Underwater cryotrap-membrane inlet system (CT-MIS) for improved in situ analysis of gases

Membrane inlet sensor techniques allow online, real-time and in situ analyses of gases during the investigation of aquatic environments. Of specific interest for research and applied objectives are quantifications of gases like methane, higher hydrocarbons, carbon dioxide, nitrogen, volatile organic compounds, and pollutants. For these objectives, membrane inlet systems are coupled to optical or solid state sensors as well as mass spectrometers. Besides the gases of interest, large quantities of water vapor are passing through the membrane and are thus introduced into the sensor system. This downgrades the detection limit, affects the ionization efficiency of mass spectrometers, or could cause the condensation of water within infrared sensors. In this study, we describe a novel robust, low-power cryotrap coupled to a membrane inlet system (CT-MIS), which is suitable to be used in harsh environments, including underwater applications. The entire system is of small size and weight, is operated at –85°C, and requires an energy consumption of less than 10 Watt. By using the cryotrap, we are able to reduce water vapor in the analytical line by more than 98%. The detection limits for major and trace gases are considerably improved this way. For the trace gas methane (CH4), the detection limit was lowered from 100 to 16 nmol/L, which allows the measurement of methane in surface and bottom waters of coastal areas and lakes. In case of membrane failure, the CT-MIS acts as a security system by shock-freezing the water, thus blocking the capillary connection to the analyzer unit. *Corresponding author: E-mail: [email protected]; phone + 49471 4831 2029 /-fax 1425 Acknowledgments The authors are grateful to Erich Dunker for technical support and advice during manufacturing the CT-MIS. The authors thank the captain and the crew of R/V Heincke for their assistance during several cruises. We are indebted to Roi Martinez for the GIS support. Sabine Kasten, Michiel Rutgers van der Loeff, and Aysel Sorensen are thanked for helpful comments on the manuscript. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP/2007-2013) under grant agreement nr 217246 made with the joint Baltic Sea research and development programme BONUS. DOI 10.4319/lom.2012.10.317 Limnol. Oceanogr.: Methods 10, 2012, 317–328 © 2012, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS (Schlüter et al. 1998). Such restrictions stimulated the development of continuous underway measurement systems (Butler and Elkins 1991; Gueguen and Tortell 2008; Johnson et al. 1999; Saltzman et al. 2009; Tortell 2005) and underwater gas analyzers for in situ measurements of O2 (Oxygen), CH4, VOCs, and CO2 (Boulart et al. 2008; Camilli and Hemond 2002; Emerson et al. 2002; McMurtry et al. 2005; Prien 2007; Short et al. 2006b; Wankel et al. 2010). Whereas optodes for in situ measurement of O2 rely on a chemical transducer and measurement of fluorescence, most other underwater gas analyzers require a gas extraction unit. Most of these units apply membrane inlet systems (MIS), which allow the phase separation of gases like CO2, CH4, DMS, radon, or VOCs from water into the gaseous phase (Boulart et al. 2010; Johnson et al. 2000; Prien 2007). The gas phase separated by the MIS is introduced into an analytical unit, like a mass spectrometer, gas chromatograph, infrared spectrometer, or solid-state gas sensors for detection of the composition of the gas mixture as well as gas concentrations. Dependent on the field of application, research as well as industrial applications make use of different membrane materials and geometries (e.g., planar, tubular, or coiled) when performing phase separation with membrane inlet systems. These membranes are made of polydimethylsiloxane (PDMS), Teflon (Teflon, AF2400) as well as materials like Nafion, Celgard, or Accurel (Melin and Tautenbach 2006). Common to these different materials are their nonporous or microporous properties, which are impermeable to the liquid phase, up to a certain pressure. The transfer of gases through solid state membranes like PDMS as well as Nafion, or through microporous membranes like Accurel or Celgard, is described and modeled by concepts like permeation or pervaporation. A detailed consideration of different membrane materials and separation mechanisms, considering analytical as well as industrial application, is provided by Melin and Tautenbach (2006). The mass transport of gas through a membrane depends on the membrane type as well as the specific properties of the gas. Most of the nonporous or microporous membranes are hydrophobic and preferentially exclude the polar water molecules over, e.g., VOCs. However, they do not completely exclude water vapor (H2Ov), and because it is the matrix, it is still the major gas species in the vacuum system (Tortell 2005). The H2Ov affects gas analyses done by mass spectrometry, as well as the IR bands of the analyte during spectroscopy, downgrades the detection limit, or could cause condensation of water inside the optical sensor section of the analyzer. Regarding membrane inlet mass spectrometry (MIMS), additional disadvantages of high water vapor contents within the vacuum line are excessive pressure in the ion source of the mass spectrometer and decreased signal stability at Ar (argon, m/z 40), O2 (m/z 32), and N2 (nitrogen, m/z 28), for example (Bell et al. 2011; McCarthy and Gardner 2003; Tortell 2005). In the laboratory, interferences due to a high amount of water vapor entering the sensor system can be avoided by absorbing agents (e.g., Dierite), molecular sieves, or cold traps. The detection limits for, e.g., VOCs, DMS, CH4, or DMSP (dimethylsulfoniopropionate) are considerably improved by enriching, and subsequently releasing, analytes within the cryotrap (Damm et al. 2008; Desmarais 1978; Mendes et al. 1996a; Simmonds 1984). For the measurement of some analytes, the trapping of gases like water vapor or hydrogen sulfide (H2S) improves the signal-to-noise ratio, the detection limit, and the measurement precision (Kolb et al. 1996; Mendes et al. 1996b). Compared to cryotraps applied in laboratories, issues like small size, low weight, small waste-heat production, robustness, and especially low energy consumption are the required specifications in underwater applications. This article describes the design and performance of a novel underwater cryotrap membrane inlet system (CT-MIS), reaching temperatures of less than –85°C to trap water vapor before entering the analytical line. The CT-MIS is self-contained, and with small modifications, can be coupled to different optical sensors in addition to mass spectrometers. For the assessment of the CTMIS, the unit was coupled to the underwater mass spectrometer (UWMS) InSpectr200-200 (Bell et al. 2007, 2011; Short et al. 2001, 2006a; Wenner et al. 2004). We applied this system in the Baltic Sea and the North Sea for the analysis of Ar, O2, N2, CH4 as well as CO2 in the water column and also at the sediment-water interface. For trapping water vapor from the gas phase before a chemical analysis using gas chromatography, MIMS, or IR spectrometry cryotraps of different types are applied. The design varies from U-shaped type capillaries submerged in Dewar flasks to advanced microjet cryotrap systems (Desmarais 1978; Kolb et al. 1996; Peters and Yakir 2010; Van Der Laan-Luijkx et al. 2010). Techniques to cool down a section of the gas stream include the application of liquid N2 or Ar contained in Dewar flasks and thermoelectric cooling by Peltier elements or high performance cryopumps. The cooling temperature depends on the mode of operation as well as the freezing point of the gases of interest. For example, to trap water vapor in a vacuum section (i.e., @0.0533 Pa = 4 ¥ 10–4 Torr) while connecting a membrane inlet system with a mass spectrometer, the cryotrap has to be operated at a temperature of about –80°C (Wexler 1977). For the application of cryotraps in laboratories, features the evaporation of cooling agents as well as size and weight of the device are of minor importance. In contrast, these features are essential for cryotraps, which are applied on research vessels or mounted in pressure housing for the use in underwater applications. In such applications, the use of cooling agents such as liquid ethanol or N2 would lead to the formation of large gas volumes and could cause security risks or are inappropriate during operation. Therefore, from a technical as well as analytical perspective, the following issues are considered mandatory for a standalone underwater cryotrap membrane inlet systems: (1) temGentz and Schlüter Underwater cryotrap-membrane inlet system