Improving the Sensitivity of Fourier Transform Mass Spectrometer (Orbitrap) for Online Measurements of Atmospheric Vapors.

Orbitrap Fourier transform mass spectrometry coupled with chemical ionization (CI) is a new-generation technique for online analysis in atmospheric chemistry. The advantage of the high resolving power of the CI-Orbitrap has been compromised by its relatively low sensitivity to trace compounds (e.g., <106 molecules cm-3) in complex gaseous mixtures, limiting its application in online atmospheric measurements. In this study, we improve the sensitivity of a Q Exactive Orbitrap by optimizing the parameters governing the signal-to-noise ratio. The influence of other parameters related to ion transmission and fragmentation is also discussed. Using gaseous compounds in an environmental chamber, we show that by increasing the number of ions in the analyzer, the number of microscans (i.e., transients), and the averaging time, the sensitivity of the CI-Orbitrap to trace compounds can be substantially improved, and the linear detection range can be extended by a factor of 50 compared to standard settings. The CI-Orbitrap with optimized parameters is then used to measure oxygenated organic molecules in the atmosphere. By improving the sensitivity, the number of detected compounds above the 50% sensitivity threshold (i.e., the signal intensity at which the sensitivity is decreased by half) is increased from 129 to 644 in the atmospheric measurements. The Q Exactive CI-Orbitrap with improved sensitivity can detect ions with concentrations down to ∼5 × 104 molecules cm-3 (1 h averaging), and its 50% sensitivity threshold is now below 105 molecules cm-3.

Online Aerosol Chemical Characterization by Extractive Electrospray Ionization-Ultrahigh-Resolution Mass Spectrometry (EESI-Orbitrap).

Current mass spectrometry techniques for the online measurement of organic aerosol (OA) composition are subjected to either thermal/ionization-induced artifacts or limited mass resolving power, hindering accurate molecular characterization. Here, we combined the soft ionization capability of extractive electrospray ionization (EESI) and the ultrahigh mass resolution of Orbitrap for real-time, near-molecular characterization of OAs. Detection limits as low as tens of ng m-3 with linearity up to hundreds of µg m-3 at 0.2 Hz time resolution were observed for single- and mixed-component calibrations. The performance of the EESI-Orbitrap system was further evaluated with laboratory-generated secondary OAs (SOAs) and filter extracts of ambient particulate matter. The high mass accuracy and resolution (140 000 at m/z 200) of the EESI-Orbitrap system enable unambiguous identification of the aerosol components' molecular composition and allow a clear separation between adjacent peaks, which would be significantly overlapping if a medium-resolution (20 000) mass analyzer was used. Furthermore, the tandem mass spectrometry (MS2) capability provides valuable insights into the compound structure. For instance, the MS2 analysis of ambient OA samples and lab-generated biogenic SOAs points to specific SOA precursors in ambient air among a range of possible isomers based on fingerprint fragment ions. Overall, this newly developed and characterized EESI-Orbitrap system will advance our understanding of the formation and evolution of atmospheric aerosols.