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

Let us consider this final equation. It says the deviation of the mass moment measuredby MALDI-TOF MS from the true mass moment is a function of the polydispersity (PD) (arisingfrom that moment) divided by a correction term arising from how far that moment is from themass M 0 around which the Taylor’s expansion to obtain k 0 and Q is centered. Equations for allthe moments of the MMD are obtained the same way.Since we shall later be gravimetrically mixing polymers to obtain estimates of Q/k 0 , let uslook at the equations relating to these mixtures. Equation [6.7], states that the MALDI-TOF MSexpG T0measured total mass, , is proportional to the true mass, :G T∑G 0 [6.15]T= MnniGexpT00= k G 1 + Q / k ( M − M ))[6.16]oT(o w 0Consider now a mixture of the chemically identical polymers with end groups havingdifferent masses, or two different average molecular mass polymers, such that they cane bedistinguish as separate series in the mass spectrum. Call them polymer A and polymer B.Then the measured ratio of the masses of each is:GGexpTAexpTBk=koAoBGGoTAoTB⎧1+( Q⎨⎩1+( QAB/ k/ koAoB)( M)( MoWAoWB− M− Moo) ⎫⎬) ⎭[6.17]Notice we perform the expansions for both polymer distributions A and B around thesameM o. By simple algebra we obtain:GGexpTAexpTBk=koAoBGGoTAoTB⎧1+ ( Q⎨⎩ 1+( QAA/ k/ koAoA)( M)( MoWAoWB− M− MoWBo) ⎫⎬) ⎭[6.18]18