Abstract: In accordance with the invention, a method for executing reactions at temperatures exceeding 50.degree. C. is disclosed using minimal quantities of fluid samples. The method uses a reaction vessel which minimizes the containment space for the fluid sample. The reaction vessel includes a sample-receiving tube and an insert. The insert engages the sample-receiving tube, forming a seal and prevents evaporation of the fluid sample during heating. The sample-receiving tube of the method is tapered to be narrow towards the bottom so that the insert can be easily inserted into the sample-receiving tube from the top and provide a tight seal in the lower extremity without increasing pressure in the reaction chamber. Equalization between the two sides of the sealing insert can occur as the insert is being introduced. Minimal displacement of the insert in the axial direction of the sample-receiving tube produces a sealing connection minimizing undesirable pressure build up.