MARS 6 In the News

Clean Water Act Methods Update Rule for the Analysis of Effluent Expands the Use of Microwave Digestion


Recently approved methodology has expanded the use of microwave digestion for the analysis of trace metals in waste water. On May 19, 2021 the United States Environmental Protection Agency (USEPA) announced an update for the Clean Water Act Methods Update Rule for the Analysis of Effluent – final rule.1 The effective final rule implementation date was July 19, 2021. Section 136 of the Code of Federal Regulations establishes test procedures and for the analysis of pollutants. Included is paragraph (b)(4)(xxiii) which states that when analyzing metals by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Graphite Furnace Atomic Absorption (GFAA) closed-vessel microwave digestion of wastewater samples is allowed as alternative heating source for Method 200.2. The newly approved methodology adds thirteen metals to the original list of approved elements. The full suite of 26 elements including Al, Sb, As, Ba, Be, B, Cd, Ca, Cr, Co, Cu, Fe, Pb, Mg, Mn, Mb, Ni, K Se, Ag, Na, Tl, Sn, Ti, V and Zn can now be prepared using microwave digestion prior to analysis.


CEM’s involvement with official USEPA methodology dates back to the late 1980’s when a group of CEM chemists approached the USEPA seeking an approval for an alternative test procedure (ATP) for microwave digestion. The USEPA response on April 20, 1987 outlined the requirements for obtaining a nationwide approval of the CEM closed vessel microwave acid digestion procedure.2 In order to obtain a nationwide National Pollution Discharge Elimination System (NPDES) approval CEM had to provide data from five different point of source discharge types and collect six samples over a varied time period. Each sample was then split and prepared and analyzed four times with the standard method and four times with the closed vessel microwave method. This rigorous test plan resulted in 240 data points for each of the approved elements. Following several months collecting, preparing and analyzing samples the data was submitted and in September of 1990 it was recommended for approval by the USEPA Environmental Monitoring and Support Laboratory (EMSL).3 On September 11, 1992 the USEPA approved the CEM Digestion Procedure for nationwide use pursuant to 40 CFR 136.5. The method was listed in the Federal Register Volume 574 and allowed for 13 metals to be tested by either AA, GFAA or ICP-OES.


A 50 mL aliquot of a well-mixed acid preserved sample is transferred into the vessel liner. Three mLs of concentrated nitric acid and two mLs of concentrated hydrochloric acid are added to each sample. The vessel is then assembled and proper torque applied to provide a complete seal. Vessels are added to the turntable and loaded into the microwave system. Up to 24 samples can be run in an analytical batch to maximize efficiency. The analytical batch is defined as a group of up to 20 samples digested together using the same techniques. The other four samples required are a method blank (MB), a laboratory control sample (LCS), a matrix spiked sample (MS) and a matrix spike duplicate (MSD). The microwave system is then programmed to ramp to 165C in fifteen minutes and then hold at that temperature for fifteen additional minutes.

Vessels are allowed to cool and then removed and vented of any remaining gases inside of a fume hood. Proper PPE is required whenever handling acid solutions especially when under pressure. If particle free, the samples are transferred to an acid cleaned centrifuge tube. If silicates or other insoluble materials are found then they must removed either by gravimetric filtration or centrifugation prior to analysis.


Laboratories can now use microwave digestion to prepare all wastewater sample types for the complete 26 element suite and analyze by ICP-MS, ICP-OES or GF-AA. Laboratories following the CEM method can prepare a batch of 24 vessels including 20 samples and 4 QC samples in thirty minutes providing a substantial time savings as compared to the several hour long hot plate or hot block analysis. In addition, a single aliquot of sample and acids are made at the beginning of the process and no additional acid additions are required. This greatly reduces labor costs by permitting technicians to attend to other tasks. The microwave audibly signals when samples are ready for further processing. Labs also save money by using far less quantities of costly high purity acids. Microwave digestion provides a lower cost per test as compared to other sample preparation methods for trace metals analysis.


1 United States Environmental Protection Agency. Clean Water Act Analytical Methods. [Online] May 19, 2021. (accessed March 1, 2022).

2 Robert L. Booth, United States Environmental Protection Agency, Office of Research and Development, Environmental Monitoring and Support Laboratory, Cincinnati, Ohio. Personal communication, 1987.

3 Nancy S. Ulmer, Inorganic Chemistry Branch, Chemistry Research Division, Unites States Environmental Protection Agency, Office of Research and Development, Environmental Monitoring and Support Laboratory, Cincinnati, Ohio. Personal communication, 1990.

4 Environmental Protection Agency, Office of the Federal Register, National Archives and Records Administration. Federal Register, Rules and Regulations. [Online] September 11, 1992, Vol. 57, Issue 177, p 41830. https:// (accessed March 1, 2022).