Statistical Uncertainty Associated for the Certified Value of a Cr (VI) ICP RM

Ralph Obenauf, PhD SPEX CertiPrep, Metuchen, NJ 08840, ROBENAUF@SPEXCSP.com 

www.SPEXCSP.com  

Abstract

Certified values of Reference Materials (CRM) are not useful unless accompanied by a stated and precisely defined uncertainty.  If certificates of analysis are examined, one often finds inconsistencies, misstatements and errors related to uncertainty (dispersion of the value that could reasonably be attributed to the measurand) and stability (change in value over time).  A precise understanding of measurement uncertainty on the part of the user is required as well as confidence that the reported uncertainty is calculated and defined correctly.  Your measured values are only as good the uncertainty of those results. 

 The paper will cover the evaluation of the total expanded uncertainty arising from all sources and covering all processes involved in determining the certified value of an ICP CRM.   This determination will be for a chromium (VI) reference material whose certified value was determined by titration against a standardized sodium thiosulfate solution which was in turn standardized against a potassium dichromate standard reference material, SRM.  Chromium (VI) is one of the six substances listed in the EU RoHS (Restriction of Hazardous Substances) directive 2002/95/EC requiring electronic equipment producers to limit these substances in their products.   

The above method was selected for discussion as it contains examples of many of the common procedures used in analytical determinations, wet or instrumental.  Therefore the uncertainty calculations here can be directly applied to other analytical methods to determine the statistical uncertainty of other common laboratory measurements such as those by ICP and ICPMS.  

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