The invention proposes preparing biological samples for spectrometry which contain cell structures and/or whole cells of human or animal origin (e.g. thin human and animal tissue sections) or prokaryotes (e.g. microorganisms), and which require constant relative humidity, in a temperature-controlled gas volume whose humidity is determined by a saturated substance solution, for example a suitable salt solution. The invention exploits a physico-chemical phenomenon called “deliquescence”, which manifests itself by keeping the relative humidity above the saturated substance solution constant with a high degree of precision when a specified temperature is maintained. Pure succinic acid exhibits deliquescence at approx. 99% relative humidity, for example. Since an enormous variety of deliquescent salts and other suitable substances are available, it is possible to find the suitable substance for almost any desired relative humidity, with adjustment of the temperature, where necessary.