Humans have been eating fish for thousands of years, enjoying its nutritional benefits. However, fish can bioaccumulate toxins and are monitored for various classes of contaminants. In extreme cases, health risks posed by contaminants can outweigh the health benefits imparted by fish consumption. Fish consumption is the primary route of mercury exposure in humans, and federal and state agencies are responsible for testing representative fish samples for harmful mercury species such as methylmercury. Methylmercury concentration data are used to develop consumption advisories for fish species residing within specific parts of U.S. waterways primarily to avert exposure risk for pregnant women and children. Concentrations of a newer class of organic contaminants, polybrominated diphenyl ethers (PBDEs), have increased rapidly in the environment. PBDEs are flame-retardant compounds used in building materials, clothing, furniture, plastics, and other consumer items. PBDEs are detected in sediments and biota, including fishes and marine mammals, but the effects of chronic PBDE exposure are poorly understood.
The National Institute of Standards and Technology (NIST, Gaithersburg, MD) has developed four Standard Reference Materials (SRMs) related to the monitoring of contaminants in fish and marine mammals: SRM 1588b Organics in Cod Liver Oil, SRM 1945 Organics in Whale Blubber, SRM 1946 Lake Superior Fish Tissue, and SRM 1947 Lake Michigan Fish Tissue. SRM 1588b is an oil sample, while the other three SRMs are cryogenically homogenized fresh-frozen materials. Material in a fresh-frozen state more closely approximates a “real-world” sample than does a freeze-dried material, and NIST research has documented that the freeze-drying process can possibly induce an interconversion of some chemical species and/or affect their endogenous complexation patterns.1 All of these natural-matrix SRMs are intended for use in validation of analytical methods and as quality control materials for the determination of contaminants. The three fish-related materials are also useful for the assessment of measurements of nutrients, primarily fatty acids.
Description of SRMs
SRM 1588b is a commercial-grade cod liver oil obtained in the mid-1980s that has been fortified with selected chlorinated dibenzodioxins and dibenzofurans. Each 2-mL amber ampule of SRM 1588b contains approximately 1.2 mL of cod liver oil under an argon atmosphere.
SRM 1945 is a frozen homogenate of whale blubber that was collected from an adult female pilot whale that stranded on Cape Cod, MA, in September 1991. The whale blubber was pulverized using a cryogenic grinding procedure,2 mixed, and aliquoted with approximately 10–15 g per bottle.3
SRM 1946 and SRM 1947 were both prepared from fillets of adult lake trout (Salvelinus namaycush) collected in October 1997 from Apostle Island in Lake Superior (SRM 1946) or at Fisherman’s Island and Big Reef near Charlevoix, MI, in northern Lake Michigan (SRM 1947). Both sets of fish fillets were cryogenically ground, mixed, and aliquoted with approximately 8 g per bottle.4
Value assignment of constituents
Table 1 - Compounds measured in fish-related SRMs
The general approach used for the value assignment of the constituents of interest in these four SRMs, using multiple independent methods, was similar to that reported for the recent certification of several other environmental-matrix SRMs.5 As summarized in Table 1, these four SRMs are characterized for a wide range of constituents, including polychlorinated biphenyl (PCB) congeners, chlorinated pesticides, PBDE congeners, fatty acids, methylmercury, total mercury, and other trace elements. SRM 1588b and SRM 1945 are characterized for organic constituents only, whereas the two fish tissues are characterized for both organic and inorganic constituents. The individual contaminant concentrations and their distributions in the two fish tissue SRMs differ due to the different levels and sources of contaminant inputs in Lake Michigan and Lake Superior.
Figure 1 - Comparison of mass fractions of selected PCB congeners in SRM 1588b Organics in Cod Liver Oil, SRM 1945 Organics in Whale Blubber, SRM 1946 Lake Superior Fish Tissue, and SRM 1947 Lake Michigan Fish Tissue. Error bars represent an expanded uncertainty about the mean at the 95% confidence level.
The mass fractions of selected constituents in the four SRMs are compared in Figures 1–4 and Table 2. In Figure 1, the mass fractions of five PCB congeners are compared. PCBs are persistent and bioaccumulative industrial insulators and lubricants that were found to cause cancer and were therefore banned in 1979. One PCB in particular, PCB 153, is typically used to monitor contamination levels. PCB 153 is the congener at the highest mass fraction in each of the materials ranging from (170 ± 9) μg/kg wet mass in SRM 1946 to (275 ± 4) μg/kg oil in SRM 1588b. SRM 1946 Lake Superior Fish Tissue has the lowest concentrations of the PCB congeners among these four materials.
Table 2 - Mass fractions (mg/kg, wet mass basis)a of arsenic, iron, total mercury, and methylmercury in SRM 1946 Lake Superior Fish Tissue and SRM 1947 Lake Michigan Fish Tissue
In Figure 2, the mass fractions of 4,4′- DDE; 4,4′-DDT; cis-chlordane; and trans-nonachlor are compared. The relative concentrations of the pesticides differ among the four SRMs, indicating that the different regions where the samples were collected have exposures to different pesticides or, in the case of 4,4′-DDT and its metabolite 4,4′-DDE, potentially a more recent exposure to 4,4′-DDT. The mass fractions of three PBDE congeners in these SRMs are compared in Figure 3. These four SRMs are the first marine tissue SRMs with values assigned for PBDE congeners, and the PBDE content is discussed in more detail elsewhere. 8 The tetrabrominated congener, PBDE 47, is the congener at the highest concentration in these materials, particularly in SRM 1947 Lake Michigan Fish Tissue. Similarly, a recent study also showed that the ratio of PBDE 47 to PBDE 99 was higher in Lake Michigan lake trout than in trout from Lake Superior.9 This similarity establishes the utility of the fish SRMs to represent current profiles of contamination. The concentrations of the other PBDE congeners are similar among the whale blubber and two fish tissue materials.
Figure 2 - Comparison of mass fractions of chlorinated pesticides in SRM 1588b Organics in Cod Liver Oil, SRM 1945 Organics in Whale Blubber, SRM 1946 Lake Superior Fish Tissue, and SRM 1947 Lake Michigan Fish Tissue. Error bars represent an expanded uncertainty about the mean at the 95% confidence level.
The mass fractions of arsenic, iron, total mercury, and methylmercury in the lake trout SRMs are compared in Table 2. The percentage of total mercury that is present as methylmercury is 91% in SRM 1946 and 92% in SRM 1947. The mass fractions of methylmercury in SRM 1946 (0.394 mg/kg ± 0.015 mg/kg) and SRM 1947 (0.233 mg/kg ± 0.010 mg/kg) bracket the U.S. EPA’s recommended limit of 0.3 mg/kg in edible fish tissue,10 making these materials particularly useful to support regulatory testing.
Figure 3 - Comparison of mass fractions of selected PBDE congeners in SRM 1588b Organics in Cod Liver Oil, SRM 1945 Organics in Whale Blubber, SRM 1946 Lake Superior Fish Tissue, and SRM 1947 Lake Michigan Fish Tissue. Error bars represent an expanded uncertainty about the mean at the 95% confidence level.
Fish-related reference materials are not only useful for the determination of environmental contaminants, but are also helpful for validating nutrient analyses. Fish are high in omega-3 fatty acids, particularly (Z,Z,Z,Z,Z)-5,8,11,14,17- eicosapentaenoic acid (EPA) and (Z,Z,Z,Z,Z)-4,7,10,13,16,19 docosahexaenoic acid (DHA), two nutritionally essential fatty acids. For this reason, fish oils are widely used as dietary supplements.
Among the SRMs discussed here, these fatty acids are relatively high in concentration in the cod liver oil SRM 1588b, as shown in Figure 4.
Figure 4 - Comparison of mass fractions of selected fatty acids in SRM 1588b Organics in Cod Liver Oil, SRM 1946 Lake Superior Fish Tissue, and SRM 1947 Lake Michigan Fish Tissue. Error bars represent an expanded uncertainty about the mean at the 95% confidence level.
SRMs 1588b, 1945, 1946, and 1947 are useful reference materials for monitoring programs examining human exposure to organic contaminants from the marine and freshwater environments. These fish and marine mammal tissue SRMs are four of the most highly characterized SRMs with values assigned in each for approximately 100–120 constituents. These biological materials complement the marine sediment SRMs, SRM 1941b Organics in Marine Sediment and SRM 1944 New York/New Jersey Waterway Sediment, available with values assigned for a wide range of organic contaminants and trace elements. More information on the SRMs is available at www. nist.gov/srm.
- Point, D.; Ignacio Garcia Alanso, J.; Davis, W.C.; Christopher, S.J.; Guichard, A.; Donard, O.F.X.; Turk, G.C.; Wise, S.A. J. Anal. Atom. Spectrom. 2008, 23, 385–96.
- Zeisler, R.; Langland, J.K.; Harrison, S.H. Anal. Chem. 1983, 55, 2431–4.
- Schantz, M.M.; Koster, B.J.; Oakley, L.M.; Schiller, S.B.; Wise, S.A. Anal. Chem. 1995, 67, 901–10.
- Poster, D.L.; Kucklick, J.R.; Schantz, M.M.; Porter, B.J.; Leigh, S.D.; Wise, S.A. Anal. Bioanal. Chem. 2003, 375, 223–41.
- Wise, S.A.; Poster, D.L.; Kucklick, J.R.; Keller, J.M.; Vander Pol, S.S.; Sander, L.C.; Schantz, M.M. Anal. Bioanal. Chem. 2006, 386, 1153–90.
- Levenson, M.S.; Banks, D.L.; Eberhardt, K.R.; Gill, L.M.; Guthrie, W.F.; Liu, H.K.; Vangel, M.G.; Yen, J.H.; Zhang, N.F. J. Res. Natl. Inst. Stand. Technol.2000, 105, 571–9.
- Guide to the Expression of Uncertainty in Measurement; ISBN 92-67-10188-9, 1st ed., ISO, Geneva, Switzerland, 1993; see also Taylor, B.N.; Kuyatt, C.E. Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results; NIST Tech. Note 1297, U.S. Government Printing Office: Washington, DC, 1994 (available at http://physics.nist.gov/Pubs).
- Stapleton, H.M.; Keller, J.M.; Schantz, M.M.; Kucklick, J.R.; Leigh, S.D.; Wise, S.A. Anal. Bioanal. Chem. 2007, 387, 2365–79.
- Streets, S.S.; Henderson, S.A.; Stoner, A.D.; Carlson, D.L.; Simcik, M.F.; Swackhaner, D.L. Environ. Sci. Technol. 2006, 40, 7263–9.
- U.S. EPA, 2001. Water Quality Criterion for the Protection of Human Health: Methylmercury, EPA-823-R-01-001.
The authors are with the Analytical Chemistry Div., National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899-8392, U.S.A.; tel.: 301-975-3106; e-mail: Michele. [email protected]; and Hollings Marine Laboratory, Charleston, SC, U.S.A.