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Article written by Thomas V. Coyner, President, Analytical Products Group
Complete Article from Edition 3 APG eNewsletter
As a PT Provider, we often receive questions on various test methods and procedures. One question that we receive is “Why are the PT Acceptance Limits so low for Total Phenolics compared to the True Value?” More frequently, we are asked, “Why did I fail Total Phenolics when all my QC Standards pass?” The method and the way acceptance limits are calculated is the key to understanding.
In the summary of the 4-aminoantipyrine Phenols method, Standard Methods reports that the method determines Phenol and several substituted Phenol compounds. However, buried deeper in the summary is a note that the response for substituted Phenols is reduced. Many industrial facilities that discharge substituted Phenols are aware of this limitation. However, the method recommends that pure Phenol be used for calibration and many labs use pure Phenol Quality Control Standards to monitor the method. This makes sense since the method recovers essential 100% of the Phenol present.
US EPA is concerned with the limitations of the method with substituted Phenol compounds and requires all PT providers to use a mixture of Phenol and substituted Phenol compounds for PT samples. They even specify the exact blend of material. This is important because only the correct blend of materials will give the correct percent recovery that the US EPA predicts for the method. The specified concentration of the sample is 0.04-5 mg/L and the sample is:
- 40% Phenol
- 20% 2-Chlorophenol
- 20% 2,4-Dinitrophenol
- 20% 2,4-Dichlorophenol
Based upon this mixture, US EPA predicts that the mean of laboratory percent recovery will range between 66% and 68%. Based upon this prediction, US EPA requires the PT provider to calculate the Acceptance Limits using a series of regression equations based solely upon the calculated True Value of the sample. If US EPA’s prediction is incorrect then good laboratories will fail their proficiency testing.
Many laboratories are unaware of the bias when testing substituted Phenol compounds because they typically analyze pure Phenol for both calibration and control samples. Similarly, laboratories with a positive bias in their method will receive higher recoveries than predicted and have issues understanding how they could possibly be wrong. However, a high bias is still a bias and an error in the performance of the method if the method is known not to respond to a compound.
This is just one of many questions we often answer. If you have a question or an issue with a method, give one of the Technical Support Chemists a call.
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