High-resolution measurement of multiple volatile organic compounds dissolved in seawater using equilibrator inlet–proton transfer reaction-mass spectrometry (EI–PTR-MS)

We developed an equilibrator inlet–proton transfer reaction-mass spectrometry (EI–PTR-MS) system for high-resolution measurement of the concentrations of multiple volatile organic compounds (VOCs) dissolved in seawater. The equilibration of five VOCs (isoprene, propene, acetone, acetaldehyde, and methanol) between seawater samples and the carrier gas, and the response time of the system, were evaluated by means of a series of laboratory experiments. Although equilibrium between the seawater sample and the carrier gas in the equilibrator was not achieved for isoprene and propene (likely because of their low water solubility), the other species did reach equilibrium. The EI–PTR-MS system was deployed during a research cruise in the western North Pacific Ocean. Evaluation of several seawater sampling methods indicated that there was no significant contamination from the sampling apparatus for the target VOCs. For isoprene, comparison of EI–PTR-MS measurements with measurements obtained with a membrane equilibrator–gas chromatography/mass spectrometry system showed generally good agreement (R2 = 0.79). EI–PTR-MS captured the temporal variations of dissolved VOCs, including small-scale variability, which demonstrates that the performance of the EI–PTR-MS system was sufficient for simultaneous and continuous measurements of multiple VOCs of environmental importance in seawater.