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

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

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

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

Compression Ratio Ion Mobility Programming (CRIMP) Accumulation and Compression of Billions of Ions for Ion Mobility-Mass Spectrometry Using Traveling Waves in Structures for Lossless Ion Manipulations (SLIM)

We report on the implementation of a traveling wave (TW) based compression ratio ion mobility programming (CRIMP) approach within structures for lossless ion manipulations (SLIM) that enables both greatly enlarged trapped ion charge capacities and also efficient ion population compression for use in ion mobility (IM) separations. Ion accumulation is conducted in a SLIM serpentine ultralong path with extended routing (SUPER) region after which CRIMP compression allows the large ion populations to be “squeezed”. The SLIM SUPER IM module has two regions, one operating with conventional traveling waves (i.e., traveling trap; TT region) and the second having an intermittently pausing or “stuttering” TW (i.e., stuttering trap; ST region). When a stationary voltage profile was used in the ST region, ions are blocked at the TT–ST interface and accumulated in the TT region and then can be released by resuming a conventional TW in the ST region. The population can also be compressed using CRIMP by the repetitive merging of ions distributed over multiple TW bins in the TT region into a single TW bin in the ST region. Ion accumulation followed by CRIMP compression provides the basis for the use of larger ion populations for IM separations. We show that over 10⁹ ions can be accumulated with high efficiency in the present device and that the extent of subsequent compression is only limited by the space charge capacity of the trapping region. Approximately 5 × 10⁹ charges introduced from an electrospray ionization source were trapped for a 40 s accumulation period, more than 2 orders of magnitude greater than the previously reported charge capacity of an ion funnel trap. Importantly, we show that extended ion accumulation in conjunction with CRIMP compression and multiple passes through the serpentine path provides the basis for a highly desirable combination of ultrahigh sensitivity and SLIM SUPER high-resolution IM separations

Development of a SLIM SUPER TWIM-MS Application Platform for Multi-Omics (ASMS 2017)

<p>Structures for lossless ion manipulations (SLIM) have been recently developed for traveling wave ion mobility (TWIM) experiments with a range of capabilities including serpentine long path separations (>100 m), trapping of large ion populations (>10⁹ ions), and compression ratio ion mobility programming (CRIMP) for increased sensitivity. These capabilities have demonstrated remarkable improvements in separation of isomeric lipids, peptides, and metabolites. </p> In this work, we evaluate the development of a SLIM TWIM-MS platform specifically designed for application in multi-omic analysis of complex biological samples. This system is designed to overcome challenges in conventional IM-MS analysis, including the ability to precisely target a narrow mobility window for extended analysis. This presentation will highlight the hardware and software innovations required for such experiments.A homebuilt SLIM system, coupled with Agilent 6538 QTOF-MS, is evaluated for its capabilities in ultrahigh resolution IM separations. The custom SLIM utilizes four different traveling wave (TW) inputs, designed for multiple accumulation regions and peak compression. Switches are placed to allow multiple passes of ions within a targeted mobility range and to control trapping events. Instrument control and data acquisition are performed with in-house developed software. Experiments involve a one pass (12.3 m path length) prescan to obtain initial drift time information and mass spectra. A targeted range is then selected for further analysis, which can include multiple separation passes for improved resolution followed by compression to reduce diffusional broadening and prevent excessive narrowing of the mobility range studied. The SLIM IM-MS applications system is evaluated for its capabilities in the analysis of complex mixtures, specifically for accumulation of a large initial ion population, concentration of a targeted mobility range, and long path separations providing improved resolution. <p>Acquisition of a prescan (small ion population, single pass separation) produces initial drift time information and mass spectra to define regions of interest. A user-defined experiment (i.e., number of passes, compression, etc.) then begins with accumulation of a large initial ion population in a 6 m region; preliminary experiments have demonstrated SLIM accumulation of >10⁹ ions using low-amplitude TWs. The ability to accumulate such a large ion population provides increased dynamic range for the analysis of complex mixtures, in which key components may be present at low concentrations. This SLIM-based trapping capacity overcomes the challenge of limited ion capacity encountered in conventional pulsed IM instrumentation. </p> <p>The in-house developed software can then control TW parameters and switches for precision control of trapping, cycling, and compression as per the defined experiment. Single pass SLIM (performed with a similar SLIM design having a 13 m separation path) has demonstrated dramatic improvements in resolution with increased path length for isomeric cis/trans lipids, leucine/isoleucine-containing peptides, reverse sequence peptides, and metabolites such as sugars. Multiple pass separations, with path lengths in excess of >100 m, provide further improvements in resolution and the detection of previously unobserved conformers. In addition, CRIMP has demonstrated increased signal intensity by merging several adjacent traveling traps. This reduces the effects of diffusional broadening that occur with long path separations and improves sensitivity for trace components in mixtures.These capabilities are integrated into a multi-pass serpentine ultra-long path for extended resolution (SUPER) SLIM TWIM-MS system that will be evaluated for analysis of complex biological samples with a multi-omic approach (e.g., metabolomics, proteomics, lipidomics).</p

Squeezing of Ion Populations and Peaks in Traveling Wave Ion Mobility Separations and Structures for Lossless Ion Manipulations Using Compression Ratio Ion Mobility Programming

In this work we report an approach for spatial and temporal gas-phase ion population manipulation, wherein we collapse ion distributions in ion mobility (IM) separations into tighter packets providing higher sensitivity measurements in conjunction with mass spectrometry (MS). We do this for ions moving from a conventional traveling wave (TW)-driven region to a region where the TW is intermittently halted or “stuttered”. This approach causes the ion packets spanning a number of TW-created traveling traps (TT) to be 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 “structures for lossless ion manipulations” (SLIM) in conjunction with MS. CRIMP with the SLIM-MS platform is shown to provide increased peak intensities, reduced peak widths, and improved signal-to-noise (S/N) ratios with MS detection. CRIMP also provides a foundation for extremely long path length and multipass IM separations in SLIM providing greatly enhanced IM resolution by reducing the detrimental effects of diffusional peak broadening and increasing peak widths

Greatly Increasing Trapped Ion Populations for Mobility Separations Using Traveling Waves in Structures for Lossless Ion Manipulations

The initial use of traveling waves (TW) for ion mobility (IM) separations using structures for lossless ion manipulations (SLIM) employed an ion funnel trap (IFT) to accumulate ions from a continuous electrospray ionization source and was limited to injected ion populations of ∼10⁶ charges due to the onset of space charge effects in the trapping region. Additional limitations arise due to the loss of resolution for the injection of ions over longer periods, such as in extended pulses. In this work a new SLIM “flat funnel” (FF) module has been developed and demonstrated to enable the accumulation of much larger ion populations and their injection for IM separations. Ion current measurements indicate a capacity of ∼3.2 × 10⁸ charges for the extended trapping volume, over an order of magnitude greater than that of the IFT. The orthogonal ion injection into a funnel shaped separation region can greatly reduce space charge effects during the initial IM separation stage, and the gradually reduced width of the path allows the ion packet to be increasingly compressed in the lateral dimension as the separation progresses, allowing efficient transmission through conductance limits or compatibility with subsequent ion manipulations. This work examined the TW, rf, and dc confining field SLIM parameters involved in ion accumulation, injection, transmission, and IM separation in the FF module using both direct ion current and MS measurements. Wide <i>m</i>/<i>z</i> range ion transmission is demonstrated, along with significant increases in the signal-to-noise ratios (S/N) due to the larger ion populations injected. Additionally, we observed a reduction in the chemical background, which was attributed to more efficient desolvation of solvent related clusters over the extended ion accumulation periods. The TW SLIM FF IM module is anticipated to be especially effective as a front end for long path SLIM IM separation modules