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Negative y intercept LCMS
Posted: Sat Sep 30, 2017 7:39 am
by SonicW
Hi all,
What are some possible causes of a negative y intercept (i.e. a positive x intercept) in a calibration curve (ESI LCMS method)?
The standards seem to have been prepared correctly (they're MeOH based standards spiked into a blank urine matrix).
Thanks for your help
Re: Negative y intercept LCMS
Posted: Sat Sep 30, 2017 10:40 am
by Peter Apps
Welcome to the forum.
This is a hardy perennial. Possible causes include non-linearity, adsorbtive (or other) losses of analyte, a signal threshold set too high, not taking the purity of the standard into account, poor repeatability, .....
What your particular problem is will depend on the details of your analysis - tell us those and we will be able to help.
A search of the archives will throw up plenty to keep you busy also.
Peter
Re: Negative y intercept LCMS
Posted: Sat Sep 30, 2017 8:54 pm
by SonicW
Hi Peter, thanks for your reply.
So the issue is with Pholcodine. I use a mass-labelled codeine as internal standard.
I'll double check the purity when I'm in the lab next week.
For adsorbtive and other losses- do you think degradation of analyte in the standard master is likely? While codeine is reasonably stable in MeOH, I'm not sure about Pholcodine.
Thanks for your help.