Underwater Mass Spectrometry (UWMS)
Technological developments in the field of Harsh Environment Mass Spectrometry (HEMS)
allow online and in situ analysis of trace gases in the ocean or lakes down to several hundred meters. For this purposes Underwater Mass Spectrometer (UWMS) like the Inspectr200-200, the THETYS or the NEREUS (Short et al., 2001, 2006; Matz et al., 1999; Hemond and Camilli, 2002; Camilli and Hemond, 2004; Wenner et al., 2004; Kibelka et al., 2004; Bell et al., 2007) were designed.
Compared to previous techniques UWMS enables the analysis of a multitude of gases like methane, CO2, nitrogen dioxide or dimethyl sulphide on a time scale of a few seconds.
Underwater Mass Spectrometer Inspectr200-200.
Underwater Mass Spectrometers, like the Inspectr200-200 (Short et al., 2001), consists of a roughing pump, an embedded PC, an Inficon mass spectrometer, a micro-controller (µC) for adjustment and control of the flow rate delivered by the gear pump as well as of the temperature of the Membrane Inlet System (MIS). By a turbo pump a pressure of less than 10-5 Torr is maintained in the vacuum section of the mass spectrometer. A more detailed consideration of this system is provided by Wenner et al. (2004).
During operation of the Inspectr200-200, water is pumped by the gear pump from the outside into the pressure housing through the membrane inlet system (MIS) where gas permeation takes place, and back into the water column. Within the MIS, the water is in contact with a polydimethylsiloxane (PDMS) tubular membrane.
The tubular membrane (polydimethylsiloxane, PDMS) separates the water phase from the vacuum section inside lumen of the PDMS tube.
The pressure inside the PDMS membrane is held at less than 10-5 Torr with the turbo pump. Gases permeating through the membrane are introduced directly into the mass spectrometer. To avoid the collapse of the PDMS tube under hydrostatic pressures of up to 20 bar (equivalent to ca. 200 m water depth), the membrane is supported at the inside by a stainless steel spring.
Deployment of the Underwater Mass Spectrometer Inspectr200-200 together with a solid state CH4 analyzer and an optical CH4 sensor.
Major objectives of our studies are the on-line analysis of methane in aquatic systems by application of membrane inlet mass spectrometry. Whereas a few studies applied MIMS for analysis of CO2, O2 or DMS concentrations in marine environments, or considered relative shifts of peak intensities indicative for CH4 concentrations (Tortell, 2004; Short et al., 1999, 2006), the calibration of an UWMS for measurement of methane concentration in coastal areas and lakes is a rather new topic. For this purpose we applied a simple and reliable volumetric calibration technique, based on the mixing of two end member concentrations.
To minimize interferences caused by the high water vapor content, permeating through the membrane inlet system into the vacuum section of the mass spectrometer, a cool-trap was designed (Schlüter and Gentz, in press).
Examples for measurements conducted during a cruise by FS Heincke to the North Sea and Baltic Sea as well as a cruise by FS Kormoran on Lake Constance are shown below.
Measurement of oxygen and CO2 in surface waters during the cruise HE260.
Methane concentrations measured in surface and bottom waters of Lake Constance.
References
Bell R. J., Short R. T, van Amerom F. H. W. and Byrne R. H. (2007). Calibration of an In Situ Membrane Inlet Mass Spectrometer for measurements of dissolved gases and volatile organics in seawater. Environ. Sci. Technol., 41, 8123–8128.
Hartnett H. E. and Seitzinger S. P. (2003). High-resolution nitrogen gas profiles in sediment porewaters using a new membrane probe for membrane-inlet mass spectrometry. Mar. Chem., 83, 23– 30.
Hemond H. and Camilli R. (2002) NEREUS: engineering concept for an underwater mass spectrometer,” Trends in Anal. Chem., 21, 526-533.
Kibelka G.P.G., Short R.T., Toler S.K., Edkins J.E. and Byrne R.H. (2004). Field-deployed underwater mass spectrometers for investigations of transient chemical systems. Talanta, 64, 961-969.
Matz, G., Kibelka, G., Dahl, J., Lennemann, F. (1999). Experimental study on solvent-less sample preparation methods-Membrane extraction with a sorbent interface, thermal membrane desorption application and purge-and-trap. J. Chromatogr., 830 (2), 365–376
Schlüter, M. and Gentz, T. (in press). Application of Membrane Inlet Mass Spectrometry for online and in situ analysis of methane in aquatic environments. J. Am. Soc. Mass Spectrom.
Short R. T., Fries, D.P. , Kerr M.L., Lembke C.E., Toler S.K., Wenner P.G., and Byrne R.H. (2001). Underwater mass spectrometers for in-situ chemical analysis of the hydrosphere, J. Am. Soc. Mass Spectrom., 12, 676–682.
Short R. T., Toler S. K., Kibelka G. P. G., Rueda Roa, D. T., Bell, R. J., Byrne, R. H. (2006). Detection and quantification of chemical plumes using a portable underwater membrane introduction mass spectrometer. TrAC, Trends Anal. Chem. 25 (7), 637–646
Tortell, P. D. (2005). Dissolved gas measurements in oceanic waters made by membrane inlet mass spectrometry. Limnol. Oceanogr. Methods 3, 2005, 24–37.
Wenner P.G., Bell P. G., van Amerom, F.H.W., Toler S.K., Edkins J.E., Hall M.L., Koehn K., Short R.T. and Byrne, R.H. (2004). Environmental chemical mapping using an underwater mass spectrometer. Trends in Analytical Chemistry, 23, 288-295.