High-frequency, accurate measurement of dimethylsulfide in surface marine environments using a microporous membrane contactor

Abstract

An automated sampling and analysis system for the shipboard measurement of dissolved dimethylsulfide (DMS) in surface marine waters is described. The system includes DMS extraction and trapping devices, a gas chromatograph (GC) equipped with a flame photometric detector (FPD), and a data acquisition system based on Visual Basic 6.0/C 6.0. The DMS is continuously and rapidly extracted from a seawater stream flowing at 100 mL min-1 across the hydrophobic wall pores of a membrane contactor. The stripping gas stream containing DMS flows at a rate of 300 mL min-1 for 1 min through an 8 cm Tenax-TA-filled glass tube controlled at -10°C by a thermoelectric cooler, within which DMS is selectively trapped. The trapped DMS is released by heating the trap to 160°C and is quantified by the GC-FPD. The detection limit of the proposed automated system is 0.2 nM of S (DMS), and the analytical precision is better than 3%. A complete analytical cycle takes 7 min from the extraction of DMS from the seawater stream to measurement of the DMS peak area, which is 3-4 times shorter than has been achieved using sparge-trap analytical setups. In field operations, the system runs on a daily cycle during which three DMS standards are measured at the beginning of the cycle and DMS derived from the seawater stream is subsequently analyzed throughout the day. The successful application of this analysis system to coastal waters suggests that the proposed DMS detection method will greatly facilitate data collection in future biogeochemical studies of the marine environment.