In 1914, Hahn and Meitner had developed a new technique for separating the tantalum group from pitchblende, which they hoped would speed the isolation of the new isotope. When Meitner resumed this work in 1917, Hahn and most of the students, laboratory assistants and technicians had been called up to serve in the armed forces, so Meitner had to do everything herself. In February, she extracted 2 grams of silicon dioxide (SiO 2) from 21 grams of pitchblende. She set 1.5 grams aside and added a tantalum pentafluoride (TaF 5) carrier to the other 0.5 grams, which she dissolved in hydrogen fluoride (HF). She then boiled it in concentrated sulfuric acid (H 2 SO 4), precipitated what was believed to be element 91, and verified that it was an alpha emitter. Hahn came home on leave in April, and together they devised a series of tests to eliminate other sources of alpha particles. The only known ones with similar chemical behaviour were lead-210 (which decays to alpha emitter polonium-210 via bismuth-210) and thorium-230.