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

Abstract

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? Intervalrepeatability 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 (r) 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 intervalrepeatability 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.

Software Package To Facilitate the Preparation of Intermediate-Range Fossil Fuel Standards from Certified Reference Materials

“…This example shows simulated data for all of the standards mixed as well as one of the unblended parent SRMs. In this example, the interval between SRM 2693 and the highest concentration blended standard falls outside the interval-repeatability limit of the method used and fails the I r QC test. This failure could be due to several causes, such as material heterogeneity, operator error, or the standard concentration being outside of the linear range of the instrument.…”

“…The QC test requires the user to enter the measured concentrations for each standard analyzed and the repeatability limit ( r ) of the method used. This measured concentration is compared to the concentration calculated for the blend and the interval repeatability limit of the measurement method, as described previously . At least one blended standard and one of the unblended certified parent standards must be prepared and measured for a QC test to be performed.…”

“…However, there are a few potential pitfalls in the mixing procedure that must be discussed to ensure that additional uncertainty is not introduced into the process. A detailed example of the blending process is reported by MacDonald et al, and this software is intended to be used in conjunction with related resources. ,, …”

“…Using masses below the minimum sample mass of a reference material can cause erroneous results, but in many cases, the minimum sample mass is too large to be practical for use with some current analytical instrumentation. This can be remedied by preparation of multiple replicates of a standard and verification of results with the I r QC test …”

“…Only total combustion methods can be used for solid mixtures, but liquid mixtures can be measured using the same analytical techniques appropriate for nonblended standards. Lastly, after analysis, the quality of the measurements must be assessed through quality control (QC) procedures and by checking the interval repeatability ( I r ) . Whenever using the mixing program discussed here, it is still important that each analyst must ensure that the measurements conform to the quality system of their organization.…”