Abstract: Process for the sequential observation of the successive states of a chemical reaction taking place in a high vacuum or under a low pressure or the determination of the nature of a substance or the composition of a mixture which can be liquid, solid or gaseous, or alternatively aerosols contained in air, by dissociation in a dissociation box or case (20) filled with a neutral gas from the molecules of the compounds formed, followed by energy filtration of the dissociation fragments in an electrostatic analyzer (22) and detector (24), characterized in that the molecules of the compounds to be analyzed are formed on a target (4), whose conductive walls are raised to a high d.c.