Microwave-accelerated extraction (MAE) was evaluated by the authors of this review in 1999 (1). Since this time, the United
States Environmental Protection Agency (USEPA) has promoted the use of SW-846 Method 3546 using MAE for the extraction of
organic compounds from solid matrices. Applications for this technique have increased and MAE equipment has been streamlined
to dramatically increase throughput and ease of use. This article reviews the latest enhancements to this technology from
a hardware and applications perspective.
In 1999, microwave-accelerated extraction (MAE) was a relatively new technique for solvent extraction, and approvals were
only obtained when the technique became mainstream. The first approval was from the state of California under its California
Environmental Technology Certification programme for the extraction of semivolatile organic compounds in soil, sediment, and
sludge (2). As the technique substantially reduces sample extraction time and solvent consumption, the sample turnaround time
for data generation improved significantly. The certification was intended to encourage use of the technique where data quality
objectives could be met by its use.
The United States Environmental Protection Agency's (USEPA's) Test Methods for Evaluating Solid Waste (SW-846) provides a
comprehensive source of information on sampling, sample preparation, analysis, and reporting for compliance with the Resource
Conservation and Recovery Act (RCRA). SW-846 outlines test procedures used to characterize solid waste in accordance with
40 CFR Part 261, Identification and Listing of Hazardous Waste. The sample preparation and analytical procedures (or determinative
steps) are categorized by the analyte, which can be inorganic or organic.
The sample matrix and analytes define the SW-846 3500 series sample extraction methods. The matrix is either aqueous, solid,
an air sampling train, or non-aqueous soluble. Analytes are characterized as either nonvolatile or semivolatile organic compounds.
All samples analyzed for non-volatile or semi-volatile organic compounds require a solvent extraction step, with the exception
of non-aqueous solvent soluble samples.
Microwave extraction (Method 3546) was formally included in SW-846 in Final Update IV of the Third Edition of the manual in
2008 (3). Microwave extraction is the process of heating solid sample–solvent mixtures in a sealed (closed) vessel with microwave
energy under temperature-controlled conditions. The temperature is elevated significantly above the atmospheric boiling point
of the solvent, accelerating extraction while giving performance comparable to the standard Soxhlet method. Solvent consumption
is only 25–50 mL per sample.
The American Society for Testing and Materials (ASTM) is an international standards organization that publishes voluntary
consensus technical standards, including test methods that define how a method is performed and the accuracy of the result.
Test results may be used to assess compliance with a Standard Specification. The ASTM has three test methods that incorporate
the MAE technique:
- ASTM 5765 – 05 (2010) is the Standard Practice for Solvent Extraction of Total Petroleum Hydrocarbons from Soils and Sediments
Using Closed Vessel Microwave Heating (4). The soil or sediment sample is extracted with acetone or hexane in a sealed microwave
transparent vessel using microwave heating to an internal temperature of 150 °C, producing an extract suitable for analysis
by gas chromatography (GC) or gravimetric techniques.
- ASTM D6010-12 is the Standard Practice for Closed Vessel Microwave Solvent Extraction of Organic Compounds from Solid Matrices.
The soil, sediment, sludge, or waste sample is extracted in an acetone-hexane mixture at 115 °C producing an extract suitable
for analysis of semi-volatile or volatile organic compounds by GC or gas chromatography–mass spectrometry (GC–MS) (5). This
standard practice reduces sample preparation time, solvent consumption, and operating costs.
- ASTM D7210-13 is the Standard Practice for Extraction of Additives in Polyolefin Plastics. This provides guidelines for extracting
phenolic antioxidants, phosphite antioxidants, UV stabilizers, antistatic agents, and slip additives from milled polyolefin
plastics (6). It now includes an MAE technique for subsequent analysis of the extract by chromatographic techniques.
Phthalates are mainly used as plasticizers to increase flexibility and durability, but they are linked to health issues and
are being phased out of use. The Consumer Product Safety Commission (CPSC) issued Test Method: CPSC-CH-C1001-09.3, a Standard
Operating Procedure for Determination of Phthalates in April 2010 (7). Phthalate content analysis in children's toys and childcare
articles was the result of standards passed in the Consumer Product Safety Improvement Act Section 108. The test method uses
a microwave-extraction technique based upon EPA Method 3546 as an acceptable method of extraction.