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Is microwave digestion an approved method?
The short answer is 'yes'.
There are several regulatory agencies that have approved methodologies for microwave sample preparation including the US EPA, ASTM, AOAC, FDA, and USDA. Microwave digestion methods are also currently under development for USP.
This is actually a difficult question to answer as there are many approvals but they depend what metals you are looking for.
The following are microwave digestion approved methods:
U.S. EPA Methods
EPA Method 3052
Microwave-Assisted Acid Digestion of Siliceous and Organically Based Matrices
EPA Method 3051a
Microwave-Assisted Acid Digestion of Sediments, Sludges, Soils, and Oils
EPA Method 3015a
Microwave-Assisted Acid Leach of Aqueous Samples and Extracts
The EPA methods can be downloaded here.
ASTM (American Society of Testing and Materials) Methods
ASTM E1645-01
Standard Practice for the Preparation of Dried Paint Samples by Hotplate or Microwave Digestion for Subsequent Lead Analysis
ASTM D5513-99
Standard Practice for Microwave Digestion of Industrial Furnace Feedstreams and Waste for Trace Element Analysis
ASTM D4309-96
Standard Practice for Sample Digestion Using Closed-Vessel Microwave Heating Technique for the Determination of Total Metals in Water
ASTM E1741-00
Standard Practice for Preparation of Airborne Particulate Lead Samples Collected During Abatement and Construction Activities for Subsequent Analysis by Atomic Spectrometry
The ASTM methods can be downloaded for a fee at www.astm.org
USCPSC (US Consumer Product Safety Commission) Methods
CPSC-CH-E1001-08.1
Standard Operating Procedure for Determining Total Lead (Pb) in Children’s Metal Products (Including Children’s Metal Jewelry)
CPSC-CH-E1002-08.1
Standard Operating Procedure for Determining Total Lead in Non-Metal Children’s Products
CPSC-CH-E1003-09
Standard Operating Procedure for Determining Lead (Pb) in Paint and Other Similar Surface Coatings
The USCPSC methods can be downloaded here.
US Pharmacopeia Methods
The following methods are draft methods and as yet not approved, but feature microwave digestion as a sample prep technique.
USP 232
Elemental Impurities – Limits
USP 233
Elemental Impurities – Procedures
USP 2232
Elemental Contaminants in Dietary Supplements