We extensively use differential mobility analysis-mass spectrometry (a type of ion mobility spectrometry-mass spectrometry pioneered by Prof. Juan Fernandez de la Mora of Yale University) to examine cluster ions, macromolecules, and proteins in the gas phase. Working at atmospheric pressure, the differential mobility analyzer (DMA) separates clusters by electrical mobility when a ramping voltage is applied. Our planar DMA, from SEADM, has a high resolution near 50, enabling cluster separation differing by as small as several atoms (such as Na+(NaCl) and Na+(NaCl)2). Because electrical mobility is proportional to collision cross section, mobility analysis with a DMA also provides information about cluster structure. For example, isomers that have the same mass but different structures may take different positions in a mobility spectrum.
A Qstar XL mass spectrometer is coupled downstream to our planar DMA, constituting a IMS-MS (ion mobility spectrometry-mass spectrometry) system. In this setup, electrical mobility and mass-to-charge ratio of charged clusters are measured in a tandem fashion. The outcome of such a measurement is 2-D spectrum with cluster mobility on one axis and mass-to-charge ratio on the other. Separation in 2 dimensions provides more information about cluster composition, structure and even reactivity in gas phase. For example, unstable clusters that dissociate in the mass spectrometer can usually preserve their identity in the DMA, therefore appearing at a different mobility from the fragments.
So far, we have used the DMA-MS to characterize ionic liquids, salt clusters, sulfuric acid clusters and ions generated by radioactive sources. A large portion of are ongoing studies are devoted to examining vapor uptake at the molecular scale by ions. We have developed a theoretical framework for analyzing such uptake experiments which relates the change in ion apparent mobility/collision cross section as a function of saturation ratio to the equilibrium binding coefficients of individual vapor molecules. We have used this theory to examine the validity of the Kelvin-Thomson model for vapor uptake by ions, as well as the extent to which Langmuir adsorption models can describe uptake at the nanoscale.
Maisser A. & Hogan C. J. (2017) Examination of Organic Vapor Adsorption onto Alkali Metal and Halide Atomic Ions via Ion Mobility-Mass Spectrometry. ChemPhysChem. In Press. 10.1002/cphc.201700747
Li C. & Hogan C. J. (2017) Vapor Specific Extents of Uptake by Nanometer Scale Charged Particles. Aerosol Sci. Technol. 51: 653–664. 10.1080/02786826.2017.1288285
Thomas J. M., He S., Larriba-Andaluz C., DePalma J. W., Johnston M. V. & Hogan C. J. (2016) Ion Mobility Spectrometry-Mass Spectrometry Examination of the Structures, Stabilities, and Extents of Hydration of Dimethylamine-Sulfuric Acid Clusters. Phys Chem Chem Phys. 18: 22962 – 22972. 10.1039/c6cp03432b
Oberreit D. R., Rawat V. K., Ouyang H., Larriba-Andaluz C., McMurry P. H., & Hogan C. J. (2015) Analysis of Heterogeneous Water Vapor Uptake by Metal Iodide Cluster Ions via Differential Mobility Analysis-Mass Spectrometry. The Journal of Chemical Physics. 143, 104204. 10.1063/1.4930278
Maisser A., Thomas J. M., Larriba-Andaluz C., He S., & Hogan C. J. (2015) The mass-mobility distributions of ions produced by a Po-210 source in air. J Aerosol Sci. 90: 36-50. 10.1016/j.jaerosci.2015.08.004
Rawat V. K., Vidal-de-Miguel G., Hogan C. J. (2015) Modeling Vapor Uptake Induced Mobility Shifts in Peptide Ions Observed with Transversal Modulation Ion Mobility Spectrometry-Mass Spectrometry. Analyst. 140, 6945–6954. 10.1039/C5AN00753D
Ouyang H.., He S., Larriba-Andaluz C., & Hogan C. J. (2015). IMS-MS and IMS-IMS Investigation of the Structure and Stability of Dimethylamine-Sulfuric acid Nanoclusters. The Journal of Physical Chemistry A. 119, 2026−2036. 10.1021/jp512645g.
Ouyang H., Larriba-Andaluz C., Oberreit D. R., & Hogan C. J. (2013). The Collision Cross Sections of Iodide Salt Cluster Ions in Air via Differential Mobility Analysis-Mass Spectrometry. Journal of the American Society for Mass Spectrometry. 24:1833-1847. 10.1007/s13361-013-0724-8
Hogan C. J., Ruotolo B. T., Robinson C. V., & Fernandez de la Mora J. (2011). Tandem Differential Mobility Analysis-Mass Spectrometry Reveals Partial Collapse of the GroEL Complex. The Journal of Physical Chemistry B. 115:3614-3621. 10.1021/jp109172k
Hogan C. J. & Fernandez de la Mora J. (2011). Ion Mobility Measurement of Non-Denatured 12 – 150 kDa Proteins and Protein Multimers by Tandem Differential Mobility Analysis – Mass Spectrometry (DMA-MS). J. Am. Soc. Mass Spectrom. 22:158-172. 10.1007/s13361-010-0014-7
Larriba C., Hogan C. J., Attoui M., Fernandez-Garcia J., Barrajo R., & Fernandez de la Mora J. (2011). The Mobility-Volume Relationship Below 3.0 nm Examined by Tandem Mobility-Mass Measurement. Aerosol Science and Technology. 45:453-467. 10.1080/02786826.2010.546820
Hogan C. J., Loo J. A., Loo R. R. O., & Fernandez de la Mora J. (2010). Ion Mobility-Mass Spectrometry of Phosphorylase B Ions Generated with Supercharging Reagents but in Charge-Reducing Buffer. Phys. Chem. Chem. Phys. 12:13476-13483. 10.1039/c0cp01208d
Hogan C. J. & Fernandez de la Mora J. (2010). Ion-Pair Evaporation from Ionic Liquid Clusters. J. Am. Soc. Mass Spectrom. 21:1382-1386. 10.1016/j.jasms.2010.03.044
Hogan C. J. & Fernandez de la Mora J. (2009). Tandem Ion Mobility-Mass Spectrometry (IMS-MS) Study of Ion Evaporation from Ionic Liquid-Acetonitrile Nanodrops. Phys. Chem. Chem. Phys. 11:8079-8090. 10.1039/b904022f