| AVS 59th Annual International Symposium and Exhibition | |
| Vacuum Technology | Monday Sessions |
| Session VT-MoA |
| Session: | Gas Flow, Leaks, Permeation and Mass Analysis |
| Presenter: | R.T. Short, SRI International |
| Correspondent: | Click to Email |
There are numerous advantages to performing in situ chemical analyses in the field rather than collecting samples for laboratory analysis. These include reducing the possibility for sample contamination or degradation, significantly improving the spatial and temporal resolution of analyses, and providing the ability to implement adaptive sampling strategies by receiving the analytical information in real-time or near real-time. Many analytical instruments, such as mass spectrometers (MSs), require high vacuum for operation. Vacuum systems for these portable instruments must be reasonably small, low-power, and often more rugged than needed for laboratory operation. Underwater instruments of this type have the additional requirements that the vacuum systems must be self-contained (or exhaust to a high-pressure environment) and the sampling interfaces must withstand pressure differentials greater than 1 atmosphere – hydrostatic pressure increases by approximately 1 bar for every 10 m depth in the ocean. Consequently, all of these constraints must be taken into account when selecting or building vacuum systems for these applications.
Several groups around the world have been working on developing and using underwater MSs for in situ chemical measurements in oceans, lakes, and rivers – at times over a mile deep. To date, all of these operational underwater MSs have employed membrane-inlet interfaces to introduce analytes into the vacuum of the MS. Analytes that are soluble in the membrane material (typically polydimethlysiloxane or Teflon) permeate through the membrane and evaporate into the ion source of the MS, where they are ionized by an electron impact source. For operation at depth, the membrane must be supported by a porous frit to withstand the increased hydrostatic pressure. Membrane inlet MSs are very effective at detecting and quantifying dissolved gases, light hydrocarbons, and volatile organic compounds, often at trace levels.
Design considerations for underwater MSs will be discussed in this presentation, along with examples of specific components chosen for the vacuum systems. In particular, the focus will be on problems with operating vacuum systems under these often extreme and harsh conditions, along with solutions to mitigate these problems. In addition, we will present examples of deployments of underwater MSs for a variety of applications on a number of unmanned deployment platforms.