A theoretical assessment of the possibility of selected-area mass-spectrometric analysis using a focused ion beam

This paper is concerned with the possibility of making selected area mass-spectrometric measurements on plane specimens of metals and rocks, with a resolution of a few micrometres, by means of a focused ion probe and a conventional mass spectrometer. The ion beam intensities expected at the detector of the mass spectrometer and the current obtainable in the primary ion beam used for bombardment of the specimen are calculated. The results obtained suggest that ratios of isotopic abundance should be measurable to a reproducibility of approximately 0·1% on areas of a few micrometres diameter when the mass concentration of the element in the specimen and the abundance of its isotopes are greater than about 10%.