Understanding Semi Volatile Extractions
Monday, 26 August 2013 09:16

Introduction – Organic compounds have become an ever growing presence in our society. Many products use some form of organic compound in their formulation and the release of these compounds in the air is an area of concern for health officials. A subset of organic chemicals known as semi volatile organic compounds (SVOCs) is also a growing concern in water and soil pollution. These semi volatile compounds are composed of pesticides, herbicides, and a laundry list of compounds guaranteed to give even the most seasoned chemist trouble pronouncing them.

Approved Methods – This LabMatters will focus solely on the preparatory extraction methods for SVOCs. There are quite a few that have been approved for use. The Clean Water Act (CWA) methods are generally for liquid samples and have an extraction procedure written into the method. The SW-846 methods are for general waste analysis, both liquid and solid, and have extraction methods written separately from the analytical methods. Because of the large number of methods available you should consult 40 CFR Part 136 and SW-846 (3000 Series) for the list of methods numbers.

Method Summary – While the methods are varied for extracting SVOCs they all follow the same basic procedure. The sample is put into contact with a solvent. The analytes of interest have a higher affinity for the solvent than the aqueous or solid matrix from the environment. Some form and combination of heat, pressure, and agitation accelerate the transfer of the analytes into the solvent. This solvent is then made ready for analysis via an approved analytical method.

What You Should Know – The organics side of the environmental laboratory is typically divided into two different sections; volatiles and semi volatiles. The actual definition of these two categories varies based on region but a good working definition is that volatiles can be analyzed directly from the sample via some form of gas chromatography. Semi volatiles must first be extracted from the sample into a solvent matrix prior to analysis. There are a variety of extraction methods from which to choose for both liquid and solid samples.

Liquid-Liquid extraction (LLE) is the oldest technique and is probably the ‘gold standard’ to which other methods are compared. This method is performed by putting your sample, usually one liter, in a large separatory funnel. The extraction solvent of choice, methylene chloride is a common one, is placed in the funnel with the sample and the funnel is shaken back and forth to mix the two liquids. Care must be taken to vent the funnel often during the shaking as solvent fumes will build up inside the glassware. The shaking procedure is repeated a total of 3 times with fresh aliquots of solvent used each time. All three extracts are combined at the end for analysis. A total of 90-100 mL of solvent is used for this. One of the major problems encountered with LLE is the formation of emulsions during the shaking.

Continuous Liquid-Liquid extraction (CLLE) is a way to set up the extraction so that it proceeds unattended, thus allowing the analyst to perform other tasks while the extraction occurs. A glassware setup is used that allows sample and solvent to be placed in a vessel with a condensing head on top. The vessel is placed on a heating station to allow the solvent to vaporize. The condenser causes the solvent to drip back down into the vessel where it falls through the sample and accomplishes the extraction. Because this is a much more passive procedure it takes 18-24 hours for a full extraction to take place. This method also requires larger volumes of solvent are used, 300-400 mL.

Solid Phase Extraction (SPE) is a two step procedure that takes advantage of different affinities of the compounds of interest. A disk or column is packed with a material that has a higher affinity for the analytes of interest than they do for water. The sample is passed through this material allowing the analytes of interest to selectively adsorb onto the solid phase material and be retained there. After the initial extraction is complete, a solvent for which the analytes have an even higher affinity is poured through the solid phase material. The solvent is then collected and sent for analysis. SPE typically has problems with samples of high solid content as it tends to clog up the extraction material and prevent the liquid from flowing through. There are also conditioning steps required for the solid phase material. This conditioning usually requires additional solvents to the one used for final extraction.

When working with solid samples there are a few more options to consider. Soxhlet extraction is similar to CLLE. The glassware used is similar except that the solid sample is held in a porous extraction thimble underneath where the solvent condenses. The solvent drips down onto the sample and percolates through before collecting in the lower flask. This method takes about the same amount of time and requires the same volume of solvent as CLLE.

Sonication can also be used to extract with solid samples. This method calls for approximately 100 mL of solvent and 30 g sample be placed in a container. An ultrasonic disruptor horn is placed so that it is submerged only in the solvent layer. It is then actuated for a short amount of time, typically 3 minutes, to agitate the sample/ solvent mixture. The solvent is removed and the entire process is repeated with fresh solvent two more times. All solvent extracts are combined for the analysis. While this method is often faster for individual samples, only one sample can be processed with each horn at a time. This method also has known limited effectiveness with certain organophosphorous pesticides.

Pressurized Fluid Extraction (PFE) is a relatively recent addition to the solid extraction arsenal. This method has the sample placed inside a specially constructed cell. The cell has openings on each end to allow the solvent to be injected through the sample inside. An instrument designed for PFE must be used to cycle solvent through the cell and hold it at specific temperatures. The heat causes a buildup of pressure inside the cell which aides in the extraction efficiency. After about 10 minutes the solvent is removed from the cell and stored in a separate holding vial. The most common problem encountered with this method is a clogging of the internal filters that prevents the solvent from being extracted at the end.

Microwave extraction uses microwave energy to produce elevated temperature and pressure similar to PFE. It is a quick method, 10-20 minutes, that requires little solvent usage, about 30 mL. Additionally multiple samples can be processed at one time, limited only by the number of vessels and how many will fit inside the microwave system at one time. Specialized microwaves and vessels are required to run this method.

A variation of the microwave method is the Soil-Cell system. This method uses a HotBlock instead of a microwave to produce the required pressure and temperature for extraction. Method 3546 contains the statement “Other systems and other types of vessels may be used, provided that the analyst demonstrates appropriate performance for the specific application.”

Method Procedure

Note – This is not intended to be a standalone method and does not address all safety or quality control aspects that may be required. Please consult your local regulations to comply with all requirements. This method will only follow the Soil-Cell adaptation of method 3546.

    2. When practical, air dry the sample for 48 hours at room temperature. Alternatively, mix the sample with an equal volume of sodium sulfate or diatomaceous earth to obtain a free flowing powder.
    4. Add 10-30 g of the sample into one of the cells.
    5. Add 30 mL of the desired solvent to the vessel. Add surrogates and/or spikes to the samples as needed.
    6. Place the stainless steel inner lid with O-ring on the cell and hand tighten the outer cap to secure the cell.
    7. Place the cells on the block for 30 minutes. This will give the cells time to heat up to and maintain the recommended temperature/pressure for 10-20 minutes.
    8. Remove cells and allow to cool.
    9. Remove the lids and rinse the inside with the appropriate solvent. Make sure to collect the rinsate in the cell.
    10. Collect the solvent from the sample. A filtering apparatus may be used to separate the solvent from the soil.
    11. Proceed with concentration, cleanup, and analysis as directed by your determinative method.

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