Introduction
Order of elution from a polar stationary phase (straight phase),
e.g. silica or aluminium oxide.
Both silica and aluminium oxide are slightly basic and will retain acidic compounds strongly unless an acidic eluent is used. Note that the ranking is very approximate!
| Least polar (least retained) |
| Chloride | -Cl |
| Alkane | -H |
| Ether | -OCH3 |
| Nitro | -NO2 |
| 3° amine | -N(CH3)2 |
| Ketone | -COCH3 |
| Ester | -OCOCH3 |
| 1° amine | -NH2 |
| 2° amide | -NHCOCH3 |
| Alcohol | -OH |
| 1° amide | -CONH2 |
| Carboxylic acid | -COOH |
| Most polar (most retained) |
Note: There is little correlation with substituent parameters such as
→ Hammett constants or
→ group electronegativities.
This is because hydrogen bonding is more important in chromatography than for chemical reactivity.