Over recent years, Ion Mobility–Mass Spectrometry (IMS–MS) measurements have become a widely used tool in a number of disciplines of scientific relevance, including, in particular, the structural characterization of mass-selected biomolecules such as proteins, peptides, or lipids, brought into the gas-phase using a variety of ionization methods. In these structural studies, the measured electrical mobilities are customarily interpreted in terms of a collision cross-section, based on the classic kinetic theory of ion mobility. For ideal ions interacting as smooth, rigid-elastic hard-spheres with also-spherical gas molecules, this collision cross-section (CCS) is identical to the true, geometric cross section. On the other hand, for real ions with non-perfectly spherical geometries and atomically-rough surfaces, subject to long-range interactions with the gas molecules, the expression for the CCS can become fairly intricate.
This complexity has frequently led to the use of helium as the drift gas of choice for structural studies, given its small size and mass, its low polarizability (minimizing long-range interactions), and its sphericity and lack of internal degrees of freedom, all of which contribute to reduce departures between measured and true cross-sections. Recently, however, a growing interest has arisen for using moderately-polarizable gases such as air, nitrogen, or carbon dioxide (among others) in these structural studies, due to a number of advantages they present over helium, including their higher breakdown voltages (allowing for higher instrument resolutions) and better pumping characteristics. This shift has, nevertheless, remained objectionable in the eye of those seeking to infer accurate structural information from ion mobility measurements and, accordingly, there is a critical need to study whether or not measurements carried out in such gases may be corrected for the finite size of the gas molecules and their long-range interactions with the ions, in order to provide cross-sections truly representative of ion geometry. A first step to address this matter is undertaken here for the special case of nearly-spherical, nanometer-sized ions.
In order to attain this goal, we have performed careful and accurate IMS–MS measurements of hundreds of electrospray-generated nanodrops of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF 4), in a variety of drift gases (air, CO2, and argon), covering a wide range of temperatures (20-100 °C, for both air and CO2), and considering nanodrops of both positive and negative polarity (the latter in room-temperature air only). Thanks to the combined measurement of the mass and mobility of these nanodrops, we are able to simultaneously determine a mobility-based collision cross-section and a mass-based diameter (taking into account the finite compressibility of the IL matter) for each of them, which then allows us to establish a comparison between the two.
Over the entire range of experimental conditions investigated, our measurements show that the electrical mobilities of these nearly-spherical, multiply-charged IL nanodrops are accurately described by an adapted version of the well-known Stokes—Millikan (SM) law for the mobility of spherical ions, with the nanodrop diameter augmented by an effective gas-molecule collision diameter, and including a correction factor to account for the effect of ion—induced dipole (polarization) interactions, which result in the mobility decreasing linearly with the ratio between the polarization and thermal energies of the ion–neutral system at contact. The availability of this empirically-validated relation enables us, in turn, to determine true, geometric cross-sections for globular ions from IMS—MS measurements performed in gases other than helium, including molecular or atomic gases with moderate polarizabilities. In addition, the observed dependence of the experimentally-determined values for the effective gas-molecule collision diameter and the parameters involved in the polarization correction on drift-gas nature, temperature, and nanodrop polarity, is further evaluated in the light of the results of numerical calculations of the electrical mobilities, in the free-molecule regime, of spherical ions subject to different types of scattering with the gas molecules and interacting with the latter under an ion–induced dipole potential. Among the number of findings derived from this analysis, a particularly notable one is that nanodrop–neutral scattering seems to be of a diffuse (cf. elastic and specular) character in all the scenarios investigated, including the case of the monatomic argon, which therefore suggests that the atomic-level surface roughness of our nanodrops and/or the proximity between their internal degrees of freedom, rather than the sphericity (or lack of it) and the absence (or presence) of internal degrees of freedom in the gas molecules, are what chiefly determine the nature of the scattering process.
|Advisor:||Mora, Juan Fernandez de la|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 76/11(E), Dissertation Abstracts International|
|Subjects:||Analytical chemistry, Physical chemistry, Engineering|
|Keywords:||Ion mobility, Ion-induced dipole interactions, Ionic liquid, Mass spectrometry, Molecule drift gas, Nanodrop|
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