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Blank Subtraction
Pat
Posted: Wednesday, February 8, 2012 3:52 PM
Joined: 2/8/2012
Posts: 2


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?

 

 


Perry Brake
Posted: Wednesday, February 15, 2012 1:10 PM
Joined: 12/17/2010
Posts: 23


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.

 

Perry

 

              
 


Pat
Posted: Friday, February 17, 2012 3:43 PM
Joined: 2/8/2012
Posts: 2


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.

Pat


Perry Brake
Posted: Sunday, February 19, 2012 11:20 AM
Joined: 12/17/2010
Posts: 23


Paragraph 1020B4 of Standard Methods (in the section on QA/QC), says you should run a reagent blank in each sample batch.  This implies that a blank should be run in an oil/grease batch, where the "reagent" is the solvent.  But none of the methods for oil/grease (the 5520-series) say you should correct for the blank.  If the only thing that caused a high blank was the solvent, it might make sense to blank correct for any sample run with the same solvent since you could assume the solvent would contribute essentially the same  amount to each waste stream sample.  However, the glassware could also be a source of contamination contributing to the positive blank, and its contribution would be expected to be different for every sample.  My conclusion is that you are correct in not correcting for the O/G blank.