Siloxane-Radical Species: Chemical Characteristics and Use for Mass Calibration in Aerosol Mass Spectrometry

Mass spectra generated by volatilized particulates often span a wide range of masses, complicating mass calibration for high-resolution time-of-flight mass spectrometers. We used synchrotron-based vacuum-ultraviolet photoionization aerosol mass spectrometry (VUV-AMS) to demonstrate that conductive silicone tubing can be used to generate a range of distinct, isolated calibration peaks over the mass range suitable for calibration of aerosol mass spectra. We generated vaporized siloxane oligomers by heating tubing composed of polydimethylsiloxane (PDMS) to temperatures of 150–230 °C. The heated tubing was placed in line with a soot source. Soot particles efficiently adsorbed these siloxane species and transported them to the detection region of an aerosol mass spectrometer. We hypothesize that siloxane radicals are catalytically formed on the heated copper oxide surface of the VUV-AMS target. This calibration source promotes the formation of three series of substituted-siloxane-oligomer radicals and twenty-one recurring mass peaks (not including isotopologues) between 73 and 799 Da. These series are associated with oligomers of the siloxane base unit attached to a trimethylsilyl group ([SiO(CH3)2]nSi(CH3)3), an SiOCH3 group ([SiO(CH3)2]nSiOCH3), and an SiOH group ([SiO(CH3)2]nSiOH). Based on our results and information from the literature, we hypothesize that the latter two series preferentially form cyclic free radicals. Without the soot source, the heated silicone tubing nucleates some particles and generates a subset of the siloxane radical peaks reported here. We verified the mass calibration from these species using a sample of five polycyclic aromatic hydrocarbons (PAHs) spanning the mass range of 178–300 Da, 2,5-dimethylfuran, which has a nominal mass of 96 Da, and known hydrocarbons from ethylene pyrolysis spanning the mass range of 165–374 Da. This calibration source provides a broad range of mass peaks for calibration with minimal modification to existing experimental designs.