Implementation of Ion Mobility Spectrometry-Based Separations in Structures for Lossless Ion Manipulations (SLIM)

Summary

Structures for Lossless Ion Manipulations (SLIM) is a powerful variant of traveling wave ion mobility spectrometry (TW-IMS) that uses a serpentine pattern of microelectrodes deposited onto printed circuit boards to achieve ultralong ion path lengths (13.5 m). Ions are propelled through SLIM platforms via arrays of TW electrodes while RF and DC electrodes provide radial confinement, establishing near lossless transmission. The recent ability to cycle ions multiple times through a SLIM has allowed ion path lengths to exceed 1000 m, providing unprecedented separation power and the ability to observe ion structural conformations unobtainable with other IMS technologies. The combination of high separation power, high signal intensity, and the ability to couple with mass spectrometry places SLIM in the unique position of being able to address longstanding proteomics and metabolomics challenges by allowing the characterization of isomeric mixtures containing low abundance analytes.

1. Introduction

Ion mobility spectrometry (IMS), originally known as plasma chromatography^{1, 2}, is a powerful technique for the analysis of gaseous ion structure. Though initially limited in its scope by available ionization sources, the development of soft ionization techniques^{3, 4} such as electrospray ionization^5-8 (ESI), made it feasible to employ IMS for a wide range of applications (e.g., detection of hazardous chemical agents,^{9, 10} environmental pollutants,^{11, 12} or illicit drugs).^13-15 Direct comparison of experimental IMS measurements with theoretical structures provided by molecular dynamics simulations has led to tremendous advancements in our understanding of biomolecular structure, ranging from small oligomers to large protein complexes or intact virus capsids.^16-20 IMS instruments are typically coupled to time-of-flight mass spectrometers, first implemented by Guevremont et al.,²¹ for powerful multi-dimensional separations that provide advantages such as sensitivity and peak capacity over mass spectrometry (MS) as a stand-alone technique. IMS separations have also been employed preceding spectroscopy measurements or ultraviolet photodissociation to enhance selectivity.^{22, 23}

In IMS, ions are generally transmitted through a neutral buffer gas under the influence of a uniform electric field (E). Each species is separated by mobility (K), a constant unique to each species that is proportional to its charge, and as a function of size and shape, which affect the frequency and nature of collisions with the buffer gas molecules.²⁴ Mobility may be calculated from the ion velocity (v_D) and electric field (Eq. 1):

$$\mathrm{K}=\frac{\mathrm{E}}{{\mathrm{v}}_{\mathrm{D}}}$$ (1)

It is also possible to calculate an experimental collision cross section (Ω) by the following relation:

$$\Omega =\frac{(18\pi {)}^{1∕2}}{16}\frac{\text{ze}}{({\mathrm{k}}_{\mathrm{b}}\mathrm{T}{)}^{1∕2}}{\left[\frac{1}{{\mathrm{m}}_1}+\frac{1}{{\mathrm{m}}_{\mathrm{B}}}\right]}^{1∕2}\frac{{\mathrm{t}}_{\mathrm{D}}\mathrm{E}}{\mathrm{L}}\frac{760}{\mathrm{P}}\frac{\mathrm{T}}{273.2}\frac{1}{\mathrm{N}},$$ (2)

where ze is the ion charge, k_B is Boltzmann’s constant, m_I and m_B are the ion and buffer gas mass, respectively, t_D is the arrival time, L is the length of the transmission region, and P, T, and N are the pressure, temperature, and neutral number density of the buffer gas, respectively.²⁴ These relationships hold true for classical drift tube IMS (DT-IMS) experiments, which utilize a uniform electric field, and have enabled precise CCS measurements of thousands of compounds which are compiled in databases.^25-27

There are many IMS techniques being employed today, each with their own advantages. DT-IMS usually consists of a series of concentric ring electrodes in a vacuum chamber operated at pressures on the order of several Torr, where a uniform electric field causes a net motion of ions against a gas flow, down the axis of the drift tube.^{28, 29} This straightforward method allows for extremely accurate collision cross section measurements, but is difficult to scale with length as the voltage required increases linearly. Many other IMS variants have been developed that attempt to alleviate the problems of size and voltage requirements, such as trapped IMS (TIMS),^{30, 31} overtone mobility spectrometry (OMS),^{32, 33} differential mobility analysis (DMA),^34-36 field asymmetric IMS (FAIMS),^37-39 and traveling wave IMS (TW-IMS).^40-42 Although each technique has advantages and is capable of high-resolution measurements, the more complicated ion motion in these IMS techniques distorts the relationship of arrival time and collision cross section such that the relationship in Eq. 2 no longer holds. For this reason, CCS measurements of well-characterized standards by DT-IMS are frequently used for calibration.^{43, 44} There are currently efforts to more accurately predict ion velocity.^{42, 45, 46}

The focus of this chapter is on a variant of TW-IMS, Structures for Lossless Ion Manipulations (SLIM), which utilizes a combination of RF and DC potentials to confine and transmit ions.⁴⁷ Although SLIM originally employed a uniform electric field to generate the axial ion motion,^{48, 49} it was quickly adapted to a TW-based design.⁵⁰ The basic operation of SLIM involves a spatially- and temporally-dynamic electric field that simulates a potential ‘wave’ to control the motion of ions as they traverse the device (Figure 1), with RF and static DC-guard electrodes to confine ions laterally. In the last few years, many SLIM technological developments have pushed the capabilities,^51-56 especially the implementation of Serpentine Ultralong Path with Extended Routing (SUPER),^{26, 57} which efficiently transmits ions around a circuitous path with the option of repeating an unlimited number of cycles for ultra-high resolution measurements (Figure 2). SLIM SUPER IMS-MS-based separations have been applied to separation of many classes of compounds, including the identification of previously indistinguishable isomers, as well as for other biological applications.^57-67

Figure 1:

Cartoon illustration depicting ions of differing mobilities (red, orange, green circles) separating in a TW device. High mobility ions (red) fall behind the wave crest (‘roll-over’) least often, so arrive first. Low mobility ions (green) roll-over most often, so arrive last.

Figure 2:

(A) Schematic diagram of a multipass SLIM IMS-MS instrument, (B) photo of one SLIM board, (C) illustration of an ion switch for sending ions to MS or to cycle another pass. Reprinted with permission from reference 26.

SLIM electrodes are manufactured on printed circuit boards (PCBs). Two boards, symmetric and with mirrored designs, are spaced millimeters apart to form the confining electric fields (Figure 3). The combination of RF, DC, and TW electrodes of SLIM allow ions to be easily directed around 90° or 180° turns in a serpentine path to provide long separation distances in a compact area.^{26, 68} Dynamically controlled electric potential ‘gates’ allow for ions at the end of the path to be cycled back to the front of the device, enabling additional passes for enhanced separation (discussed later). Optimized ion transmission is nearly 100%, even over extremely long path lengths. The maximum length of IMS separations is limited by the overall peak capacity for a given effective path length (i.e., the number of ions that can be resolved at full width half maximum in a 13.5 m path length). In some instances, the higher mobility ions (i.e., faster moving ions) will ‘lap’ the lower mobility (i.e., slower) ones, which can make the correlation between arrival time and mobility difficult to deconvolute. Additionally, axial ion diffusion can limit the overall path length for separation in ultralong separations (> 1 km). While a complex mixture may be limited to one or two passes before the fastest ions overtake the slowest, selected mobility ranges may be separated over great distances to acquire high-resolution measurements.⁶⁶ The remaining sections of this chapter serve to describe the design and operation of a SLIM IMS platform, as well as a compendium of useful notes.

Figure 3:

Photograph of two SLIM boards arranged in parallel. Ions are injected between the two boards.

2. Materials

2.1. Electrospray ionization source

1. Electrospray ionization emitter (hydrofluoric acid-etched silica capillary, Polymicro Technologies, Phoenix, AZ, USA).

2. HPLC microunion and fittings (Catalog # 502413, Millipore-Sigma, St. Louis, MO, USA)

3. Glass syringe (Hamilton, Reno, NV, USA).

4. Syringe pump (0.1 - 2 μL/min, Chemyx, Stafford, TX, USA).

5. High voltage power supply (0-8 kV DC, custom-built).

2.2. High-pressure ion optical system before SLIM

1. Inlet capillary (Figure 4)

   1. Straight = 250 – 750 μm i.d.
   2. Multi-inlet = 5 x 250 μm i.d. arranged in a cross
2. High pressure ion funnel⁶⁹

   1. 120 electrodes, 0.5 mm thickness, 0.5 mm spacing
   2. Electrodes 1 – 60 (all 25.4 mm i.d.)
   3. Electrodes 61 – 120 (25.4 to 2.5 mm i.d.)
   4. Brass conductance limit (2.5 mm i.d.)
   5. RF and DC power supply (RF = 300 V_pp, ~1 MHz, 20 V/cm, custom-built)
   6. 10 Torr nitrogen
3. Low pressure ion funnel with ion funnel trap (IFT)⁷⁰

   1. 94 electrodes, 0.5 mm thickness, 0.5 mm spacing
   2. Electrodes 1 – 24 (all 25.4 mm i.d.)
   3. Electrodes 24 – 54 (25.4 to 3 mm i.d.).
   4. Diverging region (10 electrodes, 3 to 19.1 mm i.d.)
   5. Trapping region (10 electrodes, all 19.1 mm i.d.)
   6. Converging region (20 electrodes, 19.1 to 2.4 mm i.d.)
   7. Brass conductance limit (2.4 mm i.d.)
   8. 3 independently controlled trapping grids positioned at the entrance and exit of the trapping region (95%-transmission nickel mesh, InterNet Inc., Minneapolis, MN, USA)
   9. RF and DC power supply (RF = 300 V_pp, ~1 MHz, DC = 20 V/cm, custom-built)
   10. Injection controller (Modular Intelligent Power Sources (MIPS) V.2, GAA Custom Engineering, Benton City, WA, USA)
   11. 2 - 4 Torr nitrogen or helium

Figure 4:

Experimental schematic of a typical SLIM MS system.

2.3. TW-SLIM modules

1. 2 mirror image PCBs (FR-4 material (fiber glass cloth with epoxy resin) with inlaid copper electrodes, PCB Universe, Vancouver, WA, USA)

2. Single pass & multi-pass serpentine TW-SLIM modules^{26, 58}

   1. Boards = 45.9 cm x 32.5 cm wide, spaced 2.75 mm apart
   2. Total path length = 13.5 m (1 pass, 44 U-turns, 2-90° turns, multi-pass = + 30 cm rerouting path)
   3. Electrodes = 6-RF strips interspaced with 5 arrays of traveling wave electrodes (electrode spacing = 0.13 mm)

      1. RF strips (0.43 mm wide, 300 V_pp, ~1 MHz)
      2. Traveling wave electrodes (1.03 mm long x 0.43 mm wide, 1 array = 8 electrodes, sequence = 11110000)
      3. DC guard electrodes (3.0 mm width, length of entire ion path, 5 – 15 V DC bias)

3. TW, RF, and DC all-in-one power supply (Modular Intelligent Power Sources (MIPS) V.2, GAA Custom Engineering, Benton City, WA, USA)

4. 50 mTorr higher pressure than IFT

2.4. Low pressure ion optical system after SLIM

1. Rear ion funnel

   1. 120 electrodes, 0.5 mm thickness, 0.5 mm spacing
   2. Electrodes 1 – 60 (all 25.4 mm i.d.)
   3. Electrodes 61 – 120 (25.4 to 2.5 mm i.d.)
   4. Brass conductance limit (2.5 mm i.d.)
   5. RF and DC power supply (RF = 150 V_pp, ~1.2 MHz, DC = ~20 V/cm, custom-built).
2. Short RF-only quadrupole

   1. 4-cylindrical rods, 2.6 cm length, 6.4 mm o.d.
   2. RF power supply (RF = 130 V_pp, ~1 MHz, custom-built)
   3. Pressure of 0.30 Torr

3. Time-of-flight mass spectrometer with 1.5 m flight tube (Agilent 6224 TOF MS, Santa Clara, CA, USA)

2.5. Data acquisition

1. Signal from TOF detector is sent to OP AMP-based step-down circuit (steps down TOF clock signal to 5 V, custom built)

2. 5 V signal sent to U1084A 8-bit digitizer (Acqiris, Switzerland)

3. SLIM acquisition & processing software (written in C#, custom built)

Methods

3.1. Traveling Wave IMS Operation

1. The basic operation of traveling wave IMS involves applying a sequence of spatially and temporally differentiated potentials to a series of electrodes in regular patterns (Figure 5). For example, the ‘spatial’ component of a square-wave could consist of high voltage on electrodes 1-4 and low voltage on electrodes 5-8, repeating every 8 electrodes. The ‘temporal’ component, i.e., the actual motion of the wave, is provided by stepping this pattern forward by one electrode at regular time intervals. Many waveforms (e.g., sine wave, triangle wave, sawtooth, etc.) are potentially viable and each will impact ion transmission differently.

2. In a traveling wave device, ions will experience several different types of motion depending on their mobility relative to the wave speed and amplitude. Ions that keep pace with a wave are said to be ‘surfing’ and will experience no separation from any other ions that are surfing. At very low wave amplitude, the forward motion of ions will be negligible, and diffusion will dominate their motion. Ideal separation conditions utilize wave speeds and amplitudes that result in the ions falling behind the wave, ‘rolling over’ the crest. The ability of each ion to keep pace with the wave will be proportional to its mobility, enabling separation.

Figure 5:

Top: a segment of SLIM surface showing the types of electrodes used to manipulate ions. Bottom: the application of a square-wave sequence (11110000) to sets of 8 electrodes is shown, with the first step in the sequence shown by the dashed red line. Reprinted with permission from reference 50.

3.2. Transmission Mode: Tuning for Signal Intensities of Analyte(s) of Interest

1. After the SLIM IMS-MS platform is fully connected, the initial step is to perform ion current measurements to ensure that adequate signal is obtained on the TOF-MS. By appropriately applying a voltage gradient, beginning with the high-pressure ion funnel followed by the ion funnel trap, ion current should be measured at the exit grid of the ion funnel trap (IFT). From there, by applying radio frequency (RF) and a direct current (DC) guard, ions should be confined in the SLIM serpentine path. Additionally, traveling wave amplitudes and frequencies must also be optimized to ensure that ion current remains lossless through the device. A similar voltage gradient must also be applied in the rear ion funnel so that ions can successfully transmit to the TOF-MS for detection.

2. Following tuning for ion current (average signal is on the order of a few hundred picoamperes of current), finer tuning must be done for the analyte(s) of interest. The user must know the molecular weights and possible charge states for their ions of interest, as well as if they are interested in studying ions over a broad range of mobilities or a narrower one.

3. General starting conditions for ion transmission through the SLIM IMS-MS system include: 10.5 Torr pressure of nitrogen in the high-pressure ion funnel, 2300 millitorr of nitrogen in the IFT, 2350 millitorr pressure nitrogen in the SLIM chamber, square TW profile in the 4 up 4 down sequence (11110000), 30 V traveling wave (TW) amplitude in both TW1 and TW2 sections at 200 m/s frequency, 350 V_pp RF at 900 kHz, and 15 V DC guard.

4. TOF parameters may also need to be optimized depending on the mass-to-charge (m/z) of the ions of interest.

5. If the signal intensity is lower than desired for the ions of interest, there are several parameters that are frequently adjusted to potentially increase signal. The applied TW amplitude and frequency have a large effect on the ion transmission. If the ions of interest are low in mobility (most common for very large structures of high m/z compounds), lower TW frequency and/or higher TW amplitude may be required. Conversely, the opposite settings should be applied for higher mobility species. Additionally, the use of alternate drift gases (e.g., helium or argon) should also be explored. In certain instances, we have found that lowering the pressure resulted in higher signal intensities, as well as varying the difference in pressure between the IFT and SLIM chamber. Nonetheless, optimization of signal intensity is largely an empirical task.

3.3. Ion Accumulation

1. After a sufficient signal is obtained in transmission mode for the desired ions of interest, the next step becomes to perform an IMS separation experiment. The first step is to introduce ions via accumulation.

2. The conventional approach for ion introduction is to use the ion funnel trap for accumulation. This step is performed by utilizing the series of entrance, trap, and exit grids to accumulate ions in the ion funnel trap, as well as block off the ion beam so that no leaking occurs either into the IFT or into the SLIM serpentine TW path. Filling the IFT to maximum charge capacity can occur on the order of a few milliseconds, while the release time of ions into the SLIM IMS separation path is on the order of a <1 millisecond.

3. While the IFT results in the introduction of a very narrow ion packet (<1 ms), it is ultimately limited by its charge capacity, and thus other ion accumulation approaches are desired to increase signal intensity. We have developed a new ion accumulation method that utilizes traveling waves to circumvent any charge capacity limitations. This method, termed in-SLIM ion accumulation, utilizes the actual IMS serpentine path to accumulate ions. In-SLIM accumulation is performed by leaving the IFT open at all times and introducing ions into the first traveling wave section of the SLIM IMS path. The second traveling wave section is halted (i.e., stopped) to create a potential wall so that ions can pile up in the TW1 section. The TW1 section can range in size, but most SLIM platforms contain a 9 m TW1 section, which permits over a billion ions to be accumulate with in-SLIM ion accumulation (up to 2-3 orders of magnitude more ions than accumulation via the IFT). Ions are accumulated on the order of 1-2 seconds, but we are currently exploring methods to accumulate ions for up to 5-10 seconds. After accumulation is completed, IFT voltages are applied to kill off the ion beam so that no more ions enter the SLIM IMS path. At the same point, the TW2 section resumes normal traveling wave operation so that the IMS separation can begin.

3.4. Parameter Optimization for Resolution and Sensitivity in Separation (IMS) Mode

1. Following the ion accumulation step (either via the IFT or in-SLIM), the IMS separation begins. At this stage, the primary parameters that will affect resolution of species are the TW conditions and the drift gas pressure.

2. As was discussed in the introduction, the mechanism for separation in TW IMS is that ions must experience some rollover into TW bins so that separation can occur. Thus, care must be taken for ions not to move at the same speed as the TW (i.e., surf). Furthermore, it has been demonstrated that optimum resolution occurs at the TW speed just below surfing conditions; thus, TW frequency and amplitude must be tuned to achieve these conditions. Generally speaking, to move ions closer to surfing conditions, some combination of raising the amplitude or lowering the frequency is required.

3. Additionally, it has been shown that higher drift gas pressures will permit higher resolution IMS separations. But, this may come at a tradeoff of lower signal intensity. Drift gas selection can also play a vital role in resolution; some analytes may resolve better when helium buffer gas is used instead of nitrogen, or vice versa. It remains difficult to achieve optimal resolution over a very wide mobility range, thus future work could entail utilization of a gradient TW (analogous to a gradient in liquid chromatography).

4. An example of an optimized TW-SLIM separation of Agilent Tuning Mix phosphazene ions (m/z 622, 922, 1222, 1522) is shown in Figure 6. The data were acquired using one of our 30.5 cm-long, linear TW-SLIM systems. The speed and amplitude of our traveling wave were 84 m/s and 30 V, respectively, and our guard voltage and RF amplitude were 15 V and 320 V_pp, respectively. Again, different samples will require different parameters to achieve optimal separation and transmission conditions.

Figure 6:

IMS-MS separation of Agilent Tuning Mix ions with our TW-SLIM platform. Mass spectrum (A) along with its 2D IMS-MS arrival time distribution (B and C). Reprinted with permission from reference 50.

3.5. Ion Switch for Multipass Separations

1. After optimization of IMS separation conditions for resolution, the next factor that determines how well resolved chemical species will be is the total path length of their separation. Conventional IMS devices are limited in their total path length; the longest path length in a commercial instrument is 1 m in a drift tube. Other approaches, such as the use of a counter current gas flow in trapped ion mobility spectrometry, can increase the effective path length of separation. Our conventional SLIM IMS platforms commonly have a 13.5 m serpentine path for separation in a single pass. This path length is long enough to provide baseline separation between many closely related species, such as the isomeric hexasaccharides lacto-N-hexaose and lacto-N-neohexaose (Figure 7a). These two sugars differ only in the regiochemistry of a single glycosidic bond (Figure 7 left, highlighted area in structures). The ability to easily differentiate ions based on such a small structural difference highlights the capabilities of extended path length SLIMs.

2. While 13.5 m of separation may be sufficient for some applications, more challenging separations require even longer path lengths. To circumvent the finite path length in our SLIM IMS platforms, we have enabled an ion switch at the end of the IMS path to either route ions to the TOF-MS for detection or back to the start of the serpentine path for another 13.5 m cycle. This switch operates in a high-low state, where either a DC blocking voltage is applied to route ions back to the start of the IMS path for another pass, or a TW voltage is applied to route ions to the TOF-MS for detection. After optimization of the blocking voltages applied, this ion switching can be done in a lossless fashion, thus enabling ions to be routed indefinitely around the SLIM IMS path (>>>100 meters of total IMS separation). By increasing path length through this rerouting process, it is common for new, previously unobservable spectral features to emerge. For example, performing a 121.5 m SLIM separation on the same sample of isomeric hexasaccharides resulted in the lacto-N-neohexaose splitting into two peaks and the observation of a new chemical feature (Figure 7b). Since resolving power scales as the function of the square root increase in total path length, having a nearly infinite IMS separation path length is a very attractive feature of our SLIM IMS-MS platforms.

3. Future work entails use of this ion switch to notch out a selected mobility slice and sending these ions to another region in the SLIM path for other ion manipulations (e.g., fragmentation, storage, etc.).

Figure 7:

IMS separation of isomeric hexasaccharides after 13.5 m (A) and demonstration of increased resolution from multipass IMS separations (9 total passes) enabled by SUPER after 121.5 m (B). Reprinted with permission from reference 26.

3.6. Data Processing

1. Data acquired with our homebuilt SLIM IMS-MS is in the unified ion mobility format (.uimf) and can be viewed in our custom UIMF viewer and is available freely from omics.pnl.gov or contacting the corresponding author.

2. While our homebuilt viewer can provide rapid feedback on a given experiment, further data processing is necessary so that a .uimf file can be converted into a file format that is processed by other informatics tools.

3. In ultralong path length separations, the arrival time of an ion can be very long (several seconds). However, most of the important data is contained near the end of the acquisition based on the nature of our ion switch. Thus, to speed up the data processing pipeline, the .uimf file can be truncated to exclude out any unneeded portions of the arrival time distribution.

4. While we take great care to control the drift gas pressure in both the funnels and SLIM chamber, some minor fluctuations can occur (~<5 mTorr). Such minor fluctuations can cause some drift in the arrival times of ions from separation to separation. This can become an issue when attempting to signal average (i.e., sum) multiple separations into a single output file. To overcome this, we use a homebuilt drift time aligner tool that searches for a given feature in a user-specified separation and uses the arrival time of that feature and linearly shifts all other separations to match the user-specified one. The final, processed output file will then show no deleterious effects from any such pressure fluctuations. Work is ongoing to better control such pressure fluctuations, as well as have a potential pressure feedback loop that can modulate pressure during an experiment to prevent such drift from occurring.

5. After alignment, the next step in the data processing pipeline is to convert our .uimf file into the Agilent (.d) format. We have homebuilt software that performs the conversion and is available from moics.pnl.gov. Conversion to .d format enables us to use the IMS-MS browser from Agilent, where arrival time distributions can be exported along with their corresponding mass spectra. Furthermore, .d format can then be transferred into numerous open-source software programs (e.g., Skyline) for visualization of SLIM IMS-MS data (especially when coupled to online liquid chromatography).

4. Notes

1. Pressure and temperature fluctuations affect ion arrival time, especially over long separation times. These fluctuations may make it difficult to utilize timed switches to isolate or cycle intended ions, as they may arrive at the switch earlier or later than anticipated. Therefore, minimizing pressure fluctuations is extremely important.

2. We typically hold the pressure inside the SLIM chamber at ~50 mTorr higher than the pressure inside the ion funnel trap. This creates a gas flow out of the SLIM chamber that we use to help prevent neutrals from entering the SLIM. However, it is certainly possible to use a lower pressure inside the SLIM chamber and still obtain good signal to noise ratios if all neutrals are removed prior to ions entering the SLIM chamber.

3. Encasing the ionization source in a housing helps prevent pressure fluctuations in the front funnel and improves ion transmission at the atmospheric pressure interface.

4. An alternative way to inject ions is to place a single steel grid into the straight region of an ion funnel located in front of the SLIM and apply a pulsed voltage to it.

5. We typically use conventional single capillary inlets in our SLIM systems. However, we have obtained considerably higher ion currents when using a multi-inlet capillary.

6. SLIM offers several methods for manipulating ions – CRIMP, stopping the wave, slow surf, fast surf – each of these methods have their uses, but also the potential to introduce m/z or mobility biases, or inflict substantial or complete ion loss.

7. Excessive ion activation can occur in a number of ways and places, such as: ion funnels with steep voltage gradients, high voltages in SLIM (RF, DC guard, or TW amplitude), or any interface involving these components with a momentarily strong electric field. Activation can be minimized by reducing voltages. This often involves a tradeoff between activation levels, transmission efficiency, and resolution, so it is up to the operator to decide the ‘optimal conditions’.

8. Two easy indications that ions are surfing are 1) many ions arriving at the exact same time, or 2) an ion packet with velocity equal or nearly equal to the traveling wave speed.

9. It is important to note that TW conditions (frequency, amplitude, waveform type) can play a large role in producing space-charge related effects, thus potentially biasing the m/z or mobility range of ions introduced.

10. The RF and TW voltages in SLIMs are normally biased to the same high DC voltage (e.g. +250 V) and allow the full mass range of ions traversing the SLIM to be observed (e.g. m/z 300 – 2800). However, we have occasionally observed a high mass cutoff in one of our systems that we can correct by setting the bias voltage of TW slightly higher (or lower in – ion mode) than the RF bias voltage (e.g. TW = +260 V, RF = + 250 V). We don’t normally recommend doing this since large voltage differences can result in arching that could damage the SLIM boards or associated electronics. However, if changing the bias voltage is required, it is imperative that the voltage difference be very low (< 10 V).

11. Although we typically apply RF frequencies of 1 MHz to our SLIM modules, it may be necessary to use upwards of 1.5 MHz to successfully transmit very high mobility ions.

12. At extremely long path lengths, faster (i.e., higher mobility) ions will lap the slower (i.e., lower mobility) ones. We are developing a new SLIM IMS platform that uses elevators to route ions to other levels instead of an ion switch. This elevator design will effectively be ~100 m single pass path length, thereby eliminating any lapping issues of fast and slow ions as are present in current SLIM IMS platforms.