Space-focusing condition

When reaction products are formed in a finite region, rather than a point source, it is best to extract the ions with a combination of acceleration and drift regions which are consistent with the Wiley-McLaren space focusing condition (Wiley and McLaren, 1955). Specific ratios of acceleration and drift distances for given electric fields are imposed so that the ion TOP is independent of the precise initial position of ionization. This is easily explained as follows. Suppose that two ions (with initially zero kinetic energy) are formed at positions Uj + 8 and tij — 8 as measured from the second acceleration, or drift region. Clearly the ion closer to the drift region will reach this... 

Figure 5.25 Ion TOP distributions for isotropic angular distribution of dissociation products. The mass of the fragment ion is 30 amu, the electric fields in the first two acceleration regions are 10 and 100 V/cm and the acceleration distances are 2 and 1 cm, respectively. The drift distances are indicated in the figure. The d = 55 cm corresponds to the Wiley-McLaren space focusing condition. For each drift distance, the ion TOF was calculated for three product translational energies of 0, 0.5, and 1.0 eV.

Lewis-acid-catalyzed reactions often occur with unique reactivity and selectivity, and the reactions proceed under mild conditions. Many Lewis-acid-mediated reactions are now used not only in laboratories but also in industry. This chapter summarizes these Lewis acid-mediated reactions successfully used in organic synthesis they are organized mainly by the element of the Lewis-acid catalyst. Due to limitations of space, the focus is on more recent publications.1-3... 

There are many excellent texts on nutrition. This chapter, therefore, focuses not on nutrition per se but on how biochemistry helps us understand well established and less well established aspects of nutrition and how such knowledge fits in with other subjects discussed in this text. There is now considerable medical and lay interest in what is meant by healthy and unhealthy diets. Nutrition has become a major issue in the medical sciences and in clinical practice. It is also of concern to politicians, particularly in the link between nutrition and Western diseases such as cardiovascular disease, obesity, cancer and neurological problems. In this chapter an attempt is made to provide a biochemical basis for discussion of nutrition and development of these conditions. To this end, biochemical explanations for nutritional advice and the recommendations from national bodies are provided. Similarly, explanations for the recommendations designed for different populations, different conditions and activities (physical and mental activity, the elderly, the young, during pregnancy and space flight) are discussed. Finally, the biochemistry of malnutrition, undemutrition and ovemutrition is discussed. 

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