4.1 This guide establishes basic requirements which should be met by water and environmental laboratories that generate and report test chemical analyses which the laboratory client desires to be traceable to SI units (Note 1) or certified reference materials traceable to SI units. Traceability of chemical analyses is important because it provides a uniform basis for the comparison of results from different measurement systems and because it relates those results to our current knowledge of physical laws (Note 2).

Note 1: A certified reference material traceable to SI units is a certified reference material whose value can be related with a stated uncertainty through an unbroken change of comparisons to stated references (usually national or international standards) in SI units, such as a primary measurement made in SI units or a national standard certified in SI units.

Note 2: Not all chemical analysis results can be traceable to SI units or to certified reference material's traceable to SI units, such as turbidity and or total suspended solids.

4.2 Many waters-related laboratories comply with ISO Guide 17025 and participate in Proficiency Testing Programs. Laboratories that are connected to the same accreditation bodies and Proficiency Test providers can be expected to report statistically similar results on the same sample. However, some test methods and some certified reference materials are not supported with data traceable to SI units. Therefore, fully compliant laboratories that are not connected to the same providers may report statistically different chemical analysis results if they used the same nontraceable test method on the same sample. This problem could be minimized if they used test methods, measurement devices, and certified reference materials that are traceable to SI units, where available.

4.3 Although some standard test methods and certified reference materials provide evidence of traceability to SI units, many others do not. Therefore, not all laboratories can be expected to universally meet all requests for traceable analyses until the traceability of more test methods and certified reference materials is recognized through appropriate documentation.

4.4 The primary significance of this guide is that it establishes a consensus that, in order for a laboratory to generate traceable measurements, it must (1) have a clear understanding of the needs of the user of the traceable measurements, (2) comply with the internationally accepted quality-system requirements included in ISO Guide 17025, (3) use test methods, measurement devices, and certified reference materials which have been shown to be traceable to SI units, and (4) be able to demonstrate that the measurement system was in statistical control at the time the measurements were made.

4.5 It is expected that this guide will be used by Committee D19 in setting policies for the technical content of its standards that are designated to be usable to generate traceable chemical analyses.