The system comprises a straw and a liquid dilution medium. The straw comprises a tube (11) extending between a first end and a second end and comprises a gas-permeable, liquid-tight plug, said plug being arranged in the tube (11) in the vicinity of the first end of same and extending between a first end turned towards the first end of the tube (11) and a second end turned towards the second end of the tube (11). Said plug comprises an element (14) impregnated with a reagent (22) at least in the vicinity of a first end turned towards the second end of the tube (11). The liquid medium is provided to produce, by mixing, in predefined conditions, the liquid substance (21). The reagent (22) and the medium(27) are configured such that, when the substance (21) comes into contact with the first end of the impregnated element (14), it forms a hydrogel.