The present invention concerns an automated substance preparation method comprising: - providing a first substance container containing a first substance; - a step of sampling to extract at least one portion of the first substance; - determination of a mass of the extracted at least one portion of the first substance; - encapsulating, in a first capsule, the extracted at least one portion of the first substance; and - repeating the step of sampling, mass determination and encapsulating to form a plurality of first capsules of the first substance having randomly distributed different masses of the first substance.