Ocean Science www.ocean-sci.net 1812-0784 1812-0792 5 4 2009 10.5194/os-5-537-2009 http://www.ocean-sci.net/5/537/2009/ http://www.ocean-sci.net/5/537/2009/os-5-537-2009.html http://www.ocean-sci.net/5/537/2009/os-5-537-2009.pdf 537 546 2009-11-06 A chemical ionization mass spectrometer for continuous underway shipboard analysis of dimethylsulfide in near-surface seawater E. S. Saltzman eric.saltzman@uci.edu W. J. De Bruyn M. J. Lawler C. A. Marandino C. A. McCormick University of California, Irvine, Irvine, California, USA Chapman University, Orange, California, USA now at: Leibniz Institut für Meereswissenschaften (IFM-GEOMAR), Kiel, Germany A compact, low-cost atmospheric pressure, chemical ionization mass spectrometer ("mini-CIMS") has been developed for continuous underway shipboard measurements of dimethylsulfide (DMS) in seawater. The instrument was used to analyze DMS in air equilibrated with flowing seawater across a porous Teflon membrane equilibrator. The equilibrated gas stream was diluted with air containing an isotopically-labeled internal standard. DMS is ionized at atmospheric pressure via proton transfer from water vapor, then declustered, mass filtered via quadrupole mass spectrometry, and detected with an electron multiplier. The instrument described here is based on a low-cost residual gas analyzer (Stanford Research Systems), which has been modified for use as a chemical ionization mass spectrometer. The mini-CIMS has a gas phase detection limit of 220 ppt DMS for a 1 min averaging time, which is roughly equivalent to a seawater DMS concentration of 0.1 nM DMS at 20°C. The mini-CIMS has the sensitivity, selectivity, and time response required for underway measurements of surface ocean DMS over the full range of oceanographic conditions. 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