Determination of trace impurities at the p.p.b. level in fused silica by spark-source mass spectrometry

A sample dissolution-concentration technique for the determination of trace elements in fused silica by spark-source mass spectrometry (s.s.m.s.) is described. The fused silica is dissolved by ultra-pure hydrofluoric acid, and most of the silicon tetrafluoride formed is then removed at 80–100° C, leaving behind the impurities in the solution as fluorides. After addition of ultrahigh-purity graphite and internal standards (Tl and Rb), the mixture is dried to give a sample suitable for pressing into electrodes. The method not only concentrates the trace impurities but also reduces the interferences originating from silicon oxide and carbide species in the mass spectra and thus allows the determination of a larger number of elements at p.p.b. levels. More than 30 trace impurities in 6 fused silica boules and J.T. Baker's Ultrex silicon dioxide are determined and compared with data obtained by other methods.