Sample Preparation for Metals Analysis by Microwave Digestion

Applicable Regulations
EPA Laboratory Method SW 846 3015.

Overview of Procedure
Growing concerns for the environment, increasing costs of waste disposal, and potential exposure of employees to hazardous chemicals were among the factors that led to the development of the P2 and waste minimization initiatives in the environmental laboratory.

The sample preparation and analytical methods approved by EPA and other agencies prior to 1986 often involved large sample volumes requiring significant quantities of chemicals and solvents, which result in prohibitively expensive waste storage, segregation, and disposal costs. Reduction of chemicals and solvents at the source would reduce these costs considerably.

Keeping the above objectives in mind, the U-M Occupational Safety and Environmental Health (OSEH) Environmental Laboratory developed a sample preparation procedure for metals analysis to minimize costs in labor, reagents, materials, and waste disposal. Further, the procedure described herein led to increased safety in the laboratory.

Conventional methods outlined in EPA test methods for evaluating solid waste, SW846 3010 and 3020, employ 100 mL of the sample aliquot and 10 mL of concentrated acids for metals digestion. Subsequent analysis is performed by atomic absorption or ICP spectrometry. The sample digestion is performed on a hot plate, which often takes several hours. The digestion procedure requires a dedicated acid resistant hood and several square feet of lab space to store the prepared samples. After sample preparation, the final volume is 100 mL for each sample. A good inventory of glassware, plastic standard flasks, and other supporting materials is required.

For total recoverable metals, a procedure was developed based on EPA approved microwave assisted acid digestion method, SW846 3015, which utilizes 25 mL of sample and 5 mL of concentrated nitric acid. The sample carousel can be loaded with 10 to 15 samples and the entire sample preparation task can be completed in less than 30 minutes without numerous hot plates and hazardous conditions that create corrosive acid fumes. The final volume is made up to 50 mL for each sample using deionized water. This method represents a substantial reduction in sample storage and disposal. Utilization of beakers, watch glasses and proper cleaning steps are completely eliminated thereby saving labor costs. The Teflon digestion vessels used in this procedure are reusable and the clean-up step is relatively easy and less time consuming.

Prepared samples are analyzed by an ICP/MS, an instrument of choice for metals analysis that provides a good sensitivity and requires less sample volume. Due to the specificity of ICP/MS, the results obtained are accurate and seldom require confirmation.

For dissolved metals analysis, where sample digestion is not necessary, the following waste reduction initiative was introduced. EPA methods 3010 or 3020 require a sample volume of 500 mL to be collected. Conversely, the new laboratory method by ICP/MS requires less than 2 mL of sample for analysis. Samples are collected in 50 mL wide mouth polypropylene vessels and directly placed on the ICP/MS autosampler for analysis. This final procedure reduces the extra step of transfer and sample handling.

A standard operating procedure is available for this methodology that contains a section on P2 measures associated with this protocol.

Waste Minimization Procedure
The procedure described herein works exactly the same as conventional methods, because there is no change in the chemistry. Considerable reduction in preparation time, sample size, materials required, waste generation, and potential environmental and human health threats in the work place are also realized with this procedure.

Known Limitations
None known.

Safety & Health Precautions/Personal Protective Equipment
Follow all applicable safety and health protocols and regulations as established by your institution.

Benefits
As many as 100 samples can be processed during an eight hour shift, creating an ideal situation for a high production laboratory. Time, money, and a safer work environment, than conventional methods, are all benefits of this process.

Disadvantages
None known.

Project Related Costs
Initial capital investment of $15,000 to $20,000 is involved as an associated cost for setting up the process. Conventional method of hot plate acid digestion, which requires one or two acid resistant hoods, several hot plates and a large number of glassware sets would be eliminated. The cost savings from labor, reduction of glassware and disposal costs must be accounted for when determining the cost effectiveness of implementing this technique.