There appears to be a discrepancy in EPA 7199 Revision 0 for analysis of hexavalent chromium. The method and Chapter Three of SW-846 both state that the hold time is 24 hours, yet the method indicates the use of a preservative to achieve a pH of 9‐9.5. 40 CFR Chapter 136 allows the use of EPA 218.6 in which using a buffer and adjusting the pH to 9.3‐9.7 extends the hold time to 28 days. The fact that 7199 mentions the buffer but then does not extend the hold time seems like an error. Does the Office of Resource Conservation and Recovery observe the same holding time and preservations for hexavalent chromium allowed under 40 CFR 136?
The holding times and sample preservation information found in SW-846 Chapter 3 and Method 7199 are intended as guidance. Holding times and preservation requirements a lab has to follow are typically set by data users (e.g., a regulator) or specified in a project-specific QAPP. They may specify the method be followed exactly as written, or they may allow the 28-day hold time mentioned in footnote 20 in 40 CFR Part 136.3, Table II to be followed. The footnote is reproduced below but it contains a caveat that prevents the use of the 28 day holding time as a default value in all circumstances:
"20To achieve the 28-day holding time, use the ammonium sulfate buffer solution specified in EPA Method 218.6. The allowance in this footnote supersedes preservation and holding time requirements in the approved hexavalent chromium methods, unless this supersession would compromise the measurement, in which case requirements in the method must be followed."
The caveat "unless this supersession would compromise the measurement" is important and some would argue it's intention is to use the shorter hold-time guidance in 7199 or shorter hold time specification in 218.6 unless it is proven that the longer hold time does not compromise the measurement for samples from a given site. This proof would require analyzing an aliquot of sample(s) from a site within the shorter hold time, then rerun another aliquot of the same samples later within the longer hold time and see if they had any significant change. If there is no change, the longer holding time would be acceptable. If there is a change, the laboratory is in a precarious situation if they used the longer holding time for the reported samples (i.e., the data would have to be qualified, and possibly rejected). A regulator, data user, or QAPP may choose the safest approach and just specify the shorter hold times.
Other Category: Holding Time & Preservation