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Sudden shift from linear to quadratic calibration?

Question asked by mkpbphd on May 3, 2016
Latest reply on Mar 2, 2017 by gchaplain

I work in an environmental lab doing wastewater analyses based on EPA methods which do not currently allow for any calibration model beside linear or avg response factor. Recently, we've run into some issues where two different analytes, using two different instruments and two different methods will no longer calibrate linearly but exhibit excellent quadratic calibration (R2=0.999). Are there any common factors that can contribute to this observation? We've ruled out curve preparation, autosampler performance, and column installation. In both cases inlet maintenance has been performed with no improvement (liners, gold seals, septa, etc have been changed and the columns have been trimmed).

 

Here's what we know:

 

Kepone by EPA 8081B (GC-ECD):

Problem seems to have begun after installation of new column (01/16), but did not become unacceptable until recently.

 

Benzidine by EPA 625/8270 (GC-MS):

No obvious change associated with the beginning of the problem.

New column has been installed since, but no improvement observed.

 

Has anyone observed anything like this before? I'm not sufficiently familiar with other factors that affect calibration to guess at why we are observing this.

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