Normal phase vs reverse phase HPLC

What is the difference between *normal phase* and *reverse phase* high-performance liquid chromatography?




**Normal phase**


Normal phase HPLC systems are similar to the flash-column chromatography that you might be familiar with. 


A silica stationary phase is eluted with a non-polar solvent such as hexane, or a fairly non-polar solvent mixture such as 2-propanol in hexanes. In normal phase chromatography, only organic solvents are used. 


In the normal phase, polar molecules elute slowly, and non-polar (greasy) molecules elute quickly. 


[![enter image description here](https://i.stack.imgur.com/GzU3n.jpg)](https://i.stack.imgur.com/GzU3n.jpg)


**Reverse phase**


Reverse phase is essentially the opposite of normal-phase. 


A non polar stationary phase (often silica in which the free hydroxyl groups are end-capped with something greasy, C18 chains are common but many many variants are possible) is eluted with a polar solvent such as acetonitrile/methanol, or a fairly polar solvent mixture (acetonitrile water mixtures are common, or methanol water mixtures). 


In the reverse phase, polar molecules elute quickly, and non-polar (greasy) molecules elute slowly. 


[![enter image description here](https://i.stack.imgur.com/ez6tM.jpg)](https://i.stack.imgur.com/ez6tM.jpg)


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Normal phase HPLC means the stationary phase is polar and the mobile phase is non-polar; reversed phase means the stationary phase is non-polar and the mobile phase is polar.


See [HPLC Separation Modes](http://www.waters.com/waters/en_US/HPLC-Separation-Modes/nav.htm?cid=10049076&locale=en_US) (from Waters, a manufacturer of chromatography equipment and supplies) for more insight.


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