Space-charge effects in an electrostatic multireflection ion trap

A Perspective of Multi-Reflecting TOF MS

Time-of-flight mass spectrometry (TOF MS) excels in rapid and high-sensitivity analysis, making it a cornerstone of analytical chemistry. But as sample complexity explodes in omics studies, so does the need for higher resolving power to ensure accurate results. Traditional TOF instruments face a challenge: achieving high resolution often requires a very large instrument. To overcome this limitation, scientists developed alternative designs for TOF analyzers called multi-pass TOF analyzers (MPT). These MPT analyzers come in two main configurations: multi-turn (MTT) and multi-reflecting (MRT). Drawing on the authors' extensive experience, this review describes two decades of MPT advancements. It highlights the critical development of optimized analyzer designs, tracing the evolution towards mirror-based MRT instruments, generally providing superior resolution and spatial acceptance compared to MTT. While the manuscript attempts to overview MTT advances, it primarily focuses on MRT technology. Additionally, the review explores the role of orthogonal accelerators and trap pulse converters, comparing their efficiency and the dynamic range limits imposed by space charge effects. By comparing various MRT configurations and commercially available instruments, the review sets out to inform and empower researchers so they can make informed decisions about MRT mass spectrometers.

High-resolution multi-reflection time-of-flight mass spectrometer with atmospheric pressure interface

RATIONALE

Atmospheric pressure interface multi-reflection time-of-flight mass spectrometry (API-MRTOF-MS) has the potential to be a rapid and high-resolution analytical tool for versatile applications in chemistry, biology, environmental science, and medicine.

METHODS

The ions were reflected in a mass analyzer via electrostatic mirrors and folded flight path. Therefore, flight distances were significantly increased. The ion flight path of the API-MRTOF-MS was extended from meters to over 1 km, and the mass resolution was increased. Furthermore, the mass analysis could be completed at around 10 ms due to the rapid response of TOF-MS.

RESULTS

A high-resolution API-MRTOF-MS approach is successfully developed in this study. The mass resolution could achieve 116 050 (full widths at half maximum [FWHM]) for Cs+ ions using an atmospheric pressure electrospray ionization within a total TOF of only 18 ms. An ion transmission efficiency of over 50% was achieved after 600 cycles.

CONCLUSIONS

The analytical performance of the newly developed API-MRTOF-MS demonstrated that it is suitable for high resolution and rapid analysis in many fields.

Crowd control of ions in the Astral analyzer

Space charge effects are the Achilles' heel of all high-resolution ion optical devices. In time-of-flight mass analyzers, these may manifest as reduction of resolving power, mass measurement shift, peak coalescence, and/or transmission losses, while highly sensitive modern ion sources and injection devices ensure that such limits are easily exceeded. Space charge effects have been investigated, by experiment and simulation study, for the astral multi-reflection analyzer, incorporating ion focusing via a pair of converging ion mirrors, and fed by a pulsed extraction ion trap. Major factors were identified as the resonant effect between ~103 ions of similar m/z in-flight and the expansion of trapped packets of ~104-5 ions prior to extraction. Optimum operation and compensated ion mirror calibration strategies were then generated and described based on these findings.

RECENT ADVANCES IN INSTRUMENTAL APPROACHES TO TIME-OF-FLIGHT MASS SPECTROMETRY

Time-of-flight mass spectrometry (TOFMS) is one of the simplest and most powerful approaches for mass spectrometry. Realization of the advantages inherent in TOFMS requires innovation in the theory and practice of the technique. Instrumental developments, in turn, create new capabilities that enable applications in chemical measurement. This review focuses on the recent advances in TOFMS instrumentation. New strategies for ion acceleration, multiplexed detection, miniaturized TOFMS instruments, approaches to extend the length of ion flight, and novel ion detection technologies are reviewed. Techniques that change the basic paradigm of TOFMS by measuring m/z based on ion flight distance are considered, as are applications at the frontiers of instrumental performance. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Coalescence and self-bunching observed in commercial high-resolution mass spectrometry instrumentation

RATIONALE

Self-bunching and coalescence are well-known effects in Fourier transform ion cyclotron resonance (FTICR) and multi-reflection time-of-flight (TOF) mass spectrometry. These detrimental effects can also be observed in currently more frequently used high-resolution mass spectrometry (HRMS) instruments, such as the Orbitrap and single-reflection TOF.

METHODS

A modern single-reflection TOF and a Q-Orbitrap were used to produce conditions in which self-bunching and coalescence were observed. This was done by infusion experiments of several isobaric compounds. The peak widths of some low mass isobaric ions as well as the mass resolution of such mixtures were investigated. Attention was paid to possible self-bunching and coalescence effects.

RESULTS

For the utilized TOF mass spectrometer, the measured peak widths of the ions become significantly narrower (self-bunching) when increasing the ion abundance. On the other hand, isobaric ion pairs (delta < 30 milli m/z units) became unresolvable above a certain ion abundance (coalescence). The tested Orbitrap shows similar behavior, although coalescence appeared only at delta <15 milli m/z units. Coalescence was shown to affect the quantitative data, while self-bunching can lead to biased relative isotopic ratios.

CONCLUSIONS

The conventional measurement of a peak width does not truly reflect the mass resolving power of modern HRMS instrumentation. The mass resolving power is better demonstrated by resolving a mixture of isobaric compounds. Measurements obtained at low and high ion abundances should be investigated. Coalescence and self-bunching can reduce the truly available mass resolving power and therefore negatively affect quantitative and qualitative measurements.

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 m/z 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.

In pursuit of resolution in time-of-flight mass spectrometry: A historical perspective

Time-of-flight mass spectrometry is reviewed from its inception in the 1940s to the present day. The review is concerned with fundamentals of time-of-flight analyzers and of ion sources to the extent that sources influence analyzers. The patent literature has been covered, and efforts made to bring to light less well-known papers and studies © 2015 Wiley Periodicals, Inc. Mass Spec Rev. 35:738-757, 2016.