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- Posts: 2
- Joined: Tue Feb 13, 2018 1:05 am
I have an LC-MS/MS method for a compound we have been using for the past 4 years. Recently we have had a problem where we observe a peak with the same retention time and same MRM as our compound in blanks.
I can only assume it is some form of carry-over or contamination. However, this has not been the case for the last 4 years. Cleaning the column and system did not seem to help. Perhaps the cleaning procedure was not strong enough. A new column (and on a second system) did help, with no signal at all for the MRM in blank methanol injection after 200 samples.
The problem has resurfaced when a user installed the column backwards (i.e. column then guard, in the opposite to normal flow). Now there is a peak for our MRM again.
Why I haven't been able to fix this problem: Following a brief clean with 70% isopropyl alcohol, the peak was gone. Multiple runs through the gradient with no injection show no response. All is good. Then, a fresh methanol solution was injected (10uL) and the peak is back. Subsequent injections of methanol only increase the MRM peak height.
I am very confused. Anyone with more experience have any idea what is going on?
Details:
Mobile phase typically 70%acetonitrile:water, with 0.1% formic acid.
Column is Zorbax SB-C18 2.1x50 mm 5-Micron.
System is Agilent 1200 HPLC with Agilent 6410 QQQ
Compound MRM is 602>>418 m/z
TL;DR: Stable, validated method now has response to analyte increasing with each injection of methanol (blank).
Cheers,
Matt