HPLC check standards higher than the calibration standard

[ClineJosh]

Hello guys, was wondering if someone could help me out with an issue I am having. I am fairly new with high pressure liquid chromatography. My issue is when calibration known standards, the results come out like should. However, when running a check standard of same volume and known amount it comes back higher. For example, when running a level 1 calibration standard my ethanol amount is at a 12, where it is supposed to be. Now using the same standard and using it to check the calibration I receive a 16 ethanol amount. I have not had this problem in the past, and I am just reaching out for some ideas as to what might be causing this. Thank you

[James_Ball]

If that is the lowest concentration of the calibration curve, it could be a problem with the fit of the line to the points. If you examine the points closely and that point is slightly above the line, then it will return a value slightly higher than the known amount.

Does the response remain near the same when running it the second time?

Have you quantified the actual calibration standard back against the curve to see if it does the same thing?

It could be the low concentration portion of the curve is slightly quadratic instead of linear and that would cause it to quant back high.

[DR]

...and have you gone over any factors in the data system that might need updating if you're comparing a new standard against an older check standard.

Have you compared the peak pairs to see if they are really different as compared to prior runs?