Rebecca K. Wyss

Using Guard Bands to Accommodate Uncertainty in the Spark AES Analysis of Aluminum or Aluminum Alloys When Determining Compliance with Specified Composition Limits

Every pound of aluminum or aluminum alloys cast and sold is certified to meet The Aluminum Association Inc. registered limits or other specified composition limits. Certification of aluminum and aluminum alloys to specified composition limits is typically done using Spark-Atomic Emission Spectrometry (Spark-AES) following the procedures in ASTM International (ASTM) E716 Standard Practices for Sampling and Sample Preparation of Aluminum and Aluminum Alloys for Determination of Chemical Composition by Spectrochemical Analysis and ASTM E1251 Standard Test Method for Analysis of Aluminum and Aluminum Alloys by Spark–AES. Spark-AES laboratories at major aluminum production facilities normally have excellent analytical practices and follow strict quality control protocols to provide the best results possible. However, every measurement has an associated uncertainty and the measurement of composition using Spark-AES is no exception to the rule. This paper provides a brief discussion of: 1. The uncertainty inherent in the elemental analysis of aluminum and aluminum alloys by Spark-AES. 2. The benefits of using guard bands to set internal operating limits, which are offset from specified composition limits. 3. A model of the risk for sale of out-of-specification product based on the analysis uncertainty relative to the specified composition limits. 4. The main sources of uncertainty of Spark-AES and their potential causes.