An essential component of ecosystems research is characterizing and quantifying the chemical mileu 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.