WEF Discussion Forums
Laboratory Management and Technical Issues
Does anyone know if there is a guidance document that clarifies which inorganic tests should utilize blank subtraction and which ones should not?
TSS for instance, do you subtract your blank filter?
Pat, the method generally tells you whether or not to correct for the blank, but assuming that you may be in a lab supporting a wastewater treatment plant doing of a few test, I can save you looking it up for the tests you probably run.
TSS - When you weigh the dried filter for each sample and then subtract that weight as part of the calculation, you are essentially correcting for the "blank" even though it is not called a blank in the method. If you do a "blank" as a separate QC test, its weight should NOT be used as a second correction.
BOD - The method specifically says to NOT correct for the blank. Instead, you try to eliminate it in subsequent analyses.
Chlorine - You are correcting for the blank when you "zero" your meter during the analysis of each sample. No further correction is needed.
pH - There is no such thing as a blank for pH analysis.
For other inorganic tests, carefully read the method.
Thank you Perry for the reply and good ideas. Some methods are a little vague but with many of them I can look at the recommended calculation and I hadn't thought of that. We were subtracting the blank for O & G from our OPR but I think we should not have been and have discontinued that practice.