June 11, 2015, 715004754 Rev. A
Page 60
4.3.1 Gas solubility
The amount of gas soluble in a given volume of liquid depends on these factors:
•
The chemical affinity of the gas for the liquid
•
The temperature of the liquid
•
The pressure applied to the liquid
Changes in the composition, temperature, or pressure of the mobile phase can lead to
outgassing.
4.3.1.1
Effects of intermolecular forces
Nonpolar gases (N
2
, O
2
, CO
2
, He) are more soluble in nonpolar solvents than in polar
solvents. Generally, a gas is most soluble in a solvent with intermolecular attractive
forces similar to those in the gas (“like dissolves like”).
4.3.1.2
Effects of temperature
Temperature affects the solubility of gases. If the dissolution is exothermic, the
solubility of the gas decreases when you heat the solvent. If the dissolution is
endothermic, the solubility increases when you heat the solvent. For example, the
solubility of He in H
2
O decreases with an increase in temperature, but the solubility of
He in benzene increases with an increase in temperature.
4.3.1.3
Effects of partial pressure
The mass of gas dissolved in a given volume of solvent is proportional to the partial
pressure of the gas in the vapor phase of the solvent. If you decrease the partial
pressure of the gas, the amount of that gas in solution also decreases.
4.3.2 Solvent degassing methods
Solvent degassing helps you attain a stable baseline and also improves reproducibility
and pump performance.
There are two common methods used to degas solvents:
•
Reducing pressure by vacuum
•
Sonication
You can use these methods individually or in combination.
4.3.2.1
Vacuum degassing
The in-line vacuum degasser operates on the principle of Henry’s Law to remove
dissolved gases from the solvent. Henry’s Law states that the mole fraction of a gas
dissolved in liquid is proportional to the partial pressure of that gas in the vapor phase
above the liquid. If the partial pressure of a gas on the surface of the liquid is
reduced, for example, by evacuation, then a proportional amount of that gas comes
out of solution.
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