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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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We present results for the near-real-time, on-line detection of methanol in both air and water using membrane introduction mass spectrometry (MIMS). In these experiments, we compare the sensitivity of a poly(dimethylsiloxane) (PDMS) membrane and an allyl alcohol (AA) membrane to the detection of methanol. In MIMS, the membrane serves as the interface between the sample and the vacuum of the mass spectrometer. Membrane-diffused water was used as the reagent ion (H3O+) for chemical ionization of methanol in an ion trap mass spectrometer. Linear calibration curves have been obtained for methanol using both PDMS and AA membranes. For PDMS, detection limits of methanol are 14 ppmv and 5 ppm in air and water, respectively. For AA, detection limits are 3.3 ppmv and 2 ppm in air and water, respectively. We demonstrate that the sensitivity of the analysis can be altered by the chemistry of the membrane. When the AA membrane is used, the sensitivity of MIMS is enhanced over that of PDMS by a factor of 8.5 for methanol in air and by a factor of 23.4 for methanol in water.

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 compared to signal obtained from electron ionization.

We demonstrate the advantages of utilizing a broad-band filtered noise field (FNF) for preferential mass injection of laser desorbed ions into an ion-trap mass spectrometer (ITMS). The FNF waveform is applied to the endcap electrodes of the ITMS during the laser-desorption period, and its amplitude and frequency spectrum are adjusted to resonantly eject all ions except those with selected mass-to-charge ratios. In single-pulse laser desorption, the FNF rapidly ejects desorbed matrix ions without reducing the number of target ions that are trapped. When the FNF is used in conjunction with multipole laser-desorption events, accumulation of target m/z ions results in a 40-fold increase in sensitivity over single-shot laser desorption. In addition, the application of sequential filtered noise fields for multiple stages of mass spectral interrogation is demonstrated.The combination of lasers and the ion-trap mass spectrometer has produced interesting and useful experiments ranging from fundamental studies of trapped-ion to the analysis of complex liquid and solidLaser desorption-ion trap mass spectrometry shows promise for characterization of complex matrices containing a variety of analytes including polynuclear aromatic hydrocarbons, explosives residues, transition-metal complexe~,~ and biological molecules.'o Much of the success of analytical laser-desorption methods is attributable to the flexibility of the analyzer. The ion-trap mass spectrometer (ITMS) is an ideal analyzer of laser-desorbed ions for several reasons. First, the ITMS is a multiplex analyzer capable of providing the entire mass spectrum with every laser shot. Second, due to low transmission losses, and its ability to accumulate ions over a relatively long period of time, the ion trap is among the most sensitive of all mass spectrometers." Third, the ion trap can store both positive and negative ions and can be rapidly switched from positive to negative ion detection mode.9 Last, because of its ion-storage capability, multiple levels of mass spectral interrogation can easily be carried out in the ion trap.'2.13Although laser desorption-ITMS has been successfully demonstrated for a wide variety of compounds, its implementation is frequently hampered by the generality of the desorption ionization scheme. Matrix ions can completely obscure the desired analyte and restrict detection sensitivity because of the limited ion-storage capacity of the ITMS.I4 One solution is to isolate the analyte from the matrix ions by either chemical or chromatographic methods. However, this approach can be tedious and time consuming. Furthermore, techniques such as matrix-assisted laser desorption require large excesses of matrix ions to be effective.' A second method of separating an analyte from its matrix is to form the ions external to the mass spectrometer and inject them through a mass-selective ion-optical arrangement .I5-l6 This requires tuning the injection optics for a particular mass or mass range and is subject * Author for correspondence.to ion transmission lo...

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Laser desorption in an ion trap mass spectrometer shows significant promise for both qualitative and trace analysis. In this work, we explore various combinations of time-varying DC and radiofrequency (RF) fields in order to optimize laser-generated signals. By judicious choice of timing between the laser desorption pulse and the rise in the applied RF trapping potential, we observed over an order of magnitude enhancement in the trapped ion signal. This new method for laser desorption has enabled us to observe mass spectra of many compounds (e.g., pyrene, dichlorobenzene, and ferrocene) that are barely detectable using previous laser desorption methods. Effects of laser timing and the magnitude of the steady-state RF potential are discussed.