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Peak shouldering on ACQUITY UPLC - WKB7007

Article number: 7007


  • Poor chromatographic peak shape: peaks are eluting with shoulders
  • New column, sample chemistry, and mobile phase do not resolve the issue
  • Injector valve and needle are new
  • All leak tests pass and system pressure is stable
  • The APH fittings at the column connection are properly made


  • ACQUITY H-Class System
  • ACQUITY I-Class System
  • ACQUITY M-Class System


A void is created due to improper seating of the ferrule on the Active Preheater Assembly at the valve port.

The following shows the void in the flow path that is responsible for peak shouldering.

  • Void.JPG


  1. Disconnect and reseat the APH fitting connection to reseat the ferrule into the valve port.
    • If the ferrule is seated too close to the end of the tubing, thus creating a void, replace the assembly.
  2. Replace the Active Preheater Assembly.


To assemble the new stainless steel (gold-plated) fitting to the valve port:

  1. Insert the end of a tube into the hexagonal end of the compression screw.
  2. Insert the tube into the larger end of the ferrule.
  3. Insert the tube into the fitting body.
  4. Rotate the compression screw, clockwise, into the fitting body until the screw is finger-tight.
  5. Using a permanent marker, mark the compression screw at the 12 o’clock position.
  6. Mark the fitting body at the 9 o’clock position.
  7. Ensure that the tubing makes contact with the bottom of the valve mating port, and use a 1/4-inch open-end wrench to rotate the compression screw clockwise 3/4-turn, until the two marks line up.

For column connection: See the article "How to tighten the 3 piece fitting on the APH"


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