The following are approved Direct Mercury Analysis method for the EPA and ASTM:

U.S. EPA Methods

U.S. EPA 7473

Mercury in Solids and Solutions by Thermal Decomposition, Amalgamation, and Atomic Absorption Spectrophotometry.

U.S. EPA 30B

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

ASTM  D6722-01

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