Rapid Screening of Aldehydes by SICRIT-HRMS: Understanding Ionization Behavior in Breath Analysis

Exhaled breath aldehydes represent clinically relevant biomarkers of oxidative stress and lipid peroxidation, yet their reliable online detection remains analytically challenging due to their trace concentrations, chemical reactivity, and structural similarity. This study highlights Soft Ionization by Chemical Reaction in Transfer coupled to high-resolution cyclic ion mobility mass spectrometry (SICRIT-cIMS-HRMS) for the direct detection and characterization of 16 clinically relevant volatile aldehydes in exhaled breath. A controlled offline spiking methodology using Tedlar sampling bags was developed, enabling systematic investigation of ionization behavior under realistic breath matrix conditions. Comparative analysis in ultrapure nitrogen and exhaled breath matrices revealed that SICRIT preferentially generates protonated [M+H]⁺, dehydrated [M+H-H₂O]⁺, and oxidation-derived species, with exhaled breath shifting ionization patterns toward oxygen-containing adducts. Ionization competition from N,N-dimethylacetamide, a Tedlar bag contaminant, was identified and mitigated through standardized bag usage protocols. Quantitative assessment demonstrated good linearity (R² ≥ 0.99 for 13 aldehydes), with LOD values ranging from 1.33 to 25.13 ppbv. Intraday variability was low across all adducts (CV < 11%), while interday variability was significantly lower for [M+H]⁺ species (CV < 13%) compared to oxidation-derived adducts, establishing protonated ions as the preferred targets for long-term biomarker monitoring. Cyclic ion mobility separation was evaluated for isomer differentiation. Although tentative separation of 2-ethylhexanal and octanal was observed, sensitivity losses approaching 3 orders of magnitude were encountered, likely attributed to instrument geometry and traveling wave-based ion transmission. Nevertheless, the demonstrated ability of ion mobility to discriminate VOCs from chemical noise and isobaric interferences is encouraging for future developments. This work establishes a fundamental understanding of SICRIT ionization mechanisms in complex breath matrices and identifies key analytical parameters for future method development, validation, and clinical implementation, notably motivating the investigation of SICRIT ionization mechanisms and the use of linear mobility systems for improved aldehyde isomer discrimination in respiratory disease monitoring.