Mary E. Cisper

Membrane introduction Mass Spectrometry: Trends and applications

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.

Simultaneous detection of volatile, semivolatile organic compounds, and organometallic compounds in both air and water matrices by using membrane introduction mass spectrometry

Abstract We present results for the simultaneous detection of volatile organic compounds, semivolatile organic compounds, and organometallic compounds in air and water by using membrane introduction ion trap mass spectrometry. In these experiments, a membrane composed of a microporous polypropylene hollow support fiber coated with an ultrathin (∼0.5 μm) polydimethylsiloxane layer serves as the interface between the sample and the vacuum of the mass spectrometer. Simultaneous detection of benzene, naphthalene, and ferrocene in aqueous solution is achieved by proton transfer chemical ionization using H3O+ from membrane-diffused water. With the same membrane, we also demonstrate the simultaneous detection of methyl ethyl ketone, toluene, 1-methylnaphthalene, and ferrocene in air with chemical ionization employing membrane-diffused oxygen from air as the reagent gas.

Analysis of polar organic compounds using charge exchange ionization and 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 compared to signal obtained from electron ionization.

An improved method for capturing laser desorbed ions in an ion trap mass spectrometer: dynamic r.f. trapping

Abstract We have developed an improved method, dynamic r.f. trapping, for capturing laser desorbed ions in a quadrupole ion trap mass spectrometer (ITMS). Trapping efficiency is enhanced by over an order of magnitude over previous methods. A 308 nm excimer laser pulse desorbs the sample — trimethylphenylammonium iodide (TPA-I) is used in most of the work reported — from a probe inserted through the ring electrode. The laser is fired as the r.f. trapping potential (risetime about 175 μs) is applied to the ring electrode. Laser desorbed ions penetrate the trap while the trapping potential is low, but cannot escape because the r.f. potential rises substantially during their transit across the trap. The trapping efficiency is found to depend critically on the kinetic energy of the laser desorbed ions, and on the r.f. amplitude, phase, and rate of change of the r.f. amplitude when the laser fires. Cation and anion signals are recorded as functions of coarse and fine steps in the laser-to-r.f. timing. Coarse and fine timing steps test the effects of laser-to-r.f. delay and phase respectively. We also report effects on trapping efficiency of buffer gas pressure and composition (He neat versus He:Xe mixtures) and the amplitude of the ring electrode steady state r.f. potential. The delay and phase dependence of the experimental data is analyzed with reference to an effective potential barrier model. Differences in the phase and delay dependences for anions and cations are attributed to differences in Debye shielding early in the expansion of the laser desorbed plume. Cation and anion mass spectra are presented for laser desorption/ionization of TPA-I and pyrene. For TPA-I desorption, reactions between laser desorbed cations and neutral TPA fragments in the early, high density portion of the laser plume lead to production of high mass cations.

A method of increasing the sensitivity for laser desorption in an ion trap mass spectrometer.

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.

Atmospheric analysis using a microwave plasma ionization source and ion trap mass spectrometry

Abstract The on-line detection of both positive and negative ions has been demonstrated using a microwave plasma ionization source coupled with an ion trap mass spectrometer. Real-time xenon and krypton isotope measurements in air were made using either supplemental helium or atmospheric argon for plasma generation. The halogen signature from an organic molecule was detected as a negative ion using a helium plasma. Two instrument configurations have been assembled, one combining a microwave plasma source, an injection lens, and the ion trap; in the second assembly, a radiofrequency-only quadrupole mass filter was inserted between the lens and the ion trap. Most of the data reported here were acquired with the first configuration, although preliminary data with the second configuration has been taken. General operating parameters are discussed. The precision of xenon isotope ratio measurements ranged from 0.8% to 5% relative standard deviation, depending on the magnitude of the ratio.

Air analysis using tenax collection with jet-separator enrichment and ion trap mass spectrometric analysis.

A dual adsorbent trap inlet system has been developed for an ion trap mass spectrometer (ITMS) to provide a rapid and sensitive system for screening of volatile organic compounds in air. The system employs three stages of concentration: preconcentration on an adsorbent Tenax trap, focusing in a cryogenic collection trap, and jet separator enrichment immediately prior to analysis by ITMS. Ten minute integrated samples are collected and analyzed immediately. The detection limit is 0. 9 parts-per-trillion by volume (pptrv) based on toluene as the analyte, and the reproducibility is 2% or better. Ambient air was analyzed for toluene on April 4, 1996 in Los Alamos, New Mexico, and concentrations ranged from 11–158 pptrv.