Fast atom First-order spectra

In order to obtain the mass spectrum of a compound, it must first be ionized. Early methods involved thermal evaporation of samples into the ion source of the mass spectrometer where they were ionized by electron beam bombardment or chemical reactions. Thermal instability, where present, was frequently overcome by derivatization. For large and involatile molecules, desorption techniques using high electric fields or bombardment with fission fragments, fast atoms, fast ions, or laser beams were later developed. The newest techniques, developed in the late 1980s, involve laser ionization of samples imbedded in a solid matrix and evaporation of solutions by electrospray. Both of these techniques are able to ionize large molecules such as proteins with masses of up to several hundred ki-lodaltons. This article describes these techniques in more detail and indicates the type of mass spectrometer that is suitable for analysis of the types of ion that they produce. 

In order to study the ability of the (phenylthiomethyl)silanes 1 to chelate metal complexes and the associated stereochemical problems we first prepared simple model compounds using diphenylbis[(phenylthio)methyl]silane Ic. On reaction of Ic with [PtCl2(PhCN)2] the very stable square planar chelate complex 3 was isolated. NMR investigations at variable temperature show that meso- and DL-isomers coexist in a temperature-dependent equilibrium due to a facile inversion process at the sulfur atoms [2] in solution. At higher temperature (325 K) the pyramidal inversion is sufficiently fast that only one "averaged planar" conformation is observed in the H NMR spectrum... 

Therefore, the problems which faced the would-be designers of chain reactors early in 1941 were (1) the choice of the proper moderator to uranium ratio, and (2) the size and shape of the uranium lumps which would most likely lead to a self-sustaining chain reaction, i.e., give the highest multiplication factor. In order to solve these problems, one had to understand the behavior of the fast, of the resonance, and of the thermal neutrons. We were concerned with the second problem which itself consisted of two parts. The first was the measurement of the characteristics of the resonance lines of isolated uranium atoms, the second, the composite effect of this absorption on the neutron spectrum and total resulting absorption. One can liken the first task to the measurement of atomic constants, such as molecular diameter, the second one, to the task of kinetic gas theory which obtains the viscosity and other properties of the gas from the properties of the molecules. The first task was largely accomplished by Anderson and was fully available to us when we did our work. Anderson s and Fermi s work on the absorption of uranium, and on neutron absorption in general, also acquainted us with a number of technics which will be mentioned in the third and fourth of the reports of this series. Finally, Fermi, Anderson, and Zinn carried out, in collaboration with us in Princeton, one measurement of the resonance absorption. This will be discussed in the third article of this series. 

---