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Baseline shows pressure pulsations

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Applies to

ALEXYS system, LC 100 pump


Pressure pulsations caused by the pump.


  1. Compressibility is not adjusted properly
  2. Air bubbles in the pump head or in the check valves
  3. Leaking connections, seals or damage pistons
  4. Non-miscible liquids in the pump head
  5. Clogged solvent filters
  6. Piston seal leakage
  7. Malfunction of the check valve (air or particles prevent closing of the valve)
  8. Damaged sapphire piston.

Diagnosis and Remedy

  1. Compressibility: change kappa to an optimal value.
  2. Air bubbles: in order to remove air bubbles from the pump head the purge valve is opened and the purge key is depressed for about one minute.
  3. Leakage or damage pistons: In order to check whether all connections, seals and valves in the whole HPLC system are sufficiently free of leakage the following pressure decay test may be performed. An empty column with about 1.5 ml volume is connected instead of the HPLC column to the HPLC system and then is filled with the eluent using the pump. When filled completely it is closed at the down stream end and the pressure then is increased using a very low flow rate. When the pressure has reached roughly 30 MPa the pump is stopped. Now the pressure drop with time is observed. The decay rate should not be larger than 0.2 MPa/min. If the decay rate is larger, all connections should be checked with respect to leakage before the check valves or the piston seals are interchanged.
  4. Check valves: clean check valves ultrasonically in methanol.
  5. Non-miscible liquids in the pump head: use a third liquid which is soluble in both solvents to flush the pump head.. It is good practice, to flush the pump head of an unknown pump first with water then with aceton, then with methanol and finally again with water. In this way buffer salts are dissolved first which otherwise could destroy the piston seal and then the organic eluents are washed out. Whenever buffer or other solutions of solids are used as eluents, the pump should be washed before and after use with water.
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