216 Eluent Considerations
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 controller Chart output terminal
provided for this purpose (select %A or %B). See Section 4.3.2, Setting Pump Setup
Parameters.
B.6 Eluent Degassing
Mobile phase difficulties account for most liquid chromatographic problems. Degassing
eluents that are used in the mobile phase is one of the most effective measures to eliminate
these problems. The benefits are:
• Stability in the baseline and enhanced sensitivity in some types of chromatographic
detectors
• Reproducible retention times for eluting peaks
• Reproducible inject volumes for quantitation
B.6.1 Conditions Affecting Gas Dissolution
Overview
Only a finite amount of gas dissolves in a given volume of liquid under specific
conditions. This amount depends on:
• Chemical affinity
•Temperature
• Pressure
Chemical Affinity
Generally, a gas is most soluble in an eluent where the attractive forces between molecules
of the eluent are similar to those between molecules of the gas (“like dissolves like”).
Also, when the main intermolecular attractive forces in an eluent are Van der Waals forces,
a gas is more soluble than when dipole forces or hydrogen bonding predominate. Thus,
larger amounts of helium, nitrogen, oxygen, and hydrogen dissolve in alkanes and benzene
than in water.
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