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can result in pressure changes during the run. For example, a 1:1 mixture of
water and methanol produces twice the pressure of either water or methanol
alone.
If the extent to which the pressure changes 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 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
Gas solubility
Only a finite amount of gas can be dissolved in a given volume of liquid. This
amount 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.
Effects of intermolecular forces
Nonpolar gases (N
in polar solvents. Generally, a gas is most soluble in a solvent whose
intermolecular attractive forces are 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
, O
, CO
, He) are more soluble in nonpolar solvents than
2
2
2
Mobile phase solvent degassing
July 29, 2013, 715003794 Rev. B
301
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