Quantitative determination in underwater mass spectrometry: Theoretical models and experimental insights.

Underwater mass spectrometry (UMS) enables the in-situ detection of dissolved gases based on their mass-to-charge ratios, providing strong chemical specificity, high anti-interference capability, and simultaneous multi-component analysis. Owing to its membrane-permeation-driven quantification principle, UMS achieves multi-gas quantification on a time scale of seconds, offering significant advantages in rapid response and continuous monitoring. However, permeation flux is highly sensitive to environmental parameters such as temperature and hydrostatic pressure, resulting in substantial variations in quantification under different field conditions. Although numerous UMS quantification approaches exist, their mathematical formulations, applicability, and correction strategies differ considerably, and a systematic comparison is lacking. To address this challenge, this work evaluates three representative quantification methods-the Laboratory Calibration Method, the Laboratory Calibration with Physical Correction Method, and the Reference Gas Ratio Method. The theoretical foundations and mathematical models of each method are described. Key environmental-parameter dependencies were experimentally calibrated, and comparative quantification experiments were conducted across lake transect measurements, deep-sea in-situ stationary measurements and ocean depth-profile measurements. Results reveal profound differences in the environmental robustness, accuracy, and instrument dependence of the three methods. The findings provide critical methodological guidance for the optimization and standardization of UMS-based quantitative analyses.