Sentence:  Another recent achievement was the successful development of a method for the complete combustion in a bomb calorimeter of a metal in <WSD>fluorine</WSD> when the product is relatively non-volatile . 
Meaning of the word marked with <WSD> tags:  a nonmetallic univalent element belonging to the halogens; usually a yellow irritating toxic flammable gas; a powerful oxidizing agent; recovered from fluorite or cryolite or fluorapatite