Reasons for >100% extraction recoveries

[wan]

Given that extraction recovery values had been corrected for blank values, what are the possible reasons for extraction recoveries of more than 100%?

If recovery values are between 95% to 105%, they are typically accepted. Is that extra 5% used to account for random errors?

[tlahren]

It could be just the variability in your whole analysis. I know for GC/MS methods to expect less than 5% variability for the entire methods process (including sample prep etc.) is kind of unusual. Positive bias on a GC system could be related to active sites in the chromatography system. Example: Sometimes with new GC liners and columns and for certain analytes there are active sites which cause the system to give a lower response than they will after running a few samples. i.e., If you calibrate on a new liner with new column, the samples or standards you analyze after this may be biased slightly high. I have seen this happen mostly with GC/MS analysis of PAHs and also on GC/ECD for PCBs. I would not be too concerned about a 105% recovery. If you look at U.S. EPA methods http://www.accustandard.com/asi/pdfs/ep ... /8270D.pdf you will see many tables with extraction stats for many semivolatile compounds. Note that some % recoveries are greater than 100%. This is acceptable for some compounds. If you're working with LC then I'm far from an expert but I would imagine that a similar trend would be observed as it is still related to extraction and chromatographic principles (ab/adsorption etc.)

Hope this helps.

[tom jupille]

Is that extra 5% used to account for random errors?

As a crude approximation, yes. It's outside the scope of a Forum post, so grab a statistics textbook and read up on "confidence intervals".

[wan]

tlahren: Thank you so much. That helps a lot in answering my questions.

tom: Noted. Thanks.