Waters 2475, Operator'S Manual, Page: 207 / 228

Mobile phase solvent degassing C-7
If the extent to which the pressure changes will affect the analysis is not
known, monitor the pressure during the run using the Chart Out terminal.
Mobile phase solvent degassing
Mobile phase difficulties account for 70% or more of all liquid
chromatographic problems. Using degassed solvents is important, especially
at excitation wavelengths below 220 nm. Degassing provides these benefits:
• Stable baselines and enhanced sensitivity
• Reproducible retention times for eluting peaks
• Reproducible injection volumes for quantitation
• Stable pump operation
This section explains the solubility of gases, solvent degassing methods, and
solvent degassing considerations.
Gas solubility
Only a finite amount of gas can be dissolved in a given volume of liquid. This
amount depends on:
• 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
all lead to outgassing.
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).
Effects of temperature
Temperature affects the solubility of gases. If the heat of solution is
exothermic, the solubility of the gas decreases when you heat the solvent. If
the heat of solution 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