Getting Started in HPLC

Section 2B. HPLC Pumps

   
Degassing (How do I degas thee? Let me count the ways). Four approaches are commonly used to remove excess dissolved gas from mobile phase solvents:


 
1. Sonication. When pure solvents are mixed to make up the mobile phase, excess dissolved gas escapes to form bubbles (remember, air is less soluble in solvent mixtures than in pure solvents). If the mobile phase reservoir is placed in an ultrasonic bath, the sound waves promote the coalescence of small bubbles which can escape more easily. This type of degassing works for premixed mobile phases by allowing the excess gas to escape before it enters the pump. It is not recommended for on-line mixing systems, in which bubbles form inside   the HPLC system, because it cannot reduce the dissolved gas below the saturation level of the pure solvent. Sonication is often used in conjunction with vacuum degassing.


 
2. Vacuum. This is very convenient because it can be combined with vacuum filtration of the mobile phase to remove both dissolved gas and particulate contamination. Like sonication, with which it is often combined, this technique works for premixed mobile phases, but is not recommended for on-line mixing; it is a "one-shot" technique, because the mobile phase immediately begins to re-equilibrate with air.


Vacuum filtration is often combined with sonication to remove excess dissolved gas from premixed mobile phases.


   
3. Helium sparge. A stream of helium bubbles will sweep dissolved air out of liquids (helium is virtually insoluble in most HPLC solvents, so very little helium replaces the air). Helium sparging is very effective; it can reduce the dissolved air in common solvents to levels below the saturation level of mixtures. This makes helium sparging especially suitable for use with on-line mixing systems. While helium sparging entails some risk of changing the mobile phase composition by selectively evaporating the more volatile components, this is usually not a problem if an excessively vigorous flow of helium is avoided (in other words, "don't overdo it!").


He sparge degassing system. Note the large "sinker" frit on the inlet line and the smaller helium sparging frit with the plume of He bubbles rising through the mobile phase.


   
4. On-line membrane degassing. In many respects, this is the most convenient approach to degassing. The mobile phase or solvent flows through a hollow fiber made of a semi-permeable membrane. A partial vacuum is maintained on the outside of the membrane. Because air can diffuse through the membrane while solvent vapor cannot, dissolved air is removed from the solvent before it reaches the pump. The membrane degassing module is usually plumbed into the inlet line immediately upstream of the pump, so that there is no opportunity for air to redissolve.


A membrane degasser uses a semi-permeable membrane passing through a vacuum chamber. Gasses diffuse throught the membrane and are removed. Solvents are retained.


   
 

 


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Last revised: April 06, 2001.