Frequent Questions

Holding time for SW-46 Method 8141B for organophosphorus pesticides.

The SW-46 method 8141B for Organophosphorus pesticides looks to have added a requirement for 7 day holding times for solids. This appears to be a change in practice over what’s been accepted previously as a 14 day holding time.

 

Method 8141B (Rev 2, February 2007), posted online, states in Section 8.4 "Begin extraction of either aqueous or solid samples within 7 days of collection."

This is much more specific than the language in 8141A, which only referenced Chapter 4 and gave a 48-hour requirement for the derivatization of extracts.  From the lack of specificity in earlier versions, we can see where laboratories might have assumed 14 days was the hold time as it was not spelled out clearly one way or the other in 8141A or Chapter 4 regarding organophosphorus pesticides.

Table 4-1 in Chapter 4 (Rev 5, October 2012) addresses the holding times for organochlorine pesticides and semivolatiles, but not for organophosphorus pesticides.  The reason Method 8141B calls for lowering the hold time for this specific group of target compounds was based on a study (by D. J. Munch, and C. P. Frebis, on "Analyte Stability Studies Conducted during the National Pesticide Survey," ES & T, 1992, Vol 26, 921-925), cited as Reference 12 in the method.  This study showed that the organophosphorus pesticides were not stable, with many compounds degrading before the 14-day hold time would have taken place.

Section 9.1 of 8141B states: "When inconsistencies exist between QC guidelines, method-specific QC criteria take precedence over both technique-specific criteria and those criteria given in Chapter One, and technique-specific QC criteria take precedence over the criteria in Chapter One."  Therefore, the determinative method's criteria would supersede the general guidance in Chapter 4.  If reporting 8141B results, all samples (both soils and waters) have to be extracted within 7 days to be considered acceptable.

Other category: Holding Time & Preservation

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