What is the holding time for soil samples analyzed for hexavalent chromium by Method 7196A? The method appears to say 24 hours; however, I understand that there is guidance in the SW‐846 manual that the holding time is 30 days to extraction and then 7 days from extraction to analysis.
Method 7196A is meant for TCLP extracts and groundwaters for which the hold time is 24 hours as the method specifies. These types of samples are not as stable and need to be analyzed more quickly. However, for soils there is a separate extraction step before the analytical steps described in 7196A. The extraction method gives a more stable extract than groundwater or a TCLP extract, hence the holding time can be longer before analysis. The extraction method is 3060A where Section 6.4 says, "hexavalent chromium has been shown to be quantitatively stable in field moist soil samples for 30 days from sample collection. In addition Cr(VI) has also been shown to be stable in the alkaline digest for up to 168 hours (7 days) after extraction from soil." Guidance is also given in SW-846 Chapter 3, Table 3-2 where it says aqueous samples have a holding time of 24 hours and solids have 30 days to extraction and 7 days from extraction to analysis.
In summary, for waters or TCLP extracts, the holding time is 24 hours to analyze by Method 7196A or Method 7199. For soils, the holding time is up to 30 days from sample collection before extracting by Method 3060A, then up to another 7 days to analyze the 3060A extracts by Method 7196A (colorimetry) or Method 7199 (ion chromatography).
Other Category: Holding Time & Preservation