P. H. Hemberger

Operation of a quadrupole ion trap mass spectrometer to achieve high mass/charge ratios

Abstract One of the principal limitations of the commercial quadrupole ion trap mass spectrometer is the relatively low limiting value of mass/charge ratio (650 Da per charge). This constraint limits applications of desorption ionization techniques which can produce ions of many thousand Da per charge. Several techniques for extending the mass/charge range of the quadrupole ion trap are presented. These include (i) the use of smaller electrodes, (ii) operation at lower radio frequencies, and (iii) resonance ion ejection using a voltage of appropriate frequency, applied symmetrically across the end-cap electrodes during the mass scan. The performance of each of these methods is compared using external ionization of alkali halide salts with Cs+ bombardment. Special attention is given to the effects of scan rate on resolution and a method of reducing the scan rate without loss of data is described. Mass measurement accuracy is studied in some detail, the mass shifts which occur using resonance ejection are characterized and this information is used to correct mass assignments. The relative merits of the three methods of mass range extension are assessed. Field inhomogeneities in the particular smaller electrodes used here apparently cause some loss of performance in these devices making them the least successful of the methods. Frequency reduction gives excellent results over a limited range of m/z values. However, resonance ejection can be used alone to achieve an even greater m/z extension and is the (single) method of choice. A combination of modest size and frequency reduction with axial modulation is probably an ideal solution for high mass biological mass spectrometry. Performance at high mass is illustrated by recording data on CsI clusters and on peptides. These experiments show that mass/charge measurements are accurate to better than 0.1% without external calibration, that a mass resolution of 3000 FWHM is achieved (unit resolution at 50% valley to 3000), and that a mass/charge range in excess of 70 000 Da per charge is accessible.

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.

Laser photodissociation probe for ion tomography studies in a quadrupole ion-trap mass spectrometer

Abstract Ion tomography studies based on spatially resolved photodissociation of benzoyl cation (C 6 H 5 CO + ) in an ion-trap mass spectrometer are presented. Changes in radially averaged parent ion distributions are observed as a function of applied RF potential during photodissociation and under conditions of resonant excitation with a supplementary ac voltage. This work also demonstrates stabilization of ion trajectories by helium buffer gas when the trap is used in the mass-selective instability mode of operation.

The Direct Analysis of Semi‐volatile Organic Compounds by Membrane Introduction Mass Spectrometry

We present results for direct on-line detection of semi-volatile compounds in air using membrane introduction ion trap mass spectrometry. Brief sampling periods of 10 seconds to 3 minutes produced linear and reproducible data for concentrations ranging from parts-per-trillion to parts-per-billion by volume. The method uses a composite membrane made by plasma deposition of a thin polydimethylsilicone layer on a microporous polypropylene support fiber. Charge exchange ionization was used for a variety of semi-volatile compounds and produced enhanced responses when compared to electron ionization. Among the semi-volatile species studied are dimethyl methylphosphonate, malathion, nitrobenzene, methyl salicylate, 2-chlorophenol, cyclohexanol, diethyl malonate and naphthalene. We also demonstrate direct analysis of semi-volatile compounds in aqueous solution with the composite membrane by the detection of 2-chlorophenol in water using both electron ionization and proton transfer chemical ionization.

Determination of Positions, Velocities, and Kinetic Energies of Resonantly Excited Ions in the Quadrupole Ion Trap Mass Spectrometer by Laser Photodissociation

The effects on ion motion caused by the application of a resonance AC dipole voltage to the end-cap electrodes of the quadrupole ion trap are described. An excimer laser is used to photodissociate benzoyl ions, and its triggering is phase locked to the AC voltage to follow the motion of the ion cloud as a function of the phase angle of the AC signal. Resonantly excited ions maintain a coherent motion in the presence of He buffer gas, which dissipates energy from the ions via collisions. Maximum ion displacements, which depend upon the potential well depth (qz value), occur twice each AC cycle. Axial components of ion velocities are determined by differentiating the displacements of the distributions with respect to time. The experimental data show that these velocities are maximized when the ion cloud passes through zero axial displacement, and they compare favorably with results calculated using a simple harmonic oscillator model. Axial components of ion kinetic energies are low (<5 eV) under the chosen experimental conditions. At low values of q2 (≈ 0.2), the width of the ion distribution increases as the ion cloud approaches the center of the trap and decreases as it approaches the end-cap electrodes. This effect is created by compaction of the ion trajectories when ion velocities are decreased,

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.

Selective laser ablation/ionization for ion trap mass spectrometry: resonant laser ablation

Laser ablation provides a clean, broadly applicable ionization source for ion trap mass spectrometry. However, the ion storage capacity of an ion trap mass spectrometer requires a degree of selectivity in either the ion generation or ion storage process to allow effective interrogation of minority components. This can be accomplished by low-intensity irradiation of the sample with laser pulses tuned to a one- or two-photon resonant transition in the analyte of interest. Resonant laser ablation is a multistep process involving evaporation and subsequent ionization of a solid sample component of interest. The leading edge of a tunable laser pulse vaporizes near-surface material, which forms a plume directly above the sample. The trailing edge of the pulse preferentially excites, and subsequently ionizes, the component that is in resonance with the incident photons. In this manuscript, we report on the use of resonant laser ablation with an ion trap mass spectrometer for high sensitivity, high selectivity generation of analyte ions from a solid sample.

A transportable turnkey gas chromatograph—ion trap detector for field analysis of environmental samples

Abstract A transportable gas chromatograph—ion trap detector has been developed for the in situ characterization of chemical waste sites. This instrument is based on a modular design and can be readily modified in the field for air, water, or soil sampling. A purge-and-trap gas chromatograph developed for this instrument is used for the separation of volatile organic compounds before their introduction to the ion trap for mass spectral analysis. A daughter microprocessor has been developed for the control of ancillary hardware via the ion trap software. Most analyses are accomplished in an automated 20 min procedure. The detection limit for trichloroethylene in water is in the low part-per-trillion range. The analysis of soil and water samples is demonstrated by using surrogate samples spiked with 24 volatile organic compounds. The instrument has been used under field conditions for soil analysis at a chemical waste site.

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.