Comparison of SW-846 Method 3051 and SW-846 Method 7471A for the Preparation of Solid Waste Samples for Mercury Determination


J. M. Giaquinto
A. A. Essling
J. M. Keller

The Radioactive Materials Analytical Laboratory (RMAL) located at Oak Ridge National Laboratory (ORNL) is participating in the characterization of transuranic waste targeted for the national Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico. The waste streams contain high concentrations of alpha emitting radionuclides and therefore all raw sample handling and preparations require alpha containment (radiochemical hoods, gloveboxes, or hotcells). To reduce the risk from sample exposure and increase sample throughput, it was be beneficial to prepare for all metal analysis (ICP-AES, ICP-MS, GFAA, and CVAA) in a single digestion with a minimal amount of sample handling. SW846 method 3051 (Microwave Assisted Acid Digestion of Sediments, Sludges, Soils, and Oils) is ideal for this situation. The use of this preparation method would allow for a single fast and efficient digestion for all metals targeted by the WIPP program.

The recent update (Update II) to SW-846 includes an approved method 3051 for the preparation of samples for metals analysis for techniques such as ICP-AES, ICP-MS, and GFAA, but does not include mercury analysis by CVAA. Also, SW-846 method 7471A (Mercury in Solid or Semisolid Waste (Manual Cold-Vapor Technique) does not allow for alternative preparation methods. As part of the WIPP Transuranic Waste Characterization Program the RMAL investigated in collaboration with the Analytical Chemistry Laboratory, located at Argonne National Laboratory, the feasibility of microwave digestion for mercury determination by cold vapor atomic absorption.

For this study four standard reference materials (SRM) were used to cover the range of matrices and mercury levels that most likely would be encountered during the sampling and characterization program. The laboratories independently prepared each SRM in batches of five and analyzed the digestates using method 7471A. For the microwave preparations each SRM batch was digested using three different microwave ovens (one pressure controlled, one temperature controlled, and one high pressure oven that was temperature controlled) using the pressure/temperature profiles specified in method 3051. A conventional water bath preparation as specified in method 7471A was also included.

This report presents the results of this study and examines how each method compared to the acceptance criteria established in the WIPP TWCP Quality Assurance Plan.

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