The performance of X-ray microanalysis in the variable pressure or environmental scanning electron microscope (VP-SEM or ESEM) is limited by skirting. Under certain conditions, charging effects can occur and change the X-ray emission, which affects the X-ray microanalysis. The conventional way to evaluate charging is to calculate the Duane-Hunt limit by fitting the X-ray intensity region located below the energy cut-off. Nevertheless, this method appears to have serious limitations for instance in the case of strong insulators. A perfect example of this limitation is to study the evolution of composition of an alkali glass with time. This paper reports on the evolution of the sodium X-ray intensity with time depending on accelerating voltage, pressure and presence of a surface coating. For certain conditions, a decrease of sodium X-ray intensity with time was observed but for other conditions the reverse behavior was noticed. The increase of sodium X-ray intensity with time was obtained when the force created by the surface electrons was stronger than the force generated by electrons trapped in the interaction volume, whereas the decrease of sodium X-ray intensity occurred when the force generated by electrons trapped in the interaction volume was the stronger. The variations of sodium X-ray intensity were also compared to the variation of the Duane-Hunt limit, the determination of which is studied in detail in this article.
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