CALIBRATION STANDARDS FOR DETERMINATION OF MOLECULAR MASS.NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. certified three new polyethylene standards for calibration and performance evaluation Performance evaluation The assessment of a manager's results, which involves, first, determining whether the money manager added value by outperforming the established benchmark (performance measurement) and, second, determining how the money manager achieved the calculated return of size exclusion chromatographs. Size exclusion chromatography Size exclusion chromatography (SEC) is a chromatographic method in which particles are separated based on their size, or in more technical terms, their hydrodynamic volume. It is usually applied to large molecules or macromolecular complexes such as proteins and industrial polymers. (SEC) is the most convenient and widely used method for determining the mass distribution of synthetic polymers Synthetic polymers are often referred to as "plastics", such as the well-known polyethylene and nylon. However, most of them can be classified in at least three main categories: thermoplastics, thermosets and elastomers. . The molecular mass distribution is the molecular characteristic of polymers that controls both processibility and properties. The three new mass standards, together with three previously certified standards, cover the range from 5000 g/mol to 200 000 g/mol. These provide the polymer industry with an adequate set of molecular mass calibrants for polyethylene. Calibrations of SEC are carried out with a series of narrow fraction molecular mass standards covering the mass distribution range of the polymer of interest. Narrow fraction molecular mass polymers are only available for a selected few polymers. In the absence of suitable standards, crude approximations are used which introduce uncertainties in the measurement results. Although polyethylene is the most prevalent synthetic polymer, there are no commercial standards consisting of narrow mass fractions, other than those provided by NIST. The lack of commercial standards arises from difficulties obtaining narrow molecular mass fractions of polyethylene and in measuring absolute molecular masses by light scattering or osmometry osmometry the technique of measuring osmolar concentrations. The definitive method of measuring solute:solvent ratios. at temperatures as high as 150 [degrees]C. The new standards were prepared from fractionation fractionation /frac·tion·a·tion/ (frak?shun-a´shun) 1. in radiology, division of the total dose of radiation into small doses administered at intervals. 2. of a broad distribution polyethylene that provided the fractions previously certified by NIST. |
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