In a D–T fusion plasma, not only the components of primary interest (D2, T2, 4He), but also H2 and reaction products like HD, HT, and DT have to be considered. Even traces of 3He may be found in the system. For mass-spectrometric analyses of these eight gas species, at most six data are usable out of the mass range 1 to 6. Moreover, also taking into account water and hydrocarbon molecules, the unknowns outnumber by far the available data, making the underlying equation system unsolvable. This problem is overcome by repeatedly measuring the spectrum, but with stepwise changes in the settings of selected parameters of the instrument. Of particular interest are parameters like electron energy E (offering chemical separation of helium and hydrogen) and−in case of quadrupoles−field axis potential V (allowing to distinguish between molecular ions and fragment ions). By choosing 2 parameters each of E and V, we can get up to 24 linear independent equations out of the mass range 1 to 6, good enough for spectrum decomposition by means of established fit procedures. Details of this method and results from analyzing typical gas mixtures are presented.