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MINNESOTA DEPARTMENT OF AGRICULTURE ELIMINATES METHYLENE CHLORIDE USE
IN THE LAB
Solvent Extraction Process Changes Reduce Waste
Company: Minnesota Department of Agriculture - St. Paul, Minnesota
Project: Reduce methylene chloride use in laboratory analysis
Results: Eliminated 99 gallons of methylene chloride use annually. Productivity increases and reduced costs saved $39,900 a year.
The Minnesota Department of Agriculture (MDA), Laboratory Services Division, performs chemical, microbiological and physical analyses on samples of food, water, fertilizer, plant material and other agricultural products to determine quality and compliance with state and federal laws and regulations. Its Environmental Analysis Section analyzes pesticide formulations and pesticide residues, providing laboratory results for 1,500 to 2,000 water samples per year.
Clients submit to the lab a one liter water sample for each test. A liquid-to-liquid extractor (LLE) ran 18 to 24 hours, using methylene chloride to separate pesticides out of the water sample. The extracts from LLE were concentrated on a rotary vacuum evaporator. The methylene chloride was then boiled off and the pesticides analyzed. For each one liter water sample, 300 milliliters of methylene chloride was used, creating 512 to 616 liters of methylene chloride waste annually.
Methylene chloride from the LLE process was once reused for soil analysis. This was discontinued because of possible cross contamination and a shift in analysis requests wanting methods that no longer used methylene chloride as an extraction solvent.
Incentives for Change
Methylene chloride is a potential carcinogen and a hazardous air pollutant. The U.S. Occupational Safety and Health Administration (OSHA) and U.S. Environmental Protection Agency (EPA) have regulatory limits on methylene chloride exposure and emissions. MDA’s Environmental Analysis Section wanted to reduce the volume of methylene chloride used in water analysis to help improve worker safety and to keep the hazardous chemical out of the environment. MDA was also considering alternatives to LLE, due to its lengthy extraction time.
MDA was looking at other options for extraction prior to the MnTAP intern project. A MnTAP intern organized literature on several alternative extraction methods using vendor specific equipment. By having an intern dedicated to the project, MDA was able to more quickly organize and evaluate the information.
Solid Phase Extraction
Solid phase extraction (SPE) was considered because it reduces both solvent use and extraction time. SPE is a method that uses a liquid phase and a solid phase to isolate an analyte (the substance or chemical constituent being analyzed) from a solution. Switching to SPE from LLE requires an equipment change, followed by a procedure change.
According to EPA, SPE produces nearly identical results as the conventional LLE method. Other state labs reported that SPE provides satisfactory reliability and recovery for low particulate water with advantages being no emulsions or contaminants; less sample handling and transfers, improving reproducibility; safer for the environment by reducing ﬂammable and toxic solvents; and less equipment needed for the process.
During past research on surface and ground waters that contain more particulate matter than drinking water, MDA had concerns with SPE cartridges clogging and poor recovery of the more polar compounds of certain pesticides, like atrazine metabolites. Through discussions with vendors and peers, as well as trial-and-error experience, one brand of SPE cartridges proved capable of obtaining good results. The cartridge is an extraction disk, shaped like a Buchner funnel, that contains a bed of divinylbenzene (DVB) sorbent microparticles and is engineered to process dirty samples—typical of water samples analyzed by MDA that are often from farm ﬁelds and runoff containing soil and organic matter.
Results and Benefits
Under MDA guidance, the intern evaluated alternatives to LLE and determined that SPE cartridges were the best option. SPE ﬁlter media had changed and now some types could trap analytes that would not adhere to ﬁlter media in the past.
Based on an in-house vendor demonstration of an automated extraction system and initial method development, MDA switched all LLE lab processes over to an automated SPE system that meets all in-house validation criteria. Different SPE disks are used based on the type of analysis. The samples need to be ﬁltered ﬁrst to prevent clogging. Additional validation was required to ensure that ﬁltered samples yielded the same results as unﬁltered samples.
By converting to SPE, MDA eliminated 99 gallons of methylene chloride use annually, reducing use by 85 to 90 percent and saving $3,300 in purchase and disposal costs.
Extraction time for a set of 20 samples was reduced from a minimum of 18 hours for LLE to five hours or less for a SPE batch. SPE cartridges come pre-assembled, ready to use, while LLE must be set up for use and supervised for leaks. By reducing extraction and evaporation time, the procedure change cut 13 hours labor from processing, saving $36,600 per year. Productivity also increased because the procedure can run ﬁve days a week instead of four.
Results are reproducible, with greater consistency between extractions because sample handling and transfers are reduced. The tests are fairly precise, with low coefficients of variation.
Other Options Reviewed
Off-site stills are an option for laboratories that only need occasional distillation. MDA chose not to pursue an off-site still because it was expensive and required arranging transportation.
Spinning band distillation systems separate substances based on their characteristic boiling points. Coiled bands rotate to force mixing. This system would not work for MDA because of its need to detect low concentrations.
Molecular sieves trap particles of the appropriate size into pores. The sieve can be regenerated and used again. This option was ruled out for MDA due to quality assurance problems that may arise with the purity of methylene chloride.
Decrease sample volume. This option was not feasible because MDA’s LLE required a sufficient volume of sample and methylene chloride to ﬁll the extractor and immerse the frit. Further decreasing sample volume would raise detection limits.
Challenges of Changing a Lab Procedure
When changing a lab procedure, standards regarding quality of the process and output must be met as well as any regulations or standards for the industry. MDA’s main concern was maintaining the same level of pesticide detection and having in-house method-validation criteria. Finding the time to run test experiments and validate the results can be difficult with a busy testing schedule.
Staff need to be trained in the new process and equipment. Customers need to be educated on the reliability of the new process and on any procedure changes they need to follow. Management must approve the financial investment. Gaining support of the various parties can add time to a project as additional studies and evaluations are often needed for approval.
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