In Situ Mass Spectrometer

An essential component of ecosystems research is characterizing and quantifying the chemical milieu within and around habitats. To date, there is a paucity of chemical sensors that are capable of measuring a wide range of compounds, and be left in situ for longer term investigations. Mass spectrometers are among the instruments that can detect the broadest range of compounds with a single detector, and are capable of high sensitivity and throughput.

Recently, we developed an in situ mass spectrometer (or ISMS) based around a commercially available 24 VDC mass spectrometer that can detect and quantify volatiles up to 200 daltons. It is capable of deployments down to 4500 meters, and has been laboratory-tested to 6000 meters. During a recent expedition in the Gulf of Mexico, the ISMS was deployed 7 times, including three dives to a 2900-meter coral and seep sites in the Gulf of Mexico (see Wankel 2010). More recently, the ISMS was used to interrogate subsurface hydrogen oxidation at hydrothermal vents (see Wankel 2011Geochemical flux and metabolic activity associated with the hydrothermal subsurface). The ISMS is able to measure parts-per-million concentrations of methane, alkanes hydrogen, oxygen, hydrogen sulfide, carbon monoxide, carbon dioxide, nitrogen, nitrous oxide and many other volatiles. These are of particular interest because they are substrates for microbial and macrofauanal metabolism. The ISMS will soon incorporate a recently developed gas exhaust system that will allow it to operate continuously in situ. It should also be noted that the core components of the ISMS, including the commercially available pump and mass spectrometer, perform best when operated continuously and can be expected to run for 12 to 16 months without servicing (such as lubrication and tuning).


ISMS Specifications

Mass spectrometer Quadropole, 200 amu range, 0.5 AMU resolution, faraday cup and secondary electron multiplier, 6x10-14 torr detection limit
Pumping system Hybrid turbomolecular pump, dual piston diaphragm pump, oil-free
Safety system High-speed solenoid shutoff valves seal off the instrument in 50 msec if water is detected in the line.
Power requirements 24VDC@8A on startup; 24VDC@3A nominal
Data storage/transmission Storage on board a 60GB hard drive; transmission through cable via RS-232
Data rate 28.8 to 56K baud
Dimensions/weight 25 cm OD x 90 cm L; 25 Kg dry, 13 Kg in water
Operating conditions Inlet rated to 4500 meters, tested to 6000 meters; titanium housing rated to 4500 meters; maximum instrument operating temperature is 70°C; maximum fluid sampling temperature is 350°C

Figure 1. The in situ mass spectrometer (ISMS) is deployed via human occupied submersible such as the DSV Alvin, or by an ROV such as Jason. Suni Shah (research scientist at WHOI) is shown interfacing the ISMS to the DSV Alvin during an expedition in 2009.

Figure 2. The ISMS is shown here sampling > 350° C vent fluids for analysis. The ISMS inlet is designed to be used in an “open configuration”, for seawater surveying, or with an inlet pump wherein it analyzes specific samples that are collected via a sampling wand.