Mass spectrometers have been located "under the sea" longer than most folks realize, providing some of the first examples of portable instrumentation. Mass spectrometers installed on submarines monitor air quality; Wyatt (1) provides an overview of the development of these instruments for the U.S. Navy, describing unique instrumentation dating back to 1972. Interestingly, the first submarine prototypes drew from the design of the mass spectrometry (MS) module that flew on Skylab. In the submarine environment, monitoring of carbon dioxide in the air (which many guess was the primary use of the mass spectrometer) was initially accomplished by other means and the mass spectrometer was instead used to monitor gases derived from solvents and refrigerants, as well as the volatile gases from various specialized components found exclusively on submarines. These gases could become toxic or could react with each other to form secondary contaminants in an enclosed environment filled with a great deal of heat-producing machinery. It is worth noting that the U.S. Navy only recently banned smoking on submarines; therefore, the boat atmosphere was reasonably expected to be filled with a variety of compounds, some of which may have been innocuous, but many of which required constant monitoring. The robustness of these early MS instruments was clearly a primary concern. There was also a strong need to avoid false positive conclusions that would require actions that might compromise the safety of the boat and its crew. In his article, Wyatt states that the on-board instrument was sited adjacent to a hatch through which supplies were delivered. The instrument therefore had to withstand the occasional deluge of salt water as well as dirt related to the in-and-out flux of men and supplies. Because the cost of a mass spectrometer is but a small fraction of the cost of a submarine, the research, development, and testing was expanded to include almost every conceivable instrument type for on-board installation. The basic analytical need for air monitoring aboard submarines continues, and smaller instruments based on new technology and different mass analyzers continue to be developed for this application (2,3).
Operation of a mass spectrometer on a submarine, despite the specialized specifications, could still depend on reliable power and a fixed relative location. The instrument was movable (because the submarine moved) but not subject to the same constraints as a truly portable mass spectrometer. Furthermore, despite the location of the mass spectrometer, an instrument in Skylab or aboard a submarine still operates within the human life support range of temperature and pressure. Mass spectrometers that transit interplanetary space or land on the surfaces of other planets operate in a totally different environment. The design of these voyaging instruments reflects different and stringent constraints of power load, pressure management, ionization, mass analysis, data collection and encoding, and transmission of the data to mission control.
Let's now consider MS instruments that sample from water, which may be sited in yet another physical niche, depending on the sample collection process. The water column contains dissolved gases and soluble compounds (both natural and pollutants). In addition to concentrations of such targeted compounds measured at a given depth, the vertical and horizontal variation in the concentrations might also be measured to delineate current- or temperature-driven transport processes, adding a time dimension to the measurement. Finally, certain specialized submarine environments such as thermal vents or submerged wrecks are scouted with submersibles equipped with MS (and other) analyzers, and a three-dimensional picture of plume components, some of which might be unexpected, potentially can be established.