Improving Ion Mobility Measurement Sensitivity by Utilizing Helium in an Ion Funnel Trap

Ion mobility instruments that utilize nitrogen as buffer gas are often preceded by an ion trap and accumulation region that also uses nitrogen, and for different inert gases, no significant effects upon performance are expected for ion mobility spectrometry (IMS) of larger ions. However, we have observed significantly improved performance for an ion funnel trap upon adding helium; the signal intensities for higher <i>m</i>/<i>z</i> species were improved by more than an order of magnitude compared to using pure nitrogen. The effect of helium upon IMS resolving power was also studied by introducing a He/N₂ gas mixture into the drift cell, and in some cases, a slight improvement was observed compared to pure N₂. The improvement in signal can be largely attributed to faster and more efficient ion ejection into the drift tube from the ion funnel trap

Orthogonal Injection Ion Funnel Interface Providing Enhanced Performance for Selected Reaction Monitoring-Triple Quadrupole Mass Spectrometry

The electrodynamic ion funnel facilitates efficient focusing and transfer of charged particles in the higher-pressure regions (e.g., ion source interfaces) of mass spectrometers, thus providing increased sensitivity. An “off-axis” ion funnel design has been developed to reduce the source contamination and interferences from, e.g. ESI droplet residue and other poorly focused neutral or charged particles with very high mass-to-charge ratios. In this study, a dual ion funnel interface consisting of an orthogonal higher pressure electrodynamic ion funnel (HPIF) and an ion funnel trap combined with a triple quadrupole mass spectrometer was developed and characterized. An orthogonal ion injection inlet and a repeller plate electrode was used to direct ions to an ion funnel HPIF at a pressure of 9–10 Torr. Key factors for the HPIF performance characterized included the effects of RF amplitude, the DC gradient, and operating pressure. Compared to the triple quadrupole standard interface more than 4-fold improvement in the limit of detection for the direct quantitative MS analysis of low abundance peptides was observed. The sensitivity enhancement in liquid chromatography selected reaction monitoring (LC-SRM) analyses of low-abundance peptides spiked into a highly complex mixture was also compared with that obtained using both a commercial S-lens interface and an in-line dual-ion funnel interface

Cyclable Variable Path Length Multilevel Structures for Lossless Ion Manipulations (SLIM) Platform for Enhanced Ion Mobility Separations

Ion mobility-mass spectrometry (IMS-MS) is used to analyze complex samples and provide structural information on unknown compounds. As the complexity of samples increases, there is a need to improve the resolution of IMS-MS instruments to increase the rate of molecular identification. This work evaluated a cyclable and variable path length (and hence resolving power) multilevel Structures for Lossless Ion Manipulations (SLIM) platform to achieve a higher resolving power than what was previously possible. This new multilevel SLIM platform has eight separation levels connected by ion escalators, yielding a total path length of ∼88 m (∼11 m per level). Our new multilevel SLIM can also be operated in an “ion cycling” mode by utilizing a set of return ion escalators that transport ions from the eighth level back to the first, allowing even extendable path lengths (and higher IMS resolution). The platform has been improved to enhance ion transmission and IMS separation quality by reducing the spacing between SLIM boards. The board thickness was reduced to minimize the ions’ escalator residence time. Compared to the previous generation, the new multilevel SLIM demonstrated better transmission for a set of phosphazene ions, especially for the low-mobility ions. For example, the transmission of m/z 2834 ions was improved by a factor of ∼3 in the new multilevel SLIM. The new multilevel SLIM achieved 49% better resolving powers for GRGDS1+ ions in 4 levels than our previous 4-level SLIM. The collision cross-section-based resolving power of the SLIM platform was tested using a pair of reverse sequence peptides (SDGRG1+, GRGDS1+). We achieved 1100 resolving power using 88 m of path length (i.e., 8 levels) and 1400 following an additional pass through the eight levels. Further evaluation of the multilevel SLIM demonstrated enhanced separation for positively and negatively charged brain total lipid extract samples. The new multilevel SLIM enables a tunable high resolving power for a wide range of ion mobilities and improved transmission for low-mobility ions

Simultaneous and co-located dual polarity ion confinement and mobility separation in traveling wave-based structures for lossless ion manipulations (SLIM) (ASMS 2017)

Ion mobility (IM) coupled with mass spectrometry has gained prominence as a powerful analytical tool. To advance IM technology performance to higher levels SLIM technology has recently been developed in our laboratory, and has provided the basis for large gains in IM resolution as well as sensitivity. In many applications both positive and negative ion separations provide complementary information. In this work we explore the use of traveling waves in SLIM to simultaneously confine and separate by IM co-located cations and anions. SIMION ion trajectory software was used to simulate ion confinement in SLIM, as well as ion transport. Ion-neutral collisions during ion transport simulations were modelled using the SDS collision model, which employs statistical methods to account for ion collision with buffer gas. The simulations were used to optimize the SLIM design process as well as predict possible experimental performance. MATLAB software package was also used to obtain and analyze the ion confinement potentials. Static voltages applied to guard electrodes in traditional SLIM configurations provide good lateral confinement for single ion polarity experiments, but such conditions lead to the loss of opposite polarity ions. It is well recognized that rf ion traps can simultaneously confine ions of both polarities. In this work we have explored the potential for developing instrumentation allowing the simultaneous introduction, and manipulation (including IM separation) of both positive and negative ions in a new SLIM design. Preliminary data obtained from ion trajectory simulations have shown the possibility to simultaneously confine and transport both positively and negatively charged ions. Simultaneous confinement for ions of both polarities was achieved by replacing the guard electrodes in the traditional SLIM configuration which employed static voltages (typically 5V above the travelling wave (TW) voltage) for lateral confinement of ions between the SLIM boards with RF “guards” which use dynamic voltages for the lateral confinement of the ions. Concurrent ion transport is also achieved due to the nature of the dynamic voltage profile of the TW which presents a potential minima at opposite ends of the voltage wave for each ion polarity as the wave transverses the segmented TW electrodes, and thus subsequently provide efficient ion transport. We have also shown using ion trajectory simulations, the capability to manipulate the spatial separation of ion populations in SLIM based on their polarities, by biasing the RF guards on each side of the ion conduit so as to limit the interactions between the two ion polarity populations if ion-ion interactions could lead to ion loss during transmission. This presentation will also describe our progress in experimental implementation.<p></p

Compression Ratio Ion Mobility Programming in Structures for Lossless Ion Manipulations (ASMS 2017)

Structures for Lossless Ion Manipulations (SLIM) technology has enabled very long path length IM separations using traveling waves (TW) in serpentine and multi-pass designs, but resolutions achievable are limited by peak broadening phenomena, which increasingly inhibit detection due to peak dilution. In this work we developed a new approach for spatial and temporal peak compression that can mitigate many of the negative effects of peak broadening and demonstrate its application for the collapse of the ion distributions into tighter packets to provide higher sensitivity. The nature of fields and ion dynamics enabling peak compression will be presented. The implications of compression ratio squeezing of ion packets and programming for IM separations and other applications will be discussed.Theoretical and simulation methods are used to study the process of peak compression in TW SLIM. In-house computational models were used to study effects of compression. SIMION ion trajectory simulations were used to demonstrate proof-of-concept, to predict experimental performance and optimize SLIM designs. Software package OpenFOAM was used to visualize the ion confinement fields and model the ion dynamics by treating ion motion using advection-diffusion equation. Experimental implementation was performed on a 13 m long serpentine path length SLIM device with multi-pass capability, coupled to an Agilent qTOF MS.We demonstrate peak compression using a SLIM device with a TW region (R1) and another region where a stuttering wave moves only intermittently (R2). As the ions pass the interface between R1 and R2, the ion packets spanning a number of TW-created traveling traps (TT) are redistributed into fewer TT, resulting in spatial compression. The degree of spatial compression is controllable and determined by the ratio of stationary time of the TW in the second region to its moving time. This compression ratio ion mobility programming (CRIMP) approach has been implemented using SLIM in conjunction with a TOF-MS. CRIMP with the SLIM IM-MS platform is shown to provide increased peak intensities, reduced peak widths, and improved S/N ratios with MS detection. The increase in peak height is equivalent to the applied compression ratio (CR) until such a point that space charge effects lead to ion activation and/or losses. SLIM TTs keep ions confined as long as the TW is in the surfing mode, and TW produce a ion peak bin “quantization” effect which allows peak compression with integer CR. The effect of such peak compression on IM separation and resolution will be discussed from theoretical standpoint and correlated to experimental observations. TW SLIM IM separation of milk oligosaccharide isomers in conjunction with peak compression shows that two species with very similar mobilities can be fully separated by combined application of separation and compression. Also CRIMP allows injecting a wide pulse of ions that can be separated and then compressed to enable high resolution IM separations at high sensitivity. Further, insights from ion trajectories modeling on the effects of space charge during the CRIMP process will be discussed

Development of an Ion Mobility Spectrometry-Orbitrap Mass Spectrometer Platform

Complex samples benefit from multidimensional measurements where higher resolution enables more complete characterization of biological and environmental systems. To address this challenge, we developed a drift tube-based ion mobility spectrometry-Orbitrap mass spectrometer (IMS-Orbitrap MS) platform. To circumvent the time scale disparity between the fast IMS separation and the much slower Orbitrap MS acquisition, we utilized a dual gate and pseudorandom sequences to multiplex the injection of ions and allow operation in signal averaging (SA), single multiplexing (SM), and double multiplexing (DM) IMS modes to optimize the signal-to-noise ratio of the measurements. For the SM measurements, a previously developed algorithm was used to reconstruct the IMS data. A new algorithm was developed for the DM analyses involving a two-step process that first recovers the SM data and then decodes the SM data. The algorithm also performs multiple refining procedures to minimize demultiplexing artifacts. The new IMS-Orbitrap MS platform was demonstrated by the analysis of proteomic and petroleum samples, where the integration of IMS and high mass resolution proved essential for accurate assignment of molecular formulas

Rectangular Ion Funnel: A New Ion Funnel Interface for Structures for Lossless Ion Manipulations

Structures for lossless ion manipulations (SLIM) have recently demonstrated the ability for near lossless ion focusing, transfer, and trapping in subatmospheric pressure regions. While lossless ion manipulations are advantageously applied to the applications of ion mobility separations and gas phase reactions, ion introduction through ring electrode ion funnels or more conventional ion optics to SLIM can involve discontinuities in electric fields or other perturbations that result in ion losses. In this work, we developed and investigated a new funnel design that aims to seamlessly couple to SLIM at the funnel exit. This rectangular ion funnel (RIF) was initially evaluated by ion simulations, fabricated utilizing printed circuit board technology, and tested experimentally. The RIF was integrated to a SLIM-time of flight (TOF) MS system, and the operating parameters, including RF, DC bias of the RIF electrodes, and electric fields for effectively interfacing with a SLIM, were characterized. The RIF provided a 2-fold sensitivity increase without significant discrimination over a wide <i>m</i>/<i>z</i> range and well matched to that of SLIM, along with greatly improved SLIM operational stability

Fundamentals of Ion Dynamics in Structures for Lossless Ion Manipulations (ASMS 2016)

<p>While much effort has gone into developing improved separation strategies for use with MS analysis, the extensive demands for more effective characterization of complex biological mixtures drives further efforts to meet these needs. Gas phase separations based upon ion mobility (IM) are fast, amenable to high-throughput application, and provide high reproducibility. New platforms that allow complex ion manipulations, e.g. mobility based ion selections, CID, ion/ion reactions, in addition to higher resolution separations, are of interest. Here we characterize the fundamentals of ion dynamics and consider novel ion processing approaches in Structures for Lossless Ion Manipulations (SLIM). Ion confinement, ion dynamics, heating effects and separation performance and other insights from simulations and theory will be discussed.</p

Collision Cross Section Calibration with Structures for Lossless Ion Manipulations (ASMS 2017)

Ion mobility mass spectrometry (IM-MS) is a powerful separation and structural characterization technique, providing the ability to measure collision cross sections (CCS), revealing information about the three dimensional structure of gaseous ions. In many cases, CCS can be used to identify ions in a mixture, and highly accurate and precise CCS measurements greatly expand IM-MS capabilities. Recently, long path structures for lossless ion manipulations (SLIM) traveling wave (TW) IM modules have allowed extremely high resolution IM separations. However, since SLIM do not utilize uniform low-field drift cells, CCS cannot be directly measured from experiments. To that end, we have developed a CCS calibration framework to provide high resolution CCS assignment.Travelling wave potentials and a combination of lateral DC-only electrodes (guards) and extended RF electrodes aligned with the ion path provided for TWIM separations in several Torr nitrogen in conjunction with efficient ion confinement. Ions from nanoelectrospray ionization of mixtures of multiple classes of compounds (e.g. peptide, glycan, lipid) were injected to the SLIM module. A SLIM ion switch controlled whether ions made multiple passes through the serpentine path of the module, or were sent to the TOF MS for analysis. Multiple mixtures of calibrants of different classes overlapping in CCS space with the compounds studied were prepared and infused as both external and internal calibrants. TWIM-MS features were extracted and calibrated using in-house developed software tools.Recently, multi-pass SLIM separations have been reported, showing very high IM resolutions and peak capacities for a variety of compounds, including peptides, lipids, and carbohydrates. A SLIM ion switch was positioned at the end of a long (>10 meter) serpentine ion path to allow ions to either exit to a TOF MS for mass analysis, or to be shuttled to the beginning of the ion path for addition separation. Resolutions much higher than that from conventional commercially available instruments (both TW and uniform field) have been achieved (e.g., separation powers of over 1000 for singly charged ions for 200 m multi-pass separations). Due to the abundance of information from bottom-up proteomics of many protein standards (e.g. tryptic peptide accurate monoisotopic MW), the first efforts for applying CCS calibration have utilized whole protein digests. Early results have shown baseline separations of peptides in a protein digest (serum albumin) that are inseparable by conventional IM instruments. Initially, a poly-alanine mixture was used to begin evaluating CCS calibrations for peptides and was used as external and internal calibration standards. The protein digest was then run on an Agilent 6560 IM-MS to compare the calibrated CCS values against values measured directly by a uniform low field instrument. The presentation will detail the efficacy of CCS calibration in SLIM TWIM measurements as well as effects resulting from the choice of calibrant, internal vs. external calibration, and other biological compound classes

Very Long Path Length Ion Mobility Separations using Structures for Lossless Ion Manipulations (ASMS 2016)

<p>Ion mobility-based separations are of increasing importance in conjunction with MS, not only for providing additional structure-related information, but potentially more complete analysis of complex samples, detection of lower level constituents, and much greater speeds than feasible with liquid phase separations. The benefits of mobility-based separations generally increase as separation power increases, however to date high resolution mobility separations have only been achieved in conjunction with significant ion losses and over very limited ranges of mobility, substantially limiting their practicality and range of applications. This presentation will describe progress in new approaches capable of achieving ultrahigh resolution ion mobility separations based upon utilizing traveling waves in very long serpentine path length Structures for Lossless Ion Manipulations (SLIM) modules.</p