Improving the AC-ejection method for enhanced duty cycle and wide m/z range detection of ions in oTOF-MS

Importance of the Topic

Orthogonal acceleration ejection in time-of-flight mass spectrometry (oTOF-MS) offers high resolving power but suffers from a low duty cycle, limiting sensitivity and throughput. Enhancing ion gating to extend the detectable mass-to-charge (m/z) range and improve ion transmission is critical for advanced proteomics, structural biology, and industrial quality control.

Objectives and Study Overview

This work aims to develop and validate a simplified AC-ejection method to overcome mass discrimination and duty-cycle limitations in oTOF-MS. Guided by ion optics simulations and bench experiments, the study introduces a modified lens design and optimized AC-ejection parameters to enable broad m/z coverage in a single transient without compromising resolution.

Methodology and Instrumentation

Ion optics simulations were employed to compare the traditional DC gating approach with the new AC-ejection design. Experimental validation used a timsOmni™ platform configured with:

• An AC-screen electrode integrated into the gate lens
• Simplified electronics generating a ramped AC signal (up to 1 kVpp, 2.5 MHz) and DC offset
• A collision-induced dissociation (CID) cell for fragment generation
• Orthogonal acceleration and time-of-flight detection

Optimization of transfer time, AC ramp duration, and timing delays was carried out for native proteins and peptides.

Main Results and Discussion

The new AC-ejection design achieved:

• More than 10^7 ions/s duty cycle without loss of TOF resolution
• Continuous detection of ions from m/z < 300 to > 10 000 in a single transient
• Up to 20× intensity gains for MS2 ECD and CCID fragments of carbonic anhydrase and NIST monoclonal antibody compared to DC gating

Simulation and experimental data confirmed that the AC-screen confines the high-frequency field to the gate region, reducing mass discrimination and enabling wider m/z acceptance.

Benefits and Practical Applications

The enhanced AC-ejection method offers:

• Improved sensitivity for low-abundance analytes
• Uninterrupted acquisition across a broad m/z spectrum ideal for top-down and bottom-up workflows
• Simplified hardware integration into commercial oTOF-MS platforms

Future Trends and Opportunities

Potential developments include:

• Integration with ion mobility separation for multi-dimensional analyses
• Real-time adaptive gating controlled by machine learning algorithms
• Expansion to higher-frequency AC waveforms and miniaturized electronics

Conclusion

A streamlined AC-ejection method has been demonstrated to significantly enhance ion transmission and extend the m/z detection range in oTOF-MS without sacrificing resolution. This approach supports advanced mass spectrometry applications requiring high throughput and broad spectral coverage.