Editing Isotope separation (section)

===Distillation===

Isotopes of hydrogen, carbon, oxygen, and nitrogen can be enriched by distilling suitable light compounds over long [[Fractionating column|columns]]. The separation factor is the ratio of vapor pressures of two isotopic molecules. In equilibrium such a separation results at each [[theoretical plate]] of the column and is multiplied by the same factor in the next step (at the next plate). Because the elementary separation factor is small, a large number of such plates is needed. This requires total column heights of 20 to 300 m.

The lower vapor pressure of the heavier molecule is due to its higher [[energy of vaporization]], which in turn results from its lower energy of zero-point vibration in the intermolecular potential. As expected from formulas for vapor pressure, the ratio becomes more favorable at lower temperatures (lower pressures). The vapor pressure ratio for H<sub>2</sub>O to D<sub>2</sub>O is 1.055 at 50&nbsp;°C (123 mbar) and 1.026 at 100&nbsp;°C (1013 mbar). For <sup>12</sup>CO to <sup>13</sup>CO it is 1.007 near the normal boiling point (81.6&nbsp;K), and 1.003 for <sup>12</sup>CH<sub>4</sub> to <sup>13</sup>CH<sub>4</sub> near 111.7&nbsp;K (boiling point).<ref name=":0">{{Cite book |title=Separation of isotopes of biogenic elements |date=2007 |publisher=Elsevier |author=B.M. Andreev |author2=E.P.Magomedbekov |author3=A.A. Raitman |author4=M.B.Pozenkevich |author5=Yu.A. Sakharovsky |author6=A.V. Khoroshilov |isbn=978-0-444-52981-7 |location=Amsterdam |oclc=162588020}}</ref>

The <sup>13</sup>C enrichment by ([[Air separation#Cryogenic distillation process|cryogenic]]) distillation was developed in the late 1960s by scientists at Los Alamos National Laboratory.<ref>{{cite web |url=http://www.lanl.gov/quarterly/q_w03/spotlight.shtml |title=Spotlight Los Alamos in the News |date=Winter 2003 |publisher=Los Alamos National Laboratory |access-date=2014-02-18 |archive-url=https://web.archive.org/web/20160421202634/http://www.lanl.gov/quarterly/q_w03/spotlight.shtml |archive-date=2016-04-21}}</ref><ref>{{cite web |url=http://www.lanl.gov/orgs/pa/News/080801.html |title=Laboratory alliance to put "Made in America" stamp on stable isotopes |access-date=2007-09-01 |archive-url=https://web.archive.org/web/20061012173925/http://www.lanl.gov/orgs/pa/News/080801.html |archive-date=2006-10-12}}</ref> It is still the preferred method for<sup>13</sup>C enrichment. Deuterium enrichment by water distillation is only done, if it was preenriched by a process (chemical exchange) with lower energy demand.<ref>{{Cite journal |last=Miller |first=Alistair I. |date=2001 |title=Heavy Water: A Manufacturers' Guide for the Hydrogen Century |journal=Canadian Nuclear Society Bulletin |volume=22 |issue=1 |pages=1–14}}</ref> Beginning with the low natural abundance (0.015% D) would require evaporation of too large quantities of water.