Instrument Design and Study of Operational Characteristics of a Cycloidal Coded Aperture Miniature Mass Spectrometer for Environmental Sensing

Abstract

Effluence of organic compounds like benzene, toluene, ethylbenzene and xylenes (“BTEX”), and methane from an industrial setting can have a negative impact on human health and the environment. Miniature sector mass spectrometers have the potential to acquire desirable attributes for ideal organic compound detection such as robustness, low cost, high chemical specificity, high sensitivity, and low power requirements. However, barriers to their miniaturization exist in the form of a throughput vs. resolution tradeoff. Spatially coded apertures can break this tradeoff by increasing throughput without sacrificing resolution. Cycloidal sector mass spectrometers are ideal candidates for incorporation of spatially coded apertures when used with array detectors, since they use perfectly focus the image of coded aperture at the detector due to perpendicularly oriented uniform electric and magnetic fields. A previous demonstration of a proof-of concept cycloidal-coded aperture miniature mass spectrometer (C-CAMMS) instrument employed aperture coding, a carbon nanotube (CNT) field emission electron ionization source, a cycloidal mass analyzer, and a capacitive transimpedance amplifier (CTIA) array detector to achieve greater than ten-fold increase in throughput without sacrificing resolution. However, the coded aperture image corresponding to each ion species was not constant due to a spatiotemporal variation in electron emission from CNTs, a non-uniformity in the electric field, and a misalignment of the detector and the ion source with the mass analyzer focal plane. In this work, modifications to the sample inlet, ion source, and the mass analyzer design of the previous C-CAMMS instrument were made to improve its performance. A membrane inlet enhanced the organic compound detection sensitivity of the new C-CAMMS instrument and enabled low detection limits of 50 ppm for methane and 20 ppb for toluene. A thermionic filament-based ion source produced a significantly more stable coded aperture image than the CNT based ion source. The aperture image fluctuations in the CNT-based source were determined to be likely a result of adsorption and desorption of molecules on the CNT surface that caused local work function changes and induced spatiotemporal variation in electron emission and subsequent ion generation. Modifications to the mass analyzer improved the electric field uniformity, improved the alignment of the ion source and the detector with the mass analyzer focal plane, and increased the depth-of-focus to further facilitate alignment. Finally, a comparison of reconstructed spectra of a mixture of dry air and toluene at different electric fields was performed using the improved C-CAMMS prototype. A reduction in reconstruction artifacts for a wide mass-to-charge (m/z) range highlighted the improved performance enabled by the design changes.