Method for preparing a sample, wherein the sample is provided as a thin film on a support structure; the temperature of the support structure is adjusted to a value above the dew point temperature of the environment to decrease the film thickness, light is directed at the support structure, an intensity value of the transmitted light is measured, and the support structure is automatically inserted into a liquid cryogen dependent on the measured intensity value. The application further relates to a system comprising a support structure, a temperature-controlled stage for keeping the support structure at a pre-defined temperature, a transfer mechanism for moving the support structure into a container containing a liquid cryogen, a light source, a photodetector for measuring an intensity value of the transmitted light, and a control device for triggering the transfer mechanism dependent on the measured intensity value.