The invention provides a method for the preparation of a carrier liquid which comprises the steps of: (I) preparing a single phase solution comprising: (a) a solvent or a mixture of miscible solvents, (b) a liquid carrier material, which is soluble in solvent (a), and (c) a dopant material which is also soluble in solvent (a); (II) cooling (preferably freezing) the single phase solution produced in step (I) to a temperature at which at least both the solvent (a) and carrier material (b) become solid; and (III) removing solid solvent (a) from the cooled (frozen) single phase solution in vapor form, such that the remaining cooled (frozen) carrier material (b) and dopant material (c) are returned to ambient temperature thus providing a product of liquid carrier material (b) having dopant material (c) dispersed therein.