Collision frequency, common gases

Coefficient of Friction, 15-47 to 48 Coercivity, magnetic materials, 12-105 to 113 Collision diameter of gases, 6-37 Collision frequency, common gases, 6-37 Combustion, heat of fuels, 5-71... 

For polyatomic gases in porous media, however, the relaxation rate commonly decreases as the pore size decreases [18-19]. Given that the relaxation mechanism is entirely different, this result is not surprising. If collision frequency determines the Ti, then in pores whose dimensions are in the order of the typical mean free path of a gas, the additional gas-wall collisions should drastically alter the T,. For typical laboratory conditions, an increase in pressure (or collision frequency) causes a proportional lengthening of T1 so the change in T, from additional wall collisions should be a good measure of pore size. 

The Maxwellian representation of the collision frequency of one molecule of type mi with similar molecules (2.173) is commonly rewritten using the ideal gas law ... 

The triple quadrupole mass spectrometer consists of a linear combination of three quadrupole mass analyzers. Figure 9.15 [2,56,111]. Only the first and third quadrupole have scan capability. The middle quadrupole, which is sometimes a multipole device of higher order, is a radio frequency-only, gas-filled collision cell with ion focusing properties. All ions above a certain threshold m/z value are transmitted by the middle quadrupole. An offset voltage between the ion source and collision cell can be adjusted to allow the collision energy to be varied between zero and about 200 eV. Inert gases (e.g. He, Ar, Xe) at a pressure of about 2-4 mTorr are commonly used as targets to... 

The quadrupole MS detector was the first, and is still the most common, detector used for LC/MS, but a number of other mass spectrometers have been adapted to this application. Both three-dimensional spherical (ITD) and linear (LIT) ion trap detectors offer tremendous potential for general, inexpensive LC/MS systems. They both offer the ability to be used as either a mass spectral detector or as a MS/MS detector. The 3D ITD (Fig. 15.5) allows ions to be trapped in the ion trap where they can be fragmented by heavy gas collision and the fragments released by scanning the dc/RF frequency of the trap. 

Collisions between two species (a bimolecular collision) are a far more common occurrence than the simultaneous collision of three (trimolecular) or more species. For example, in air at ordinary laboratory conditions, where less than 0.1 percent of the gas volume is occupied by gas molecules, one molecule hits another (bimolecular collision) approximately 10 times per second. Three molecules collide simultaneously (trimolecular collision) at a rate of about 10 times per second." We can reason that on the basis of the much greater frequency of bimolecular collisions, it is more likely that these are usually responsible for chemical reactions. 

Because fluorescence detection by repeated absorption-emission cycles is not applicable to trapped molecular ions in UHV, that is, in the absence of collisions with a buffer gas [68], different techniques are required for their reliable identification. A commonly used destructive technique for molecular ions is time-of-flight (TOP) mass spectroscopy. We have used a simplified variant in the Ba+ apparatus. The trapped ions are extracted from the trap by reducing the radio-frequency amplitude, in the presence of a finite dc quadrupole potential Vo, which causes the ion trajectories to become unstable (the Mathieu -parameter enters the instability region). Heavy and hot ions escape first. Upon leaving the trap, the ions are guided to and attracted by the cathode of a channel electron multiplier (CEM) and counted. 

Tandem-in-Space Spectrometers. In tandem-in-space instruments, two independent mass analyzers are used in two different regions in space. The triple quadrupole mass spectrometer is the most common of these instruments. In commercial triple quadrupole instruments, such as the instrument illustrated in Figure 20-2,3, the sample is introduced into a soft ionization source, such as a Cl or FAB source. The ions are then accelerated into quadrupole 1 (Q), which is an ordinary quadrupole mass filter. The selected fast-moving ions pass into quadrupole 2 (q), which is a collision chamber where dissociation of the ions selected by quadrupole 1 occurs. This quadrupole is operated in a radio-frequency-only mode in which no dc voltage is applied across the rods. This mode basically traps the precursor and product ions in a relatively high concentration of collision gas so that CAD can occur. Quadrupole 3 (Q) then allows mass analysis of the product ions formed in the collision cell. The configuration is known as the QqQ configuration. 

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