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

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

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

Experimental Evaluation and Optimization of Structures for Lossless Ion Manipulations for Ion Mobility Spectrometry with Time-of-Flight Mass Spectrometry

We report on the performance of structures for lossless ion manipulation (SLIM) as a means for transmitting ions and performing ion mobility separations (IMS). Ions were successfully transferred from an electrospray ionization (ESI) source to the TOF MS analyzer by means of a linear SLIM, demonstrating lossless ion transmission and an alternative arrangement including a 90° turn. First, the linear geometry was optimized for radial confinement by tuning RF on the central “rung” electrodes and potentials on the DC-only guard electrodes. Selecting an appropriate DC guard bias (2–6 V) and RF amplitude (≥160 V_p‑p at 750 kHz) resulted in the greatest ion intensities. Close to ideal IMS resolving power was maintained over a significant range of applied voltages. Second, the 90° turn was optimized for radial confinement by tuning RF on the rung electrodes and DC on the guard electrodes. However, both resolving power and ion transmission showed a dependence on these voltages, and the best conditions for both were >300 V_p‑p RF (685 kHz) and 7–11 V guard DC bias. Both geometries provide IMS resolving powers at the theoretical limit (<i>R</i> ∼ 58), showing that degraded resolution from a “racetrack” effect from turning around a corner can be successfully avoided, and the capability also was maintained for essentially lossless ion transmission

Characterization of Traveling Wave Ion Mobility Separations in Structures for Lossless Ion Manipulations

We report on the development and characterization of a traveling wave (TW)-based Structures for Lossless Ion Manipulations (TW-SLIM) module for ion mobility separations (IMS). The TW-SLIM module uses parallel arrays of rf electrodes on two closely spaced surfaces for ion confinement, where the rf electrodes are separated by arrays of short electrodes, and using these TWs can be created to drive ion motion. In this initial work, TWs are created by the dynamic application of dc potentials. The capabilities of the TW-SLIM module for efficient ion confinement, lossless ion transport, and ion mobility separations at different rf and TW parameters are reported. The TW-SLIM module is shown to transmit a wide mass range of ions (<i>m</i>/<i>z</i> 200–2500) utilizing a confining rf waveform (∼1 MHz and ∼300 V_p‑p) and low TW amplitudes (<20 V). Additionally, the short TW-SLIM module achieved resolutions comparable to existing commercially available low pressure IMS platforms and an ion mobility peak capacity of ∼32 for TW speeds of <210 m/s. TW-SLIM performance was characterized over a wide range of rf and TW parameters and demonstrated robust performance. The combined attributes of the flexible design and low voltage requirements for the TW-SLIM module provide a basis for devices capable of much higher resolution and more complex ion manipulations

Structures for Lossless Ion Manipulations Device as an Ion Mobility Filter (ASMS 2017)

Structures for Lossless Ion Manipulations (SLIM) allow confining and manipulating ions utilizing a combination of radio frequency (RF) and direct current (DC) fields or traveling waves (TW). TW can be employed in SLIM devices to separate ions based on their mobility. We have been exploring concepts for the continuous filtering of ions for the selection of specific and narrow mobility ranges. Such a device would be an IM analog of a e.g. quadruple mass filter. In this presentation we show the supporting simulations and the experiments to demonstrating the filtering capability of the SLIM device.The SLIM filter module (30.5 cm) was designed having two parallel arrays of electrodes, namely the rung and guard electrodes. Ions are confined laterally by the applied DC voltage to the guard electrodes, while confined between the surfaces by effective potentials created by applying alternating 1800 out of phase RF voltages. In the current design, ions are guided by a combination of TW and opposing DC drift fields. The SLIM was segmented into two mirror-image sections where the TW and opposing DC are applied. By choosing the suitable combination of DC gradient and TW parameters for the two sections, it is possible to transmit ions of certain mobility while filtering out other ions.In this presentation, we demonstrate a SLIM ion mobility filter allowing ions of specific mobility to be efficiently transmitted. Ion trajectories simulations showed the SLIM devices can filter ions according to their mobilities when opposing TW and DC drift fields are combined. By choosing the suitable combination of DC gradient and TW parameters for the two sections, we found it is possible to transmit ions of certain mobility while filtering out other ions. The SLIM filter is operated by combining a positive DC gradient in the first half and a negative DC gradient in the second half of the SLIM. Two TW were used, one moving forward in the first section, while with the other is moving in the reverse direction in the second section of the SLIM module. The filtering is determined by DC gradient and the TW parameters, such as frequency, amplitude and the sequence (or in other words, the duty cycle of the travelling waveTW). Experiments show that filtering with minor loss of ions could can be achieved by adjusting proper selection of TW frequencies. The difference in frequency, frequency window, determines the range of mobilities transmitted through the filter, which can be explained by the relative ion velocity obtained from the applied DC and TW potentials. The sequence of the TW was found to affect the sensitivity of the device. The velocity of the ions due to TW and that due to the DC field were extracted from the simulations. The filtering is due to the opposing effects of the TW and the DC gradient. Those ions whose mobility due to TW is higher than that due to the DC gradient will successfully pass the first section. While in the second section ions having a higher mobility due to DC gradient will be transmitted

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

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

Petroleomic Characterization using an Ion Mobility-Orbitrap Platform (ASMS 2016)

<p>Ion mobility spectrometry (IMS) is a fast gas-phase separation technique that separates ions in part based upon their shape. Different chemical classes form shape-related ‘trend lines’ in the 2D drift time-m/z chromatogram. These trend lines can be utilized as a quick diagnostic for chemical classes, and hence information that augments that from mass spectrometry. Herein we present a new IMS-Orbitrap platform that couples the benefits of drift-tube IMS with a high mass resolution Orbitrap MS. The new platform was utilized to analyze and profile an array of petroleum products, and highlights the utility of the IMS -Orbitrap platform for analysis of highly chemically complex substances.</p