OPTIMIZING THE ELECTRON-MICROPROBE ANALYSIS OF HYDROUS ALKALI ALUMINOSILICATE GLASSES

The time-dependent loss of NaK alpha X-ray intensity during electron-b eam irradiation of hydrous alkali aluminosilicate glasses is apparentl y more significant during the initial few seconds of beam exposure tha n it is for anhydrous glasses, and it is pronounced for incident beam currents >2-5 nA (using 15-20 mu m beam diameters). Exponential fits o f NaK alpha intensity vs. lime show a progressive decrease in the appa rent 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 co ntents by almost 10% (relative) for higher beam currents. Similar expo nential fits to the intensity-time data for AlK alpha and SiK alpha sh ow that ''grow-in'' is greater for Al than for Si. For incident curren ts greater than or equal to 5 nA, the magnitudes of all intensity chan ges also increase with total H2O content of glass. On the basis of the se observations, the optimal conditions for analysis of hydrous alkali aluminosilicate glasses include a 2 nA beam with 20 pm diameter and c ounting times of 20-40 s for the analysis of alkali aluminosilicate co mponents, 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 accu racy, and work with a wide variety of standard materials (i.e., glass standards with compositions and H2O contents comparable to the unknown s are not needed). For complete analysis of more complex multicomponen t systems, two beam conditions are recommended: an initial 2 nA, 20 mu m diameter beam for analysis of alkali aluminosilicate components, fo llowed by a 20 nA, 20 mu m diameter beam for analysis of. all other co mponents. With the use of these methods, the H2O contents of hydrous g lasses (H2O as the only unknown) can be determined by difference with uncertainties mostly <5% (relative to FTIR values) for glasses contain ing up to 10 wt% H2O. At beam currents >5 nA, corrections for Na loss ignoring Al (and Si) grow-in underestimate H2O contents by about 10-50 % of concentration.