High mass accelerator

After acceleration through an electric field, ions pass (drift) along a straight length of analyzer under vacuum and reach a detector after a time that depends on the square root of their m/z values. The mass spectrum is a record of the abundances of ions and the times (converted to m/z) they have taken to traverse the analyzer. TOP mass spectrometry is valuable for its fast response time, especially for substances of high mass that have been ionized or selected in pulses. 

The mass range requirement invariably means that FAB is used in conjunction with a magnetic sector instrument. Conventional detectors, such as the electron multiplier, are not efficient for the detection of large ions and the necessary sensitivity is often only obtained when devices such as the post-acceleration detector or array detector are used. Instruments capable of carrying out high-mass investigations on a routine basis are therefore costly and beyond the reach of many laboratories. 

Using a homogeneous model proposed by Owens (1961) for low void fractions (a < 0.30) and high mass flux, as is usually encountered in a water-cooled reactor, the momentum change (or acceleration) pressure gradient term is obtained from... 

It should be noted that the acceleration component is dominant in the last part of the pipe, where, because of the rapid pressure drop and the low absolute pressure, the specific volume of the gas increases sharply. This effect is more pronounced at high mass flow rates with large values of mass flow ratio, 3 (= mjm,). As shown in Figures 3.44a and 3.446, the average friction coefficient is affected by the mixture mass flow rate m, the mass flow ratio 3, and the diameter of the pipe D. The Re is defined as... 

Discrimination of slower ions as compared to faster ions is observed with SEMs, [243] CEMs, and MCPs as well. [239,244,245] This means a reduction in sensitivity upon reduction of the acceleration voltage of a mass spectrometer, and of course, with increasing ion mass (Eq. 4.3). In order to reduce such effects and especially to improve sensitivity for high-mass ions, post-acceleration detectors have been developed. [236,245] In post-acceleration detectors the ions are accelerated immediatly in front of the detector by a voltage of 10-30 kV before they hit the first dynode or the first MCP. 

The essential independence of mean ion velocities on the molecular weight of the analyte leads to an approximate linear increase of the mean initial kinetic energies of the analyte ions with mass. High-mass ions therefore carry tens of elec-tronvolts of translational energy before ion acceleration. [33,41,50] The initial velocity of the ions is superimposed onto that obtained from ion acceleration, thereby causing considerable losses in resolution with continuous extraction TOP analyzers, in particular when operated in the linear mode. 

Currently used magnetic analyser mass spectrometers constitute a logical evolution of the previously described instrument. They provide very accurate m/z values, but are limited for high mass analyses (problems produced in the magnetic sector). They also include an electrostatic sector E placed after ion acceleration and before the magnetic field, B (Fig. 16.3). 

The use of solid electrolytes in batteries and fuel cells is another important application. Examples are zirconia based fuel cells and sulphur batteries with Na-/ -A1203 as electrolyte. Many other interesting and practical aspects of solid electrolytes are worth mentioning, for example, the possibility to detect stresses, to build up high pressures, or to monitor mass accelerations. Also, solid electrolytes have recently been used to investigate the interface kinetics in crystals (Section 10.4.2). 

AMS directly measures the number of 14C atoms, and the ratio of 14C to 13C and/ or 12C, using a high-energy accelerator as an inlet to a mass spectrometer. The key characteristics of 14C-AMS are the electron stripping and ion acceleration, which allow 14C to be distinguished from isobars and molecules that would confuse a standard mass spectrometer. AMS requires only a fairly small sample of lOOpg to 1 mg of C. In addition, the measurement only takes minutes per sample. 

Upper limits on neutrino masses come from laboratory experiments, such as tritium decay and high-energy accelerator experiments, and are (see Review of Particle Physics, Hagiwara et al.(2002))... 

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