No pure uranium-235 was available, so experiments at Liverpool were conducted with natural uranium. The results were inconclusive, but tended to support Frisch and Peierls. By March 1941, Alfred Nier had managed to produce a microscopic amount of pure uranium-235 in the United States, and a team under Merle Tuve at the Carnegie Institution of Washington was measuring the cross section. The uranium-235 was too valuable to send a sample to Britain, so Chadwick sent the Americans a list of measurements he wanted them to carry out. The final result was that the cross section was smaller than Frisch and Peierls had assumed, but the resulting critical mass was still only about eight kilograms.