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

6.0. Signal Axis Calibration ProcedureMost of the systematic uncertainties in converting from the MMS to the MMD arise in thesignal axis calibration. Uncertainties arise from inconsistent, irreproducible samplepreparation (particularly when using hand spotting methods), from drift in the machineparameters (especially variability in the laser energy over various time scales), from unequaldetector sensitivity as a function of ion mass, from ad hoc data analysis methods (primarilybackground subtraction and peak integration), and from many other minor effects. With allthese complications it seems impossible that we can find a simple way to look at quantitation onthe signal axis; however, by taking the clue from many published papers (discussed below) thatthe polydispersity of a polymer must be fairly low to get a proper MMD from the MMS, amathematical model was created to build on this observation. This entailed gravimetricblending of narrow polydispersity polymers to control the mixture polydispersity and discovera limiting case for narrow polydispersity materials. The literature in this area will bereviewed in the next section.6.1 Gravimetric Blending for Signal Axis QuantitationThere are several works in the literature that use blending of polymers of differentmolecular mass to obtain some estimate of how polydispersity affects the predicted versusfound properties of the MMD or moments thereof. Shimada et al. 10, 11 fractionated lowmolecular mass polystyrenes and polyethylene glycols into a single oligomers usingpreparative supercritical fluid chromatography (SFC). Oligomers with degree ofpolymerization up to 25 were fractionated. Then with five or six isolated oligomers from agiven polymer they obtain a calibration curve to convert the MMS to the MMD as a functionof machine parameter. This is certainly unique but depends on being able to fractionate thepolymers into oligomers at any molecular mass, which as of this writing is only true for lowmass oligomers. Furthermore, as we shall note later, it is important to be working underexperimental conditions where there is a linear relationship between signal intensity andanalyte concentration. As will be shown, this is necessary to make quantitative measurementsof the MMD. Linearity was not demonstrated in the work of Shimada, et al.12