Trace metals reference material

The primary and immediate need is for a trace metal reference material, but a certified reference material would provide even greater benefits. A technique based on isotope dilution with detection by inductively-coupled plasma mass spectrometry (ICP-MS) (Wu and Boyle, 1998) most clearly meets the traceability criteria required for a certified reference material. Although useful for iron and several other metals, isotope dilution is not possible for monoisotopic elements like cobalt, so other techniques must also be used. Indeed, it is advisable that several techniques be used to certify a trace metal reference material. 

National Science Foundation (NSF), and the Chemical Oceanography program invests substantial resources in that area. The upper water column in the center of Lake Superior might be a good place to collect water for preparation of a freshwater trace metal reference material. 

Approaches to the Preparation of Seawater Trace Metal Reference Materials... 

The optimal size for a portion of a trace metal reference material is 1L, so large volumes (greater than 1000 L) must be collected and prepared, which is feasible given currently available Teflon storage containers, sampling systems, and pumps. More experience is required, however, in collecting large volumes of uncontaminated water. 

Two trace metal reference materials (one based on surface ocean water and one based on deep ocean water) are urgently needed to further the research community s ability to investigate the role of trace metals in ocean biogeochemistry. Thus these two materials are also assigned a high priority. 

Brown, S.S. (1977) Trace metals reference materials and methods. In Brown, S.S. (Ed.) Clinical Chemistry and Chemical Toxicology of Metals, Elsevier/North-Holland, Amsterdam, p. 381. 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

Berman SS, Sturgeon RE, Desaulniers lAH and Mykttiuk AP (1983) Preparation of the sea water reference material for trace metals, NASS-i. Mar Pollution Bull 14 69-73. 

Berman SS (1984) Marine biological reference materials for trace metals. In WolfWR, ed. Biological Reference Materials, PP79-88. John Wiley Sc Sons. 

Berman SS, Sturgeon RE (1988) A new approach to the preparation of biological reference materials for trace metals. Fresenius Z Anal Chem 332 546-548. 

National Research Coundl Canada (1992) Certified Reference Material NASS-4 Open Ocean Seawater Reference Material for Trace Metals. National Research Council of Canada. Ottawa. National Research Council Canada (1995) Certified Reference Material CARP-i Groimd Whole Carp Reference Material for Organochlorine Compounds. National Research Council of Canada, Ottawa. 

Quevauviller Ph, Lachica M, Barahona E, Gomez A, Rauret G, Ure A, Muntau H (1998m) Certified reference material for the quality control of EDTA- and DTPA-extractable trace metal contents in calcareous soil (CRM 600). Fresenius J Anal Chem 360 505-511. 

DE Boer J (1997) The preparation of biological reference materials for use in interlaboratory studies on the analysis of chlorobiphenyls, organochlorine pesticides and trace metals. Mar Poll Bull 35 84-92. 

One approach to providing secondary RMs is the NIST Traceable Reference Material (NTRM) program for gas standards (Jenks et. al. 1998 NIST 1997). The NTRM program is to be extended to metals, trace elements and pure substances. It may be possible to extend this concept to other types of RM, but it is difficult to see how it can easily be applied to the production of complex matrix CRMs. 

Committee members were chosen for their wide variety of scientific expertise and experience in production and certification of reference materials. In addition, members with proficiency in the use of reference materials for the analysis of trace metals, radioisotopes, nutrients, carbon, and organic matter were represented. The committee met on four sepa-... 

Seawater reference materials are recommended for nutrients and for trace metals (especially iron). There is an urgent need for a certified reference material for nutrients. Completed global surveys already suffer from the lack of previously available standards, and the success of future surveys as well as the development of instruments capable of remote time-series measurements will rest on the availability and use of good nutrient reference materials. The reference materials for trace metals— though initially characterized only for the important micronutrient, iron— should ultimately be useful for the analysis of the other metals and some dissolved organic materials. The committee also recommends the devel-... 

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

The collection of a reference material for trace metals could complement collection of samples for other priority analytes including DOM and DOC. Seawater used for DOM analysis can also be collected in Teflon , for example. Although samples for DOM are not normally stored in plastic bottles (as the iron reference material would be), releases of plasticizers are minimal and are not expected to interfere with the detection of individual organic compounds such as amino acids and sugars. 

At present, soil derived humic matter and fulvic acids extracted from freshwater are available commercially and are commonly used to test techniques for DOM detection and also used as model compounds for trace metal chelation studies. The results obtained using these model compounds are frequently extrapolated to the natural environment and measurements on "real" samples provide evidence that this DOM is a good model compound. In the past, some investigators also made available organic matter isolated from marine environments using C18 resins. While these compounds come from aquatic sources, this isolation technique is chemically selective and isolates only a small percentage of oceanic DOM. Reference materials are not currently available for these compounds, which inhibits study of the role they play in a variety of oceanographic processes. 

TABLE 4.5 Distribution of Mineral, Trace Metal, and Organic Analytes in the Target Biological Matrices Recommended as Reference Materials (+ = present and - = absent). 

Seawater studies require certified reference materials for biologically important dissolved components such as carbon (both inorganic and organic), nutrients, and trace metals, as well as for salinity, which is hydro-graphically important. A number of the committee s key recommendations therefore explicitly address these parameters. There is also a striking need for reference materials based on particulate matrices, where many of the analytical techniques used are matrix dependent and differ markedly... 

The committee recommends the development of two reference materials for seawater trace metal analysis ... 

A significant proportion of the needs for reference materials for seawater trace metal studies would be addressed by the preparation of these materials. Although the total iron concentration of these reference materials should be provided, these materials clearly will be useful for studies of other important metals such as zinc, manganese, copper, molybdenum, cobalt, vanadium, lead, aluminum, cadmium, and the rare earth elements. With careful planning, such water samples should be useful for analysis of dissolved organic substances as well. The collection sites should be chosen carefully to provide both a high and a low concentration reference material for as many metals as possible. 

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