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

From classical polymer methods of analysis of the MMD one obtains one or twomoments of the MMD. M w can be obtained by light scattering or ultracentrafugation, and M ncan be obtained by the measurement of colligative properties like osmotic pressure, or by endgroup analysis as by nuclear magnetic resonance (NMR) or titration techniques. Generallythese moments give an incomplete description of the overall MMD. Properties like meltviscosity, tensile strength, and impact strength often depend on the tails of the MMD ratherthan the central portion as defined by the two central moments. The polydispersity index canbe used as a poor surrogate in determining the effect of the tails of the MMD. Thus, it iscritically important that we make an effort to obtain the entire MMD. Furthermore, it is notuncommon, due to purposeful blending or to the polymer chemistry (resulting from twodifferent mechanisms (intentionally of unintentionally) competing during preparation), thatthe polymer MMD will be bimodal; thus the central moments, M n and M w , are exceedinglypoor representations of such MMD. Size exclusion chromatography (SEC) and matrixassistedlaser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) eachoffers the prospect of obtaining the entire MMD. SEC is not an absolute technique in that italways requires calibration with another polydisperse polymer analyte. Also, it is difficult todetermine the Type B (systematic) uncertainty of an SEC measurement. MALDI-TOF MS,particularly at molecular masses below about 25 000 u for polystyrene (PS), can obtain amolecular mass spectrum (MMS) with distinct oligomer peaks. Such equivalent averages asdescribed above are then applied to MMS of polymers where it is assumed that the area undera peak of an oligomer is proportional to the number of molecules at a given mass m j . From theMMS one can, with proper understanding of the limitations and corrections of the spectrum,derive a good approximation to the true MMD of the polymer. It is the central purpose of thiswork to consider how one can use the MMS to approximate the MMD. For this we shall needto consider carefully the repeatability, accuracy, and precision of MALDI-TOF MS forpolymer analysis.One can ask, what is the precision and accuracy of the method? How well does the methodmeasure the quantities it is purported to measure (in this case the MMD)? From one point ofview, this can be rephrased as, how well does it agree with other methods measuring the samequantity? These other values can be obtained from an independent measurement on the9