Electrostatic Ion Trap Research Articles

Realization of Higher Resolution Charge Detection Mass Spectrometry.

Charge detection mass spectrometry (CD-MS) allows mass distributions to be measured for heterogeneous samples that cannot be analyzed by conventional MS. With CD-MS, the m/z and charge are measured for individual ions using a detection cylinder embedded in an electrostatic linear ion trap (ELIT). Imprecision in both the m/z and charge measurements contribute to the mass resolution. However, if the charge can be measured with a precision of <0.2 e the charge state can be assigned with a low error rate and the mass resolving power only depends on the m/z resolution. Prior to this work, the best resolving power demonstrated experimentally for CD-MS was 700. Here we demonstrate a resolving power of >14,600, 20-times higher than the previous best. Trajectory simulations were used to optimize the geometry and electrostatic potentials of the ELIT. We found conditions where the energy dependence of the oscillation frequency becomes parabolic, and then operated with a nominal ion energy at the minimum of the parabola. The 14,600 resolving power was obtained with a beam collimator before the ELIT. With the collimator removed, the resolving power of the optimized ELIT is 7300, which is still an order of magnitude higher than the previous best. The resolving power was demonstrated by resolving the isotope distributions for peptides and proteins. High resolution CD-MS measurements were then used to resolve the glycans on a monoclonal antibody and applied to the analysis of hepatitis B virus capsids. The results indicate that procedures for adduct removal need to be improved for the full benefit of the higher resolving power to be realized for higher mass species. However, these results represent a key step toward using CD-MS to analyze very complex protein mixtures where charge states are not well resolved in the m/z spectrum because of congestion from numerous overlapping peaks.

Tandem mass spectrometry using continuous-wave infrared multiphoton dissociation in an electrostatic linear ion trap.

The electrostatic linear ion trap (ELIT) can be operated as a multi-reflection time-of-flight (MR-TOF) or Fourier transform (FT) mass analyzer. It has been shown to be capable of performing high-resolution mass analysis and high-resolution ion isolations. Although it has been used in charge-detection mass spectrometry (CDMS), it has not been widely used as a conventional mass spectrometer for ensemble measurements of ions, or for tandem mass spectrometer. The advantages of tandem mass spectrometer with high-resolution ion isolations in the ELIT have thus not been fully exploited. A homebuilt ELIT was modified with BaF2 viewports to facilitate transmission of a laser beam at the turnaround point of the second ion mirror in the ELIT. Fragmentation that occurs at the turnaround point of these ion mirrors should result in minimal energy partitioning due to the low kinetic energy of ions at these points. The laser was allowed to irradiate ions for a period of many oscillations in the ELIT. Due to the low energy absorption of gas-phase ions during each oscillation in the ELIT, fragmentation was found to occur over a range of oscillations in the ELIT generating a homogeneous ion beam. A mirror-switching pulse is shown to create time-varying perturbations in this beam that oscillate at the fragment ion characteristic frequencies and generate a time-domain signal. This was found to recover FT signal for protonated pYGGFL and pSGGFL precursor ions. Fragmentation at the turnaround point of an ELIT by continuous-wave infrared multiphoton dissociation (cw-IRMPD) is demonstrated. In cases where laser power absorption is low and fragmentation occurs over many laps, a mirror-switching pulse may be used to recover varying time-domain signal. The combination of laser activation at the turnaround points and mirror-switching isolation allows for tandem MS in the ELIT.

Surface-induced dissociation of protein complex ions in a modified electrostatic linear ion trap

The electrostatic linear ion trap (ELIT) is a device that can provide mass analysis via frequency measurement, followed by Fourier transformation from the frequency domain to the time domain, or via time measurement in a multi-reflection time-of-flight measurement. ELIT devices have been demonstrated to be capable of relatively high mass measurement resolution as well as high-resolution ion isolation capabilities. We have been exploring the possibility of developing the ELIT geometry as a stand-alone high-performance tandem mass spectrometer for native mass spectrometry (MS) studies. In this context, it is desirable to use an activation method capable of dissociating the high-mass complexes encountered in native MS. To this end, we describe the implementation of surface-induced dissociation (SID) in an ELIT and describe initial results for protein complexes generated under native conditions in which ion isolation, fragmentation, and product ion mass analysis all occur within the ELIT.

Electrostatic Linear Ion Trap Optimization Strategy for High Resolution Charge Detection Mass Spectrometry.

Single ion mass measurements allow mass distributions to be recorded for heterogeneous samples that cannot be analyzed by conventional mass spectrometry. In charge detection mass spectrometry (CD-MS), ions are detected using a conducting cylinder coupled to a charge sensitive amplifier. For optimum performance, the detection cylinder is embedded in an electrostatic linear ion trap (ELIT) where trapped ions oscillate between end-caps that act as opposing ion mirrors. The oscillating ions generate a periodic signal that is analyzed by fast Fourier transforms. The frequency yields the m/z, and the magnitude provides the charge. With a charge precision of 0.2 elementary charges, ions can be assigned to their correct charge states with a low error rate, and the m/z resolving power determines the mass resolving power. Previously, the best mass resolving power achieved with CD-MS was 300. We have recently increased the mass resolving power to 700, through the better optimization of the end-cap potentials. To make a more dramatic improvement in the m/z resolving power, it is necessary to find an ELIT geometry and end-cap potentials that can simultaneously make the ion oscillation frequency independent of both the ion energy and ion trajectory (angular divergence and radial offset) of the entering ion. We describe an optimization strategy that allows these conditions to be met while also adjusting the signal duty cycle to 50% to maximize the signal-to-noise ratio for the charge measurement. The optimized ELIT provides an m/z resolving power of over 300 000 in simulations. Coupled with the high precision charge determination available with CD-MS, this will yield a mass resolving power of 300 000. Such a high mass resolving power will be transformative for the analysis of heterogeneous samples.

Effects of Molecular Size on Resolution in Charge Detection Mass Spectrometry.

Instrumental resolution of Fourier transform-charge detection mass spectrometry instruments with electrostatic ion trap detection of individual ions depends on the precision with which ion energy is determined. Energy can be selected using ion optic filters or from harmonic amplitude ratios (HARs) that provide Fellgett's advantage and eliminate the necessity of ion transmission loss to improve resolution. Unlike the ion energy-filtering method, the resolution of the HAR method increases with charge (improved S/N) and thus with mass. An analysis of the HAR method with current instrumentation indicates that higher resolution can be obtained with the HAR method than the best resolution demonstrated for instruments with energy-selective optics for ions in the low MDa range and above. However, this gain is typically unrealized because the resolution obtainable with molecular systems in this mass range is limited by sample heterogeneity. This phenomenon is illustrated with both tobacco mosaic virus (0.6-2.7 MDa) and AAV9 (3.7-4.7 MDa) samples where mass spectral resolution is limited by the sample, including salt adducts, and not by instrument resolution. Nevertheless, the ratio of full to empty AAV9 capsids and the included genome mass can be accurately obtained in a few minutes from 1× PBS buffer solution and an elution buffer containing 300+ mM nonvolatile content despite extensive adduction and lower resolution. Empty and full capsids adduct similarly indicating that salts encrust the complexes during late stages of droplet evaporation and that mass shifts can be calibrated in order to obtain accurate analyte masses even from highly salty solutions.

Design Of An Electrostatic Ion Trap And Time-of-Flight Tube Using SIMION Software

Electrostatic traps are a class of ion traps that rely on electric fields to confine charged particles in a region of space. It uses an electric field to trap the incoming ions from an ion source, confine them for a certain period of time, and then release them either as a continuous or pulsed ion beam. Ions once released from the trap are made to fly through a Time of Flight (ToF) tube for segregation based on their energies and mass. The time taken to travel the length of the ToF tube is equated to the mass of the ion, which can further be used to calculate the energy and mass of the ions. SIMION software simulates the trajectories of charged particles through electric and magnetic fields of different geometries. In this proposed paper, we will be presenting simulations of an (a) electrostatic ion trap and (b) Time of Flight tube using SIMION software. Experimental data of confinement time of ions in the trap and the time of flight of the ions in a 1m and 2m long ToF tube are simulated and tabled. The results from the simulations reported in this paper can be used as a template to design and develop custom electrostatic traps in conjunction with ToF tubes for specific instruments like spectrometers, medical devices, etc.

Genetic algorithm parallel optimization of a new high mass resolution planar electrostatic ion trap mass analyzer.

The study and design of high-resolution mass analyzers is a very important task in mass spectrometry. A planar electrostatic ion trap (PEIT) mass analyzer with image charge detection and FT-based data processing has been developed, theoretically simulated, and experimentally validated. However, the 10 ring electrode configuration (PEIT-10) is difficult for mechanical construction and voltage tuning; moreover, few methods have been reported for optimizing the performance of multi-electrode mass analyzers. In this article, a simplified PEIT-7 mass analyzer was designed, and a genetic algorithm parallel optimization (GAPO) method was developed for optimizing multiple voltage settings of the new PEIT-7 mass analyzer to achieve spatial and energy isochronicity as well as iso-coordinate property. The automatic voltage optimization processes for the reduction of time aberration and spatial aberration showed that the developed GAPO method can significantly improve the optimization efficiency (the optimal voltage set being found within 5 hours with a maximum time aberration of 15 ps and a maximum z aberration of 0.10 μm). Based on the results obtained from the GAPO method, the resolving power of the PEIT-7 mass analyzer for six groups of ions with closely packed masses (m/z = 117.000 Th to 117.010 Th) was demonstrated, and a mass resolution of 171k was achieved at an acquisition time of 200 ms. The established GAPO method facilitates the design and optimization of high-resolution mass analyzers and may be useful for the design of other multi-electrode ion optical devices.

Protein Analysis by Electrospray-Orbital Frequency Analyzer with Charge Detection Mass Spectrometry Algorithm.

Orbital frequency analyzer (OFA) is one of the electrostatic ion trap mass analysers for FTMS, which offers ultrahigh mass resolution and high ion charge capacity. In order to analyze multiply charged proteins and other large biological particles by means of charge detection mass spectrometry, a data processing algorithm was created to suit the image charge signal of nonharmonic waveform nature. The algorithm is capable of detecting collisions between ions and residual gas molecules, to determine lifetime of ions, and to evaluate the charge and mass values for ions having lifetime above a threshold. With the filtering of the lifetime and charge value, the chemical noise from small molecules and protein fragments can be eliminated in the reconstructed spectrum, facilitating measurement of protein content at a very low concentration, down to tens of nanomolars. The standard deviation of charge measurement is between 1.1 to 1.8 e for ions with oscillation lifetimes from 1 to 0.4 s, and this in turn determines the CDMS spectrum mass peak width. It has been found that the lower voltage setting of the OFA results in a larger population of ions surviving for longer times and thus produces narrower mass peak width. While OFA is able to run multiplexed CDMS without ion motion interference, coexisting ions of the same or very close m/z can cause interference between their image charge signals, which increases the error in charge determination.

Higher Resolution Charge Detection Mass Spectrometry.

Charge detection mass spectrometry is a single particle technique where the masses of individual ions are determined from simultaneous measurements of each ion's m/z ratio and charge. The ions pass through a conducting cylinder, and the charge induced on the cylinder is detected. The cylinder is usually placed inside an electrostatic linear ion trap so that the ions oscillate back and forth through the cylinder. The resulting time domain signal is analyzed by fast Fourier transformation; the oscillation frequency yields the m/z, and the charge is determined from the magnitudes. The mass resolving power depends on the uncertainties in both quantities. In previous work, the mass resolving power was modest, around 30-40. In this work we report around an order of magnitude improvement. The improvement was achieved by coupling high-accuracy charge measurements (obtained with dynamic calibration) with higher resolution m/z measurements. The performance was benchmarked by monitoring the assembly of the hepatitis B virus (HBV) capsid. The HBV capsid assembly reaction can result in a heterogeneous mixture of intermediates extending from the capsid protein dimer to the icosahedral T = 4 capsid with 120 dimers. Intermediates of all possible sizes were resolved, as well as some overgrown species. Despite the improved mass resolving power, the measured peak widths are still dominated by instrumental resolution. Heterogeneity makes only a small contribution. Resonances were observed in some of the m/z spectra. They result from ions with different masses and charges having similar m/z values. Analogous resonances are expected whenever the sample is a heterogeneous mixture assembled from a common building block.

Deceleration of Metastable Li+ Beam by Combining Electrostatic Lens and Ion Trap TechniqueSupported by the Scientific Instrument Developing Project of the National Natural Science Foundation of China (Grant Nos. 11934014, 11622434, and 11804373), the Scientific Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ201552), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos.

Ion deceleration has played a critical role in ion-related research when the ions are produced in the form of a high-energy beam. We present a deceleration method combining electrostatic lens and ion trap technique, which can effectively decelerate ions to energy below the trapping potential of a typical ion trap. The experiments were performed on metastable 1s2s 3S1 Li+ ions, and demonstrated that the kinetic energy could easily be reduced from ∼450 eV to a few eV, with the latter being confirmed using the Doppler-shifted fluorescence spectra.

Dynamic Calibration Enables High-Accuracy Charge Measurements on Individual Ions for Charge Detection Mass Spectrometry.

Charge detection mass spectrometry (CDMS) depends on the measurement of the charge induced on a cylinder by individual ions by means of a charge-sensitive amplifier. For high-accuracy charge measurements, the detection cylinder is embedded in an electrostatic linear ion trap (ELIT), and the ions oscillate back and forth through the cylinder so that multiple measurements are made. To assign the charge state with a low error rate, the charge of each ion must be determined with an uncertainty (root-mean-square deviation) of around 0.2 elementary charges. We show here that high-accuracy charge measurements can be achieved for large ions by dynamic calibration of the charge measurement using an internal standard. The internal standard is generated by irradiating the detection cylinder, by means of a small antenna, with a radiofrequency signal. Using this approach, we have obtained a relative charge uncertainty of around 5 × 10-4, allowing charge-state resolution to be achieved for single ions with up to 500 charges. In another application of this approach, the detection cylinder is irradiated with a signal that counteracts the transients generated when the potentials on the ELIT end-caps are switched to trapping mode. Using this approach, the dead time after switching (during which the signal cannot be analyzed) has been reduced by more than an order of magnitude. With charge-state resolution for ions with up to 500 charges, we were able to calibrate the charges precisely. The results show that the response of the charge-sensitive amplifier with dynamic calibration is linear to within a small fraction of an elementary charge.

Segmented electrostatic trap with inductive, frequency based, mass-to-charge ion determination

We propose a new type of mass analyser - an electrostatic ion trap utilising a quadro-logarithmic trapping potential comprising concentric ring electrodes and inductive mass-charge detection in which packets of ions oscillating in a harmonic potential are arranged to be spatially focused as they pass the detection electrodes. Image-current detection is performed by two thin wire electrodes at the centre of the device. The combination of maximised ion velocity and ion focussing improves the signal-to-noise characteristic, allowing the device to operate at high resolution with short transient times. In order to fully exploit the sharp, localized features in the resulting transients, we utilise a Bayesian signal processing algorithm to fully exploit the sharp, localised features in the resulting transients, and to avoid the strong harmonics that would be produced by Fourier transform methods. The geometry of the mass analyser and possible ion introduction methods are described and results from an in-silico model are presented. The properties of ion motion in an ideal quadro-logarithmic potential are reviewed and integration of ion trajectories in the perturbed field created by the device is performed. The detection electronics and the signal processing approach are described and simulated. The potential instrument resolution and detection limits are presented. A simulated mass-to-charge doublet of 100 Th and separation 3 mTh can be resolved in 3.1 μs when there are 250 ions of each species. This corresponds to a simulated resolving power of 33,000 in 3.1 ms.

Ion-Ion Interactions in Charge Detection Mass Spectrometry.

Charge detection mass spectrometry (CDMS) is a single-particle technique where the masses of individual ions are determined by simultaneously measuring their mass-to-charge ratio (m/z) and charge. Ions are usually trapped inside an electrostatic linear ion trap (ELIT) where they oscillate back and forth through a detection cylinder, generating a periodic signal that is analyzed by fast Fourier transforms. The oscillation frequency is related to the ion's m/z, and the magnitude is related to the ion's charge. In early work, multiple ion trapping events were discarded because there was a question about whether ion-ion interactions affected the results. Here, we report trajectory calculations performed to assess the influence of ion-ion interactions when multiple highly charged ions are simultaneously trapped in an ELIT. Ion-ion interactions cause trajectory and energy fluctuations that lead to variations in the oscillation frequencies that in turn degrade the precision and accuracy of the m/z measurements. The peak shapes acquire substantial high and low m/z tails, and the average m/z shifts to a higher value as the number of trapped ions increases. The effects of the ion-ion interactions are proportional to the product of the charges and the square root of the number of trapped ions and depend on the ions' m/z distribution. For the ELIT design examined here, ion-ion interactions limit the m/z resolving power to several hundred for a typical homogeneous ion population.

Dramatic Improvement in Sensitivity with Pulsed Mode Charge Detection Mass Spectrometry.

Charge detection mass spectrometry (CDMS) is emerging as a valuable tool to determine mass distributions for heterogeneous and high-mass samples. It is a single-particle technique where masses are determined for individual ions from simultaneous measurements of their mass-to-charge ratio (m/z) and charge. Ions are trapped in an electrostatic linear ion trap (ELIT) and oscillate back and forth through a detection cylinder. The trap is open and able to trap ions for a small fraction of the total measurement time so most of the ions (>99.8%) in a continuous ion beam are lost. Here, we implement an ion storage scheme where ions are accumulated and stored in a hexapole and then injected into the ELIT at the right time for them to be trapped. This pulsed mode of operation increases the sensitivity of CDMS by more than 2 orders of magnitude, which allows much lower titer samples to be analyzed. A limit of detection of 3.3 × 108 particles/mL was obtained for hepatitis B virus T = 4 capsids with a 1.3 μL sample. The hexapole where the ions are accumulated and stored is a significant distance from the ion trap so ions are dispersed in time by their m/z values as they travel between the hexapole and the ELIT. By varying the delay time between ion release and trapping, different windows of m/z values can be trapped.

Simultaneous Isolation of Nonadjacent m/z Ions Using Mirror Switching in an Electrostatic Linear Ion Trap.

Simultaneous isolation of ions of disparate mass-to-charge (m/z) ratios is demonstrated via appropriately timed pulsing of entrance and exit ion mirrors in an electrostatic linear ion trap (ELIT) mass spectrometer. Manipulation of the voltages of the entrance and exit mirrors, referred to as "mirror switching", has been demonstrated as a method in which ions can be both captured and isolated. High resolution isolation (>35 000) was previously demonstrated by selective gating of trapping electrodes to avoid ion lapping while closely spaced ions could continue to separate [ Johnson et al. Anal. Chem. 2019 , 91 , 8789 ]. In this work, we demonstrate that advantage can be taken of the ion lapping phenomenon in an ELIT to enable the simultaneous isolation of ions of disparate m/z ratios using mirror switching. This process is demonstrated with minimal ion loss using isotopologues of three carborane compounds ranging in m/z from 320 to 1020. Simultaneous isolation is demonstrated with the isolation of two and three peaks in separate isotopic distributions as well as with the isolation of alternating isotopologues within the same distribution. Such simultaneous isolation experiments are particularly useful when conducting experiments in which a mass calibrant is needed or when multiplexing in a tandem MS workflow.

Mirror Switching for High-Resolution Ion Isolation in an Electrostatic Linear Ion Trap.

Ion isolation was achieved via selective pulsing of the entrance and exit ion mirrors in an electrostatic linear ion trap mass spectrometer (ELIT). Mirror switching has been described previously as a method for capturing injected ions in ELIT devices. After ion trapping, mirror switching can be used as a method for ion isolation of successively narrower ranges of mass-to-charge (m/z) ratio. By taking advantage of the spatial separation of ions in an ELIT device, pulsing of the entrance and/or exit mirrors can release unwanted ions while continuing to store ions of interest. Furthermore, mirror switching can be repeated multiple times to isolate ions of very similar m/z values with minimal loss of the stored ions, as is demonstrated by the isolation of protonated l-glutamine and l-lysine (Δ m/z = 0.0364) from a mixture of the two amino acid ions and the isobaric mixture of [PC P-18:0/22:6] and [PC 19:0/19:0] (Δ m/z = 0.0575). As isolation is accomplished due to the spatial/temporal separation of ion packets within the ELIT, multiple reflection-time-of-flight (MR-TOF) mass spectra are shown to demonstrate separation in the ELIT at the time of isolation. An isolation resolution of greater than 35 000 fwhm is demonstrated here using a 5.25 in. ELIT. This resolution corresponds to the fwhm resolution necessary to reduce contaminant overlap of an equally abundant adjacent ion to 1% or less of the isolated ion intensity.

High-Capacity Electrostatic Ion Trap with Mass Resolving Power Boosted by High-Order Harmonics.

A form of electrostatic ion trap mass analyzer, named the orbital frequency analyzer (OFA), has been developed. The ions in the analyzer are trapped around the middle plane and orbit around the central axis perpendicular to the middle plane with high-ellipticity and precessing trajectories. The orbital frequency of the ions in this device has been optimized to be independent of ions' energy so that the image charge signal picked up by some of the field-forming circular/ring electrodes can be used to produce a mass spectrum after Fourier transform data processing. Spectra acquired by the OFA are rich of high-order harmonics, which offer higher mass resolving power than that for fundamental frequency components. The experiment shows that the resolving power is proportional to the harmonic order and exceeds 150 k for mass-to-charge ratio ( m/ z) of 526 Th and the transient length of 500 ms. Using high-order harmonics, an isotopic cluster of a heavy protein was resolved with a shorter transient length. The transient signals from different pick-up electrodes give different waveform shapes, and therefore, their harmonic peak distributions in a frequency spectrum are different, thus allowing the removal of unwanted harmonic peaks. The preliminary results also show a wide dynamic range of the analyzer.

A Miniaturized Fourier Transform Electrostatic Linear Ion Trap Mass Spectrometer: Mass Range and Resolution.

Mass resolution (M/ΔMFWHM) was increased by reducing the axial length of a Fourier transform electrostatic linear ion trap (FT-ELIT) mass spectrometer. The increase in mass resolution corresponds directly to increased axial ion frequencies in the FT-ELIT. Increased mass resolution was demonstrated for equivalent transient lengths in a 5.25″ versus 2.625″ ELIT using the isotopes of [bradykinin+2H]2+ and [insulin+5H]5+ as test ions. Both bradykinin and insulin show mass resolution increases of ~ 90% allowing baseline resolution of the [insulin+5H]5+ isotopes after only 300ms of data acquisition. Relative changes in mass/charge range were explored using mirror switching to trap ions injected axially into the ELIT. When trapping ions using mirror switching, the mass/charge range in a FT-ELIT mass spectrometer for a given switch time is determined by the time required for fast ions to enter and exit the trap after one reflection versus the time it takes for slow ions to enter the trap. By reducing the length of the FT-ELIT mass spectrometer while maintaining a constant distance from the point from which ions are initially accelerated to the entrance mirror, only the low m/z limit is affected for a given mirror switching time. For the two ELIT lengths examined here, the effective mass/charge range at any given switch time is reduced from m/zlow-8.9*m/zlow for the 5.25″ ELIT to m/zlow-5.2*m/zlow for the 2.625″ ELIT. Graphical Abstract.

Effects of Individual Ion Energies on Charge Measurements in Fourier Transform Charge Detection Mass Spectrometry (FT-CDMS).

A method to correct for the effect of ion energy on charge measurements of individual ions trapped and weighed with charge detection mass spectrometry (CDMS) is demonstrated. Ions with different energies induce different signal patterns inside an electrostatic ion trap. The sum of the amplitudes of the fundamental and second harmonic frequencies in the Fourier transform of the induced signal, which has been used to obtain the ion charge, depends on both ion energy and charge. The amplitudes of the fundamental frequencies of ions increase over time as ions lose energy by collisions with background gas and solvent loss from larger ions. Model ion signals are simulated with the same time-domain amplitude at different energies and frequencies and the resulting fundamental frequency amplitudes are used to normalize real ion signals for energy and frequency effects. The fundamental frequency amplitude decreases dramatically below 20kHz and increases by ~ 17% from the highest energy to lowest energy that is stable with a given trap potential at all frequencies. Normalizing the fundamental frequency amplitude with the modeled amplitudes removes the systematic changes in the charge measurement of polyethylene glycol (PEG) and other ions and makes it possible to signal average the amplitude over long times, which reduces the charge uncertainty to 0.04% for a PEG ion for a 500-ms measurement. This method improves charge measurement accuracy and uncertainty, which are important for high-accuracy mass measurement with CDMS. Graphical abstract ᅟ.

Optimized Electrostatic Linear Ion Trap for Charge Detection Mass Spectrometry.

In charge detection mass spectrometry (CDMS), ions are passed through a detection tube and the m/z ratio and charge are determined for each ion. The uncertainty in the charge and m/z determinations can be dramatically reduced by embedding the detection tube in an electrostatic linear ion trap (ELIT) so that ions oscillate back and forth through the detection tube. The resulting time domain signal can be analyzed by fast Fourier transforms (FFTs). The ion's m/z is proportional to thesquare of the oscillation frequency, and its charge is derived from the FFT magnitude. The ion oscillation frequency is dependent on the physical dimensions of the trap as well as the ion energy. A new ELIT has been designed for CDMS using the central particle method. In the new design, the kinetic energy dependence of the ion oscillation frequency is reduced by an order of magnitude. An order of magnitude reduction in energy dependence should have led to an order of magnitude reduction in the uncertainty of the m/z determination. In practice, a factor of four improvements was achieved. This discrepancy is probably mainly due to the trajectory dependence of the ion oscillation frequency. The new ELIT design uses a duty cycle of 50%. We show that a 50% duty cycle produces the lowest uncertainty in the charge determination. This is due to the absence of even-numbered harmonics in the FFT, which in turn leads to an increase in the magnitude of the peak at the fundamental frequency. Graphical Abstract ᅟ.