A Comparison of Methods for the Determination of Trace Coloring Oxides in Optical Glasses
At concentrations of 100 ppm and lower, each of 13 potential contaminating elements, viz., Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Pt, Pd, Rh, Nd, and Pr, has a deleterious effect on the optical transmittance and often the color of glasses for refractive optics. Determination of these impurities in complex glasses poses an unusually difficult trace analysis problem. The applicability of several leading trace analysis techniques is examined and realistic detection limits for the elements of particular concern are estimated. Assuming minimal sample treatment, the detection limits, in most instances, are inadequate. Indeed, chemical pretreatment and sample preconcentration are presently essential for adequately dealing with this analysis problem. Utilizing chemical separations and preconcentrations, optical emission and colorimetry, although not ideal, would appear to be the methods most capable of coping with trace analysis of complex glasses. Solids mass spectrometry has some merit, but at present the method lacks the desired precision. Single-sweep polarography also appears to hold promise. A real need exists for more sensitive methods for chromium, iron, cobalt, nickel, the platinum metals, and the rare earths.
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