Evidence of Feed Contamination Due to Sample Handling and Preparation During a Mass Balance Study of Dioxins in Lactating Cows in Background Conditions
In 1997, the United States (US) Environmental Protection Agency (EPA) conducted a mass balance study of polychlorinated dibenzo-p-dioxins (CDDs) and dibenzofurans (CDFs) in lactating cows in background conditions. The field portion of the study occurred at the US Department of Agriculture's Research Service (ARS) facility in Maryland, and the analysis of samples occurred at EPA's Environmental Chemistry Laboratory (ECL) in Mississippi. The purpose of the study was to confirm that feed is the primary source of dioxin exposure for the dairy cattle under study. The study was comprised of three sampling periods using four dairy cows well into lactation such that steady state conditions could be expected. At steady state, intakes of dioxins in feed would be comparable to outputs in milk and feces. Significant departure from this expectation could suggest other sources of dioxin intake not accounted for (if outputs were much higher than inputs as measured only by feed), a state of disequilibrium (if either inputs or outputs were much higher their counterpart), or a methodological problem in sample handling, preparation, or chemical analysis such that the data are not representative of what the cow is ingesting or excreting. The mass balance results suggested that feed was the principal source of dioxins to the cows; dioxin toxic equivalent (TEQ) outputs ranged from 60 to 75% of TEQ inputs for all three periods, reasonably consistent with similar CDD/F mass balance studies in the literature. While TEQ concentrations in feed and feed components were at or less than 0.50 pg/g (ppt; all results are expressed on dry weight basis) during all three periods, consistent with literature measurements of TEQ in vegetation in background settings, there was variability in the concentrations between periods. The concentrations for mixed feed and feed components during the first and third sampling period were in the range of 0.10 to 0.20 ppt, while the concentrations for some of the feed components in the middle period were in the range of 0.50 ppt TEQ. In these studies, the ARS facility oven dried and ground samples prior to shipping them to the ECL for analysis. Drying was in a forced air oven at 60 ?C until a constant weight was attained on two consecutive days; usually 72 hrs were required. In order to evaluate whether this variability is typical in the feed in the ARS facility where the study was conducted, mixed feed samples that were collected (but not prepared or analyzed) in 1996 from another study5 were retrieved from cold storage and sent to the ECL lab in 1999. These raw mixed feed samples were dried and ground at the ECL instead of the ARS facility. The oven at the ECL was a gravity convection oven, which doesn't use a fan like a forced air oven. The procedure for drying at 60 ?C until a constant weight is obtained was used at the ECL, similar to ARS facility. Following analysis of these samples, evidence surfaced which suggested low levels of dioxin-like compounds were introduced into the 1996 and 1997 feed samples during sample preparation at the ARS research facility, not during analysis at the ECL.
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