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

2.2 Initial Homogeneity and End-Group Composition TestingBefore certification, the within-lot homogeneity and approximate MMD of the candidatepolymer were verified by size exclusion chromatography (SEC) to be what was requested byNIST and claimed by the manufacturer. Its end-group composition and number-averagemolecular mass were obtained by nuclear magnetic resonance (NMR). NMR found the expectedn-octyl and proton end groups, an M n in agreement with the value found by SEC, and indicationsof a trace (< 1 % by mass) of an aromatic solvent, likely a residual of the synthesis. Since thecertification of SRM 2881depends on comparisons between gravimetric and mass spectrometriccompositions of polymer mixtures, all polystyrenes used were vacuum dried at 60 °C for 72 h toremove excess solvent.From the above results we concluded the polymer was of the correct molecular structureand of the necessary purity to serve as a candidate for certification.2.3 Bottling and SamplingA total of 69 individual samples of SRM 2881 were prepared in screw-cap glass vials eachcontaining about 0.25 g of polymer. The vials were divided into five subsets. One vial wasrandomly selected from each subset for bottle-to-bottle homogeneity testing of the packagedmaterial. The first and last bottles were also taken for study. Furthermore, one bottle (containingabout 2.5 g) was retained for use in future experiments. This bottle was also sampled forhomogeneity as a matter of course.3. Bottle-to-Bottle Homogeneity TestingBottle-to-bottle homogeneity testing was accomplished using size-exclusionchromatography (SEC). An Alliance 2000 GPC Liquid Chromatograph (Waters Corp., Milford,MA) with a differential refractive index detector and two Styragel 300 mm x 7.5 mm ID 10 μmHT6-E columns, and one Styragel 300 mm x 7.5 mm ID 10 μm HT-2 column (Waters Corp.,Milford, MA) was used in this study. The chromatography was run at a 1.0 mL/min solvent flowrate. The injector and column compartment of the chromatograph were controlled at 40 °C for allmeasurements. Tetrahydrofuran (Mallinckrodt Specialty Chemicals, Paris, KY) with addedantioxidant, 2,6-di-tert-butyl-4-methyl phenol (commonly known as butylated hydroxytoluene or5