Solvent degassing D-7
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 controller’s Chart
output provided for this purpose (select %A or %B).
Solvent degassing
Mobile phase difficulties account for 70% or more of all problems in liquid
chromatography. Degassing solvents used in the mobile phase is one of the
best 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 injection volumes
• Stable pump operation
Only a finite amount of gas is dissolved in a given volume of liquid under
specific conditions. This amount depends on the temperature of the liquid,
pressure on the liquid, and the chemical affinity of the gas for the liquid.
Generally, a gas is most soluble in a solvent where the intermolecular
attractive forces between 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 helium, nitrogen, oxygen, or
hydrogen dissolves in alkanes and benzene than in water.
Temperature affects the solubility of gases in two ways. First, the higher the
boiling point of a gas, the more soluble the gas is in a given solvent. Second,
increasing the temperature of the gas/liquid solution affects the percentage of
gas in solution. If the heat of solution results in an exothermic reaction, the
percentage of gas in solution reduces. If the reaction is endothermic, the
percentage increases. For example, solubility of helium in water decreases
with an increase in temperature, but the solubility of helium in benzene
increases in direct proportion to the temperature.
The mass of gas dissolved in a given volume of solvent is proportional to the
partial pressure of the gas in the vapor phase of the solvent. If the gas
pressure decreases, the amount of that gas in solution also decreases.
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