QUANTIFYING CLOPYRALID
IN Phase II of the Oregon Department of Environmental Quality (DEQ) study, concentrations of clopyralid in the compost samples were determined by two commercial laboratories, using two different analytical methods. One method, used by Lab A, was labeled the EPA method because it is based on an approved EPA procedure, EPA 8151A. The second method (Lab B), referred to as the “GC/MS” method by the DEQ fact sheet, is an abbreviation for “Gas Chromatograph/Mass Spectrometry”, the analytical instruments used to detect clopyralid in the sample extracts. Actually both methods used in the study are modifications of the EPA procedure and both can use gas chromatographs and mass spectrometers to detect clopyralid. However they employ different extraction steps and solvents in an attempt to recover more of the clopyralid from the samples of compost. They are not so much different methods as they are different variations, each developed and used by different commercial laboratories. As such, they are referred to here as Lab A and Lab B methods respectively instead of the EPA and GC/MS methods.

It is difficult to determine the concentration of any particular compound in any solid media, but a highly organic environment like compost is especially troublesome. Compounds targeted for analysis are extracted from solid media with solvents and then the concentration is measured from the extract (solvents plus extracted compounds). Chemical compounds in organic media can escape the solvents by binding within the many reactive surfaces that organic substances offer. Therefore, the extraction step is a key part of the procedure and can greatly influence the results. When very low concentrations must be detected, as in the case of clopyralid, extraction becomes even more critical.

In the Oregon study, the Lab B (GC/MS) results consistently indicated higher concentrations of clopyralid compared to those obtained by the Lab A (EPA) procedure. Differences in results between the two laboratory methods for clopyralid have been found before. A recent study conducted by King County Solid Waste Division and Seattle Public Utilities in Washington sent identical compost samples to three different laboratories using three different analytical methods. One laboratory detected no clopyralid in any of the samples. However, the two other laboratories detected clopyralid at levels ranging from 9 to 18 ppb and 16 to 43 ppb (on a dry weight basis) for each lab respectively. The second lab was “Lab B” from the Oregon work. As in Oregon, the concentrations found by this lab (using the “GC/MS” method) were consistently higher.

According to the Oregon DEQ fact sheet (see main article), the extraction procedures used by Lab B have been “optimized for clopyralid’s removal.” The commercial laboratory developed this method with input from Dow AgroSciences. It follows that this method would detect more clopyralid. Consequently, the technical committee overseeing the Oregon study has concluded that the Lab B (“GC/MS”) method is a more reliable method for measuring clopyralid in compost and composting feedstocks. However, neither method has been officially accepted or validated.

Nevertheless, the availability of a more efficient method of analysis raises questions about how to interpret analytical data. For example, suppose that some university conducts greenhouse studies and sends samples of the compost to Oregon’s Lab A to determine the clopyralid concentrations. Further suppose that based on a comparison of the bioassay results and the analytical data, the university recommends that compost with clopyralid levels above 20 ppb (for the sake of argument) should not be sold for general horticultural use. Now, what if a compost producer sends compost samples to Lab B, which finds clopyralid levels of 30 ppb, using the more sensitive test. Should the sale of that compost be restricted based on the university’s recommendation? Or should the compost producer assume that the compost is still okay because a more sensitive analysis was used? The answers to these questions are not apparent. The required research is not yet available, although a few studies are beginning to provide some guidance, such as the research conducted by Woods End Research Laboratories (see “States Take Actions to Protect Compost Against Clopyralid”, July 2002). Presumably, the methods of analysis will need to be either stipulated or identified.

Analytical methods may not be the best option for determining the performance of potentially contaminated compost. Bioassays more predictably determine whether or not a compost product will cause damage. Unfortunately, there also are differences among bioassay techniques. The King County study, referred to above, examined bioassay methods from three different institutions in Washington. The study found variations in results among the three. As a result, the Washington Department of Ecology facilitated the development of a standard bioassay protocol for use by composting facilities in Washington. The standard bioassay is still in the draft stage. A description is available at the web site: www.css.wsu.edu/compost/bioassay.pdf.

Contributors: David Bezdicek, Mary Fauci of Washington State University; Gabriella Uhlar-Heffner, Seattle Public Utilities and Marti Roberts–Pillon with Oregon Department of Environmental Quality.