Chemical Reference Materials: Setting the Standards for Ocean Science (2002)

In this new millennium, during which human activity will bring about unprecedented change in the natural world, ocean scientists are called upon to address a variety of important challenges. The National Research Council (1998) chose to highlight three related research areas in a report, Opportunities in Ocean Sciences: Challenges on the Horizon. The focal areas selected were:

• Improving the health and productivity of coastal oceans,

• Sustaining ocean ecosystems for future generations, and

• Predicting climate variations over a human lifetime.

This report also notes that the oceans still remain too vastly undersampled in time and space to adequately address these global-scale long-term challenges.

In response to an earlier recognition of the growing need for a comprehensive characterization of ocean systems over time and space, the 1990s saw the start of various large research programs, such as the Joint Global Ocean Flux Study (JGOFS), designed to intensively study processes that cycle biologically active elements and compounds over key regions and time periods. New expedition-based sampling programs are presently in the planning stages. In addition, numerous observatories have been established over the last 10 to 15 years at coastal and open

ocean sites to record the extensive time-series observations needed to recognize decadal-scale trends. These observatories include the Hawaii Ocean Time-series (HOT) and the Bermuda Atlantic Time Series (BATS) stations. Observations made at these sites have been particularly important in identifying changes in processes that regulate carbon cycling through large areas of the ocean (e.g., Karl, 1999). The specific scientific opportunities that can result from long-term observations over broad spatial scales have been highlighted in several recent reports (e.g., Illuminating the Hidden Planet: The Future of Seafloor Observatory Science (NRC, 2000); Ocean Sciences at the New Millennium (NSF, 2001); and Discovering Earth’s Final Frontier: A U.S. Strategy for Ocean Exploration (President’s Panel for Ocean Exploration, 2000).

In addition to the need for adequate sample coverage, comprehensive time-series studies of the global ocean require accurate, precise, and comparable chemical measurements of a daunting variety of dissolved and particulate constituents. Chemical characterizations have been a mainstay of oceanography for over a century. For example, precise measurements of total salt content are used to identify ocean waters of different origins and to predict how they will interleave and circulate. Analyses of dissolved nutrients such as nitrate (NO₃), phosphate (PO₄), and silicate (Si(OH)₄) indicate the potential for phytoplankton growth as well as the flow of food and energy to zooplankton and fish. Stable isotopes, trace elements, and organic molecules provide tags and tracers of events in the water column and sedimentary record, and radioactive isotopes serve as “clocks” to aid in determining the rates of the processes that produce these distribution patterns.

This work has relied on the skills and dedication of the individuals involved, who have typically carried out scientific studies of extremely high quality. However, even using modern techniques, chemical analyses of ocean waters and particles remains challenging given that they often involve low concentrations and complex sample matrices. Oceanographic measurements are often particularly prone to “matrix effects,” where the target analyte is influenced by a variety of other sample components. In this time of global change, a central challenge for the oceanographic community is to synthesize data sets obtained at different times, by different laboratories using a variety of analytical techniques that are applied to contrasting sample matrices.

However, this is not a recent development. A 1971 report of the Marine Chemistry Panel of the National Academy of Sciences Committee on Oceanography wrote that:

The need for oceanographic standards and interlaboratory comparisons was again reiterated more than 20 years later in the 1993 report of the Ocean Studies Board Committee on Oceanic Carbon:

In response to this clear and continually growing need, the current National Research Council Committee on Chemical Reference Materials for Ocean Science was empanelled in 2001 to:

• compile from available sources a list of important oceanographic research questions that may benefit from chemical reference standards;

• create a comprehensive list of reference materials currently available for oceanographic studies; (See Appendix E)

• identify and prioritize the reference materials needed to study the identified research questions;

• determine for each priority analyte whether reference materials and/or analytic methods should be standardized; and

• identify the most appropriate approaches for the development and future production of reference materials for ocean sciences.

Reference samples and reference materials have served a critical role in analytical chemistry since its inception. The reliability of all analytical results is completely dependent on the availability of suitable reference materials, and now nearly all branches of analytical chemistry declare an

urgent need for such standards (Zschunke, 2000). Specific terminology has evolved for different types of reference materials and is highlighted (Box 1.1) for use in later discussion.

There is often some confusion between the terms standards and reference materials. Primary standards represent the top-tier of chemical standards and, in principle, provide a means of establishing the traceability of analytical data to the SI measurement units (e.g., the kilogram, mole, meter, and second). A limited number of pure chemicals are recognized as primary standards (and thus can constitute certified reference materials). Most certified reference materials are not of themselves primary standards; rather, the property values assigned to them are traceable to primary standards where practical.

For the purpose of this report, the committee focused on reference materials for chemical composition measurements used in the ocean sciences. Although a number of these materials are simple mixtures or

solutions, the majority of materials discussed are “compositional” or matrix reference materials. Such materials are based on “natural” substances (e.g., seawater, sediments, or biological materials such as phytoplankton), and offer an advantage over primary standards by providing a better match to sample composition. Thus they offer a tool to minimize matrix effects and to identify problems in the application of analytical methods to natural samples. If the reference material is a close match to the sample being analyzed, the measurement on the reference material will provide useful information on the quality of the laboratory’s overall process. Conversely, analysts should be aware that matrix reference materials can contain constituents not present in their samples. If present at sufficiently high concentration, these constituents could prevent true matrix-matching between the reference material and the sample.

The highest quality reference materials are certified for the concentration values of the constituent(s) of interest, reflecting high confidence in the value’s accuracy and the thorough investigation of all known or suspected sources of bias. Used appropriately (e.g., Roper et al., 2001; Zschunke, 2000), these certified values provide an effective means to ensure comparability (both among laboratories and over time). It is not always practical, however, to undertake the work required to produce a certified value; in many such cases, a value can be carefully determined, but insufficient information exists to assess the associated uncertainty. This information value is nevertheless of substantial interest to other users of the reference material. It can, for example, allow laboratories to compare results even though full traceability is impractical.

In this report, this distinction between certified and information values will be drawn on a number of occasions, usually to emphasize the cost of establishing the detailed uncertainty of analytical techniques used to certify materials relative to the perceived benefit of certification. In many situations, a particular analyte—though important scientifically—is studied by a limited number of researchers, in which case it is most practical to establish an information value for that particular reference material by consensus (provided the necessary conditions of stability and homogeneity are met).

It is impossible to have a discussion of the “quality” of chemical analyses made in the support of ocean science without a clear recognition that, in large part, this depends on the skills and dedication of the individuals involved. Indeed, almost all the scientific progress made to date in the ocean sciences has been achieved without the benefit of reference materi-

als. Furthermore, reference materials are costly to produce—particularly if they are certified for a number of constituents—and it has not always been clear that this cost is repaid with significant added value.

However, it is also appropriate to consider the cost of not using reference materials. Reference materials provide the benefit of comparability—between results obtained at different times, in different places, by different people, and using contrasting methods. Almost every sub-discipline of ocean science research faces the need to understand complex dynamical processes that require large-scale, time-series data sets for effective study. When such data sets are acquired without using suitable reference materials, significant effort is expended later on efforts to adjust data to a common scale. In many cases this adjustment is impossible and the measurements have then, at best, limited value. Indeed, the costs may be far higher if erroneous management decisions are later based on such measurements.

The establishment of observatories constitutes a paradigm shift in the conduct of ocean sciences research—away from the traditional expeditionary mode—and coupled to this shift in observational modes is a need to ensure that an adequate infrastructure exists to support these observing systems and to provide the necessary quality assurance (NRC, 2000). In addition, the analytical tools needed for many large and small oceanographic studies are currently in a rudimentary form. Creating reference materials that can be exchanged between different laboratories will enable researchers to better understand their own techniques and the information they provide. Unfortunately, at present few certified reference materials exist for seawater properties, and many common marine matrices such as opal and carbonate bearing phytoplankton and sediments are not available as reference materials at all. Regardless, many research problems in the ocean sciences would benefit from the availability of suitable reference materials.

The recent Future of Ocean Chemistry in the U.S. (FOCUS) report (NSF, 2000) identified eight key themes for future chemical oceanographic research (Box 1.2). Clearly, oceanographic analyses over the coming decades will involve a wide variety of dissolved and particulate constituents from coastal ocean, open ocean, and seafloor settings, and thus a variety of reference materials will be required to represent these key sample matrices.

These eight research initiatives will require new strategies to ensure uniform analytical quality and comparability, and each of these areas could benefit to a greater or lesser extent from the availability of appropriate reference materials. Discussions at the Workshop (Islamorada, Florida in September 2001) held as part of this process helped the committee to clarify the ocean science community’s perspectives on this, and

provided a basis for future committee discussions during the preparation of this report.

However, the costs of developing new certified reference materials are substantial and new projects should not be entered into lightly. In choosing areas of ocean science to highlight by proposing new reference materials, the committee aimed to strike a balance between emphasizing a clear need for certified reference materials to assist in the quality control of long-term, multi-investigator observing programs, and identifying a parallel need for reproducible reference materials that will assist new communities of ocean scientists in improving their understanding of matrix problems in difficult and sophisticated analyses.

The committee thus recommends that a limited number of carefully chosen matrix reference materials be prepared (based on seawater, sediments, and algal materials) to respond to a significant portion of the current pressing needs. Such materials, certified for a limited number of key

analytes, would represent stable homogeneous samples for a far wider range of additional constituents than is currently available. The committee recommends that these reference materials be investigated further by the scientific community to ascertain their usefulness for a wide range of constituents such as trace metals, organic compounds, and radioisotopes. Subsequently, consensus values for the concentrations of particular constituents should be assigned to many of these reference materials, further enhancing their usefulness.

Substantial discussion arose in deciding how to arrange this report as a consequence of the two-pronged nature of the problem. Should reference materials be thought of (and hence grouped for discussion) by analyte, or by matrix? Each of the natural materials occurring in the ocean system—seawater, biological materials, and sediments—provides a highly complex mixture of constituents. It is neither practical nor desirable to provide a complete analysis of all these constituents. Nevertheless, each matrix addresses particular scientific questions and provides particular challenges; the committee thus opted to arrange its discussion in terms of sample matrices.

Chapter 2 sets the stage with a brief statement of the present use of reference materials by the oceanographic community. A number of case studies have been included both to illustrate the value of such materials and to indicate the potential limitations of reference materials as a cure-all for oceanic measurements.

Chapter 3 discusses the analysis of seawater constituents and reiterates the scientific problems that necessitate a number of seawater-based reference materials.

Chapter 4 discusses the analysis of particulate materials such as sediments and biological materials. It outlines the current state of the science in some of these areas, and suggests that a limited number of particulate reference materials could play a significant role in moving these fields forward.

Chapter 5 provides specific suggestions for the production and distribution of reference materials for ocean science, noting some of the potential challenges and indicating possible strategies to avoid or mitigate these problems.

Chapter 6 provides a summary of the committee’s principal recommendations that arise throughout the report and indicates those actions that the committee found to be of the utmost priority.

Appendix A contains biographical information on committee members. Appendix B contains the names and affiliations of the participants

who attended the workshop in Islamorada, Florida in September 2001. Appendix C contains a glossary of technical terms used in this report. In addition, because this report relies on discussion of chemical elements, compounds, isotopes and radionuclides, a standard form of nomenclature was required. For this reason, where practical, the names of elements are spelled out throughout the report (e.g., nitrogen, carbon). The names of chemical substances are spelled out and the abbreviation given on their first mention (e.g., carbon dioxide [CO₂]). All isotopes and radionuclides are referred to by their standard chemical abbreviation (e.g., ¹⁴C, ²²⁸Ra). A list of the elements, compounds, radionuclides and acronyms referred to in this report, is also provided in Appendix D for clarification. Appendix E provides information about all currently available reference materials identified in this report, as well as sources for obtaining these (and other) reference materials.