applications as they require the sample to be liquid. Ultrasonics also require liquid samples and impart large amounts of heat during processing.
