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- Posts: 3475
- Joined: Mon Jan 07, 2013 8:54 pm
- Location: Western Kentucky
I had trouble setting up the MRM for 2,4-DB-d3 as I could not get the molecular ion in any abundance. Normal transition would be 250/164, but the 250 or any other mass around it was not present, but the 164 was, even at infusion. I build a method using 164 as the parent then breakdown masses from that in the transition. It holds very stable throughout all my calibration and sample runs.
I had built the MRM for 2,4-D-d3 using 222/164 and it was working well. Now when performing analysis the response will hold through the curve then drop to half as the run progresses, if I schedule it to run again back to back with the first run, the response will begin where it was at the end of the first pass, then fall about half after the second run of the calibration curve.
Now I am noticing that there is also a peak for the same transition as the 2,4-DB-d3 based on 164 as parent stacked on top of the 222/164 transition. It is as if I am losing molecular ion and only getting the major daughter ion. The longer the instrument runs, the worse it gets.
Has anyone ever experienced something like this? The mobile phase is water and methanol with acetic acid as modifier. Just not sure why the loss of molecular ion as the run proceeds. Chemically I would not think it is lost while the sample sits on the autosampler. Matrix is 50/50 water methanol with a small amount of formic to keep it acidic. Would there be something about the TurboV source in ESI mode changing to cause it?
The target analytes seem to follow the same process as the recoveries fall near 100% as the run progresses, yet the 2,4-DB-d3 recovery will increase to over 200% as the internal standard falls to 1/2 its initial response, which is expected as it does not lose sensitivity like the rest.