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

We close this section saying that the gravimetric calibration of the signal axis usingchemically identical polymers can avoid the issues discussed in Section 4 on the uncertaintiesarising from ablation, ionization, and detection. However, the issues arising fromuncertainties in repeatable sample preparation, and data analysis, still affect the gravimetriccalibration techniques. In sample preparation, the problem of phase separation as a functionof molecular mass (each component of the blend is usually of a different molecular mass)appears. In solution, the solubility of some polymers is very molecular-mass dependent (e.g.,polyethylene glycol solubility in tetrahydrofuran). As the solvent evaporates in the drieddroplet technique the oligomers of different molecular masses come out of solution atdifferent rates and thus may find themselves at different matrix:polymer ratios. In dataanalysis the issues of determining the baseline consistently and drawing areas consistently isstill an issue and may incorrectly represent the amount of mass in one or the other polymer inthe blend. Further, if one polymer is a small fraction of the other, the experimental noise willcause major problems with peak integration.7. Experimental Method for the Calibration of the Signal AxisWe wish to look at the calibration of the signal axis using the expression derived insection 6. Calibration of the signal axis has been done before, specifically by Shimada, et al.10, 11 by fractionating the polymer into distinct oligomers with supercritical fluidchromatography (SFC) and then using these materials to calibrate the spectrometer and obtainthe MMD of the whole polymer from these calibrations. Our method does not depend on theSFC fractionation but rather uses narrow MMD polymers without fractionation with differentend groups so they can easily be distinguished in the mass spectrum.7.1 Samples and ReagentsThree octyl-initiated (M n of 6200 u, 9180 u, and 12 600 u (as measured by thevendor using SEC with multiple-angle laser light scattering) and one butyl-initiated (M nof 9100 u) polystyrene samples were synthesized by Scientific Polymer Products, Inc.(Ontario, NY). These four polymers will be referred to as OctylB, OctylA, OctylC, andButylA, respectively. SRM 2888, a PS of about 6800 u made by Polymer Source(Dorval, Québec, Canada) was used as received. Two other PS with butyl end groups21