Article written by Jeffrey Stewart, QC Chemist, Analytical Products Group, Inc.
Complete article from Edition 31 APG eNewsletter
Why do I fail the Orthophosphate in the Drinking Water Nutrients?
The problem may not lay with your laboratory results. The problem could be contributed to the EPA acceptance limits. The EPA acceptance limits are based on the true value multiplied by a series of regression constants. In the past, the regression constants were setup in such a way that the acceptance range was very tight. This tight range would cause a lab with a reasonable recovery to fail because its result may not have been within the acceptance range.
Just recently, the EPA accepted the new NELAC regressions for all analytes list in the combined EPA and NELAC Fields of Testing (FoT) table. One of the changed analytes was Orthophosphate as Phosphorous. With the new regressions, the acceptance range is much wider. As a result, more laboratories that have failed in the past may now pass.
For example, if the true value for orthophosphate was 0.5mg/L, the original acceptance limits were 0.431mg/L-0.546mg/l. With the new regressions, that same sample will have an acceptance range of 0.399-0.614mg/L.
My Orthophosphate result is three times the true value on the Certificate of Analysis?
What is going on with the result? The answer is right at your fingertips. You must understand how the instrument is calculating the Orthophosphate result. Some instruments manufactured by Hach calculate Orthophosphate as Phosphate (as PO4). APG's sample for Orthophosphate is calculated as phosphorous (as P). The conversion between Orthophosphate and Phosphorous is the Orthophosphate result multiplied by 0.3322. This would give the result for Orthophosphate as P.
Another potential source of error to consider when determining the orthophosphate result is how the instrument was calibrated. If the instrument was calibrated using orthophosphate as PO4 standards, then the result must be converted to phosphorous or P. If the instrument was calibrated using an Orthophosphate as P standard, then no conversion is necessary because the conversion has already been factored into the calibration material.
With the new regressions for orthophosphate from the EPA and NELAC and knowing how the instrument calculates Orthophosphate results, you should have a better chance of passing the analysis.
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