Standard Reference Material 2881 - National Institute of Standards ...

anticipated to be a useful material for laboratories to check their proficiency in applying thesestandard methods.Various averages, or molecular moments, can be defined as useful summaries of the MMD.Measuring and computing these summary statistics has comprised a major part of traditionalpolymer analysis. The three most common measures of the MMD are the number averagerelative molecular mass, M n , the mass average relative molecular mass, M w and the z-averagerelative molecular mass, M z .Mn= ∑ mjnj/ ∑ nj[4.1]jj2Mw= ∑mjnj/j∑jm njj[4.2]PD = M w/ M n[4.3]Mz= ∑mj3jnj∑j2j/ m n[4.4]jwhere m j is the mass of a discrete oligomer, n j is the number of molecules at the given massm j. The experimental moments from MALDI-TOF MS are defined as M n , M w , and M z , whilethe “true” values are given as M 0 n, M 0 w, and M 0 z. PD defines the polydispersity index whichis a measure of the breadth of the polymer distribution. When PD is equal to one (i.e., instatistical terms the variance of MMD is zero), all of the polymer molecules in a sample are ofthe same molecular mass and the polymer is referred to as monodisperse. Natural polymers,such as proteins, are typically monodisperse. This is seldom the case for synthetic polymers,which have a non-point-mass MMD, whose width depends in a complex fashion on thepolymerization chemistry, specifically its mechanism, kinetics, and reaction conditions. Ingeneral, no two polymerization reactions yield precisely the sample MMD no matter howcarefully conditions are controlled. This means that the product each polymerization reactionmust be measured to obtain a reliable MMD.8