Dual-Trap Sequential Enrichment System with Photochemical Ionization TOFMS for Automated Simultaneous Determination of DMS and Isoprene in Seawater and Atmosphere

Abstract For the detection of dimethyl sulfide (DMS) and isoprene in marine environments, existing methods suffer from limitations in sensitivity, temporal resolution, or the ability to analyze gas–liquid biphasic phases. To address these issues, we developed a dual-trap sequential enrichment (DTSE) system with a highly sensitive photochemical ionization TOFMS (PCI-TOFMS). The DTSE is based on dynamic headspace extraction and dual-trap enrichment, enabling simultaneous efficient extraction and alternating automatic analysis of seawater and air samples. A series of innovative functional designs are embedded into this system, such as in-cycle self-cleaning, stepwise gradient desorption, and dynamic dilution calibration. PCI generates O2+ reagent ions through photoelectrons, and O2+ subsequently ionizes DMS and isoprene via chemical ionization to M+ and [M–H]+, which are subsequently separated and detected with TOFMS. After optimization, the DTSE-PCI-TOFMS system enables continuous extraction and enrichment of seawater and air in a single 5 min analytical cycle, achieving recoveries of 88–90%, enrichment factors of 19–45, and detection limits as low as sub-pM and sub-ppt (S/N = 3), along with good stability (RSD = 1.2–4.2%) and linearity (R2 > 0.99). Compared to conventional P&T-GC-MS methods, the temporal resolution for DMS was reduced from 20 to 5 min, while the detection sensitivity was enhanced by a factor of 17–44. Finally, the practical performance of the DTSE-PCI-TOFMS system was validated by continuously monitoring DMS and isoprene metabolism in the model diatom over 48 h under varying temperature, light, and pH conditions.