Reproducibility and Solvent Degassing 169
C
Volume of the Sample Loop
The drag from the tubing, viscosity, and the elasticity of the fluids (sample and mobile
phase) affect the ability of the sample to follow the syringe withdrawal. If you use the
auxiliary loop with injections of less than 200 µL, you increase the effects of drag and
elasticity, and decrease the reproducibility of the injection.
Note: Always use the smallest sample loop that can contain the injection volume.
C.2 Solvent Degassing
Degassed solvents improve reproducibility in chromatography by providing:
• Accurate sample volume injections
• Stable baselines and enhanced sensitivity
• Reproducible retention times for eluting peaks
• Stable pump operation
This section presents the theory involved in solvent degassing, solvent degassing
methods, and solvent degassing considerations.
C.2.1 Theory of Solvent Degassing
Only a finite amount of gas can be dissolved in a given volume of liquid under specific
conditions. This amount depends on the temperature of the liquid, the pressure applied to
the liquid, and the chemical affinity of the gas for the liquid.
Effects of Intermolecular Forces
Generally, a gas is most soluble in a solvent where the intermolecular attractive forces
between the molecules of the solvent are similar to those of the gas (“like dissolves like”).
If the main attractive forces are Van der Waals forces, the gas is more soluble in this
solvent than in one with dipole forces or hydrogen bonding. Thus, a larger amount of
hydrogen (H
2
), nitrogen (N
2
), oxygen (O
2
), or helium (He) dissolves in alkanes and alcohol
than in water.
Effects of Temperature
Temperature affects the solubility of gases in two ways:
• The higher the boiling point of the solvent, the more soluble the gas is in a given
solvent.
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