Peak shouldering on ACQUITY UPLC Systems - WKB7007

SYMPTOMS

• 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

ENVIRONMENT

• ACQUITY UPLC I-Class 
• ACQUITY UPLC I-Class PLUS 
• ACQUITY UPLC H-Class PLUS
• ACQUITY UPLC H-Class PLUS Bio
• ACQUITY UPLC Premier

CAUSE

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

FIX or WORKAROUND

1. Replace the Active Preheater Assembly 

ADDITIONAL INFORMATION

Before attempting any part replacement, reseat the Active Preheater Assembly in the valve port.

The following picture shows an example of the void in the flow path that is responsible for peak shouldering:

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"