Abstract: A method of preparing samples of a liquid the composition of which varies from one region to another within a given volume, for subsequent storage or use, in which a first liquid is introduced into a long narrow tube which has particular dimensions and which may or may not be provided with a particulate filling, and subsequently a second liquid is introduced into the tube in circumstances such that in the tube the second liquid has a varying composition from point to point along the tube. The variation in composition may be caused by absorption by a packing if the tube has one, by dispersion of the front between the first and second liquids if they are miscible, or the second liquid may already have such a variable composition when introduced into the tube. The ends of the tube are then sealed if the sample is to be stored.