Over the past few
years, structures for lossless ion manipulations
(SLIM) have used traveling waves (TWs) to move ions over long serpentine
paths that can be further lengthened by routing the ions through multiple
passages of the same path. Such SLIM “multipass” separations
provide unprecedentedly high ion mobility resolving powers but are
ultimately limited in their ion mobility range because of the range
of mobilities spanned in a single pass; that is, higher mobility ions
ultimately “overtake” and “lap” lower
mobility ions that have experienced fewer passes, convoluting their
arrival time distribution at the detector. To achieve ultrahigh resolution
separations over broader mobility ranges, we have developed a new
multilevel SLIM possessing multiple stacked serpentine paths. Ions
are transferred between SLIM levels through apertures (or ion escalators)
in the SLIM surfaces. The initial multilevel SLIM module incorporates
four levels and three interlevel ion escalator passages, providing
a total path length of 43.2 m. Using the full path length and helium
buffer gas, high resolution separations were achieved for Agilent
tuning mixture phosphazene ions over a broad mobility range (K
0 ≈ 3.0 to 1.2 cm2/(V*s)).
High sensitivity was achieved using “in-SLIM” ion accumulation
over an extended trapping region of the first SLIM level. High transmission
efficiency of ions over a broad mobility range (e.g., K
0 ≈ 3.0 to 1.67 cm2/(V*s)) was achieved,
with transmission efficiency rolling off for the lower mobility ions
(e.g., K
0 ≈ 1.2 cm2/(V*s)).
Resolving powers of up to ∼560 were achieved using all four
ion levels to separate reverse peptides (SDGRG1+ and GRGDS1+). A complex mixture of phosphopeptides showed similar coverage
could be achieved using one or all four SLIM levels, and doubly charged
phosphosite isomers not significantly separated using one SLIM level
were well resolved when four levels were used. The new multilevel
SLIM technology thus enables wider mobility range ultrahigh-resolution
ion mobility separations and expands on the ability of SLIM to obtain
improved separations of complex mixtures with high sensitivity.
We determined absolute rate coefficients for the recurrent fluorescence (RF) process in C 6 − anions at excitation energies above the adiabatic electron attachment energy of 4.18 eV. The experiment was performed by extracting C 6 − ions from a sputter ion source and storing them in a bent electrostatic ion beam trap. After 1 s of storage, during which the anions cooled down to temperatures close to room temperature, they were excited by a short laser pulse and the neutralization rate due to vibrational autodetachment (VAD) was measured as a function of time at several wavelengths. Due to the different energy dependence of the two competing decays via the RF and the VAD process, their contributions to the measured total decay rate coefficients could be disentangled. For excitation energies ≲4.6 eV, the decay is found to be dominated by the RF process with decay rate coefficients on the order of 5 × 10 4 s −1 . The result clearly demonstrates the presence of the RF process in C 6 − and illustrates the importance of this process in the production and cooling of isolated molecules of astrophysical interest.
This article compares the experimental results obtained in metal-assisted secondary ion mass spectrometry of polymers, with molecular dynamics simulations involving hybrid metal-organic surfaces. The theoretical sputtering yields are in agreement with the trends highlighted in recent experiments, in which different projectiles (Ga + , C + 60 ) were used to bombard pristine and Au-covered polymer samples. In the experiments, the link between the organic ion yield enhancement/decrease and the fraction of the surface covered by the metal is clearly established. On the other hand, the simulations show that the position of the impact point on the metal-covered surface critically influences the calculated yields of metal and organic material, in a manner that depends on the projectile. The discussion analyzes the information obtained from the simulations and the experiments to propose a mechanism of yield enhancement.
Here we describe instrumental approaches for performing dual polarity ion confinement, transport, ion mobility separations and reactions in Structures for Lossless Ion Manipulations (SLIM). Previous means of ion confinement in SLIM based upon rf- generated pseudopotentials and dc fields for lateral confinement cannot trap ions of opposite polarity simultaneously. Here we explore alternative approaches to provide lateral confinement of both ion polarities. Traveling wave ion mobility (IM) separations experienced by both polarities in such SLIM cause ions of both polarities to migrate in the same directions and exhibit similar separations. The ion motion (and relative motion of the two polarities) under both surfing and IM separation conditions are discussed. In surfing conditions, where the traveling wave speed is less than the ion velocity, the two polarities are transported losslessly and non-reactively in their respective potential minima (higher absolute voltage regions confines negative polarities and lower absolute potential regions are populated by positive polarities). In separation mode, where the traveling wave speed is greater than the ion velocity, the two polarities can interact during rollovers over the traveling wave. Strategies to minimize overlap of the two ion populations to prevent reactive losses during separations are presented. A theoretical treatment of the time scales over which two populations (injected into a dc field-free region of the dual polarity SLIM device) interact is considered, and SLIM designs for allowing ion/ion interactions and other manipulations with dual polarities at 4 torr are presented.
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