Are methods 3545 and 3546 acceptable substitutions for sonication as the first step of Method 8151A, assuming of course that proper initial and ongoing QC are acceptable?
Method 8151A was last updated in 1996. Methods 3545A and 3546 were last updated in 2007. Therefore, the herbicide analysis method (8151A) was published 10 years earlier than either extraction method.
Method 3545A expressly allows the extraction of chlorinated herbicides in Section 7.7.5 which states, “Chlorinated herbicides may be extracted with an acetone/methylene chloride/phosphoric acid solution or an acetone/methylene chloride/trifluoroacetic acid solution".
Research for Method 3546 has demonstrated the effectiveness of microwave extractions for chlorinated herbicides. As with any technique, a demonstration of effectiveness at your laboratory, with your specific equipment would be needed though before the procedures are used on samples.