Several analytical mass spectrometric and optical spectroscopic methods require a step during which a nonvolatile substance is desorbed by a laser pulse. It is, however, very difficult to use these methods for quantitative measurements because an accurate control over the amount desorbed by the laser pulse is generally not possible, especially when mixtures of several substances are used. We report a new fast and convenient sample preparation procedure that solves these problems. A solution of the analytes is mixed with a solution of poly(vinyl chloride) to obtain a homogeneous and vacuum-stable thin polymer membrane after the solvent has evaporated. Laser ablation is then performed directly from this membrane, allowing an accurate control of the amount of ablated analytes and excellent reproducibility. Quantitative laser desorption mass spectrometry over three orders of magnitude as well as optical spectroscopic measurements using this sample preparation method are demonstrated for polycyclic aromatic hydrocarbons. ©1998 American Institute of Physics.