In a tour de force of measurement science, researchers at the National Institute of Standards and Technology (NIST) have developed and issued for sale a new test material for calibrating quality control equipment used extensively by the polymer industry. The product of a 5-year effort that also produced significant improvements in mass spectrometry techniques for analyzing polymers, the new NIST reference material is the first to offer manufacturers a test sample with a known—and verified—distribution of molecular masses.
Almost all industrial polymers—"plastics" in common parlance—are manufactured in continuous processes. Raw ingredients are fed into chemical reactors that cause the individual base units—called monomers—to link together into the long repeating chains that make them polymers. By its nature, the process produces polymer molecules having a variety of lengths (and therefore molecular weights), varying from a few tens of monomers long to hundreds or thousands, depending on a variety of process variables. This molecular mass distribution is tricky to control, but the product's physical properties depend heavily on getting the correct distribution, so manufacturers are intensely interested in ways to monitor it accurately.
Typically this is done by periodically sampling the product using chromatography, a measurement technique that separates molecules out by length. For years NIST has provided polymer mass reference materials for use in calibrating these instruments, but the earlier reference materials were certified by the laboratory only for an average distribution—essentially, they would pin down only a couple of values across the total mass distribution.
The new material, NIST Standard Reference Material (SRM) 2881, "Polystyrene Absolute Molecular Mass Distribution Standard," provides the absolute molecular masses of 43 different lengths of polystyrene molecules, and their cumulative contribution to the total mass of the sample from 1 to 99 percent. Developing the new SRM, according to project leader William Wallace, involved a long process of analyzing and optimizing all aspects of the sample preparation, an international comparison of results from different laboratories, and developing improved tools for measuring individual molecular masses using a technique called Matrix Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI TOF). (For related information on using MALDI for polymer measurements, see "Mass Spectrometry Methods Database Gets Major Update," NIST Tech Beat, May 25, 2006.)
The NIST reference material is polystyrene, and it can be used to calibrate mass distribution measurements for a broad range of polymers. Technical details and ordering information for SRM 2881 can be found at https://www-s.nist.gov/srmors/view_detail.cfm?srm=2881. Standard Reference Materials are among the most widely distributed and used products from NIST. The agency prepares, analyzes and distributes more than a thousand different materials that are used throughout the world to check the accuracy of instruments and test procedures used in manufacturing, clinical chemistry, environmental monitoring, electronics, criminal forensics and dozens of other fields. For more information, see NIST's SRM Web page at http://ts.nist.gov/measurementservices/referencematerials.