Ion mobility studies

Bushnell and co-workers [117] employed extensive molecular modelling to understand the nature of cis and trans isomerism in tetrahedral p-phenylene vinylene oligomers, and to aid the interpretation of time of flight mass spectrometry and ion mobility studies. Molecules such as T4R, shown in Figure 18, with four equivalent arms can be used to control the crystallinity in thin films. The authors reported the observation of a species in the mass spectrum resulting from the loss of an arm from the central carbon. This species will be referred to as P4R. 

May, J.C. Russell, D.H., A mass-selective variable-temperature drift tube ion mobility-mass spectrometer for temperature dependent ion mobility studies, J. Am. Soc. Mass Spectrom. 2011, 22(7),1134-1145. 

In spite of the advantages cited above, ion mobility spectrometers operating at atmospheric pressure have been used infrequently to obtain physical chemical data, kinetic and thermodynamic, in the study of ion/molecule chemistry. In this chapter, an overview is given on the type of information obtainable from ion mobility studies at atmospheric pressure and the variety of experimental methods employed in such studies. The data obtained under weU-defined conditions agree favorably with those from other more frequently used methods, for example (i) pulsed high pressure mass spectrometry (PHPMS), which is operated at well-defined temperatures but at pressures ca 200 times lower than IMS and (ii) FT-ICR and ion trap mass spectrometers, which are operated under vacuum. 

The drift velocity is a function of the parameter E/N, which is a frequently used parameter in ion mobility studies and is expressed in Townsend (1 Td = 10 Vcm ). De Gouw et al. showed that this calculation is in excellent agreement with experiment [6]. The reaction time f = L/Ud is around 110 ps, at 120 Td for a drift tube length of L = 10 cm. 

In summary, the synergy of experiment and theory extends from ion mobility studies to equilibrium studies covered here and provides important insight into energetic and structural aspects of fully desolvated, partially solvated, and—together with other methods—fully solvated biomolecules. 

PB A thanks Drs. Chad Rue and Chris Iceman for their substantive contributions to our ion mobility studies, which have been funded by the National Science Foundation. 

Creaser, C. S. Griffiths, J. R. Stockton, B. M., Gas-phase ion mobility studies of amines and polyether/amine complexes using tandem quadrupole ion trap/ion mobility spectrometry , European Journal of Mass Spectrometry 2000, 6, 213-218. 

An Arrhenius form was assumed for the rates, with activation energies given by a nonlinear function of the reaction free energy, the only aspect of the model specific for ion type. Activation energies are substantial for free energies below 60-90 kJ/mol. The collisional prefactor introduces a dependence on the analyte size and, hence, molecular weight. Effective molecular radii can be estimated from ion mobility studies. In this form the secondary reaction model has no adjustable parameters beyond those necessary for primary ionization. 

Most ion-molecule techniques study reactivity at pressures below 1000 Pa however, several techniques now exist for studying reactions above this pressure range. These include time-resolved, atmospheric-pressure, mass spectrometry optical spectroscopy in a pulsed discharge ion-mobility spectrometry [108] and the turbulent flow reactor [109]. 

The above methods measure ion transport rates as ionic conductivities. By varying the parameters of the experiment, it is often possible to indirectly identify the mobile ion(s),173 and in some cases to estimate individual ion mobilities or diffusion coefficients.144 Because of the uncertainty in identifying and quantifying mobile ions in this way, EQCM studies that provide the (net) mass change accompanying an electrochemical process36 have played an important complementary role. 

We have performed ionic mobility studies on mica and in alkali halide surfaces. Here we shall describe some results obtained on mica with different surface ions. Alkali halides will be discussed in detail in the next section. 

Principles and Characteristics Ion mobility spectrometry (IMS) is an instrumental technique for the detection and characterisation of organic compounds as vapours at atmospheric pressure. Modern analytical IMS was created at the end of the 1960s from studies on ion-molecule chemistry with mass spectrometers and from ionisation detectors for vapour monitoring. An ion mobility spectrometer (or plasma chromatograph in the original termininology) was first produced in 1970 [272],... 

In a combined MALDI-ToF MS, ion mobility experiment and molecular modelling study, Baker and co-workers [120] studied the sequencing in copolymers of glycidyl methacrylate and butyl methacrylate, Figure 20, with different termination groups. 

Enhancement of Ion Mobilities in Liquid Silicates from Computer-Simulation Studies to 800-Kilobars. 

ESR spectroscopy can be used with adsorbed paramagnetic ions to study the liquid associated with mineral surfaces. Cu(II) and Mn(II) have been used in his type of investigation, although difficulties are encountered with observing a resonance from Mn(II) in distorted environments. Measurements of Cu(II) on silica at room temperature and above have shown that adsorbed water behaves in the same manner as bulk water, but at lower temperatures it experiences a decreased mobility (61). On freezing two types of water are found one which is freezable and undergoes crystallization and the other which is unfreezable, in which the ice structure cannot be formed because of the surface interaction. NMR, IR and differential thermal... 

Earlier, Gavach et al. studied the superselectivity of Nafion 125 sulfonate membranes in contact with aqueous NaCl solutions using the methods of zero-current membrane potential, electrolyte desorption kinetics into pure water, co-ion and counterion selfdiffusion fluxes, co-ion fluxes under a constant current, and membrane electrical conductance. Superselectivity refers to a condition where anion transport is very small relative to cation transport. The exclusion of the anions in these systems is much greater than that as predicted by simple Donnan equilibrium theory that involves the equality of chemical potentials of cations and anions across the membrane—electrolyte interface as well as the principle of electroneutrality. The results showed the importance of membrane swelling there is a loss of superselectivity, in that there is a decrease in the counterion/co-ion mobility, with greater swelling. 

We have recently explored the use of an ion mobility spectrometer (IMS) for the study of negative ioinnolecule reactions at atmospheric pressure. This instrument, shown in Figure 6, consists of three major components. They are an ion mobility spectrometer, a mass spectrometer, and a gas-handling plant (GHP). 

Figure 14. A pulsed electron beam ion mobility spectrometer for the study of CPIC over the pressure range of 0.01 to 1.0 atm.

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