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Good analytical instruments are prerequisite to unravel the chemical complexity of atmospheric organic compounds. My research pursues the development of new mass spectrometry methods to fully characterize the chemical composition of atmospheric organic compounds.
Benzene+ CIMS
Benzene+ CIMS
We explored the capability of Chemical Ionization Mass Spectrometry (CIMS) using benzene cations as reagent ions for detecting atmospheric trace gases. We introduce a thermodynamics-based framework that classifies analytes into three categories based on their ionization energy (IE). Each class exhibits distinct ionization mechanisms and product ions and predictable sensitivities. It is cool that the benzene CIMS can measure NO, VOCs, and their oxidation products all at once!
Related Publications
Puttu, U.G; Kamp, J. R.; Chen, X.; Chen, J. H.; Li, J.; Gonzalez-Meler, M. A.; Wang, J.; Xu, L.* Chemical ionization mass spectrometry utilizing benzene cations for measurements of volatile organic compounds and nitric oxide. EGUsphere 2025, 2025, 1-29. DOI: 10.5194/egusphere-2025-4103.
NH4+ CIMS
NH4+ CIMS
We developed and characterized a Chemical Ionization Mass Spectrometry (CIMS) using NH4*H2O+ as reagent ion. NH4*H2O+ is a versatile reagent ion for measurements of a wide range of oxygenated organic compounds. The instrumental sensitivities of analytes from the diverse chemical class depend on the binding energy of the analyte-NH4 cluster, which can be estimated using voltage scanning. This offers the possibility to constrain the sensitivity of analytes for which no calibration standards exist.
Related Publications
Xu, L.; Coggon, M. M.; Stockwell, C. E.; Gilman, J. B.; Robinson, M. A.; Breitenlechner, M.; Lamplugh, A.; Crounse, J. D.; Wennberg, P. O.; Neuman, J. A.; Novak, G. A.; Veres, P. R.; Brown, S. S.; Warneke, C., Chemical ionization mass spectrometry utilizing ammonium ions (NH4+ CIMS) for measurements of organic compounds in the atmosphere. Atmos. Meas. Tech. 2022, 15 (24), 7353-7373.
Photo of compact GC
Figure source: Vasquez et al. 2018 AMT
GC-CIMS
GC-CIMS
Our group at Caltech is currently working on the development and characterization of a Gas Chromatography Chemical Ionization Mass Spectrometry (GC-CIMS). Through combining the isomer separation ability of GC and fast response of CIMS, we are able to achieve isomer-resolved quantification of oxygenated volatile organic compounds (OVOCs) in real time. Further, we use low-pressure gas chromatography to address the analyte degradation problem in traditional GC methods, but maintain good performance in peak separation. We use CF3O- as reagent ion for selective measurement of a variety of OVOCs with little fragmentation. Last but not least, our customized GC is compact (130mm×130mm×5mm) and field-deployable. Exciting results from this novel GC-CIMS include, but are not limited to:
Quantify the unimolecular reaction rate of RO2 based on isomer-specific measurements of organic hydroxy nitrates in the oxidation of VOCs.
Reveal the different hydrolysis rate of isoprene hydroxy nitrate isomers and the subsequent impacts on NOx cycle and ozone formation.
Reveal the complex and dynamic behavior of isoprene RO2.
Related Publications:
Xu, L.; Møller, K. H.; Crounse, J. D.; Otkjær, R. V.; Kjaergaard, H. G.; Wennberg, P. O. Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of Α-Pinene and Β-Pinene by Hydroxyl Radicals. The Journal of Physical Chemistry A 2019, 123, 1661-1674 (link).
Vasquez, K. T.; Allen, H. M.; Crounse, J. D.; Praske, E.; Xu, L.; Noelscher, A. C.; Wennberg, P. O., Low-pressure gas chromatography with chemical ionization mass spectrometry for quantification of multifunctional organic compounds in the atmosphere. Atmos. Meas. Tech. 2018, 11 (12), 6815-6832 (link).
Møller, K. H.; Praske, E.; Xu, L.; Crounse, J. D.; Wennberg, P. O.; Kjaergaard, H. G., Stereoselectivity in Atmospheric Autoxidation. The Journal of Physical Chemistry Letters 2019, 6260-6266 (link).
Figure source: Xu et al. 2017 EST
PILS-AMS
PILS-AMS
Water-solubility is an important property of organic compounds. For direct and online characterization of the water-solubility of organic aerosol, I developed a Particle-Into-Liquid-Sampler - Aerosol Mass Spectrometer (PILS-AMS) system. Using this novel system, I measured the water-solubility of OA factors (surrogate for OA sources) and provided improved understanding of the complex OA sources in the atmosphere.
Related Publications:
Xu, L.; Guo, H.; Weber, R. J.; Ng, N. L., Chemical Characterization of Water-Soluble Organic Aerosol in Contrasting Rural and Urban Environments in the Southeastern United States. Environ Sci Technol 2017, 51 (1), 78-88 (link).
Figure source: Xu et al. 2016 ACP
TD-AMS
TD-AMS
The volatility is an important property of organic compounds, which greatly determines the potential to produce organic aerosol and the lifetime of species. However, the measurements of these properties are sparse in the literature. I deployed a Thermal-Denuder - Aerosol Mass Spectrometer (TD-AMS) system to measure the volatility of aerosol in both laboratory chamber experiments and field measurements. Using this instrumentation in the lab, I found that the isoprene SOA has a non-linear dependence on the NOx level. From field measurement, I revealed that OA with different O:C ratios may have similar volatility, suggesting that the O:C ratio is not a good proxy for OA volatility.
Related Publications:
Xu, L.; Kollman, M. S.; Song, C.; Shilling, J. E.; Ng, N. L., Effects of NOx on the Volatility of Secondary Organic Aerosol from Isoprene Photooxidation. Environ Sci Technol 2014, 48, (4), 2253-2262 (link).
Xu, L.; Williams, L. R.; Young, D. E.; Allan, J. D.; Coe, H.; Massoli, P.; Fortner, E.; Chhabra, P.; Herndon, S.; Brooks, W. A.; Jayne, J. T.; Worsnop, D. R.; Aiken, A. C.; Liu, S.; Gorkowski, K.; Dubey, M. K.; Fleming, Z. L.; Visser, S.; Prévôt, A. S. H.; Ng, N. L., Wintertime aerosol chemical composition, volatility, and spatial variability in the greater London area. Atmos. Chem. Phys. 2016, 16, (2), 1139-1160 (link).
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