Isotopes as Environmental Tracers in Archived Biological Specimens

There is increasing interest in the analytical and environmental communities in using mercury isotopic fingerprinting to identify source patterns and to better understand the biogeochemical cycling of mercury in the environment. The research activities described here will advance the application of these techniques by improving our understanding of fractionation of mercury in the environment, and will complement previous isotopic mercury measurements that NIST has made on seabird eggs in the Alaskan arctic and subarctic.

NIST collaborates with other government agencies to provide analytical support for environmental applications and specimen banking, typically related to quantification of analytes relevant to monitoring or health assessments. Recent improvements in analytical capabilities have made it possible to make high-resolution isotopic measurements on elements (e.g., mercury) that exhibit relatively low degrees of fractionation in the environment that previously could not be detected. This has caused rapid growth in this field, and prompted an interest in applying these techniques to environmental samples that are available through NIST other-agency activities.

The first such project is the Seabird Tissue Archival and Monitoring Program (STAMP). STAMP is a multi-agency effort between the U.S. Fish and Wildlife Service (USFWS), the U.S. Geological Survey (USGS), the Bureau of Indian Affairs, and NIST to collect, bank, and analyze seabird tissues from the Alaskan arctic and subarctic to monitor contaminants in the marine ecosystem. Collections date back to 1999, and analyses for mercury have been performed on collections through 2005. A collaboration with the North Pacific Research Board focuses on the Norton Sound region of the Bering Sea where previous analyses showed elevated levels of mercury in eggs relative to the rest of the Bering Sea. Norton Sound is greatly influenced by discharges from the Yukon and Kuskokwim Rivers and has localized areas of historically heavy gold and mercury mining, all of which might contribute to the observed elevated mercury levels in biota. The current work expands on the previous study by investigating whether mercury isotopic patterns in eggs from Norton Sound differ from eggs from other colonies, and whether different fractionation patterns can be resolved among colonies within Norton Sound that may be subject to different local sources and biogeochemical cycling.

As described elsewhere, NIST also supports marine animal health programs conducted by the National Oceanic and Atmospheric Administration (NOAA). NOAA is currently performing an ecosystem health comparison between a superfund site on the Turtle River near Brunswick, Georgia, and the nearby Sapelo Island National Estuarine Research Reserve Site (NERRS). Dolphin skin biopsies and terrapin shell samples have been collected from both of these sites to assess mercury exposures in these indicator species. There is increasing interest in the analytical and environmental communities in the possible use of mercury isotope fractionation patterns to identify source patterns of mercury in the environment. This NOAA collaboration is ideally suited to test whether a large point source (a historical chlor-alkali plant in Brunswick) creates an isotopically distinct mercury signature in the vicinity of its impact. Isotopic patterns in these two indicator species will be assessed near the source and 40 km distant from the source, where atmospheric and natural mercury inputs are presumed to dominate. Potential shifts in the isotopic signature at the two trophic levels represented by these species will also be investigated to assess the consistency of the source fractionation pattern through the food web.

All samples were collected by collaborators involved in field sampling campaigns. Egg samples undergo a standard homogenization procedure prior to aliquoting and banking. However the dolphin skin biopsies are often completely consumed in the analysis due to their small mass and the need to measure mercury plus carbon/nitrogen in the same sample. Preliminary work will assess the heterogeneity of these skin biopsies and develop a sample preparation method involving freeze-drying, de-fatting, and micro-homogenization that is appropriate for both mercury analysis (concentration and isotopes) and carbon/nitrogen (isotopes). The carbon/nitrogen isotope data provide valuable information on trophic level and food web status that will improve interpretation of the mercury isotopic data, and of the mechanisms involved in mercury fractionation. 

A similar analytical approach will be used for the isotopic analysis of all tissues.   A standard microwave-assisted acid digestion will be performed on a subsample of the tissue. The digest will then be split into two aliquots: one for total mercury quantification by cold vapor isotope dilution inductively coupled plasma mass spectrometry (CV-ID-ICP-MS), and the other for isotopic analysis by CV-multicollector (MC)-ICP-MS. The sample will be injected as a vapor (after reduction with tin chloride) with inline mass bias correction using thallium injected by micro-nebulizer. SRM 3133 Mercury Standard Solution will serve as the isotopic reference standard, providing additional in-house measurements of its isotopic composition to complement the information values currently provided on the certificate. SRM 1947 Lake Michigan Fish Tissue will also be analyzed concurrently with these batches to provide additional mercury isotopic characterization of this material and to monitor instrument reproducibility and drift.

Major Accomplishments:

• Methods were developed for mercury isotopic analysis of biological matrices, including testing the effects of freeze-drying, matrix interference, etc. 
• Approximately 40 seabird egg samples were analyzed in 2007 in collaboration with University of Pau and  Laboratoire des Mécanismes et Transferts en Géologie (LMTG) in Toulouse, France.
• Generated preliminary mercury isotopic data for SRM 3133 and SRM 1947.
• Collaborated with the North Pacific Research Board to perform the mercury isotope analyses described here on archived Alaskan seabird eggs.