Underwater mass spectrometry systems can be used for direct in situ detection of volatile organic compounds and dissolved gases in oceans, lakes, rivers and waste-water streams. In this work we describe the design and operation of (1) a linear quadrupole mass filter and (2) a quadrupole ion trap mass spectrometer interfaced, in each case, with a membrane introduction/fluid control system and packaged for underwater operation. These mass spectrometry systems can operate autonomously, or under user control via a wireless rf link. Detection limits for each system were determined in the laboratory using pure solutions. The quadrupole mass filter system provides detection limits in the 1-5 ppb range with an upper mass limit of 100 amu. Its power requirement is approximately 95 Watts. The ion trap system has detection limits well below 1 ppb, an upper mass limit of 650 amu and MS/MS capability. Its power consumption is on the order of 150 Watts. The present membrane limits analysis to non-polar compounds (Ͻ300 amu) with analysis cycles of 5-15 minutes. Deployments of both types of instruments are described, along with a discussion of the challenges associated with in-water mass spectrometry and descriptions of alternative in-water mass spectrometer configurations. (J Am Soc Mass Spectrom 2001, 12, 676 -682 ) © 2001 American Society for Mass Spectrometry S tandard methods for analysis of aqueous systems typically involve collection of samples and delivery to a laboratory for analysis [1]. Inherent in this practice is the possibility of both contamination and loss of analytes, particularly in the case of highly reactive or volatile species. In addition, this type of sampling severely limits both spatial and temporal sampling densities. A particularly important limitation of long collection/analysis cycles is the inability to implement adaptive sampling. Appropriate monitoring of dynamic biogeochemical systems requires rapid sampling adaptations in response to rapidly varying analyte distributions [2]. Intelligent sampling strategies are required to adequately characterize vast bodies of water that influence, and are influenced by, human activities. Since mass spectrometry is arguably the most versatile of chemical sensors, we have undertaken development of in situ mass spectrometry systems capable of real-time, adaptive in-water analyses.Although mass spectrometry has been used in the laboratory for an extremely wide variety of chemical analyses, from precise isotopic ratio measurements [3] to DNA sequencing [4 -6], no single configuration of mass analyzer and sample interface is appropriate for all types of measurements. This is evidenced in recent reviews of field analytical techniques [7] and advances in miniaturization of mass spectrometers [8]. Accordingly, we have chosen a modular approach for development of immersion mass spectrometers. Initially, simpler designs are used to integrate available components, and simultaneously new subsystems are being developed for evolving reconfigurations capable of accessing a w...

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