CDMS mode of FTMS Orbital Frequency Analyzer

Significance of the Topic

This work integrates Charge Detection Mass Spectrometry (CDMS) with an Orbital Frequency Analyzer (OFA) to enable accurate single-ion charge and true mass measurements at high mass ranges. The approach boosts mass resolving power and throughput without extending acquisition time, addressing critical needs in proteomics, polymer science, and high-mass biomolecule analysis.

Objectives and Study Overview

• Implement CDMS mode on an OFA platform to measure individual ion charges and true masses.
• Develop data processing workflows for frequency and lifetime extraction, charge calculation, and mass histogram generation.
• Assess methods for interference elimination, frequency domain fitting, scoring, and mass‐loss calibration to enhance charge accuracy and multiplexing.

Methodology

• FTMS principle with an OFA detects non-sinusoidal image charge signals at high-order harmonics.
• Folded time-domain acquisition compresses data and enables high multiplexing (Ninj up to 200, Tacq 250–1000 ms).
• Frequency domain fitting accounts for interference among adjacent ion frequencies.
• Scoring algorithm computes optimal charge assignments based on detected frequencies, lifetimes, and predicted m/z values.
• Mass‐loss calibration (mlCDMS) adjusts charge estimates using measured neutral losses during collision events.
• Data processing pipeline: extract frequency and lifetime → calculate charge Q → derive mass M = (m/z)·Q → compile mass histograms.

Used Instrumentation

• Electrospray Ionization (ESI) probe with skimmer, heated capillary, and ion transfer optics (Q-array, Q1 pre-rod/post-rod).
• Quadrupole collision cell (Q2) with He/Ar cooling for controlled collisional activation.
• Orbital Frequency Analyzer equipped with image-charge pickup electrodes for FTMS detection.
• Pumping system: rotary pump, dual-inlet and triple-inlet turbo molecular pumps maintaining 3–4×10⁻¹⁰ Torr vacuum.
• Data acquisition electronics supporting folded time-domain signals and high-order harmonic detection.

Main Results and Discussion

The study demonstrates that resolving power increases with harmonic order, exemplified by Verapamil (455 Da) achieving high resolution without longer acquisition. Interference elimination reduces charge inaccuracy to the electronic noise limit (σ≈1.1 kDa for lifetimes >800 ms). The scoring and mass-loss methods yield improved charge assignments, enabling reliable CDMS at high multiplexing levels. Protein analyses at nanomolar concentrations (e.g., myoglobin 40 nM, aldolase 70 nM) achieved isotopic resolution and clear mass histograms. Collisional activation experiments (Emax up to 3.8 keV/Q) revealed fragmentation patterns while maintaining mass accuracy.

Benefits and Practical Applications

• True mass determination of individual ions enhances confidence in high-mass biomolecule analysis.
• High resolving power and multiplexing reduce sample consumption and analysis time.
• Applicable to proteomics, polymer characterization, QA/QC in pharmaceutical and biotechnological industries.
• Capability to detect isotopic distributions and fragmentation patterns at low concentrations.

Future Trends and Applications

Advances in signal processing algorithms promise further improvements in charge accuracy without hardware modifications. Integration of machine learning for real-time data interpretation, expansion to larger biomolecules and nanoparticles, and enhanced multiplexing strategies will broaden CDMS-OFA applications in structural biology, materials science, and clinical diagnostics.

Conclusion

The implementation of CDMS on an OFA platform delivers high-accuracy single-ion mass measurements with exceptional resolving power and throughput. The combination of interference elimination, frequency domain fitting, scoring, and mass-loss calibration enables robust analyses of proteins and other high-mass species at low concentrations. Ongoing algorithmic and hardware enhancements will extend the technique’s applicability across diverse analytical challenges.

Reference

• Li Ding, Ranjan Badheka, Zhengtao Ding, Hiroaki Nakanishi. A Simulation Study of the Planar Electrostatic Ion Trap Mass Analyzer. J Am Soc Mass Spectrom. 2013 Mar;24(3):356–364.
• Li Ding, Aleksandr Rusinov. High-Capacity Electrostatic Ion Trap with Mass Resolving Power Boosted by High-Order Harmonics. Anal Chem. 2019;91(12):7595–7602.
• Aleksandr Rusinov, Li Ding, Sergey Smirnov, Patrick Knight, Roch Andrzejewski, Hiroaki Waki. Protein Analysis by Electrospray-Orbital Frequency Analyzer with Charge Detection Mass Spectrometry Algorithm. J Am Soc Mass Spectrom. 2021;32(5):1145–1154.