Chromatography Forum

Dear all,

I am facing a big problem with my LC-ESI-QQQ.
I am running a multi-method with ~20 analytes. All of them exhibit linear calibration curves over the working range.

Accidentally I measured 2 samples that were very high in 4 of the analytes.
Since then, the calibration curves for those compounds are "flat" (no slope), i.e., all concentrations give more or less the same signal.

I first thougt of a contamination somewere in the LC-system, as the blanks were a little compromised, however, after cleaning the system, the blanks are clean, still the calibration standards give the same response for any concentration (10 to 70 µg/L).

Can there be a phenomenon such as a memory effect for those ions? all other analytes behave normal...

Did anybody experience this?

Thanks
Georg

Perhaps the problem is not at the LC side. Look at the signals for those particular before and after the injection of high concentration: is the baseline elevated? It is possible that those analytes are being detected continually, and just give a slight bump when they elute.

I would start with performing normal ESI maintenance: cleaning the cone and the chamber.

For troubleshooting, you could manually check the MS signals when there's a flow of mobile phase over the column. Check if you can find the compounds like when you're tuning, but without introducing them.

Thanks for the hint.

The baseline was elevated before HPLC maintainance, now the baseline is acceptable.

The signal for any standard (independent of its concentration) is about the response for the medium standard (40 µg/L) of the calibration before the problem, however, the blanks are clean... looks weird.

Anyway, I will do a extended ESI-source and ion focusing unit maintainance, maybe it helps.

BR
Georg