Hi,

I am new to nitrate testing, and I had some questions concerning the calculations for method 353.3. I hope to certify the lab for the new Hach method during our next audit, but in the mean time I'm using the cadmium reduction method.  The method states that the mg/L calculations need to be calculated from computing sample concentrations from the standard curve (absorbance of standards against nirate concentrations).  I am using a DR 2800 Spectrophotometer, and have the option for the equipment to give me the results in mg/L. For quality control purposes, I can create a standard curve plotting mg/L against concentration, and the results are the same.   Is it acceptable to incorporate a method modification to get my results directly from the spectrophotometer?

Also, are there any bench sheet/calculation examples??  I think the calculations are incorrect for my lab bench sheets?

Thanks for your help!!

Jessica,

Unfortunately, the question of what's acceptable during an audit is up to the auditor.  The Hach manual should tell you how the instrument calculates sample concentrations.  Things to watch out for are:  (1) does the instrument force the "zero" cal standard through zero?  (2) does the instrument use any kind of weighting to calculate the slope? (3) rounding.

An easy thing to do might be to calculate your own slope and intercept off a cal curve using absorbances, then use that data to calculate the results for a couple of made-up samples.  Then let the instrument calculate the results on its own and see if you get the same numbers.  If you do, I can't imagine an auditor making you do all the math by hand.

As far as bench sheets being "wrong," the question can't be answered in the forum w/o actually seeing an example bench sheet.

Chuck Lytle