Membrane introduction mass spectrometry: Trends and applications

Recent advances in membrane introduction mass spectrometry (MIMS) are revie wed. On-line monitoring is treated by focusing on critical variables, inclu ding the nature and dimensions of the membrane, and the analyte vapor press ure, diffusivity, and solubility in the membrane barrier Sample introductio n by MIMS is applied in (i) on-line monitoring of chemical and biological r eactors, (ii) analysis of volatile organic compounds in environmental matri ces, including ail; water and soil, and (iii) in more fundamental studies, such as measurements of thermochemical properties, reaction mechanisms, and kinetics. New semipermeable membranes are discussed, including those consisting of th in polymers, low vapor pressure liquids, and zeolites. These membranes have been used to monitor polar compounds, selectively differentiate compounds through affinity-binding, and provide isomer differentiation based on molec ular size. Measurements at high spatial resolution, for example, using sili cone-capped hypodermic needle inlets, are also covered, as is electrically driven sampling through microporous membranes. Other variations on the basi c MIMS experiment include analyte preconcentration through cryotrapping (CT -MIMS) or trapping in the membrane (trap-and-release), as well as different ial thermal release methods and reverse phase (i.e., organic solvent) MMS. Method limitations center on semivolatile compounds and complex mixture ana lysis, and novel solutions are discussed. Semivolatile compounds have been monitored with thermally assisted desorption, ultrathin membranes and deriv atization techniques. Taking advantage of the differences in time of membra ne permeation, mixtures of structurally similar compounds have been differe ntiated by using sample modulation techniques and by temperature-programmed desorption from a membrane interface. Selective ionization techniques that increase instrument sensitivity towards polar compounds are also described , and comparisons are made with other direct sampling (nonchromatographic) methods that are useful in mixture analysis. (C) 2000 John Wiley & Sons, In c.