Large Ion Population Research Articles

Distinguishing enantiomeric amino acids with chiral cyclodextrin adducts and structures for lossless ion manipulations.

Enantiomeric molecular evaluations remain an enormous challenge for current analytical techniques. To date, derivatization strategies and long separation times are generally required in these studies, and the development and implementation of new approaches are needed to increase speed and distinguish currently unresolvable compounds. Herein, we describe a method using chiral cyclodextrin adducts and structures for lossless ion manipulations (SLIM) and serpentine ultralong path with extended routing (SUPER) ion mobility (IM) to achieve rapid, high resolution separations of d and l enantiomeric amino acids. In the analyses, a chiral cyclodextrin is added to each sample. Two cyclodextrins were found to complex each amino acid molecule (i.e. potentially sandwiching the amino acid in their cavities) and forming host-guest noncovalent complexes that were distinct for each d and l amino acid pair studied and thus separable with IM in SLIM devices. The SLIM was also used to accumulate much larger ion populations than previously feasible for evaluation and therefore allow enantiomeric measurements of higher sensitivity, with gains in resolution from our ultralong path separation capabilities, than previously reported by any other IM-based approach.

Ion behavior in low-power magnetically shielded and unshielded Hall thrusters

Magnetically shielded Hall thrusters achieve a longer lifespan than traditional Hall thrusters by reducing wall erosion. The lower erosion rate is attributed to a reduction of the high energy ion population impacting the walls. To investigate this phenomenon, the ion velocity distribution functions are measured with laser induced fluorescence at several points of interest in the magnetically shielded ISCT200-MS and the unshielded ISCT200-US Hall thrusters. The center of the discharge channel is probed to highlight the difference in plasma positioning between the shielded and unshielded thrusters. Erosion phenomena are investigated by taking measurements of the ion velocity distribution near the inner and outer wall as well as above the magnetic poles where some erosion is observed. The resulting distribution functions show a displacement of the acceleration region from inside the channel in the unshielded thruster to downstream of the exit plane in the ISCT200-MS. Near the walls, the unshielded thruster displays both a higher relative ion density as well as a significant fraction of the ions with velocities toward the walls compared to the shielded thruster. Higher proportions of high velocity ions are also observed. Those results are in accordance with the reduced erosion observed. Both shielded and unshielded thrusters have large populations of ions impacting the magnetic poles. The mechanism through which those ions are accelerated toward the magnetic poles has so far not been explained.

Multipurpose dissociation cell for enhanced ETD of intact protein species.

We describe and characterize an improved implementation of ETD on a modified hybrid linear ion trap-Orbitrap instrument. Instead of performing ETD in the mass-analyzing quadrupole linear ion trap (A-QLT), the instrument collision cell was modified to enable ETD. We partitioned the collision cell into a multi-section rf ion storage and transfer device to enable injection and simultaneous separate storage of precursor and reagent ions. Application of a secondary (axial) confinement voltage to the cell end lens electrodes enables charge-sign independent trapping for ion-ion reactions. The approximately 2-fold higher quadrupole field frequency of this cell relative to that of the A-QLT enables higher reagent ion densities and correspondingly faster ETD reactions, and, with the collision cell's longer axial dimensions, larger populations of precursor ions may be reacted. The higher ion capacity of the collision cell permits the accumulation and reaction of multiple full loads of precursor ions from the A-QLT followed by FT Orbitrap m/z analysis of the ETD product ions. This extends the intra-scan dynamic range by increasing the maximum number of product ions in a single MS/MS event. For analyses of large peptide/small protein precursor cations, this reduces or eliminates the need for spectral averaging to achieve acceptable ETD product ion signal-to-noise levels. Using larger ion populations, we demonstrate improvements in protein sequence coverage and aggregate protein identifications in LC-MS/MS analysis of intact protein species as compared to the standard ETD implementation.

Tandem Ion Trap Design with Enhanced Mass Analysis Capabilities for Large Populations of Ions

A new arrangement consisting of two separate radio frequency (rf) quadrupole ion traps is used to analyze large populations of ions over a wide mass-to-charge (m/z) range. The setup consists of an "accumulation" trap that is maintained at a higher pressure than the second high-performance "analyzer" trap. The two traps are scanned simultaneously, with a mass difference between that determines the residence time and mass range of ions in the analytical trap. Initially, all ions are trapped in the accumulation trap and then mass-selectively ejected into the analyzer trap. As ions arrive in the analyzer trap, they cool through collisions with the buffer gas and then are mass selectively ejected toward the detector. This concurrent linked mass scanning reduces the total number of ions present in the analyzer trap during mass analysis, thereby reducing space charge effects and leading to improved resolution and mass accuracy of analytical spectra.

Ion acceleration regimes in underdense plasmas

Acceleration of large populations of ions up to high (relativistic) energies may represent one of the most important and interesting tools that can be provided by the interaction of petawatt laser pulses with matter. In this paper, the basic mechanisms of ion acceleration by short laser pulses are studied in underdense plasmas. The ion acceleration does not originate directly from the pulse fields, but it is mediated by the electrons in the form of electrostatic fields originating from channeling, double layer formation and Coulomb explosion.

High-frequency electron-gas secondary neutral mass spectrometry: evaluation of transient effects

In electron-gas secondary neutral mass spectrometry (SNMS), a low-pressure plasma serves both as an ion source for sputter depth profiling the target and for post-ionizing the sputtered neutrals. In its high-frequency mode, a rectangular RF bias is applied to the target. We investigate by PIC/MC kinetic simulation the processes occurring in the vicinity of the substrate as a consequence of the voltage jumps: sheath expansion and contraction, as well as flux and energy of the ions impinging onto the substrate. In particular, we determine the enhancement of the ion current shortly after negatively charging the substrate; this enhancement is due to the acceleration of the large ion population in the expanding sheath. Our results indicate that already at a switch frequency of only 1 MHz the surface treatment by rectangularly shaped RF potentials is dominated by transient effects.

Prediction of a Space Charge Induced Upper Molecular Mass Limit Towards Achieving Unit Mass Resolution in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Cyclotron phase locking arises when two ion clouds have similar mass-to-charge ratios and a sufficiently large ion population such that the relative cyclotron dynamics are dominated by the mutual Coulomb E x B drift dynamics. Once two or more ion clouds are phase locked, a Fourier transform ion cyclotron resonance mass spectrometer cannot distinguish them by image current detection since the ion clouds have identical detected cyclotron frequencies. A simple analytical model, based on the assumption of rigid ion clouds, predicts that the maximum number of ions having equal charge states and closely spaced masses, which can be contained in two clouds before phase locking occurs, is proportional to their mass difference and to the square of the magnetic field strength divided by the molecular mass, Δm(B/M) 2 , and is independent of the charge state. This molecular mass dependence establishes an upper molecular mass limit for resolving closely spaced peaks due to cyclotron phase locking. The rigid ion cloud model, supported by numerical simulations, demonstrates that phase locking causes the 1 Da spacing of the isotopic envelope for large molecules to be unresolvable past a high molecular mass limit (M max ). M max is directly proportional to B and independent of the charge state for adjacent ion clouds with equal charge state. An order of magnitude estimate predicts that M max 1x10 4 (in units of Da and Tesla), independent of charge state for peaks having a 1 Da spacing. This estimate concurs with present instrumental capabilities. Full three-dimensional numerical stimulations on realistic ion clouds, which include the combined effects of internal and external ion cloud dynamics, Z-oscillation, linear dipolar excitation and trap potential anharmonicity, demonstrate the qualitative validity of the assumptions inherent in the rigid ion cloud model predictions.

Cyclotron motion of two Coulombically interacting ion clouds with implications to Fourier-transform ion cyclotron resonance mass spectrometry.

This work investigates the cyclotron motion of two Coulombically interacting ion clouds with different mass-to-charge ratios. Trap geometry as well as the shape of each ion cloud determines the maximum number of ions that can be confined in a Penning-like ion trap. We consider the two important cases of either spherically or cylindrically shaped ion clouds. These models exhibit the most important space charge effects in Fourier-transform ion cyclotron resonance mass spectrometry including frequency shifts, amplitude and phase modulation, and phase locking. Both positive and negative frequency shifts are possible for spherical ion clouds when their cyclotron radii differ. Due to the Coulombic ineraction between ion clouds, both cyclotron-radius and phase modulation occur. This modulation increases inversely to the cyclotron frequency difference. Cyclotron phase locking results when two ion clouds have similar mass-to-charge ratios and a sufficiently large ion population, at which point a mass spectrum shows only a single peak. Spherical ion clouds are usually more likely to phase lock than cylindrically shaped clouds. The phase-locking threshold sets limits on the maximum resolution, mass accuracy, and dynamic range achievable by mass spectrometry. Phase locking is treated in detail and our results are compared to previously published experimental data. The present model quantitatively describes the onset of phase locking as well as the general trends in frequency shifts and measured abundances just before phase locking.