Measurement Uncertainty

Using an ICP instrument, Damon reported his method uncertainty as ±10%. To facilitate a higher degree of confidence, Damon's management asked him to report the standard deviation associated with his three triplicate measurements of the sample as his uncertainty or confidence in the number. Fearing this may not be the best approach, Damon sought Paul's advice on the measurement uncertainty associated with ICP.

	RE: Measurement Uncertainty Dear Damon,      The measurement uncertainty is 2(SD)/1.716, where SD is the standard deviation. The 2 is referred to as the coverage factor and the 1.716 is the square root of 3 (for three measurements).      The analytical community and quality standard community support the idea that all data should be reported with a defined uncertainty. A more realistic uncertainty is to include the uncertainty of the standard used for calibration and the weight and volume measurements made for the sample preparation. The standard deviations of each of these operations must be determined. Then the overall standard deviation should be calculated by taking the square root of the sum of the squares, instead of just the measurement standard deviation. Thank you for contacting Inorganic Ventures. I hope this helps.     Paul R. Gaines, Ph.D. Serving you in chemistry