Dionex AXP/AXP-MS Manual
technique for degassing is to sparge the solvent with standard
laboratory-grade (99.9+%) helium. Helium is only sparingly soluble in
HPLC solvents, so other gases dissolved in the solvent diffuse into the
helium bubbles and are swept from the system. Solvent filtration is not
an effective alternative to helium degassing.
It is recommended that you sparge the solvent vigorously for 10 to 15
minutes before using it. Then, maintain a trickle sparge during use to
keep atmospheric gases from dissolving back into the mobile phase.
The sparged solvent must be continually blanketed with helium at 2 to
3 psi. Non-blanketed, sparged solvents will allow atmospheric gases to
dissolve back into the mobile phase within four hours.
Solvent mixtures using water and organic solvents (like methanol or
acetonitrile) hold less dissolved gas than pure solvents. Sparging to
reduce the amount of dissolved gas is therefore particularly important
when utilizing solvent mixture.
Even with sparging, some outgassing may occur. Installation of a
backpressure regulator after the detector flow cell will help prevent
bubbles from forming and thus limit baseline noise.
WARNING: Always release pressure from the pump slowly.
A rapid pressure release could cause the pulse damper
diaphragm to rupture.
2.4.2 Cavitation
Cavitation occurs when inlet conditions restrict the flow of solvent and
vapor bubbles are formed during the inlet stroke. The key to
preventing cavitation is to reduce inlet restrictions. The most common
causes of inlet restrictions are crimped inlet lines and plugged inlet
filters. Inlet lines with tubing longer than 48 in. or with tubing of less
than 0.085-in. ID may also cause cavitation.
Placing the solvent reservoirs below the pump level also promotes
cavitation. The optimal location of the reservoirs is slightly above the
pump level, but it is adequate to have them on the same level as the
pump.
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