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The time-dependent loss of NaKα X-ray intensity during electron-beam irradiation of hydrous alkali alumino silicate glasses is apparently more significant during the initial few seconds of beam exposure than it is for anhydrous glasses, and it is pronounced for incident beam currents &gt; 2-5 nA (using 15-20 μm beam diameters). Exponential fits of NaKα intensity vs. time show a progressive decrease in the apparent zero-time intercepts for incident beams from 2 to 20 nA, and thus methods for correcting Na concentrations solely on the basis of curve fitting and extrapolation to zero-time values may underestimate Na contents by almost 10% (relative) for higher beam currents. Similar exponential fits to the intensity-time data for AlKα and SiKα show that “grow-in” is greater for Al than for Si. For incident currents ≥ 5 nA, the magnitudes of all intensity changes also increase with total H 2 O content of glass. On the basis of these observations, the optimal conditions for analysis of hydrous alkali aluminosilicate glasses include a 2 nA beam with 20 μm diameter and counting times of 20-40 s for the analysis of alkali aluminosilicate components, with Na and Al analyzed first (simultaneously, if possible). These methods minimize Na loss and grow-in for Al and Si to the point that little or no correction is needed, provide good statistical accuracy, and work with a wide variety of standard materials (i.e., glass standards with compositions and H 2 O contents comparable to the unknowns are not needed). For complete analysis of more complex multicomponent systems, two beam conditions are recommended: an initial 2 nA, 20 μm diameter beam for analysis of alkali alumino silicate components, followed by a 20 nA, 20 μm diameter beam for analysis of all other components. With the use of these methods, the H 2 O contents of hydrous glasses (H 2 O as the only unknown) can be determined by difference with uncertainties mostly &lt;5% (relative to FTIR values) for glasses containing up to 10 wt% H 2 O. At beam currents &gt; 5 nA, corrections for Na loss ignoring Al (and Si) grow-in underestimate H 2 O contents by about 10-50% of concentration.

Optimizing the electron microprobe analysis of hydrous alkali aluminosilicate glasses