Abundance of the chemical elements of the volatile elements hydrogen, helium, neon, and nitrogen, as well as carbon which has been lost as volatile hydrocarbons. The remaining elemental composition is roughly typical of the "rocky" inner planets, which formed in the thermal zone where solar heat drove volatile compounds into space. The Earth retains oxygen as the second-largest component of its mass (and largest atomic-fraction), mainly from this element being retained in silicate minerals which have a very high melting point and low vapor pressure. The mass-abundance of the nine most abundant elements in the Earth's crust is approximately: oxygen 46%, silicon 28%, aluminum 8.2%,
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.