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Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply necessarily the best available for the purpose. According to ISO 31-8, the term "Molecular Weight" has been replaced by "Relative Molecular Mass," symbol M r Thus, if this nomenclature and notation were followed in this publication, one should write /W rw instead of the historically conventional M w for the weight average molecular weight with similar changes for M n , M z , and M v. M w would be called the "Mass Average Relative Molecular Mass." The conventional notation, rather than the ISO notation has been used in this publication.

The recertification of a narrow molecular weight distribution poly (ethylene)

The characterization of a pol 3 rurethane standard reference material, SRM 1480, is described. The weight-average molecular weight of SRM 1480 by light scattering was determined to be 4.7 x lO'* g rnole"^. The intrinsic viscosity of SRM 1480 in THF was also measured and found to be 43.8 mL/g. The Size Exclusion Chromatography (SEC) of SRM 1480 in THF as received in the bottle was found to exhibit concentration dependent peak positions, even at low concentrations. The addition of 0.01 moles per liter LiBr to the THF was found to alleviate this problem. This low concentration of LiBr did not change the Polystyrene calibration of the SEC columns.