The National Institute of Standards and Technology (NIST, Gaithersburg, MD) issued a new series of Standard Reference Materials® (SRMs) targeting the ever-increasing list of potentially toxic substances in the aquatic environment that challenge environmental monitoring efforts and require new and improved analytical technologies for organic pollutants and trace metals. To support rapidly developing sectors of environmental analysis for the determination of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyl (PCB) congeners, and chlorinated pesticides, as well as for trace metal analysis, the NIST collection of SRMs was supplemented with the renewal material SRM 1941b Organics in Marine Sediment and the new materials SRM 2702 Inorganics in Marine Sediment and SRM 2703 Sediment for Solid Sampling (Small Sample) Analytical Techniques.
Figure 1 - Sediment collection in the Chesapeake Bay.
SRMs 1941b, 2702, and 2703 were developed from the same source material collected from the Chesapeake Bay at the mouth of the Baltimore (MD) Harbor near the Francis Scott Key Bridge (39°12.3'N and 76°31.4'W) (see Figure 1). This location is very near the site where SRM 1941 and SRM 1941a, the precursors to SRM 1941b, were collected. The sediment was collected using a PVDF-coated modified Van Veen-type grab sampler. A total of approximately 3300 kg of wet sediment was collected from the site. The sediment was freeze-dried (56% solid mass fraction), sieved at 70 µm (100% passing), homogenized in a cone blender, radiation-sterilized at a 33–45 kGy (60Co) dose, and then packaged in screw-capped amber glass bottles, each containing approximately 50 g. The total amount had been divided into a lot for SRM 1941b and a lot for SRM 2702, with 20 kg set aside from the latter for SRM 2703.
The processing of the collected material yielded a very uniform material with a median particle size of about 5 µm and nearly 100% smaller than 40 µm. Particle sizing analysis illustrates the uniformity of the material throughout the lots (Figure 2). The 20-kg batch was further prepared to form SRM 2703 for small sample analytical techniques. Further processing with jet-milling and size classification yielded a very fine particulate material with essentially no particles larger than 20 µm. The material for SRM 2703 was then packaged in screw-capped amber glass bottles, each containing approximately 5 g. The processed material has a more uniform particle size, which may improve the already very good homogeneity parameters of the sediment.
Figure
2 - Particle size distributions in sediment SRMs determined in aqueous
suspension via laser light scattering instrumentation (Mastersizer 2000,
Malvern Instruments, Worcestershire, U.K.).
SRM 1941b is one of the most widely characterized materials for organic constituents. It was issued in 2002 with concentrations assigned for 67 PAHs, 42 PCB congeners, and 9 chlorinated pesticides. SRM 1941b complements the other sediment SRM available with organic constituents characterized, SRM 1944 New York/New Jersey Waterway Sediment. The concentrations of the PAHs, PCB congeners, and pesticides in SRM 1944 are generally 10–20 times higher than those in SRM 1941b. SRM 1941b represents a moderately contaminated sediment collected from an urban area, and SRM 1944 represents a highly contaminated sediment collected from an urban industrial area.
SRM 2702 is a replacement for SRM 2704 Buffalo River Sediment with improved certified values based on a modified and cost-effective analytical approach. The analyses for value assignment involved NIST analysts as well as analysts from the U.S. Geological Survey (USGS) and National Research Council Canada (NRC). This provides a broader basis for the best estimates of the true values. Assigned expanded uncertainties are generally less than 5% relative and less than the older uncertainties of SRM 2704 issued in 1988. The lower uncertainties are probably due to the high homogeneity of the material and the improved analytical methodology. Certified concentration values are assigned for all elements from the U.S. EPA priority pollutant list, except for beryllium, as well as for a number of other trace and matrix elements. Certified mass fraction values are provided for 25 elements, reference values for 8 elements, and information values for 11 elements. This elemental coverage provides high utility to the users.
SRM 2703 was specifically developed for microsampling techniques. Analysts using direct and slurry sampling, as well as dissolution techniques for milligram size samples, can employ this SRM to benchmark their procedures. A thorough homogeneity assessment of the material was conducted using instrumental neutron activation analysis (INAA). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), microbeam X-ray fluorescence (µXRF), and microproton beam induced X-ray emission (µPIXE) techniques complemented the INAA assay with homogeneity results for significantly smaller sample sizes. Additional measurements with several solid sampling procedures and dissolution procedures were provided by collaborating laboratories with the following techniques: atomic absorption spectrometry (AAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and ICP-MS, and X-ray fluorescence (XRF). All elemental assays were designed to establish comparability of values between the conventionally certified parent material SRM 2702 and the microsampled SRM 2703. These measurements confirmed that the elemental composition of the material had not changed in processing and that the values in SRM 2703 represent the material as it was initially prepared for analysis. Certified mass fraction values are provided for 22 elements, reference values for 7 elements, and information values for 9 elements.
Additional reading
Certificate of Analysis available at www.nist.gov/srm.
Zeisler, R. Anal. Bioanal. Chem.2004, 378, 1277–83.
Wise, S.A.; Poster, D.L.; Schantz, M.M.; Kucklick, J.R.; Sander, L.C.; De Alda, M.L.; Schubert, P.; Parris, R.M.; Porter, B.J. Anal. Bioanal. Chem.2004, 378, 1251–64.
The authors are with the Analytical Chemistry Div., National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899-8392, U.S.A.; tel.: 301-975-6290; e-mail: [email protected]. Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedures in adequate detail. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.