Advancing Low flow LC/MS for single cell proteomics with variable flow and 50 cm microfabricated pillar array columns

Significance of the Topic

Single-cell proteomics addresses critical needs in life sciences by enabling study of cell subpopulations, detection of rare circulating tumor cells, and analysis of minimal sample material such as exosomes or needle biopsies. Overcoming limitations of bulk proteomics, this approach reveals cellular heterogeneity and advances precision diagnostics and targeted therapies.

Objectives and Study Overview

This study aims to optimize a low-flow liquid chromatography–mass spectrometry (LC-MS) workflow for single-cell proteomics. Key goals include:

• Maximizing sensitivity through microfabricated column technology and variable flow strategies.
• Maintaining high instrument productivity while handling nL-level sample volumes.
• Demonstrating deep proteome coverage on Thermo Scientific Orbitrap platforms.

Instrumentation Used

• Vanquish Neo UHPLC system: precise injection down to 10 nL and trap-and-elute capabilities.
• µPAC Neo Plus 50 cm microfabricated pillar array columns for ultra-low flow separations.
• FAIMS Pro Duo interface for gas-phase separation.
• Orbitrap Exploris 240 and Orbitrap Astral mass spectrometers for high-resolution detection.
• CellenONE and Tecan Uno single-cell dispensing systems.

Methodology and Workflow

A backflush trap-and-elute setup with vial-bottom detection ensures full aspiration of 1–10 µL single-cell samples. A variable-flow gradient initiates elution at 750 nL/min to transport analytes rapidly, then reduces to 100 nL/min at the elution onset to enhance sensitivity. Single cells are isolated into narrow size bins (1–3 µm) and digested in a one-pot protocol within LoBind 384-well plates. Data acquisition employs 120 k MS2 resolution, 60–80 Th windows, and 400–800 m/z range, with processing in Spectronaut 19 using match-between-runs.

Main Results and Discussion

• Variable-flow LC achieved 83 % instrument productivity, balancing throughput and sensitivity.
• Bulk HeLa dilutions at true low flow confirmed method consistency in triplicate analyses.
• Size-selected single HeLa cells showed strong correlation between cell diameter and proteome depth, highlighting the importance of narrow selection for data reproducibility.
• Orbitrap Exploris 240 enabled identification of ~2 000 protein groups per cell at 30 samples/day.
• Employing Orbitrap Astral with FAIMS yielded ~5 000 protein groups per cell, doubling both throughput and depth compared to Exploris alone.

Benefits and Practical Applications

Implementing microfabricated columns and variable flow LC-MS offers:

• Enhanced analyte concentration and ionization efficiency at low flow rates.
• Minimal sample loss through precise nanoliter injections and trap-and-elute design.
• Scalable throughput suitable for large single-cell studies in biomedical research and clinical diagnostics.

Future Trends and Possibilities

Advances expected in the coming years include integration of automated sample preparation, further miniaturization of LC systems, novel column materials for even lower flow robustness, and coupling with AI-driven data analysis pipelines. Multiplexing strategies and real-time quality control will expand applications in personalized medicine and high-content screening.

Conclusion

This optimized low-flow LC-MS workflow, combining variable flow gradients with 50 cm microfabricated pillar columns and advanced Orbitrap detection, delivers unprecedented sensitivity and throughput for single-cell proteomics. It provides a robust platform for exploring cellular heterogeneity and supports diverse applications from basic research to clinical diagnostics.