A Statistic that Identifies Errant Standard Preparation and Instrument Nonlinearity Demonstrated with Mercury Standards Prepared by Blending NIST Fossil Fuel CRMs of Similar Matrices

Two coal certified reference materials (CRMs) for mercury content were blended, and their results checked, resulting in National Institute of Standards and Technology (NIST) traceable standards at mercury concentrations intermediate to parent certified values. How are CRM blends checked? Interval-repeatability is a new statistic that can be employed as a quality control test to identify incorrectly prepared working standards or nonlinear instrument response over a targeted concentration interval based upon the established repeatability limit (<i>r</i>) of the analytical method used. It is applicable to working standards prepared by CRM dilution, as well as those prepared by CRM blending. Interval-repeatability is unique because it measures the precision between standards of different concentrations. For the measurements of standards to comply with the method precision requirements, the absolute value of observed interval-repeatability must be less than or equal to the analytical method’s calculated interval-repeatability limit for a given concentration interval. Interval-repeatability can provide valuable information to the CRM user regarding the internal consistency of standards, measurement precision, the success of standard sample preparation, and instrument linearity. The theory behind this approach is discussed, detailed instructions on its implementation are given, and interval-repeatability quality control test data for mercury in coal standards created by the CRM user are presented