The electron microprobe has been used in petrographic research for almost 20 years and more recently as a tool for investigations of clay minerals. However, to the author's knowledge no information has been published concerning the reliability of such analyses―especially those of the alkali elements. Since the alkali contents of clays are often critical, e.g. in determining the smectite content of mixed-layered minerals, it is important to have some idea of the reliability of an analysis made using an electron microprobe.

Three variables can be controlled in electron microprobe analysis—time, beam-current intensity and beam size (the diameter of the beam which excites the sample). Exaggerated conditions of any one of these variables tend to diminish the signal received by the detectors for the alkali elements, especially sodium. As an example, the effect of varying two of these parameters on the intensity of sodium radiation for sodium-aluminumsilicon glass is shown in Fig. 1. The signal decreases (% loss Na) over a 200 s counting time. Beam intensity and beam diameter were varied in this example.