Isotopes of helium a thousand times lower than helium-4 (2.17 K). This difference is explained by quantum statistics, since helium-3 atoms are fermions, while helium-4 atoms are bosons, which condense to a superfluid more easily. The most common isotope, , is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized nuclei. is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis. Terrestrial helium consists almost exclusively (99.99986%) of this isotope. Its boiling point of 4.2 K is the lowest of
Bioseparation of 1,3-propanediol and proteins. Of the three chemicals water has the lowest boiling point (see the water, 1,3-propanediol, and glycerol articles for boiling point information), so it is removed as distillate in the first column. The bottoms of the first column is then sent to a second column, where BioPDO is removed as distillate because of its lower boiling point. Both columns operate under low pressure (55 mm Hg in the first column; 20 mm Hg in the second column) to lower the boiling points of the distillate and bottoms streams, thereby using a lower pressure steam than that for atmospheric columns.