Liquid chromatography mobile phase properties requirements
An ideal liquid chromatography mobile phase solvent should have low viscosity, good compatibility with the detector, easy to obtain pure product and low toxicity.
After the filler (stationary phase) is selected, the strong solvent reduces the adsorption of the solute on the surface of the filler, and the corresponding capacity factor k decreases; while the weaker solvent increases the adsorption of the solute on the surface of the filler, and the corresponding capacity factor k increases. Therefore, the k value is a function of the composition of the mobile phase. The number of trays N is generally inversely proportional to the viscosity of the mobile phase. So the following aspects should be considered when choosing a mobile phase:
1 Flow does not change any of the properties of the filler. Low cross-linking ion exchange resins and exclusion chromatography cartridges sometimes encounter certain organic phases that swell or shrink, thereby changing the nature of the column packing. The alkaline mobile phase cannot be used in silica gel column systems. The acidic mobile phase cannot be used in the column system of adsorbents such as alumina and magnesia.
2 purity. The lifetime of the column is related to the passage of a large amount of mobile phase, especially when the impurities contained in the solvent accumulate on the column.
3 must match the detector. When using a UV detector, the mobile phase used should have no absorption at the detection wavelength or a small absorption. When using a refractive index detector, a solvent with a large difference between the refractive index and the sample should be selected as the mobile phase to improve the sensitivity.
4 viscosity is low (should be <2cp). The high viscosity solvent will affect the diffusion and mass transfer of the solute, reduce the efficiency of the column, and increase the pressure drop of the column to prolong the separation time. It is preferred to select a mobile phase having a boiling point below 100 °C.
5 The solubility of the sample should be appropriate. If the solubility is poor, the sample will precipitate on the column head, which will not only affect the purification separation, but also deteriorate the column.
6 samples are easy to recycle. Volatile solvents should be used.
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