Sub‐surface charging, a source of error in microprobe analysis

Qualitative microprobe analysis of sodian potassian hydroxonian meta‐autunite failed to reveal any sodium, although nearly 3% was present. Potassium, magnesium and calcium were underestimated to a smaller extent in related natural and synthetic structures. The effect is due to the migration of these interlayer cations from the analysed region—almost instantaneously in the case of sodium—and a consequent decrease in count rates, under the influence of the space‐charge resulting from the incident electron beam. Experiments with the autunites and with a soda‐glass subjected to various coating treatments showed that the effect of charging can be dramatic. Such charging also occurs when alkali metals are not present, a fact which has important implications for all microprobe analyses.