The selection and use of a solvent has to be based on several factors. The solvent must be a substance that will dissolve and separate the metabolite. At the same time, the solvent and metabolite must be immiscible (will not blend or mix) so it doesn't create a new or different compound.
A second purpose of solutions is inactivation of the cells for examination. This process is known as quenching. After one series of experiments, an ammonium carbonate solution was considered best for this purpose. The solution provided low cell leakage, an energy charge indicating good condition of the metabolism and easy removal of the cellular components.
Alternatively, a mechanical means of extracting the metabolite is to remove the organism with a syringe and spin it to remove debris.