TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Conditions: The hexafluorosilicate is heated at 400degreesC or more. The tetrafluorosilane gas containing hexafluorodisiloxane is reacted with fluorine gas at 100-350degreesC. The tetrafluorosilane gas is reacted with fluorine gas or metal fluoxide at 50-350degreesC, in presence of a fluorine gas. Preferred Compound: The hexafluorosilicate is selected from group of alkali metal hexafluorosilicate and alkaline earth metal hexafluorosilicate. The high purity tetrafluorosilane comprises 0.1 volume ppm or less of hexafluorodisiloxane. Preferred Process: The hexafluorosilicate is pulverized and dried before heating. The production of tetrafluorosilane further involves contacting silicon with tetrafluorosilane gas at 50degreesC or more. The silicon is heat-treated at 400degreesC or more in presence of an inert gas before contacting with tetrafluorosilane gas. The production process further involves contacting tetrafluorosilane gas with a gas separation membrane and/or a molecular sieving carbon. Method for analyzing impurities in a high purity tetrafluorosilane involves introducing tetrafluorosilane containing hexafluorodisiloxane as an impurity into a cell with the material of window being composed of a metal halide, and analyzing the hexafluorodisiloxane and/or hydrogen fluoride by infrared spectrometry.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Metal: The high valent metal fluoxide is supported on a support obtained by fluorinating at least one compound selected from alumina, titania and zirconia. Preferred Membrane: The gas separation membrane is an silicon dioxide-zirconium dioxide ceramic membrane and/or a poly(4-methylpentene-1) heterogeneity membrane. Preferred Property: The molecular sieving carbon has a pore size of 5 Angstrom or less. Preferred Adsorbent: The adsorbent is an activated carbon, petroleum pitch spherical activated carbon and/or molecular sieving carbon having a pore size of 6 Angstrom or more. SPECIFIC COMPOUNDS - The high valent metal fluoxide is at least one compound selected from cobalt trifluoride (CoF3), manganese trifluoride (MnF3), manganese fluoride (MnF4), silver fluoride (AgF2), cerium fluoride (CeF4), lead fluoride (PbF4) and K3NiF7.    EXAMPLE - Sodium hexafluorosilicate having an average particle size of about 1 mum was dried at 120degreesC. The dried sodium hexafluorosilicate (1500 g) was charged into a reactor. Then, while controlling the temperature of sodium hexafluorosilicate to less than 400degreesC, nitrogen gas was passed at 1000 ml/minute, and when the reduction of the hydrogen fluoride concentration in the exhaust gas to 1 ppm or less was confirmed, the drying of sodium hexafluorosilicate was completed. Subsequently, the flow rate of nitrogen was controlled to 200 ml/minute and the temperature was raised to 700degreesC. Consequently, tetrafluorosilane having concentration of 30 volume% was generated. The generated sample was introduced into a fluorine reactor and was mixed with fluorine gas (3 ml), and hexafluorodisiloxane contained in the tetrafluorosilane was reacted with fluorine at 300degreesC. The resulting sample gas was introduced into a reactor equipped with silicone chips having size of 8-10 mesh, treated with nitrogen. The gas generated after reaction with fluorine was freeze-recovered, cooled to -121degreesC with liquid nitrogen. The tetrafluorosilane recovered was introduced into an adsorption tower, and outlet gas from the tower was analyzed and was found to have less than 0.1 volume ppm of hexafluorodisiloxane, oxygen, nitrogen, carbon monoxide, carbon dioxide and hydrogen fluoride.