Abstract: In order to analyze gaseous compounds of a mixture which have a mass below a given value, a fraction containing a number of phases is continuously sampled in a loop mounted in parallel with a main circuit under conditions of pressure such that the Reynolds numbers in the main circuit and the sampling loop are equivalent. The mixture is passed through traced pipes, at least one dust remover and at least one heat exchanger in which a heat-transporting fluid is circulated at an adjustable temperature T.sub.1 ; condensed and gaseous fractions of the mixture are collected in a flash drum placed in an enclosure at an adjustable temperature T.sub.2 ; the products of the mixture are then passed to an analytical instrument, their mass being such that they are in gaseous form at the temperature T.sub.2.