How do you choose a mobile phase for chromatography?

A mobile phase for chromatography is chosen based on its polarity and the nature of the sample to be separated.

In chromatography, the mobile phase is the solvent that carries the sample through the stationary phase. The choice of the mobile phase is crucial as it directly affects the separation of the components in the sample. The main factor to consider when choosing a mobile phase is its polarity, which should match the polarity of the sample.

If the sample is polar, a polar mobile phase like water or methanol should be used. This is because polar compounds tend to dissolve well in polar solvents (like dissolves like). On the other hand, if the sample is non-polar, a non-polar mobile phase like hexane or chloroform would be more suitable.

Another factor to consider is the pH of the mobile phase. Some compounds can change their charge state (ionise) at different pH levels, which can affect their interaction with the stationary phase and hence their separation. Therefore, the pH of the mobile phase should be adjusted to ensure optimal separation.

The choice of the mobile phase also depends on the type of chromatography being used. For example, in gas chromatography, the mobile phase is a gas (usually helium or nitrogen), while in liquid chromatography, the mobile phase is a liquid.

Lastly, the mobile phase should be compatible with the detection method used. For instance, if UV detection is used, the mobile phase should not absorb UV light at the same wavelength as the sample.

In conclusion, the choice of the mobile phase in chromatography is a critical step that requires careful consideration of several factors, including the polarity and pH of the mobile phase, the nature of the sample, the type of chromatography, and the detection method.