The following are approved Direct Mercury Analysis method for the EPA and ASTM:
U.S. EPA Methods
Mercury in Solids and Solutions by Thermal Decomposition, Amalgamation, and Atomic Absorption Spectrophotometry.
Determination of Total Vapor Phase Mercury Emissions from Coal-Fired Combustion Sources using Carbon Sorbent Traps.
The EPA methods can be downloaded here.
ASTM (American Society of Testing and Materials) Methods
Standard Test Method for Total Mercury in Coal and Coal Combustion Residues by Direct Combustion Analysis
The ASTM methods can be downloaded for a fee at www.astm.org