Waters 2489, Обзор и руководство по обслуживанию

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June 11, 2015, 715004752 Rev. A
Page 145
that occur as the solvents are mixed in different proportions 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 will affect the analysis is not known,
monitor the pressure during the run using the Chart Out terminal.
C.6 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,
• stable baselines and enhanced sensitivity;
• reproducible retention times for eluting peaks;
• reproducible injection volumes for quantitation.
• stable pump operation.
C.6.1 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.
C.6.1.1 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).
C.6.1.2 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 temperature, but the solubility of
He in benzene increases with an increase in temperature.