Decomposition of geochemical samples by an acid mixture in re-usable sealed glass tubes has been shown to give results comparable to standard methods, at a lower cost and with no large-scale evolution of corrosive fumes. Samples are exposed to the acid mixture at a temperature well above its normal boiling point, in a layered block that cools the tops of the tubes to protect the caps. Volatile analytes are retained, and the test solution is suitable for a variety of instrumental methods of analysis.