Drift field, electrostatic

Rate constants for the first-order dissociation of symmetrical proton-bound dimers, M H + —> MH + + M, have been determined for organophosphorus compounds (M = 2,4-dimethylpyridine (DMP) and dimethyl methylphosphonate (DMMP)), where the shapes of the mobility spectra are of the form shown in Figure 13.2d [44]. Some proton-bound dimers decompose in the time taken for the ions to travel between the shutter and the detector plate, and this residence time was varied by changing the electrostatic drift field strength. Typical ion mobility spectra obtained at different field strengths are shown in Figure 13.5 and peaks were mass identified as first peak, H + (DMP), the protonated monomer and second peak H (DMP)2, the proton-bound dimer. The raised baseline between the peaks was due entirely to (DMP)H +, from the decomposition of the proton-bonnd dimer as in Equation 13.25... 

According to this approximation, the drift velocity is proportional to the square of the electric field. This is a clear indication of the importance of the electric field inside an electrostatic precipitator. Equation (13.60) is a valid approximation for large particles [dp > 0.5 m), provided that particle charge is close to the saturation level. In the case of small particles, the effect of diffusion charging must be taken into account. 

Also in 1950 Sakliarov and Tamm proposed an idea for a controlled thermonuclear fusion reactor, the TOKAMAK (acronym for the Russian phrase for toroidal chamber with magnetic coiF ), which achieved the highest ratio of output power to input power of any fusion device of the twentieth centuiy. This reactor grew out of interest in a controlled nuclear fusion reaction, since 1950. Sakharov first considered electrostatic confinement, but soon came to the idea of magnetic confinement. Tamm joined the effort with his work on particle motion in a magnetic field, including cyclotron motion, drifts, and magnetic surfaces. Sakharov and Tamm realized that... 

The mass separated, pulsed, and focused primary ions with the energy of 1 -25 keV, typically liquid metal ions such as Ga, Cs, and O", are used to bombard the sample surface, causing the secondary elemental or cluster ions to emit from the surface. The secondary ions are then electrostatically accelerated into a field-free drift region with a nominal kinetic energy of ... 

Ion mobility spectrometry (IMS) [3,12] is the most widely used instrument for drug detection. The sample is heated to vaporize the analyte, which is then ionized by atmospheric (ambient) pressure chemical ionization (APCI) [3]. The resulting gas-phase ions travel through a drift tube and are separated by their distinct velocities (mobilities) in a weak electrostatic field. IMS instruments use ambient air or nitrogen as the carrier gas, making it particularly adaptable to field applications. 

If we place an ionic conductor between parallel-plate blocking electrodes that produce an electric field E parallel to the x-axis, the electrostatic potential varies as — xE on passing from one electrode at x = 0 to the other. At equilibrium, the mobile-ion concentration Cj(x) is proportional to exp(qEx/kT), and the ionic drift-current density (7(E in the field is balanced by a diffusion current due to the concentration gradient (Fick s law) ... 

Let us apply the interpolation procedure to a case involving an electric field. It is well known that the efficiency of the granular bed filters can be significantly increased by applying an external electrostatic field across the filter. In this case, fine (<0.5-/rm) particles deposit on the surface of the bed because of Brownian motion as well as because of the electrostatically generated dust particle drift [51], The rate of deposition can be calculated easily for a laminar flow over a sphere in the absence of the electrostatic field [5]. The other limiting case, in which the motion of the particles is exclusively due to the electric field, could also be treated [52], When, however, the two effects act simultaneously, only numerical solutions to the problem could be obtained [51],... 

Since pulsed laser ionization produces well defined packets of ions and electrons, TOF analysers (which essentially are magnetically shielded, electric-field-free drift tubes with apertures and an electron multiplier) can readily be used. TOF resolution for slow electrons can approach 3 meV, and throughput is similar to that of electrostatic analysers operating without an extraction field (i.e. a detection efficiency < 1%). The kinetic energy is obtained from the flight time, which is proportional to the reciprocal velocity, (KE) /2, whereas the resolution varies as (KE)/2. Thus, the resolution for 1-5 eV electrons is comparable to that for electrostatic analysers, but degrades seriously for 5-10 eY electrons. 

In biology, we are typically concerned with the study of electrostatic systems in which electromagnetic interactions are ignored and the electric field is the gradient of the electrostatic potential E = — V. The differential equation for electrokinetic drift follows from Equation (3.45). 

There is, however, another method of deriving the diffusion potential. One takes note of the fact that when a steady-state electroneutrality field has developed, the system relevant to a study of the diffusion potential hangs together in a delicate balance. The diffusion flux is exactly balanced by the electric flux the concentrations and the electrostatic potential throughout the interphase region do not vary with time. (Remember the derivation of the Einstein relation in Section 4.4.) In fact, one may turn a blind eye to the drift and pretend that the whole system is in equilibrium. 

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