Chromatography Forum

This might seem a very basic question but...............
If you've validated your method is linear- say between 5 and 1000ppm. You run some calibrants - say between 10 and 100ppm. Your samples come out at say 7.5ppm or 120ppm - is it Ok to quote these figures given that you know your method is linear over that range?
I've had some difficulty with some QA people over this.

John

What I have seen, it is acceptable to use single point standards in the middle of a proven linear range. I think it is more important to repeat the standard after 4 or 6 samples than to run multiple points.

I think you should be fine.

If you have proven that the method is linear, then a single point calibration is suitable. For the best accuracy, it is better for the single point concentration to be similar to your expected sample concentration.

I agree, that replication of your calibration injection is important to confirm that your system is stable throughout the run.

Actually I'd never do a single point calibration. What if the intercept was non-zero? I was just talking about a multi-point calibration where the samples were just outside the calibration but not the validated linear range. I've had difficulty explaining that to QA people. But I suppose if they understood science they'd never have gone into QA in the first placed would they? The SOP wielding fascists!
John

At the risk of sounding pedantic, tell your QA people that results within the validated range are, by definition, valid.

That said, I am always more comfortable with interpolated than with extrapolated values. If I were regularly getting high or low values, I'd add an additional calibratant or two to bracket the wider range.

That approach probably best describes what I do.
Apologies to our brothers and sisters in QA for my strong remarks yesterday........... It must have been the beer!
John