A semi-permeable membrane, mounted in a direct insertion probe, is used to introduce aqueous samples into a triple quadrupole mass spectrometer while a simpler version of this device is used to introduce samples into an ion trap detector. Both instruments are equipped with flow injection analysis fluid handling systems and this allows on-line monitoring of aqueous solutions at low levels. Low ppb level detection limits are achieved for some typical compounds of environmental interest, and response to changes in analyte concentration is very rapid, especially when the membrane is heated. This means that reacting systems can be monitored on-line, a capability demonstrated for the interconversion of the chloramines NH2Cl, NHCl2 and NCl3. This methodology is also well-suited to determination of organochloramines at sub-ppm levels in water and the successive chlorinations can be followed as the solution pH is adjusted. Typical aliphatic amines are chlorinated at nitrogen but aniline is ring chlorinated. The reactions which accompany ozonation of contaminated water are also studied and the pentafluorobenzyl hydroxylamine derivatives of aldehydes are studied by negative chemical ionization using membrane introduction mass spectrometry. Membrane introduction mass spectrometry is also demonstrated to be applicable in air monitoring
