Static SIMS: metastable decay and peak intensities

Abstract

The decay of a metastable ion to a daughter ion along the flight path of a time-of-flight (ToF) mass spectrometer leads to well-defined peak structure in the mass spectrum. Through interference, these daughter ion peaks can reduce the detection limits in static SIMS and lead to uncertainties in both the true peak area intensity and the peak position. The area of the peak for the metastable parent ion is reduced to an extent which depends on its half life, the analyser design and the chosen instrument settings. These intensity changes directly affect the quality and reproducibility of spectra. The decay of metastable ions is analysed for the reference material, PTFE, used in a recent inter-laboratory study. A method is developed, for a ToF reflectron analyser, to characterise the decay process of the parent ion so that both parent and daughter are accurately identified. This method involves measuring the transit time shift of the metastable peak as a function of the reflectron voltage. To illustrate the method, the mass of the C 4F 4 + parent ion is determined to an uncertainty of 0.6 amu. This parent ion decays by emission of CF 2 0 to C 3F 2 +. The choice of instrument parameters to avoid interference from metastable peaks and to improve data transferability is discussed.