Bioequivalence and Bioavailability Forum

It is a great example of something being or going out of whack, thanks, Shuanghe.

Perhaps the plot shows why it is such a good idea to use an IS. It saves the day.
And yes, that run may be acceptable by many standards for exactly that reason.

Yet, it is also appearing sick to the bone.

It may be that instrument sensitivity varies (check the level of IS across runs, try and plot means or medians for the types of interest), contrast this run with the subsequent and prior ones on the same instrument (most informative if they were done using the same working/spiking/stock solutions). If the trend is not a one-off case then one of the next runs may face issues with saturation or other. The between-runs IS plots whichever way you create them can be very, very informative and highly predictive for the purpose of telling if the method or instrument is about to quit on you, but note I started a tread on this some months ago and no-one on this forum acknowledged what a genius I am

If another instrument on the same power line in the lab was active at the time this happened, even if it was on another project and with other transitions, try and look if you saw a fluctuation there, too. the lab may not grant you permission to look at this if they are independent.

The injector may be unstable. From this plot it cannot be determined if the injected volume has an upward trend or if is the instrument sensitivity which varies (or, both, or hypothetically a case of within-run variations in (matrix effect on) ionisation properties, something I have not manifestly faced ever). At the investigation stage, I do not recommend to speculate too heavily into the root cause when defining what to investigate, because then you might be restricting yourself a bit in terms of the data collected. But I know this is difficult and certainly impossible to completely avoid.

And check the various temperatures over time, as applicable (vaporiser, heat block, etc).
That info may not be easily accessible but it is usually extractable on systems provided by the big vendors.

If you figure the cause out, I'd love to learn of the outcome.

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Edit: Merged with a later (now deleted) post. [Helmut]