Environ. Sci. Technol. 2009, 43, 5014–5021
Characterizing Spatial and Temporal Variability of Dissolved Gases in Aquatic Environments with in situ Mass Spectrometry RICHARD CAMILLI* AND ANTHONY N. DURYEA Woods Hole Oceanographic Institution MS#7, Woods Hole, Massachusetts 02543, and Monitor Instruments Co., 290 East Union Road, Cheswick, Pennsylvania 15024
Received December 31, 2008. Revised manuscript received March 30, 2009. Accepted April 28, 2009.
The TETHYS mass spectrometer is intended for long-term in situobservationofdissolvedgasesandvolatileorganiccompounds in aquatic environments. Its design maintains excellent low mass range sensitivity and stability during long-term operations, enabling characterization of low-frequency variability in many trace dissolved gases. Results are presented from laboratory trials and a 300-h in situ trial in a shallow marine embayment in Massachusetts, U.S.A. This time series consists of over 15 000 sample measurements and represents the longest continuous record made by an in situ mass spectrometer in an aquatic environment. These measurements possess sufficient sampling density and duration to apply frequency analysis techniques for study of temporal variability in dissolved gases. Results reveal correlations with specific environmental periodicities. Numerical methods are presented for converting mass spectrometer ion peak ratios to absolute-scale dissolved gas concentrations across wide temperature regimes irrespective of ambient pressure, during vertical water column profiles in a hypoxic deep marine basin off the coast of California, U.S.A. Dissolved oxygen concentration values obtained with the TETHYS instrument indicate close correlation with polarographic oxygen sensor data across the entire depth range. These methods and technology enable observation of aquatic environmental chemical distributions and dynamics at appropriate scales of resolution.
Introduction Mass spectrometry (MS) is a versatile analytical technique for determining chemical and isotopic compositions. The principles of MS are well understood and have become a standard analytical technique for diverse types of investigations, including environmental exploration and monitoring. As the field of MS has matured specialized instruments have been developed for in situ use as payload aboard spacecraft (1-4), aircraft (5-7), and terrestrial mobile platforms (8-12). Over the past decade research efforts have appreciably advanced underwater MS to a state that instruments have been successfully deployed on a range of in situ underwater platforms, including human occupied submersibles, robotic underwater vehicles, and ship-tethered systems (13-20). * Corresponding author phone: 1-508-289-3796; fax: 508-289-2191; e-mail:
[email protected]. 5014
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 13, 2009
Underwater mass spectrometric analysis is largely motivated by the need for improved sensing of trace dissolved chemicals in aquatic environments. Laboratory-based analyses are generally labor intensive and time-consuming, inherently limiting the spatial or temporal resolution of measurements. The resulting data sparseness can obscure spatially localized or alias temporally transient chemical features. Furthermore, care must be taken in sample collection, transport, storage, and handling to avoid contamination or chemical transformation. In situ underwater chemical sensors avoid the drawbacks of off-site analysis but are typically limited to detecting single or relatively few chemical species and can lack specificity or exhibit cross sensitivity. By comparison MS is inherently able to detect wide ranges of individual chemicals within complex mixtures. This is advantageous for rapid exploration, monitoring, and diagnosis of marine environments, particularly when there is limited or no a priori knowledge as to the type and levels of chemicals present. In situ underwater MS is well suited to numerous types of environmental analyses. Example applications include identification of point and nonpoint source anthropogenic pollution such as petroleum spills or nutrient loading, estimation of primary productivity and respiration, and investigation of naturally occurring chemical phenomena such as hydrothermal venting and greenhouse gas cycling dynamics. Despite the potential utility of in situ MS, capability has remained restricted to short-term deployment because of technological hindrances such as power demand, operational endurance, and component fragility. These limitations are largely because in situ underwater MS designs have relied on components developed for operation in controlled environmental conditions. To overcome these limitations the TETHYS (tethered yearlong spectrometer) membrane inlet mass spectrometer design uses components designed specifically for in situ operation in uncontrolled aquatic environments, with the goal of enabling continuous highendurance water column chemistry monitoring.
Instrumentation Design and Laboratory Experimentation TETHYS development was carried out through a research partnership between the Woods Hole Oceanographic Institution and Monitor Instruments Co. The instrument is fully self-contained within a 16 L cylindrical metal pressure housing with hemispherical end-caps (60 cm long by 22 cm external diameter) and is optimized for long-term unattended environmental monitoring of low molecular weight (2-200 amu) gases and volatile chemicals in a variety of hostile environments. The instrument’s high mass resolution at low masses enables quantitative measurement of substances such as hydrogen and methane without interference from “zero blast”, a phenomenon common to quadrupole mass filters, or adjacent ion peaks. The TETHYS design has no moving parts and can be operated across a wide range of temperatures (near freezing to 45 °C), pressure (to 500 atm), and vibration/ physical shock conditions (such as those generated during cable towed platform operations and shipboard launch/ recovery of submersible platforms). It is capable of amphibious operation as well, permitting use in a variety of terrestrial applications including monitoring air quality and mixed phase exhaust streams. Power draw is 25 W for the instrument and 29 W with an integrated CTD and sampling pump (model SBE49 FastCAT, SeaBird Electronics Inc., Bellevue, Washington, U.S.A.). The instrument’s size, power, and endurance characteristics permit extended deployment on 10.1021/es803717d CCC: $40.75
2009 American Chemical Society
Published on Web 05/20/2009
moorings, cabled observatory nodes, and mobile submersible platforms. Mounting orientation does not affect the instrument’s operation, allowing it to be integrated according to available platform space or vehicle trim requirements. Analyte ionization is via electron impact by a closed-source thoriated tungsten filament operating at 70 eV. The mass analyzer uses a novel inverted prolate trochoidal ion path (historically described as a cycloidal ion path) based on equations originally set forth in 1938 by Bleakney and Hipple (21). Unlike the Robinson design (22) used in the NEREUS instrument (15), which is unable to continuously scan lower than approximately 11 AMU, the TETHYS analyzer’s inverted prolate trochoidal ion trajectory extends its lower mass range limit to 1 amu. The TETHYS cycloid uses a homogeneous fixed magnetic field generated by a rare-earth magnet that is superimposed upon an orthogonal electric field. The electrical field is supplied with low power (
