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Complete Article from Edition 43 APG eNewsletter
Article written by Jeffrey Stewart, Quality Control Chemist, APG
According to EPA method 524.2, a continuing calibration check must be analyzed. If the value for each analyte is within 30% of the true value, the instrument is considered still in check and analysis may continue. This is fine for routine analysis, but there is a gap between method quality control and proficiency test sample evaluation.
If the concentration of the analyte is below 10ppb, the acceptance range for the analyte is + 40% according to the NELAC FOT tables. If the continuing calibration check is within the 30% set forth by the method, the lab results for that particular analyte should fall within the acceptance range.
The issue begins when the concentration of the analyte is above 10ppb. According to the NELAC FOT tables, the acceptance range is + 20%. If the continuing calibration check for a given analytes falls between 20%-30% of the true value, the data for the PT sample quite possibly is outside the NELAC acceptance range. One would not know for sure if the data for the PT analyte is acceptable within 20% even if the continuing calibration check is acceptable.
It’s a touchy subject to suggest a laboratory add an additional quality control check to routine analysis or set internal criteria tighter than a method requires. A lab could make a continuing calibration check at 10ppb and analyze it. If the results are within 20% of the true value, the lab should feel more confident in their data. However, in a NELAC audit this may be considered special treatment of a PT sample. A more effective solution would be to have internal criteria on continuing calibration check standards at ± 20%.
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