An alternative method for the certification of the sulfur mass fraction incoal Standard Reference Materials

The S mass fractions of coal SRMs 2682b, 2684b, and 2685b are certified by direct comparison with coal SRMs 2682a, 2684a, and 2685a, respectively, usi ng high-temperature combustion analysis with infrared (IR) absorption detec tion. The S mass fractions of the "a" materials used for calibration were p reviously determined by means of isotope-dilution thermal-ionization mass s pectrometry (ID-TIMS). Therefore, the comparisons performed with the combus tion-IR absorption method establish direct traceability links to accurate a nd precise ID-TIMS measurements. The expanded uncertainties associated with the certified S mass fractions are of approximately the same magnitude as would be expected for the ID-TIMS methodology. An important aspect of these certifications is that each "b" material is essentially identical with the corresponding "a" material, because both were produced from the same bulk, homogenized coal. As a test of the efficacy of the new certification appro ach when calibrant and unknown are not identical, the S mass fraction of co al SRM 2683b has been determined by direct comparison to coal SRM 2683a. Th ese two coals, which have both previously been analyzed with ID-TIMS, are d iffer ent in terms of S content and other properties. Whereas the S mass fr action for SRM 2683b determined with the new methodology agrees statistical ly with the ID-TIMS value, there is reason for caution in such cases. In ad dition to the usefulness of the alternative approach for certification acti vities within NIST, this approach might also be an excellent way of establi shing NIST traceability during the value assignment process for reference m aterials not issued by NIST. Further research is needed, however, to unders tand better the scope of applicability.