Development and Validation of the New EPA Microwave-Assisted Leach Method 3051A

An updated, optimized method has been developed to minimize the chemical biases and inefficiencies between two EPA SW-846 alternative leach methods, microwave-assisted Method 3051 and open vessel Method 3050B, for the determination of metals in solid samples. The microwave method is more easily transferred and provides more reproducible leach results, mainly from more precise control of the temperature during the extraction process. Previously, reagent limitations originally placed on Method 3051 did not allow these alternative methods to yield similar results for some analytes. This work describes the development and validation of EPA Method 3051A, which includes an option to add HCl to improve the chemistry of the microwave method, making it comparable to open vessel Method 3050B. Including HCl provides complexation and stabilization of some analytes by Cl- species, leading to improved recoveries. Optimizing the acid combination involved adding either 1, 3, or 5 mL of HCl to 9 mL of HNO3 and performing a series of leaches on standard reference materials (SRMs). Optimum recoveries are achieved for “problem” analytes, such as Ag and Sb, by adding 3 mL of HCl. Recovery of other metals, such as Cu and Ni, are unaffected. Method validation was provided by leaching and analysis of SRMs.