Design and Qualification of a Coded Aperture Cycloidal Mass Spectrometer to Detect Perfluorocarbon Tracer Molecules for Environmental Applications

Abstract

In urban, dense, and/or environmentally sensitive regions, underground high-pressure fluid-filled (HPFF) transmission cables are used to transport electricity at high voltages to prevent electrical losses. With age and use, these HPFF cables degrade and can leak petroleum-based dielectric fluid (DF) into the surrounding environment. Maintaining adequate DF in these cables is required for safe and reliable operation. Currently, detecting and locating underground DF leaks is challenging, time intensive, and inconsistent. One leak location method exists utilizing perfluorocarbon (PFC) tracer molecules and a mobile gas chromatograph. This instrument is sensitive enough to detect the atmospheric background levels of PFC in the ppqv (parts per quadrillion by volume, 10-12) range; however, this instrument is prone to analyte saturation, is not fully portable, nor does it produce real-time results.An improved mobile and highly sensitive PFC detection method is required. A cycloidal coded aperture miniature mass spectrometers (C-CAMMS) could be such a method due to its integration of three core technologies that help to overcome the throughput versus resolution tradeoff that has historically hampered the miniaturization of mass spectrometers. The C-CAMMS prototype combines: a cycloidal mass analyzer, aperture coding, and a focal plane array detector, to enable a mobile instrument capable of detecting PFC tracers with high resolution, sensitivity, and selectivity. The cycloidal mass analyzer utilizes perpendicularly-oriented overlapping electric and magnetic fields to linearly separate ions by mass-to-charge ratio (m/q). The C-CAMMS platform uses aperture coding to increase throughput without sacrificing resolution which commonly occurs during miniaturization. Finally, a capacitive transimpedance amplifier (CTIA) array ion detector offers sensitive, simultaneous ion detection across a wide mass range.A detailed understanding of the detection process for this instrument was obtained through extensive simulations and experiments. From this knowledge, a set of design considerations and a six-step design approach have been established for reproducibly developing a cycloidal mass analyzer that uses a focal plane array detector. This design knowledge relates the magnetic and electric field homogeneity of the cycloidal mass analyzer to the performance of the complete mass spectrometer. The efficacy of this design roadmap is demonstrated by designing the PFC-CAMMS instrument for the specific use case of PFC tracer detection and location.The design knowledge was implemented to create a PFC-CAMMS instrument with high resolution, high sensitivity, a large mass range, and a small form factor. PFC-CAMMS achieved a resolution of 0.071 u at m/q 69 and 0.27 u at m/q 331 and demonstrated an overall mass range of m/q 29 – 502. Additionally, using PMCH (C7F14) as the test molecule, the PFC-CAMMS instrument achieved a detection limit of 20 ppbv (parts per billion by volume, 10-9) with a response time of less than 5 s from sample introduction using a capillary inlet. There is no evidence of a hysteresis effect when detecting PFCs for this benchtop (66 x 46 x 30 cm3 and ~50 kg) laboratory prototype. This work describes not only the understanding that was generated about designing electromagnetic components for a PFC-CAMMS, but also explains the fabrication, alignment, and assembly details that enable this improved baseline performance. The reproducible procedure for developing a cycloidal mass analyzer with an array detector facilitates improved hardware designs that produce a more consistent system response function across the intended mass range. Following reconstruction of the coded mass spectrum, the PFC-CAMMS instrument can overcome the throughput vs. resolution tradeoff due to its stronger yet more uniform field.