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Mass spectrometry is on the move. It used to be confined to the lab because of the large instrumentation employed and the requirement for high vacuum but advances in miniaturisation have enabled the development of mobile systems. In one extreme case, mass spectrometry has gone where no man has gone before - to Mars. Missions to the red planet have taken mass spectrometers with them to search for organic compounds and to sample the Martian atmosphere. This type of deployment illustrates the flexibility of mass spectrometers. If it is too dangerous or simply impossible for people to accompany them, they can be sent off on their own to perform experiments and transmit data back to base. Mass spectrometers have been designed to be taken to volcanoes to measure emissions, to the Antarctic, into battlefields to search for chemical warfare agents, on rockets to detect leaks, and into space, in a branch of the discipline dubbed harsh environment mass spectrometry. In this scenario, water is also regarded as a "harsh environment", because the mass spectrometry is performed in situ and the conditions are severe compared with the lab. One of the leading teams developing submersible systems is that led by Tim Short, formerly based at the Center for Ocean Technology at the University of South Florida but now part of the newly formed Marine Technology Program of SRI International. Their systems use a membrane inlet to isolate dissolved gases and volatile organic compounds from water and pass them to the mass spectrometer for measurement. The team has used both remotely operated vehicles and autonomous underwater vehicles carrying a membrane inlet mass spectrometer (MIMS) which have been able to measure low-molecular-mass compounds in bodies of water like Tampa Bay. The early systems were operated at shallow depths, down to about 30 m, but the latest ones are capable of deep sea deployment down to 2000 m. Membrane inlets are rugged, reduce the gas loads on vacuum pumps, and permit the simultaneous analysis of many analytes. However, they present some problems when used in water, due to the effects of temperature, pressure, and hydrodynamics at the water-membrane interface. The latest instrument developed by Short's team has an in-built heater block to control the temperature around the membrane and is operated at constant flow. This leaves variable hydrostatic pressure (HP) as the principal uncontrollable factor that affects the membrane permeability. It would be possible to impart some control over HP, but that would likely reduce sample throughput and complicate the MIMS systems. However, the problem must be solved if underwater mass spectrometry is to be extended to the deep ocean to study phenomena like hydrothermal vents. So, Short and his colleagues have studied the effects of HP on the permeability of the poly(dimethylsiloxane) membrane used in their MIMS. The experiments were carried out in the lab, using their underwater MIMS system, which was fitted with a quadrupole residual gas analyser operated in electron ionisation mode. Samples were introduced at controlled temperature and flow rate and the changes in signal intensity with changing HP were measured up to 20 MPa, corresponding to the approximate HP at a depth of 2000 m. For a selection of analytes (methane, nitrogen, oxygen, hydrogen sulphide, argon, carbon dioxide, dimethyl sulphide, chloroform, toluene), the ion currents were found to vary non-linearly with HP. For simple gases, the permeability decreased with increasing HP, due to decreases in membrane permeability. Larger and non-polar analytes decreased initially, but increased as the HP was increased further, due to a combination of diffusion and partition coefficients. This behaviour could not be fitted adequately to published models of membrane behaviour, so the researchers developed a new, simple equation relating the HP to permeability. The equation was applied to real data collected for water, nitrogen and oxygen in the Gulf of Mexico to generate vertical profiles at depths up to 500 m. It could not be tested on volatile organics in the open sea since their concentrations are below the instrumental detection limits. The results, when corrected for HP, were in good agreement with those determined by conventional oceanographic methods using oxygen and CTD (conductivity, temperature and depth) sensors. This is the first in situ measurement of dissolved gas profiles in the ocean obtained with a MIMS. It demonstrates the applicability of the system for the marine environment and will allow detailed depth profile studies of the concentrations of gases and volatile organic compounds down to the sea bed. Related links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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 The underwater mass spectrometer system being deployed
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