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

Substantially increased awareness and availability of reference materials offers many benefits to the ocean sciences. The regular use of such materials can provide a much-needed basis for interlaboratory and international comparison of results, making it possible to acquire accurate, meaningful global data sets that can be used to study problems requiring observations on large space and time scales. However, reference materials are costly to produce—particularly if they are certified for a number of constituents—and it has not always been clear to the ocean science community that this cost will be repaid with significant added value.

In the case of the acquisition of large-scale data sets, the benefits of using reference materials are self-evident. In the past, whenever such data sets have been acquired without using suitable reference materials, a great deal of effort has subsequently been needed to adjust the data to a common scale. But the benefit of comparability is not restricted to large programs. Matrix-based reference materials that can be exchanged between different laboratories will enable researchers to better understand their own techniques and the information they provide.

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 hydrographically 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

in their implementation from laboratory to laboratory. In response, the committee proposes both the preparation of such materials as well as the development of a vision for community involvement in reference material production, characterization, and use (see Chapter 5). This involvement will ultimately create a better understanding of the future needs for certified reference materials.

Any new effort to provide reference materials to the ocean sciences community must also put in place a number of educational and advertising efforts to make researchers aware of the existence of appropriate reference materials and to provide instruction on how to make best use of what is bound to be, at best, a finite resource. The committee, therefore, also identifies education as a critical need.

The development of new reference materials should not be undertaken lightly. Producing the prototype of each new material—including testing of the material for homogeneity and stability—requires substantial investment, and still further investment is required to provide accurate analyses of certain properties (as discussed in Chapter 5). Reference materials recommended in this report have been selected from a wide range of possible candidates. Their availability to the ocean science community will enable meaningful progress across a range of scientific questions.

In the past, a limited number of reference materials have been explicitly developed for ocean science (as discussed in Chapter 2): salinity, ocean CO₂, and DOC. Although salinity reference materials are available on a commercial basis from Ocean Scientific International Ltd. in the United Kingdom, the others are presently supported through grants from the U.S. National Science Foundation. The widespread use of such materials and their success in enhancing the scientific return on ocean studies is clear, and it is essential that such materials remain available.

In addition, NIST presently prepares a number of standard reference materials that are of immediate use to the ocean science community. These include materials for ¹⁴C (SRM 4990C) and ³H (SRM 4361C) as well as ²³⁸U, ²³⁴U, ²³⁵U (SRM 4321C), ²³⁰Th (SRM 4342—presently out of stock), ²²⁶Ra (SRM 4969), ²²⁸Ra (SRM 4339B), ¹⁰Be (SRM 4325), ocean sediment (SRM 4357), and river sediment (SRM 4350B). It is important to assure the continued availability of these materials.

The committee recommends the development of a seawater-based reference material containing the nutrient elements: nitrogen (as NO₃), phosphorus (as PO₄), and silicon (as Si(OH)₄) at concentrations similar to those in oceanic deep waters (40 µM for NO₃, 3 µM for PO₄, and 150 µM for Si(OH)₄) and certified for these constituents.

There is an urgent need for a certified reference material for these 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 committee recommends the development of two reference materials for seawater trace metal analysis:

• A seawater-based reference material with concentrations of metals corresponding to oceanic deep water, certified for total iron concentration.

• A seawater-based reference material with concentration of metals corresponding to open ocean surface water, with an information value for total iron concentration.

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.

The committee recommends, but assigns a lower priority to, the preparation of reference materials from other locations. For example a standard for dissolved iron in a coastal seawater matrix containing high concentrations of dissolved organic material would be particularly useful in addressing matrix effects associated with such materials.

The committee recommends the development of three radionuclide reference solutions:

• A synthetic certified reference material based on an acidic solution containing ²³⁸U and ²³⁵U (ca. 20 Bq/g and ca. 1 Bq/g respectively) with daughters in secular equilibrium through ²²⁶Ra and ²²³Ra.

• A synthetic certified reference material based on an acidic solution containing ²³²Th (ca. 20 Bq/g) with daughters in secular equilibrium through ²²⁴Ra.

• A synthetic certified reference material based on an acidic solution containing ²¹⁰Pb (ca 20 Bq/g) with daughters in secular equilibrium through ²¹⁰Po.

The presence of ²²²Rn in the ²³⁸U series makes the extension of the series through ²¹⁰Pb very difficult due to the escape of radon gas (see Chapter 5 for further preparation details).

Solid matrix-based reference materials are recommended for ten important matrix types: three based on algal materials and seven on sediments (details of possible preparation techniques are given in Chapters 4 and 5). Furthermore, the committee recommends that each of these materials be certified for both inorganic and organic carbon concentrations, total nitrogen concentration, δ¹³C of both the inorganic and the organic carbon components, and δ¹⁵N for the total nitrogen component. These are fundamental parameters whose measurements are not yet fully agreed upon.

All of these sediment reference materials would provide stable homogeneous materials containing a wide variety of chemical constituents that could be studied at the discretion of the ocean sciences community, and may ultimately be assigned consensus values for a number of important additional organic and inorganic analytes.

The committee recommends the development of three types of algalbased reference materials:

• A reference material based on a freeze-dried culture of the diatom Thalassiosira pseudonana.

• A reference material based on a freeze-dried culture of the dinoflagellate Scrippsiella trochoidea.

• A reference material based on a freeze-dried culture of the haptophyte Emiliania huxleyi.

These three cultures represent three major biogenic matrices: opal, carbonate, and organic matter, thus providing materials that can be used to investigate a variety of matrix-dependent effects (Table 4.5).

The committee recommends seven types of sediment-based reference materials:

• Open-ocean, carbonate-rich, sediment-based reference material.

• Open-ocean, silicate-rich, sediment-based reference material.

• Open-ocean, clay mineral-rich, sediment-based reference material.

• Coastal, carbonate-rich, sediment-based reference material.

• Coastal, silicate-rich, sediment-based reference material.

• Coastal, clay mineral-rich, sediment-based reference material.

• Deltaic sediment-based reference material (that has not contacted seawater).

Table 4.6 lists locations where such sediment types can be found. Taken together with the algal-based materials, the coastal and open-ocean sediments will provide material encompassing a wide range of early diagenetic states. The algal materials provide a fresh organic matrix, while the open-ocean red-clay material represents some of the most degraded sedimentary material that exists in the marine environment. The other proposed sediments lie between these two extremes. These matrix types (opal, carbonate, and aluminosilicate) provide end members for analysis and could be blended to produce any mixture desired.

The various reference materials described above are listed together in Box 6.1. Each of these is considered by the committee to have the potential for significant impact on an important area of ocean science and, as such, is assigned a high priority (however, see Statement of Top Priorities below).

In addition to making new reference materials available to the ocean sciences community, it is essential that a number of other strategies be put in place. These strategies will ensure optimal use of such materials, thus increasing their value to the ocean sciences and justifying this investment.

The use of appropriate reference materials should be a key feature of the quality assurance/quality control structure in any future ocean science project involving chemical measurement. Reference materials use should be explicitly addressed in the project planning stages, proposals, and publications. In the event that appropriate reference materials are not already available, the committee recommends that the proposed project develop a strategy for preparation to assure the ultimate value of measurements made as part of the research.

It is essential to develop and maintain a searchable, user-friendly database that ocean scientists can access to learn about those reference materials that are of particular interest to their research. A number of materials are currently available, but are rarely employed, in part because individual scientists may not be aware of their existence.

Many ocean scientists may be unaware of the clear gains to be garnered by the appropriate use of reference materials, and of the best way to use such materials to ensure quality control of their analytical measurements. The best way to address these deficiencies is to encourage the presentation of short courses on this topic in conjunction with national meetings for ocean scientists. Ideally, such courses would be co-taught by individuals from both the ocean science and the reference material provider communities (such as NIST or NRC-Canada).

A substantial impetus for improvements in analytical quality control within individual laboratories comes from participation in round-robin exercises in which participating laboratories independently analyze samples of a particular test material for specified analytes. It is important

that round-robin exercises be organized using materials and analytes relevant to the ocean sciences and that laboratories be encouraged to participate, even at an early stage in their experience with the relevant analytical techniques.

Most of the reference materials proposed in this report are based on natural matrices (seawater, algal cells, sediment) and will initially only be certified for a limited number of constituents. Nevertheless, it is apparent that such materials provide a resource for the investigation of a much wider variety of constituents, and it is important that the ocean science community be encouraged to investigate these materials further. In particular, these materials would enable a wide variety of necessary interlaboratory method comparisons that have been neglected to date, and ultimately additional consensus values for further constituents will be assigned to various research materials.

If the ocean sciences are to move forward and adopt the regular use of reference materials, the advantages of using such materials (and the pitfalls of not using them) need to be more broadly disseminated. Proposal and journal article reviewers should question the analytical quality control of measurements made without the benefit of reference materials. Outreach focused on how to use reference materials in the ocean sciences will further increase the awareness of the individual investigators and provide a focus for proper handling and application of a laboratory’s internal reference materials.

The recommendations made in this section are listed together in Box 6.2.

The committee recognizes that, by current standards, these recommendations will require a significant investment in reference materials for the ocean sciences. While such an investment is warranted, and the consequent gains to the ocean sciences will fully justify such expenditures, there is a need to assign additional priority to a subset of these materials to guide future planning.

The committee agreed that it was essential to ensure that reference materials for salinity, ocean CO₂, and DOC be kept available. These materials are now used regularly and are contributing to improvements in the science that can be achieved. The next highest priority is the development of a seawater-based nutrient reference material. Work on this material is presently in progress at NRC-Canada, and should be encouraged.

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.

Although each of the ten materials proposed as solid reference materials provides clear benefits (each represents a unique matrix), the following subset was considered the highest priority: reference materials based on Thalassiosira pseudonana cells (providing an opal matrix), on Emiliania huxleyi (providing a carbonate matrix), and on the three open-ocean sedi-

ments (providing opal, carbonate, and aluminosilicate matrices). These priority materials should be initially certified for organic carbon and nitrogen.

To take advantage of these solid reference materials (and to a lesser extent the trace metal seawater-based materials), it will be essential to establish an infrastructure for the collation and redistribution of the information that is accumulated about these materials through community-wide use.

The final, and yet perhaps the highest priority of all, is to improve the awareness and knowledge base within the ocean science community as to the availability and optimal use of reference materials. To ensure quality control of measurements in ocean sciences, it is paramount to establish and maintain a database of reference materials, and to make available short courses in reference material use.