Solvent Considerations
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Vacuum Degassing
Vacuum degassing operates on the principle of Henry’s law to remove dissolved gases
from the solvent. According to Henry’s law, the mole fraction of a gas dissolved in a liquid
is proportional to the partial pressure of that gas in the vapor phase above the liquid. If the
partial pressure of a gas on the surface of the liquid is reduced, for example, by
evacuation, then a proportional amount of that gas comes out of solution.
Note:
Vacuum degassing may change the composition of mixed solvents.
Inline Degassing
Inline degassing removes gases from the eluent as it passes through a gas-permeable
membrane enclosed in a vacuum chamber. The vacuum in the chamber accelerates the
rate at which the dissolved gas diffuses through the gas-permeable membrane. This
method provides an automatic, continuous method of removing dissolved gasses, and
allows for quick eluent changeover. Waters makes two inline degassers: part numbers
186001273 (2 Eluent) and 186001272 (4 Eluent). Contact Waters for details.
Using an Inline Degasser
The longer a solvent is exposed to a vacuum, the more dissolved gases are removed. Two
factors affect the amount of time the solvent is exposed to the vacuum:
•
Flow rate – At low flow rates, most of the dissolved gas is removed as the solvent
passes through the vacuum chamber. At higher flow rates, lesser amounts of gas per
unit volume of solvent are removed.
•
Surface area of the degassing membrane – The length of the degassing
membrane is fixed in each vacuum chamber. To increase the length of membrane,
you can connect two or more vacuum chambers in series.
Heating
Heating to remove dissolved gas is usually not effective unless you boil the eluent, and this
is not practical or safe for mixed, flammable, or volatile eluents. However, raising the
temperature even slightly raises the partial pressure of the eluent, and thereby reduces
the rate of resolubilization of a gas.
Vacuum Sonication
Sonication in combination with a vacuum degasses eluents very quickly. This technique
does not change the composition of mixed eluents appreciably.
Caution:
Apply vacuum only to suitable vessels. The brown gallon bottles in which
eluent is shipped are not designed for vacuum degassing. There is a high risk of
implosion if these bottles are used for vacuum degassing.
