Mass spectrometry of solids ? with special emphasis on probe sampling

Abstract

In mass spectroscopic elemental analysis a fraction of the sample atoms — those which become ionized in the sampling procedure — are separated on the basis of their isotopic masses. Simultaneous detection of elements is possible without an inherent background. Therefore, mass spectrometry ranks among the most sensitive methods for analysis of solids, with respect to both trace concentrations and trace quantities. A mass resolution of 250 is required to separate all elemental isotopes having different mass numbers. At this resolution peak identification and quantification are frequently hampered by interfering molecular peaks. With a mass resolution of 2500, peaks from organic contaminants are separated from elemental peaks, but a still higher mass resolution (25000) is necessary to separate hydride, oxide and hydroxide peaks from elemental peaks at the same mass number. Solid samples can be analysed without chemical preparation. Sample ionization has been traditionally accomplished with a spark or arc source. While the elemental sensitivities lie within an order of magnitude, the main problem has been reproducibility of ionization conditions. Since the early sixties other ways of sample ionization have become important, namely by lasers or beams of ions or electrons. The aim of these developments has been not only to provide reproducible or steady state sample ionization, but also to permit highly localized sampling. With laser- or electron-beam sampling the solid is locally vaporized and a fraction of the atoms ionized. With ion-beam sampling the atoms are released from the sample surface by momentum transfer, without heating. This technique, termed ion-probe analysis, has found widespread use in the past few years. Its main drawback lies in the severe matrix effects which cause elemental sensitivities to vary over several orders of magnitude. Normally, ion-detection has involved either simultaneous multi-element detection with an ion-sensitive photographic plate or sequential single-peak electrical detection. Promising trends towards the ideal of simultaneous multiple electrical detection have been reported recently by researchers using electro-optical devices.