Box 3-1 Standard Reference Materials

Systematic error is a consistent error that can be detected and corrected. Box 3-1 describes Standard Reference Materials designed to reduce systematic errors. 

Precision describes the reproducibility of a result. If you measure a quantity several times and the values agree closely with one another, your measurement is precise. If the values vary widely, your measurement is not precise. Accuracy describes how close a measured value is to the true value. If a known standard is available (such as a Standard Reference Material described in Box 3-1), accuracy is how close your value is to the known value. 

Analyze a known sample, such as a Standard Reference Material. Your method should reproduce the known answer. (See Box 15-1 for an example.)... 

You purchased a Standard Reference Material (Box 3-1) coal sample certified by the National Institute of Standards and Technology to contain 3.19 wt% sulfur. You are testing a new analytical method to see whether it can reproduce the known value. The measured values are 3.29, 3.22, 3.30, and 3.23 wt% sulfur, giving a mean of 3r = 3.260 and a standard deviation of, v = 0.04,. Does your answer agree with the known answer To find out, compute the 95% confidence interval for your answer and see if that range includes the known answer. If the known answer is not within your 95% confidence interval, then the results do not agree. 

Specifications could include required accuracy and precision, reagent purity, tolerances for apparatus, the use of Standard Reference Materials, and acceptable values for blanks. Standard Reference Materials (Box 3-1) contain certified levels of analyte in realistic materials that you might be analyzing, such as blood or coal or metal alloys. Your analytical method should produce an answer acceptably close to the certified level or there is something wrong with the accuracy of your method. 

Analyze a Standard Reference Material (Box 3-1) in a matrix similar to that of your unknown. Your method should find the certified value for analyte in the reference material, within the precision (random uncertainty) of your method. 

Only one isotope, Pu-244, is presently available as a NBS Standard Reference Material and can be obtained from the DOE New Brunswick Laboratory. A second NBS/SRM as a 1 1 mixture of Pu-239 and Pu-242 is in preparation and a third, Pu-239, is planned for the future. All the isotopes are available from the ORNL Isotopes Sales Office, Oak Ridge National Laboratory, P. 0. Box X, Oak Ridge, Tennessee, 37830. 

Case 1 Box Practical Tips on the Subject Materials, Reagents, Reference Standards... 

---