Remote Monitoring of Volatile Organic Compounds in Water by Membrane Inlet Mass Spectrometry

This text is the first detailed account of the application of new and established mass spectrometric techniques utilizing trapped or confined ions for prolonged investigation and increased sensitivity. Each chapter contains complete references and utilizes a consistent format and writing style, with all terms, acronyms, procedures, and equations thoroughly explained. The strong editorial input to the diverse sections enables readers to readily appreciate the commonalities of topics ranging from theory of instrument operation to proteins, flavonoids, atomic clocks, and single ion mass spectrometry. This chapter deals with the monitoring Volatile Organic Compouns (VOCs) in water samples. VOCs are a very important class of water pollutants because of their persistence and suspect carcinogenicity for many of them. There are about sixty VOCs, including benzene, toluene, ethylbenzene, and xylenes (BTEX), halomethanes, haloethanes. A real-time, on-line, continuous monitoring of such compounds would allow prompt actions in order to avoid the diffusion of pollutants in the water system or to take appropriate countermeasures, thus restoring safe conditions in case of accidental contamination. Membrane Inlet Mass Spectrometry (MIMS) has been extensively studied for the determination of VOCs in various environmental matrices, especially water and air samples. MIMS allows the introduction of VOCs in the mass spectrometer through a thin (some tenths of millimeter) hollow-fiber polymeric membrane, which is selective towards organic compounds. When the membrane is in contact with the sample and an ion trap mass spectrometer is used as detector, such as in the present instance, VOCs are extracted in the membrane, concentrated in its small volume, and swept into the mass spectrometer by a gentle stream of helium carrier gas. The whole process is called pervaporation. When the membrane is exposed to a sample containing the target compounds, and the ions characteristic of each target compound are detected by mass spectrometry, the inherent ion current increases up to reaching a plateau. In brief, the characteristic ions are the qualitative information which allows identification of the analytes, while the plateau height is the quantitative information, with sensitivity in the sub-ppb levels and dynamic range of up to four decades for many VOCs. Although many papers have demonstrated that MIMS is a potentially excellent technique for continuous VOC monitoring given its simplicity and sensitivity, as to our knowledge no experiment has been published to check whether MIMS can be really implemented in a device able to work unattended for months. The present chapter reports on laboratory and field tests of MIMS instruments built in our laboratory as to hardware and software. Four instruments were deployed in unmanned sites, where they monitored VOCs in natural- and waste waters during a period exceeding 1 year/instrument. Laboratory performance – in terms of sensitivity, reproducibility, linearity tests, and comparison with P&T/GC/MS – and field performance – in terms of data output, most frequent maintenance operations and technical failures, and overall stability of the four remotely controlled instruments will be discussed.