According to ISO 31-8, the term "Molecular Weight" has been replaced by Relative Molecular Mass, symbol M r .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 M r,w 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.Final report prepared for the sponsors, the NIST Standard Reference Materials Program (SRMP).NISTIR 6456