R.S. Mason

Linked scanning and metastable ion mapping

Abstract A method has been developed for the ripid and routine acquisition of metastable ion data for all decompositions occurring in the field-free region between the accelerating and electrostatic-analyser regions of a forward-geometry double-focussing mass spectrometer. It is based on a previous idea of scanning the “B.E plane”, but makes use of modern computing facilities to generate control, collect and store the data in the form of a metastable ion decomposition map. A microcomputer controls E and V, the electric-sector field strength and the accelerating voltage, respectively, whilst a commercial “data system” controls the rapid and repetitive scanning of the magnetic field B. The data are collected and stored by the commercial data system, which also performs a time-to-mass conversion, avoiding the use of a Hall probe. Although the data can be displayed as a map using standard GC—MS software, special computer programs were written to display the data in the form of more easily interpretable —fragmentation maps. Details in the map can be more readily appreciated bygenerating simulated “linked-scan” spectra from stored data. Illustrative examples are given for methanol, decanol, acetyl salicylic acid. caffeine, and a mixture. All the features of decomposition throughout the mass spectrometer up to the field-free region between the electric and magnetic sectors are seen. Their appearance is easily interpreted without recourse to complicated multiparameter plots. This technique is an efficient and rapid method of obtaining data that would normally have to be collected by many different linked-scan experiments, and hence is a potentially more powerful method not only in studies of fundamental aspects of unimolecular decomposition and structure analysis, but also in applications to the analysis of mixtures without recourse to prior separation.

Angular distribution of fragment ions arising from the collision-induced decomposition of polyatomic primary ions

Abstract The angular distributions of the relative intensities of fragment ions produced by the collision-induced decomposition of a number of selected ions have been measured. The distributions have been discussed in terms of contributions from the scattering angle and from the release of internal energy as translational energy of the fragment ions. The relative importance of the two contributions appears to vary with the ion undergoing fragmentation but in general, it is possible to vary the average internal energy of the decomposing ion by varying the angle at which products are observed.

The temperature dependence of ion-molecule association reactions

Abstract The temperature dependence of five ion-molecule association reactions and three ion-molecule equilibria have been determined using a high pressure source in which the electron beam is pulsed. The temperature dependence of the rate constants takes the form k=CT−m and values of m have been compared with literature values and theoretical predictions. No reason for the discrepancies between the various values can be given. Thermodynamic data derived from the variation of equilibrium constants with temperature are in excellent agreement with literature data.

Experimental Methods to Study the Thermochemistry of Gas Phase Ions and Ion-Molecule Reactions

Ionic species and ion–molecule reactions provide, in one way or another, a major contribution to the Earth’s chemistry. Although the most familiar reactions occur in solution (where they are often significantly moulded by solvent interaction) there are a number of important areas where ions exist and react in the gas phase. The upper atmosphere is an obvious and important example (0(2). An even more hospitable environment for free ions lies outside the Earth, in outer space. It is only just being revealed how important such reactions are in the creation and propagation of interstellar clouds (3). Gas phase ion–molecule reactions also occur in flames (4) electrical discharges (50). and are sometimes a corrosive nuisance in the coolant gases of nuclear reactors, Finally, they have become an important tool in analytical mass spectrometry (6).

Ion decomposition maps

Abstract All the ions from metastable decompositions occurring in the first field free region of a forward geometry double focussing mass spectrometer (magnetic field B, electrostatic analyser field E) can be collected automatically and rapidly by a sweep of the B,E plane, with the aid of a microcomputer and data system. The results are conveniently displayed in the form of an ion decomposition map of parent versus daughter ion mass. These data are amenable to further computer manipulation. Here ion decomposition maps are examined as a means of sequencing small peptides.

THE THERMOCHEMISTRY OF GAS PHASE ION-MOLECULE REACTIONS

In the previous chapter1 a description was given of how thermo-chemical measurements on ions have been extended to gas phase ion– molecule reactions. One of the major advantages of making these measurements is that intrinsic effects on the reactivity of molecules can be studied in the absence of the solvent effects usually present in solution chemistry. This has led to the collation of a large amount of self-consistent the rmo chemical data, by many different laboratories, from which it is possible to build a self-consistent picture of molecular reactivity in relation to molecular structure. It also provides a bank of reliable data from which to compute ionic heats of formation and against which to test increasingly sophisticated theoretical calculations. In addition, much effort has gone into studying the solvation process itself, not only by direct comparison of gas and solution phase data, but also through the investigation of ion “clustering” or “solvation” reactions in the gas phase, perhaps even giving an insight into nucleation phenomena and the phase transition process itself. It is these aspects of gas phase ion thermochemistry to which this chapter is devoted.