Crystallization of Proteins: Principles and Methods - Molecules: Nucleation, Aggregation and Crystallization - page 120

Modified microbatch
This is similar to the method mentioned above, the only difference being
the use of either silicone fluid, a polymeric compound composed of
repeating dimethylsiloxane units in place of the paraffin oil, or alterna-
tively a 1:1 ratio of silicon oil and paraffin oil.
Diffusion methods
In diffusion methods the samples are equilibrated against the crystallization
solutions. During this equilibration process the sample becomes more con-
centrated, increasing the relative supersaturation of the sample in the drop.
Vapor diffusion
Vapor diffusion (Fig. 5) is by far the most widely employed crystallization
method. The principle of evaporation and diffusion of the volatile species
(water or organic solvent) is utilized by this technique. The initial reagent con-
centration in the droplet is less than that in the reservoir. Vapor exchange
occurs between the precipitant and the drop until equilibration is reached.
During this equilibration process the sample becomes more concentrated,
increasing the relative supersaturation of the sample in the drop. Vapor diffu-
sion trials can be carried out using any one of the following three techniques:
Hanging drop
A small droplet of the sample mixed with the precipitant is pipetted out
onto a siliconized glass cover slide and inverted over a reservoir contain-
ing the precipitant.
Sitting drop
A small droplet of the sample is mixed with the precipitant and placed on
a platform to allow the drop to undergo equilibration against a reservoir
solution.