[SpiritAnalytics]

Hello Everybody

I work in a Distillery analysing our products and incoming raw Materials with an GC using FID detection.

Recently i recallibrated the System and afterwards an anomaly appeared which i cannot make sense of.
I have checked and double checked my Steps, calculations and Settings but found nothing wrong.
So i turn to You in hope of getting an idea whats going wrong here.

Usually i force the calibration Curve thru the Zeropoint. It makes Sense to me that Zero Concentration should equal Zero Detector Signal above Baseline. This has until now worked nicely for me as the callibration Curve did match up with the zeropoint with minimal deviation.

In my latest Callibration one of 34 Substances in my Standard came out different. The 36 Datapoints (6x6) show a linear Line but the Slope in no way fits the Zeropoint.

I made some Pictures so you can see my Problem:





With this i have two Options and none of them is really satisfying:
1. Ignore the Zeropoint and work with The Linear callibration Curve providet by the Data. This results in negativ Values for lower Concentrations. I can hardly send out Analysis-Reports containing negative Concentrations as this is plain false.
2. Force Zero and have R-Squarred at 0,79 when it should be at least 0,99. This callibration Curve does not really have any relation to real Concentrations and is thereby unusable.

Basically this Callibration is not usable and i dont know what caused it.
I have done two Callibrations recently and both came up with this problem so it aint a one time Thing either.

In my earlier Callibrations this Problem did not occur, therefor the error whereever it lies must have come with this new Callibration and it only happens for one out of 34 Substances.
Furthermore i have run Blankruns before and after the Calibrations which show nothing but a Baseline.

Im at my wits end and i hope one of You see´s where the Problem lies or has encountered this himself in the past and can help me get it fixed.

P.S.: i read your topic about "Use of origin/zero in calibration curve" with interesst but all i got from it was you shouldnt force but if you have to you got to do it. And im not really asking about if its right/wrong but more looking if anybody knows what could cause this behaviour.

Thanks for your efforts,

SpiritAnalytics

[P]

I am sorry if this is way out - I do not know your system. If the baseline is showing a positive (rather than zero) - could it be that there is some contamination? If there was a trace of methylacetate there already in the machine then it could give a positive reading even if your test/calibration sample has zero in it. Could the calibration sample have got some contamination even?

As I said - sorry if this is way off - just my first thoughts/guesses. This is for 1 sample in particular, the other standards still zero fine yes?

[marquis]

The first step with troubleshooting GC is to look at the chromatograms.  Is anything weird there? A coeluting peak for example? Take a look at the chromatograms and get back to us.  Good luck.

[SpiritAnalytics]

Sorry for the late answer but work had me busy.

@P "Could the calibration sample have got some contamination even?"
You nailed it. I wouldn´t have thought it possible but this was the case. Dont understand how it happened but never the less it did.

@marquis I had checked the Chromatograms before posting the first time. But thanks for the Idea

The Culprit has benn found!

Our Solvent Ethanol was contaminated with the Substance in Question. Thereby the Concentrations in each Calibration-Solution was not what i had calculated but what was calculated plus a Value that was equal in all Solutions. This explains why the Calibration Curve was off.

To clarify the Blankruns i made before my first post where runs without any injection. Just to check if my System is Contaminated somehow. After the post was made, i made a run with our Solvent solution water-Ethanol mixture which clearly showed a Peak for the Substance in Question.

Hopefully this Thread can help other who may encounter the same Problem in the Future.



Good bye and thanks for your Help

SpiritAnalytics

[P]

Well worked out. Thanks for coming back and letting us know what happened!

Good luck in the future.