PT compounds running high

[eauchem]

I am attempting to run a PT sample for carbamates (method 531.2). I ordered the PT and the QC samples. I ran my normal QC: CCV, QCS, LRB, and a QC standard I ordered from the same PT company - all of these passed. I submitted my PT results and they all came back running high (123-140% of expected value).
I made up the QC and PT standards and post-column reaction reagents, as well as mobile phases new the day I ran.
I have re-ran my cal curve and ran newly prepared standards again with the same results.
I am running a Waters Alliance HPLC and Waters 2475 FLD.
I would appreciate any suggestions.
Thanks!
Liz

[Alexandre]

Dear Liz,

To troubleshoot PT is not easy, especially when your results not so bad 23-40% above the mean.
There are a lot of things that can happen and you should check every step of your procedure and try to understand its influence.

We do PT for carbamates but by LCMS. Your derivatization yield may be affected by matrix of this particular sample.
Secondly, the PT’s mean is established as average of all submitted results. If those were derived using other methods then there is a chance that your method may have a bias compare to the mean of PT.

If any sample left try standard additions and/or matrix matching calibration.

[Peter Apps]

Hi Liz

Wow ! ten acronyms in 2 lines, some of them I know, one I worked out, and the rest are a mystery. I also have no clue what process you follow in Method 531.2, so I can only offer generic advice.

Your quality control passed - but was the result higher than expected ? - in other words did it show the same trend as the proficiency test ? If so there is something systemic wrong with the method, the instrument, the sample prep or the calibration.

If the QC and PT results do not show the same trend, or if the trend is the same but the discrepancy from expected is markedly different, then examine in minute detail what you did - is one material in a different solvent, is there an extra dilution step with one of them, larger or smaller volumes measured with different techniques (syringe vs pipette for e.g.). Are the concentrations in the QC and PT approx the same so that you end up with peaks that are about the same size, and reading from the same part of the calibration curve. Did you run QC and PT as part of the same sample batch ? Etc etc.

Peter