WEF Discussion Forums : Laboratory Management and Technical Issues : Matrix spike recovery calculations

Wed, 04 Apr 2012 23:45:21 GMT

Does anyone have a reference for calculating matrix spike recoveries when the analyte in the unknown sample is at the method detection limit? All the calculations I have seen instruct you to subtract the 'measured concentration of analyte before spiking' from the 'measured concentration of analyte after spiking'. What happens when the analyte is recovered below your PQL or, more important, below your MDL? Do you subtract the actual concentration, even if it is a slightly negative number (which does sometimes happen on the ICP-AES) or do you subtract the MDL concentration or do you not subtract anything at all?

Fri, 06 Apr 2012 12:36:34 GMT

Ruby,

Here's a quote from the EPA Inorganics CLP Statement of Work. It's in Section 12.6.7 of Exhibit C:

"When the sample concentration is less than the Method Detection Limit (MDL)...use zero...for purposes of calculating percent recovery."

There may be other references, but this particular one happened to be on my desk.

Chuck Lytle

City of Portland, OR

Tue, 30 Oct 2012 17:26:29 GMT

We use zero in this case as well.