the present invention comprehends an extremely sensitive apparatus which can be used for the detection of electronegative particles and provide data as to their elemental composition . a mass spectrometer selects negative ions of the required mass coming from an ion source . these ions are then directed into a dissociator which fragments complex molecules and strips electrons from the resulting products producing positively charged ions . these positively charged ions are filtered by a series of elements which independently measure some combination of the quantities : energy / charge , energy , momentum / charge , velocity , charge . such measurement allows the actual mass of each particle to be uniquely defined and , if necessary , over - determined for reduction of backgrounds .