A Novel High Pressure, High Space Charge Capacity Ion Separation Device Based on Opposing Travelling Wave and DC potentials

Importance of the Topic

Ion mobility separations coupled to mass spectrometry are critical for high-resolution analysis of complex mixtures in proteomics, metabolomics and industrial QA/QC. Conventional TWIMS devices offer extended drift lengths but suffer from limited ion capacity and constrained control over elution timing. The novel device combining opposing travelling wave and DC fields addresses these limitations, enabling compact high-throughput separations and tunable operation modes.

Study Objectives and Overview

The study aimed to design, theoretically model and experimentally validate an ion separation device that uses counteracting travelling wave (TW) and DC potentials. Key goals included achieving high space charge capacity, flexible control of ion elution, scalable multi-channel architecture and the ability to operate in both mobility-dominant and m/z-dominant modes.

Methodology and Instrumentation

A comprehensive analytical framework was developed to describe ion motion under opposing DC and TW fields, incorporating convection-diffusion effects in a multi-segment structure (pre-trap, trapping region, analytical region). An eight-channel prototype replaced the IMS cell in a Waters SYNAPT XS platform. Experiments were conducted at 1 mbar N2 or 3 mbar He using:

• Waters SYNAPT XS mass spectrometer
• Keysight B2985B electrometer for ion current measurements
• AIM-TTi TGA12104 arbitrary waveform generator
• Tabor Electronics 9400A four-channel high-voltage amplifier

Key Results and Discussion

• Mobility resolution of ~45 FWHM for a 100 ms separation at 480 m/s TW velocity and 1 V/mm DC field, with peak resolution increasing to ~120 FWHM for slower scans.
• Space charge capacity of ~4 × 10^7 charges with linear response across m/z 300–3000.
• At elevated TW velocities (1680–1920 m/s), separation shifts to an m/z-dominated regime, yielding narrower arrival time distributions and improved peak capacity.
• Calibration using raffinose, melezitose, polyaniline and ADH digest demonstrated residual CCS and m/z errors below 2% at moderate TW speeds, rising to ~17% at highest speeds.
• Theoretical predictions of ion transit and resolution agree closely with experimental data, validating the analytical model.

Benefits and Practical Applications

• High duty-cycle separations with tunable mobility or m/z selectivity
• Scalable architecture via parallel channels to boost charge capacity
• Compact footprint enables retrofit on existing MS platforms
• Potential to synchronize with downstream quadrupole for enhanced DDA/DIA MS/MS performance

Future Trends and Applications

Ongoing development may focus on increasing field strength or analytical region length, implementing lighter buffer gases (e.g., H2) for higher resolving power, scaling channel count for greater throughput and integrating upstream of analytical quadrupoles to maximize MS/MS duty cycle.

Conclusion

The opposing TW/DC device demonstrates a novel separation modality that combines high space charge capacity, flexible control over elution timing and dual mobility/mass-based operation. Theoretical and experimental results are in strong agreement, highlighting its potential to enhance ion mobility-MS workflows across research and industrial applications.

References

1. Richardson K., Langridge D., Giles K., Fundamentals of travelling wave ion mobility revisited I. Smoothly moving waves. Int. J. Mass Spectrom. 428, 71–80 (2018).
2. Wildgoose J. et al., Novel Operating Modes of an Ion Mobility Quadrupole Time-Of-Flight Hybrid Instrument. Proc. 63rd ASMS Conf. Mass Spectrom. Allied Topics, St. Louis, MO (2015).