| A novel sub-miniature double-focusing mass spectrometer for in-situ environmental gas monitoring applications has been fabricated at the University of Minnesota using a combination of conventional machining methods and microfabrication techniques typically used in the sensor technology industry. Its design is based on the mass separation capabilities of a 90° cylindrical crossed electric and magnetic sector field analyzer (ro, = 2cm ), which under proper conditions is able to effectively cancel the angular and chromatic dispersion of the ion beam, thus improving the resolving power of the instrument. Ion simulations using finite element analysis and computer modeling were employed to verify and optimize the performance of the proposed instrument before it was actually fabricated. The fabricated lab prototype was able to attain a resolving power of 106, a detection limit of about 10 parts per million, and a mass range up to 103 atomic mass units. Its overall size, including the magnet assembly, is 3.5cm wide, 6 cm long and 7.5 m tall, its weight is 0.8 kg and its power consumption was measured to be 2 W, which makes this miniature mass spectrometer ideal for portable analytical instrumentation. Its performance was found to be comparable to commercial residual gas analyzers.;The fast in-situ mass spectrometer (MS) measurements of volcanic gaseous emissions were conducted at several fumarolic sites, demonstrating the usefulness of this analytical technique for real-time continuous volcanic gas monitoring applications. The gas concentrations obtained by the MS were in agreement with the values obtained using GC at the Hawaiian Volcano Observatory. In addition, the C/S ratios measured by the MS, using both traditional bottle sampling and in-situ sampling, were found to be very similar. Other gases like helium, which is not possible to monitor with any other continuous technique currently available to the volcanologists, were easily detected by the mass spectrometer. The field measurements also provided the instrumental performance parameters for our "Compact Double Focusing Mass Spectrometer" (CDFMS) prototype, as well as the ideal characteristics for a field-deployable mass spectrometer instrument. |
