Reverse-Flow Engineering of a Liquid Microjunction Surface Sampling Probe for Ambient Ionization Mass Spectrometry Using Computational Fluid Dynamics.

The liquid microjunction surface sampling probe (LMJ-SSP) is a powerful ambient ionization interface for localized extraction and mass spectrometric analysis. However, the low spatial resolution (∼1.0 mm) and the potential for particle-induced clogging can limit its broader application. We integrate computational fluid dynamics (CFD) modeling with experimental measurements to systematically examine how flow direction governs liquid microjunction stability, shear stress distribution, and clogging propensity. A reverse-flow configuration where solvent is introduced through the inner fused-silica capillary and aspirated through the intercapillary annular gap was applied to overcome the limitations of the traditional design. CFD modeling revealed a pronounced reorganization of streamlines, enhanced shear forces at the liquid-surface interface, and suppressed stagnation zones, producing a more confined microjunction meniscus than the normal flow. Experimental clogging tests with silica microspheres (5, 16, 25 μm) and biological debris indicated a roughly 3-fold reduction in clogging potential under reverse flow, consistent with particle-tracking simulations showing outward radial trajectories that help prevent central blockage. Real-world applicability was confirmed through direct analysis of metabolites from Taraxacum officinale pollen and leaf tissues, where reverse flow enabled sampling from complex, particulate-rich surfaces. Together, these findings establish reverse-flow engineering as a robust strategy for enhancing LMJ-SSP performance, providing improved operational stability, reduced clogging, and consistent surface sampling under mass spectrometric operating conditions.