Directory: 04 Poor Sensitivity
File name: 03_Post_column_leak

Note: There was no problems with mobile phase or needle wash or any method changes.

Symptom #

It's our benzodiazepine high calibrator. The retention time is the same, the peak shape is the same. When overlaying peaks from today with a peak from before, the retention times are the same, but the highest peak is only about 12% of the typical performance.

Investigation, Troubleshooting Tool and Root Cause #

Investigation and Tools used:

1. Q: Check the system suitability tests results.
   A: test passed.
2. Q: Had a feeling of something not right with the LC.
   A: There was a pool in the tray underneath the post column switch.

Root Cause: Leak underneath the post column switch. The reason that chromatography was normal is that the leak was post column, the separation had already occurred. And the nature of this leak I guess, was such that 90% of the flow ended up in the tray beneath the valve and 10% of the flow went to the mass spec without any distortion in the peak.

Lesson Learned: why did the system suitability test pass if there was a leak present? Well, we had set the threshold too low.

Question for follow-up: Could pressure trace be handy for detecting post-column leak?

Reference: MSACL. The Basics of LC-MSMS Troubleshooting: Tools, Strategy, Cases

Go to “POOR SENSITIVITY”