Recent advances in membrane introduction mass spectrometry (MIMS) are reviewed. On-line monitoring is treated by focusing on critical variables, including the nature and dimensions of the membrane, and the analyte vapor pressure, diffusivity, and solubility in the membrane barrier. Sample introduction by MIMS is applied in (i) on-line monitoring of chemical and biological reactors, (ii) analysis of volatile organic compounds in environmental matrices, including air, 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 thin 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 molecular size. Measurements at high spatial resolution, for example, using silicone-capped hypodermic needle inlets, are also covered, as is electrically driven sampling through microporous membranes. Other variations on the basic MIMS experiment include analyte preconcentration through cryotrapping (CT–MIMS) or trapping in the membrane (trap-and-release), as well as differential thermal release methods and reverse phase (i.e., organic solvent) MIMS.

Method limitations center on semivolatile compounds and complex mixture analysis, and novel solutions are discussed. Semivolatile compounds have been monitored with thermally assisted desorption, ultrathin membranes and derivatization techniques. Taking advantage of the differences in time of membrane permeation, mixtures of structurally similar compounds have been differentiated 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. © 2000 John Wiley & Sons, Inc., Mass Spec Rev 19: 1–37, 2000

Membrane introduction mass spectrometry: trends and applications.

The purpose of this review is to highlight the versatility of membrane introduction mass spectrometry (MIMS) in environmental applications, summarize the measurements of environmental volatile organic compounds (VOCs) accomplished using MIMS, present developments in the detection of semi-volatile organic compounds (SVOCs) and forecast possible future directions of MIMS in environmental applications.

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Environmental applications of membrane introduction mass spectrometry.

Air analysis by gas chromatography

Recently, a new and efficient method for total homocysteine (tHcy) quantitation in plasma using trap and release membrane introduction mass spectrometry (TR hence combined tCys and tHcy quantitation in plasma or serum can be achieved. The method employs Cys and Hcy derivatization with ethyl chloroformate (after disulfide bond reduction with dithiothreitol and protein precipitation with trichloroacetic acid), preconcentration in a capillary silicone membrane, and their thermal desorption to the gas phase inside the ion source region of a mass spectrometer, at a point exactly between the two ionization filaments. Thermal desorption uses the uniform heat radiation provided by the two
ionization filaments. The analytes are then ionized by electron ionization and both Cys and Hcy are quantitated by mass spectrometry using selected ion monitoring. For tCys quantitation, good linearity and reproducibility was observed for concentrations ranging from 5 to 350 µM, recovery was near 95%, and the limit of detection (LOD) was of 2 µM. This LOD is well below the mean Cys concentration in plasma, and serum samples from a large group of healthy people showed a mean tCys concentration of 132 ± 45 µM.

Combined cysteine and homocysteine quantitation in plasma by trap and release membrane introduction mass spectrometry

The solubilization of ethanol and the aromatic hydrocarbons benzene, toluene and the isomeric ethylbenzene and xylenes (BTEX) in water in contact with commercial samples of ethanol-containing Brazilian gasoline was monitored on-line by membrane introduction mass spectrometry (MIMS) and BTEX and ethanol concentrations in water after prolonged contact were measured by flow injection analysis (FIA) coupled with MIMS (FIA–MIMS). The cosolvent effect of ethanol, which is known to increase BTEX solubility in water, was also found to speed up BTEX solubilization as compared with ethanol-free gasoline. The ethanol cosolvent effect was also compared with that of methyl tert-butyl ether (MTBE). Chemometric multivariate analysis applied to FIA-MIMS mass spectra of the ethanol–BTEX aqueous solutions formed after prolonged contact with gasoline samples showed that identical samples cluster closely whereas small variances in BTEX composition occurring among and within the three
different types of ethanol-containing Brazilian gasoline are easily detected as they cause great dispersion in the chemometric plots.

Water solubilization of ethanol and BTEX from gasoline: on-line monitoring by membrane introduction mass spectrometry

Charge exchange ionization in conjunction with membrane introduction mass spectrometry provides a sensitive method for the detection of polar volatile organic compounds and semivolatile compounds in air. Sample introduction into an ion trap mass spectrometer was accomplished with a hollow fiber silicone membrane assembly. Atmospheric oxygen, which diffuses through the membrane, was used as the charge exchange reagent. Chemical ionization parameters were optimized using methyl ethyl ketone (2-butanone) standards in air. Several other oxygen-containing compounds, including acetone (2-propanone), methyl isobutyl ketone (4-methyl-2-pentanone), propanal, isopropyl alcohol (2-propanol), cyclohexanol, dimethyl sulfoxide (sulfinylbismethane), 2-(diethylamino)ethanol, and dimethyl methylphosphonate were analyzed with this technique. This method was used to obtain mass spectra for a variety of classes of compounds and produced a 4−20-fold improvement in response for all of the polar compounds we examined when compa...

and D. Cameron

Papers

Analysis of polar organic compounds using charge exchange ionization and membrane introduction mass spectrometry.