Abstract: When vaporizing a sample substance consisting of big molecules, in particular for the purpose of mass-spectroscopic examinations, the energy introduced for the vaporization process may lead to thermolytic decomposition of the sample substance. In order to prevent such decomposition, the invention proposes that the sample substance be mixed, prior to its irradiation, with a matrix material which is easily decomposed under the influence of the laser beam pulses. The matrix may consist of a material which absorbs the radiation and which is easily decomposed thermolytically, or else of a material which is permeable to laser radiation, but mixed with a metal powder. When the mixture is exposed to laser beam pulses, the instable matrix material will decompose first whereby the embedded molecules of the sample substance are set free. It is possible in this manner to prevent, practically completely, the molecules of the sample substance from being destructed.