Accuracy results

[smartin]

Hi,

Developing a LC-MS/MS method. The standard calibration with n=7 points is ok. The acceptance criteria for precision and accuracy are ok.

I repeat the analysis and now the standard calibration is ok, and also the precision. But the acceptance criteria for accuracy is not ok!!! (out of the 15%, except for the LLOQ, when it should be 20%)!!!!!!

On the other hand I have also problems with the accuracy for quality samples....

I would like to share with you this:

1.- Some reasons for bad accuracy for the calibration standard? The main could be to use a wrong calculation?
2.- Could be a problem of the extraction method? (in fact, I'm using a precipitation method).
3.- Could be something related to the MS/MS itself?

What do you think about it?

Thank you very much.

[Don_Hilton]

Without additional information, it is hard to say. Could it be stability of reagent, or analytes? Could it be the time solutions stand during preparation? Could it be..

A description of the experiments and results would give something to go on. How have you QC'ed the instrument for performance independently from samples coming throught the sample prepapartion. Are your analytes stable under storage condtions and under sample prep conditions?

[smartin]

Great, no problem, of course!!
What kind of additional data do you want?
In fact, I have 2 main questions:
1.- Why the back calculations of my calibration samples are out of the limits? It means, which can be the main sources of it?
I can understand that problems in accuracy for QC samples can be done by a bad preparation…. But for calibration samples? I can’t understand it.

2.- For QCs samples, of course, I have to check the preparation of them has been done correctly….

[Don_Hilton]

A calibration can fail because of non-llinearity or scatter in the data. We need more detail in how the calibration is failing. Did you run multiple injections at each level? What conentration range are we tlking about. How do the peaks look? Does the curve hook or do the data points scatter to both sides of the line?

More detail coming out of these might lead us to ask more questions about the preparation of the standards or of the performance of the instrument.

And knowing something about the anayte and LC conditions might be helpful.