Recent advances in underwater in situ mass spectrometry: instrumentation, deep-sea applications, and challenges

The ocean is a major component of the Earth system and contains abundant biological, mineral, and energy resources. Conventional laboratory-based analytical methods are limited in underwater environments because sample collection, preservation, and transportation can alter the original chemical composition. In contrast, underwater in situ mass spectrometry enables adaptive sampling and analysis across temporal and spatial dimensions, achieving real-time feedback for in situ , long-term monitoring while significantly reducing labor and material resource consumption. These technologies facilitate qualitative and quantitative monitoring of dissolved gases in aquatic systems, characterized by rapid response and high sensitivity. These systems provide important analytical support for marine environmental monitoring, biogeochemical studies, and resource exploration. This review outlines the core components of underwater in situ systems, including membrane introduction systems, ionization systems, and miniaturized mass spectrometers. Recent advances in membrane introduction, water-vapor removal, ionization strategies, miniaturized mass analyzers, deep-sea field deployment, and recent integration of micro gas chromatography with mass spectrometry are critically summarized. Current challenges, including water-vapor interference, membrane fouling, calibration drift, power consumption, and long-term autonomous operation, are also discussed.