Editing Column chromatography (section)

== Mobile phase (eluent) ==
[[File:Column chromatography sequence.png|250px|thumb|Column chromatography proceeds by a series of steps.]] 
The ''mobile phase'' or ''[[eluent]]'' is a [[solvent]] or a mixture of solvents used to move the compounds through the column. It is chosen so that the [[retention factor]] value of the compound of interest is roughly around 0.2 - 0.3 in order to minimize the time and the amount of eluent to run the chromatography. The eluent has also been chosen so that the different compounds can be separated effectively. The eluent is optimized in small scale pretests, often using [[chromatography|thin layer chromatography]] (TLC) with the same stationary phase, using solvents of different polarity until a suitable solvent system is found. Common mobile phase solvents, in order of increasing polarity, include [[hexane]], [[dichloromethane]], [[ethyl acetate]], [[acetone]], and [[methanol]].<ref>{{cite book |last1=Furniss |first1=Brian S. |last2=Hannaford |first2=Antony, J. |last3=Smith |first3=Peter W. G. |last4=Tatchell |first4=Austin S. |date=1989 |title=Vogel's Textbook of Practical Organic Chemistry |publisher=Longman Scientific & Technical |page=203 |isbn=978-0582462366}}</ref> A common solvent system is a mixture of hexane and ethyl acetate, with proportions adjusted until the target compound has a retention factor of 0.2 - 0.3. Contrary to common misconception, methanol alone can be used as an eluent for highly polar compounds, and does not dissolve silica gel.

There is an optimum [[Volumetric flow rate|flow rate]] for each particular separation. A faster flow rate of the eluent minimizes the time required to run a column and thereby minimizes diffusion, resulting in a better separation. However, the maximum flow rate is limited because a finite time is required for the analyte to equilibrate between the stationary phase and mobile phase, see [[Van Deemter's equation]]. A simple laboratory column runs by [[gravity]] flow. The flow rate of such a column can be increased by extending the fresh eluent filled column above the top of the stationary phase or decreased by the tap controls. Faster flow rates can be achieved by using a pump or by using compressed gas (e.g. air, [[nitrogen]], or [[argon]]) to push the solvent through the column (flash column chromatography).<ref>{{Cite journal|last=Still|first=WC|last2=Kahn|first2=M|last3=Mitra|first3=A|date=1978|title=Rapid chromatographic technique for preparative separations with moderate resolution|journal=[[Journal of Organic Chemistry|J Org Chem]]|publisher=[[American Chemical Society|ACS]]|volume=43|issue=14|pages=2923–2925|doi=10.1021/jo00408a041}}</ref><ref name="HarwoodMoodyEOCPAP">{{cite book|title=Experimental organic chemistry: Principles and Practice|url=https://archive.org/details/experimentalorga00harw|url-access=registration|last=Harwood|first=Laurence M.|last2=Moody|first2=Christopher J.|date=13 Jun 1989|publisher=Blackwell|isbn=978-0-632-02017-1|edition=Illustrated|location=London|pages=[https://archive.org/details/experimentalorga00harw/page/180 180–185]|oclc=1079261960|name-list-style=vanc}}</ref>

[[File:Cromatografia su colonna.jpg|thumbnail|250px|Photographic sequence of a column chromatography]]
The particle size of the stationary phase is generally finer in flash column chromatography than in gravity column chromatography. For example, one of the most widely used silica gel grades in the former technique is mesh 230 – 400 (40 – 63&nbsp;μm), while the latter technique typically requires mesh 70 – 230 (63 – 200&nbsp;μm) silica gel.<ref>{{Cite web|url=http://www.materialharvest.com/welcome/silica_products/silica_gels_chromatography.html|title=Material Harvest Silica Gel for Normal Phase Column Chromatography|date=2008|publisher=Material Harvest|access-date=3 Jan 2019}}</ref>

A spreadsheet that assists in the successful development of flash columns has been developed. The spreadsheet estimates the retention volume and band volume of analytes, the fraction numbers expected to contain each analyte, and the resolution between adjacent peaks. This information allows users to select optimal parameters for preparative-scale separations before the flash column itself is attempted.<ref name="FairKormos2008">{{cite journal|last=Fair|first=JD|last2=Kormos|first2=CM|year=2008|title=Flash column chromatograms estimated from thin-layer chromatography data|journal=[[Journal of Chromatography A|J Chromatogr A]]|volume=1211|issue=1–2|pages=49–54|doi=10.1016/j.chroma.2008.09.085|issn=0021-9673|pmid=18849041}}</ref>