Ion mobility calculations

Table 2-2 Ion mobilities 1. in S cm mol for calculating specific conductivity with Eq. (2-12) between 10 and 25°C, conductivity increases between 2 and 3% per °C...

Wong MW (2003) Quantum-Chemical Calculations of Sulfur-Rich Compounds. 231 1-29 Wrodnigg TM, Eder B (2001) The Amadori and Heyns Rearrangements Landmarks in the History of Carbohydrate Chemistry or Unrecognized Synthetic Opportunities 215 115-175 Wyttenbach T, Bowers MT (2003) Gas-Phase Confirmations The Ion Mobility/Ion Chromatography Method. 225 201-226... 

Gilb, S., Weis, P., Furche, F., Ahlrichs, R. and Kappes, M.M. (2002) Structures of small gold cluster cations (Au u< 14) Ion mobility measurements versus density functional calculations. Journal of Chemical Physics, 116, 4094—4101. 

Diffusion of ions can be observed in multicomponent systems where concentration gradients can arise. In individnal melts, self-diffnsion of ions can be studied with the aid of radiotracers. Whereas the mobilities of ions are lower in melts, the diffusion coefficients are of the same order of magnitude as in aqueous solutions (i.e., about 10 cmVs). Thus, for melts the Nemst relation (4.6) is not applicable. This can be explained in terms of an appreciable contribntion of ion pairs to diffusional transport since these pairs are nncharged, they do not carry cnrrent, so that values of ionic mobility calculated from diffusion coefficients will be high. 

The ion mobility coefficients pj are calculated similarly. First, the ion mobility of ion j in background neutral i is calculated using the low- -field Langevin mobility expression [219]. Then Blanc s law is used to calculate the ion mobility in... 

Approximate single ion mobilities may be calculated by assuming that the cation and anion mobilities of a selected electrolyte are the same and equal to... 

The ion hopping rate is an apparently simple parameter with a clear physical significance. It is the number of hops per second that an ion makes, on average. As an example of the use of hopping rates, measurements on Na )3-alumina indicate that many, if not all the Na" ions can move and at rates that vary enormously with temperature, from, for example, 10 jumps per second at liquid nitrogen temperatures to 10 ° jumps per second at room temperature. Mobilities of ions may be calculated from Eqn (2.1) provided the number of carriers is known, but it is not possible to measure ion mobilities directly. 

The p0 dependence of oxygen nonstoichiometry (8) was determined by using coulometric titration. The data were analyzed using a simple point defect model and thermodynamic quantities were calculated. From this model, the standard enthalpy for oxidation (AH0f) and disproportionation (A77D) were determined to be -140.7 and 228.7 kJ/mol, respectively. The mobilities of the electron holes, electrons, and oxygen ions were calculated from the conductivity data using the defect concentrations determined from the stoichiometry and point defect model. 

Since the theory of operation is similar, with some subtle difference, for both instruments the absolute sensitivity (mass detected) for both instruments should be similar. The critical attribute where the two instruments vary, which drives the utility for swab determinations at extremely low levels is in sample introduction. To reach the submi-crogram/swab regime, direct swab analysis may be required. Recall the acceptance limits calculated in Table 15.2. Using the assumptions in Table 15.1, a 1 mg tablet will drive the acceptance limit to 2.0 xg/swab, a 0.1 mg tablet will drive the acceptance limit to 0.2 xg/swab and a 0.01 xg tablet will drive the acceptance limit to 0.02 p,g/ swab. With direct swab analysis, ion mobility should be able to attain the required sensitivity for most compounds with this limit as the absolute amount on the swab would be 20 ng. However, if a typical dilution is required and deposition of a small aliquot... 

We have shown that combining ion mobility spectrometry (IMS) equipment with mass spectrometry (MS) provides a powerful tool to examine the three-dimensional structure of polyatomic ions by measuring collision cross sections of mass identified ions. The technique is particularly useful in conjunction with molecular modeling or electronic structure calculations. Further, we have reviewed applications where the IMS-MS equipment is used to obtain kinetic and thermo chemical data of ions. 

In systems where ion-pair formation is possible, the mobility calculated from the diffusion coefficient =D/kT is not equal to the mobility calculated from the equivalent conductivity u yZieo = (A/ZjeQ)F and therefore the Nernst-Einstein equation, which is based on equating these two mobilities, may not be completely valid. In practice, one finds a degree of nonapplicability of up to 25%. 

Ion mobility is based on the measurement of the amount of time it takes for an ion to drift through a buffer gas under the influence of a weak electric field. This drift time inherently contains information about the conformation of the ion. Differently shaped ions have various collision cross sections and hence different mobilities (and drift times) when drifting through the gas. Thus, various computational methods are then used to generate model structures of the ions and calculate their cross sections for comparison to experiment. For instance. X-ray crystallography and NMR spectroscopy are usually used to obtain structural data on POSS molecules. However, POSS-polymer systems can be difficult to examine with these methods since synthetic polymers exist as a mixture of chain lengths data can thus only be obtained for the entire polymer distribution as a collective using these methods. In this respect, detailed information about how POSS interacts with one particular... 

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