Applications of a travelling wave-based radio-frequency-only stacked ring ion guide

The use of radio-frequency (RF)-only ion guides for efficient transport of ions through regions of a mass spectrometer where the background gas pressure is relatively high is widespread in present instrumentation. Whilst multiple collisions between ions and the background gas can be beneficial, for example in inducing fragmentation and/or decreasing the spread in ion energies, the resultant reduction of ion axial velocity can be detrimental in modes of operation where a rapidly changing influx of ions to the gas-filled ion guide needs to be reproduced at the exit. In general, the RF-only ion guides presently in use are based on multipole rod sets. Here we report investigations into a new mode of ion propulsion within an RF ion guide based on a stack of ring electrodes. Ion propulsion is produced by superimposing a voltage pulse on the confining RF of an electrode and then moving the pulse to an adjacent electrode and so on along the guide to provide a travelling voltage wave on which the ions can surf. Through appropriate choice of the travelling wave pulse height, velocity and gas pressure it will be shown that the stacked ring ion guide with the travelling wave is effective as a collision cell in a tandem mass spectrometer where fast mass scanning or switching is required, as an ion mobility separator at pressures around 0.2 mbar, as an ion delivery device for enhancement of duty cycle on an orthogonal acceleration time-of-flight (oa-TOF) mass analyser, and as an ion fragmentation device at higher wave velocities.     

Fluorescence reabsorption in anthracene single crystals: Lifetime variations with emission wavelength and temperature

Fluorescence spectra and lifetimes of anthracene melt-grown single crystals and sublimation flakes have been examined at 298 and 77°K, using a mono-photon counting technique for the lifetime measurements. The observed emission decay times were nearly independent of the excitation wavelength, though a small dependence of the fluorescence spectrum on the excitation wavelength was noted. By contrast, large variations of fluorescence lifetimes in thick crystals were found as a function of emission wavelength. For thick melt-grown single crystals at 298°K the lifetime was found to increase from 9.8 nsec at 405 nm to 20.4 nsec at 445 nm. For sublimation flakes at 77°K and at 298°K and for thick melt-grown crystals at 77°K, the lifetimes were less than 10 nsec and were nearly independent of emission wavelength. Despite these relatively large variations in lifetimes, the decay rates at each separate wavelength remained exponential, within experimental error. Theoretical calculations were made of emission lifetimes based on a model with one reabsorbing state. The calculations are in substantial agreement with the experimental results.     

Quantitative autoradiographic imaging using gas-counter technology

Gas counters based on the multiwire proportional counter (MWPC) and multistep avalanche (MSA) technologies can combine the functions of imaging and quantitation (or assay) of radioactively labelled electrophoretograms and blots. Submillimeter spatial resolutions combined with good quantitation have been achieved for the most common beta-emitting radio labels. The current state of development of this technology is reviewed and assessments of the performance of these devices relative to the more traditional systems (X-ray film and the scintillation counter) presented. Particular attention is paid to the potential of these devices to enhance the productivity of the scanning of blots and the performance of assays.     

Separation of beta2-microglobulin conformers by high-field asymmetric waveform ion mobility spectrometry (FAIMS) coupled to electrospray ionisation mass spectrometry

An investigation into the use of high-field asymmetric waveform ion mobility spectrometry (FAIMS) coupled to electrospray ionisation mass spectrometry (ESI-MS) for the differentiation of co-populated protein conformers has been conducted on the amyloidogenic protein beta(2)-microglobulin (beta(2)m). Accumulation of beta(2)m in vivo can result in the deposition of insoluble fibrils whose formation is thought to originate from partially folded protein conformers; hence, the folding properties of beta(2)m are of significant interest. We have analysed beta(2)m using ESI-FAIMS-MS under a range of pH conditions and have studied the effect of the ion mobility spectrometry parameters on the behaviour of the various protein conformers. The data show that different protein conformers can be detected and analysed by ESI-FAIMS-MS, the results being consistent with observations of pH denaturation obtained using complementary biophysical techniques. A variant of beta(2)m with different folding characteristics has been analysed for comparison, and the distinctions observed in the data sets for the two proteins are consistent with their folding behaviour. ESI-FAIMS-MS offers significant opportunities for the study of the conformational properties of proteins and thus may present valuable insights into the roles that different conformers play in diseases related to protein folding.     

Ion dispersion near parallel wire grids in orthogonal acceleration time-of-flight mass spectrometry: predicting the effect of the approach angle on resolution

Ions experience small deflections in the vicinity of grids in accelerators and ion mirrors in time-of-flight (TOF) mass spectrometers. Recent experiments with an orthogonal acceleration (oa) TOF instrument have verified that the effect can significantly degrade resolution when ions approach grids at an angle deviating from 90 degrees. The phenomenon becomes significant only when ions have components of velocity at right angles to the wires of the grids. A model is presented in this study to predict this phenomenon for parallel wire grids. The fractional energy spread of ions (calculated in the static TOF-spectrometer frame of reference) scales directly with the approach angle of ions to the grid (as measured from normal approach). The energy spread also scales with the range of angles that is a consequence of the focusing effect in each gap between the wires of the grid. The equations imply that closely spaced parallel wire grids are best for deployment in oa-TOF systems where non-zero approach angles are unavoidable. Such grids are relatively impractical to manufacture and support but rectangular repeat cell grids with relatively few wires at right angles to the source axis are shown experimentally to introduce minimal energy spread. When these grids are rotated by 90 degrees, the resolution measured in a Q-TOF spectrometer is degraded in approximate agreement with the parallel wire model. A practical implication of this work is that grid transmissions in oa-TOF systems may be significantly increased without loss of resolution. Improvements of approximately 200% (V-mode) and approximately 400% (W-mode) in ion transmission were obtained in this study without compromising resolution. This was achieved with approximately 73% transmission grids and greater potential improvements in transmission are being realised since this study with approximately 89% transmission grids having similar geometry.     

Bacterial siderophores: unusual 3,4,5,6-tetrahydropyrimidine-based amino acids in pyoverdins frompseudomonas fluorescens

Two unusual natural amino acids have been identified in the pyoverdins occurring from two different strains ofPseudomonas fluorescens. They contain a 3,4,5,6-tetrahydropyrimidine moiety resulting from the condensation of 2,4-diaminobutyric acid with the car☐yl group of respectively serine and glutamine.     

Quantitative-qualitative data acquisition using a benchtop orbitrap mass spectrometer

Current approaches to discovery-stage drug metabolism studies (pharmacokinetics, microsomal stability, etc. ) typically use triple-quadrupole-based approaches for quantitative analysis. This necessitates the optimization of parameters such as Q1 and Q3 m/z values, collision energy, and interface voltages. These studies detect only the specified compound and information about other components, such as metabolites, is lost. The ability to perform full-scan acquisition for quantitative analysis would eliminate the need for compound optimization while enabling the detection of metabolites and other non-drug-related endogenous components. Such an instrument would have to provide sensitivity, selectivity, dynamic range, and scan speed suitable for discovery-stage quantitative studies. In this study, a prototype benchtop Orbitrap-based mass analyzer was used to collect both quantitative and qualitative data from human microsomal incubation samples as well as rat plasma from pharmacokinetic studies. Instrumental parameters such as scan speed, resolution, and mass accuracy are discussed in relation to the requirements for a quantitative-qualitative workflow. The ability to perform highly selective quantitative analysis while simultaneously characterizing metabolites from both in vitro and in vivo studies is discussed.     

Authority in an agency-centered, inquiry-based university calculus classroom

Authority roles among teachers and students have traditionally been hierarchal and centered with the expertise and power of the teacher limiting opportunities for students to act with autonomy to build and justify mathematics. In this paper we discuss authority roles for teachers and students that have been realized in an inquiry-based university, honors calculus classroom. We define and discuss four types of authority we identified from video data: institutional, expertise, mathematical, and performative. We suggest that the institutional authority of the instructor is present whether or not she is acting with the intention to bear authority, thereby interfering with the instructor's ability to share authority with her students. Further, we believe that the traditional focus on the bearer of authority limits our ability to create environments wherein authority can be shared. Instead we suggest that a focus on empowering the receiver to recognize expertise and mathematical authorities may be more fruitful.