Hi Ladies & gentlemen, Does combining old (500 microlitres(400 microgram/mL) and new(400 microgram/mL)( stock 16 US EPA Mix PAH affect my calibration plot? Am NOT getting a good R squared value. Could the problem be as a result of my combining old and new stock solutions? Or Is my HPLC machine not operating correctly?
You should have checked the new solution with the old one (old calibration) before even thinking about mixing the standards, In this way you can check if the new standard is equivalent to the old one (whithin your QC limits), otherwise you have to investigate about the mismatch (conservation, evaporation, light exposure...whatever).
Your bad R has nothing to do with that, unless one of the stock solutions is so degraded that you actually are not measuring a PAH but something else...
I suppose that each batch of calibration mixtures has a specific certificate with the exact (not the nominal) concentrations of each PAH. This should be checked. If the two standards had somewhat different concentrations, this may cause a problem. But if you mixed the two (old and new) solutions and made dilutions after mixing, the linearity should be maintained, although the exact concentrations in the "mixed (old/new)standard" will depend on the initial concentrations and the respective amounts of the old and new stock solutions.
You should have checked the new solution with the old one (old calibration) before even thinking about mixing the standards, In this way you can check if the new standard is equivalent to the old one (whithin your QC limits), otherwise you have to investigate about the mismatch (conservation, evaporation, light exposure...whatever).
Your bad R has nothing to do with that, unless one of the stock solutions is so degraded that you actually are not measuring a PAH but something else...
In general, it is bad practice to mix "Old" and "New" solutions of calibration mix together. In doing so you create a non-certified mixture. Document the old mix, run the new mix.
You have answered your own question; your calibration WAS affected by mixing old and new stock.
Would mixing old and new stock be expected to affect a calibration ? - yes.
Should you have mixed old and new stock ? - no.
Nonetheless, as Jean-Francois Gal says, if you dilute the mixture properly its being a mixture should not affect linearity per se.
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