A sample for analysis by Laser Desorption Mass Spectrometry is prepared by applying a substrate material to a target for the mass spectrometer, dissolving the sample material in a solvent and applying the solution to a substrate material on the target to be absorbed thereby. A matrix material is also dissolved in a solvent and this matrix material in solution is applied to the substrate material on the target. The solvent used to dissolve the matrix material is chosen such as to release the sample material from the substrate so that, after evaporation of the matrix material, there is provided on the substrate material an intimately mixed deposit of sample material and matrix material.