Abstract: A method and apparatus for decomposing a layer of silicon oxide on a silicon wafer is described which employs the application of a heated mist of aqueous HF to the cooled wafer surface. The technique is applied to the analysis of silicon wafers for trace impurities using a scanning fluid drop to collect the residue containing the impurities after the silicon oxide has been decomposed. The novel method offers an order of magnitude increase in the rate of silicon oxide decomposition over the prior art which uses a vapor phase decomposition technique. In addition the novel method provides better control and safer disposition of the corrosive vapors over the prior art. The apparatus comprises a movable dome fitted with a carrier gas supply and a means for injecting a heated aqueous HF mist generated by a specially designed mist generator into the carrier gas flow.