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.