Ion-mobility

Ion mobility spectroscopy is based on the migration of a charged chemical species through a weak electric field in a gas phase. The mobility is calculated from the ion velocity within the drift tube of the ion mobility spectrometer. The rapid analysis time and portability of IMS make it a popular 

In similar work, Rearden and Harrington [9] demonstrated the use of SPME-IMS for the analysis of alkyl phosphonic acid degradation products in soil samples. Detection limits of 

10 Xgg were achieved with an analysis time of 30 min [9]. More recently, Kolakowski et al. 128] demonstrated the use of atmospheric pressure ionization-high field asymmetric waveform ion mobility mass spectrometry for the analysis of nerve agent degradation products in food samples. 

Positive and negative ions in liquid xenon were produced by positive charge transfer and electron attachment, respectively (Hilt, 1995 Hilt et al., 1994). Positive 

Negative ions of oxygen and sulfur hexafluoride were produced by electron attachment. 

In many earlier mobility studies, liquid samples were employed which still contained traces of water. Recently, it was unambiguously demonstrated that ions interact with water molecules through the formation of cluster ions (Balakin et al., 1993). The mobility of negative oxygen ions, O2, in tetramethylsilane was found to decrease from 1.85 to 1.410 cm V- s in the range of concentrations from 0 to 2 x lO M of water (see also Section 3.8.2). 

According to the transition state theory [337] the prefactor is given by kT/h. When we set T = D we find the direct connection with the discussion above. For d — 500K we get 10 /s. 

Let us now consider, in what follows, the three elementary jump mechanisms [343] In the case of the vacancy mechanism in Fig. 6.4a a regular particle (Aa) hops into the vacancy of effective charge number Zv which is of opposite sign to the ionic charge of particle A, and leaves behind a vacant position (V )  

This mechanism operates stochastically in the absence of correlations and external driving forces. If the vacancy, for instance, is effectively positively charged and an external field is applied, then the vacancy migrates to the side of the negative pole (opposite to the direction of A-migration). This mass transport can be described very much more simply from the point of view of the vacancy defect, rather than by considering the actual trajectories of the substantial A-particles involved. 

Two mechanisms are of importance in the case of the migration of an interstitial particle (i) The defect (charge number Zj) jumps directly from one interstitial site to the next (Fig. 6.4b) 

It is expected that, in the case of large ions in close-packed structures, the second (interstitialcy or indirect interstitial) mechanism is associated with a lower transition barrier compared to the direct interstitial jnechanism which involves pushing aside neighboirring A particles (see Fig. 6.4). We can see that the electrochemical information is then transmitted from particle to particle, and a single substantial particle under consideration does not move any great distance, just as in the vacancy mechanism . 

In its simplest form, the drift-tube ion-mobility system measures how fast a given ion moves in a uniform electric field through a given atmosphere. Thus, an ion-mobility system separates ions by shape and charge. The flow drift technique can be apphed to determine quantities like ion mobility and diffusion coefficient, as these ate functions of the nonreactive attractive and repulsive ion-neutral interactions [129, 130]. Ion mobilities have been measured for a wide range of ions in several buffer gases (He, N2, Ar). With Ar as buffer gas, the mobility can be predicted with reasonable accuracy, whereas the measured mobility in He shows poor agreement with theoretical predictions [131]. 

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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]. 

McFarland M, Albritton D L, Fehsenfeld F C, Ferguson E E and Schmeltekopf A L 1973 Flow-drift technique for ion mobility and ion-molecule reaction rate constant measurements. I. Apparatus and mobility measurements J. Chem. Phys. 59 6610-19... 

Conductometric Analysis Solutions of elec trolytes in ionizing solvents (e.g., water) conduct current when an electrical potential is applied across electrodes immersed in the solution. Conductance is a function of ion concentration, ionic charge, and ion mobility. Conductance measurements are ideally suited tor measurement of the concentration of a single strong elec trolyte in dilute solutions. At higher concentrations, conduc tance becomes a complex, nonlinear func tion of concentration requiring suitable calibration for quantitative measurements. 

Applied voltage Gos density Ion mobility Oust thickness Sectionolizotion Collection oreo Wire rodius Collector rodius... 

The quotient is called the electrochemical mobility and is tabulated along with ion mobilities. It is important to pay attention to the units because of possible confusion. Values of /, are given in Table 2-2. Raising the temperature usually increases ion mobility, while increasing the concentration reduces the conductivity due to interactions ... 

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...

The random thermal motion or the diffusion of gas ions is characterized by the diffusion coefficient D, which is related to the ion mobility Z. by... 

For iron in most oxidising environments, the PBR is approximately 2.2 and the scale formed is protective. The oxidation reaction forms a compact, adherent scale, the inner and outer surfaces of which are in thermodynamic equilibrium with the metal substrate and the environment respectively, and ion mobility through the scale is diffusion controlled. 

According to the nine assumptions and approach a) for the diffusion potential inside the membrane the selectivity coefficient Kg , can be expressed by other parameters. Table 3 shows the results for the different kinds of membranes 66). In some cases the expressions for K J i contain ion-mobilities inside the membrane... 

Finally some assumptions could not be verified as, e.g., the complete co-ion exclusion necessary for the treatment of the phase boundary potential as a Donnan potential, or the constant ion mobility through glass membranes with hydrated layers76). 

The explicit mathematical treatment for such stationary-state situations at certain ion-selective membranes was performed by Iljuschenko and Mirkin 106). As the publication is in Russian and in a not widely distributed journal, their work will be cited in the appendix. The authors obtain an equation (s. (34) on page 28) similar to the one developed by Eisenman et al. 6) for glass membranes using the three-segment potential approach. However, the mobilities used in the stationary-state treatment are those which describe the ion migration in an electric field through a diffusion layer at the phase boundary. A diffusion process through the entire membrane with constant ion mobilities does not have to be assumed. The non-Nernstian behavior of extremely thin layers (i.e., ISFET) can therefore also be described, as well as the role of an electron transfer at solid-state membranes. 

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. 

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... 

Note that a number of complicating factors have been left out for clarity For instance, in the EMF equation, activities instead of concentrations should be used. Activities are related to concentrations by a multiplicative activity coefficient that itself is sensitive to the concentrations of all ions in the solution. The reference electrode necessary to close the circuit also generates a (diffusion) potential that is a complex function of activities and ion mobilities. Furthermore, the slope S of the electrode function is an experimentally determined parameter subject to error. The essential point, though, is that the DVM-clipped voltages appear in the exponent and that cheap equipment extracts a heavy price in terms of accuracy and precision (viz. quantization noise such an instrument typically displays the result in a 1 mV, 0.1 mV, 0.01 mV, or 0.001 mV format a two-decimal instrument clips a 345.678. .. mV result to 345.67 mV, that is it does not round up ... 78 to ... 8 ). 

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. 

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... 

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