Instrumental Dispersion Evaluation on Supercritical Fluid Chromatography-Medium Vacuum Chemical Ionization Tandem Mass Spectrometry

Instrumental dispersion in the ion source can severely distort fast chromatographic peaks in supercritical fluid chromatography (SFC)-mass spectrometry (MS). Despite this importance, the dispersion characteristics specific to medium-vacuum chemical ionization (MVCI) sources have not been quantitatively investigated. In this work, we combine targeted experiments with established computational tools-computational fluid dynamics (CFD) and electrostatic field simulation-to characterize ion transport in the MVCI flow tube. Arrival profiles of vitamin K₁ (VK₁) ions monitored by selected ion monitoring consistently showed a reproducible two-component structure consisting of an early narrow bandwidth ion packet followed by a delayed shoulder. CFD calculations reproduce this tailing peak profile, and electrostatic modeling further revealed that applying the same potential to the MVCI flow tube and inner cylinder generates lateral potential walls that inhibit long-residence-time ions from entering the skimmer. Introducing an appropriate potential difference between the MVCI inner cylinder and the skimmer orifice isolates the MVCI flow field from the ion guide region and suppresses long residence-time trajectories, which narrows the VK₁ peak width by more than threefold and restores the intrinsic column efficiency of a sub-2-μm SFC column (from the theoretical plate (N) = 3120 to 12079). These results provide a practical diagnostic and mitigation framework for ion-source derived dispersion in MVCI, demonstrating that modest electrostatic confinement is effective in maintaining chromatographic fidelity in high-speed SFC-MVCI-MS. The present work also highlights the utility of CFD for evaluating proposed MVCI flow designs and their influence on peak dispersion.