Mobile phase
A mobile phase, as its name suggests, is a phase that travels through the stationary phase.
Different mobile phases are used depending of the working method:
- Non polar solvents for Normal-phase HPLC. The most common ones are hexane, heptane or iso-octane. They can be combined with slightly polar solvents, such as isopropanol, ethyl acetate or chloroform.
- Polar solvents for Reverse-phase HPLC. Normally, water is used as a base solvent and is combined with a polar organic solvent (such as methanol or acetonitrile).
Similar to the stationary phase, mobile phase polarity is also a scale (see Figure 1). Water is an example of a polar mobile phase and on the opposite end is Hexane which is a non polar mobile phase.
Figure 1. Mobile phase polarity scale.
| Solvent | Polariy index (P') | |
|---|---|---|
| Water | 9 | |
| Acetonitrile | 5.8 | |
| THF | 4.0 | |
| Hexane | 0.1 |