Polydispersity
In principle the distinction of broad and narrow standards is based on the PolyDispersityIndex, PDI =Mw/Mn.
Narrow standards
Narrow polymer standards are made by living ionic polymerisation techniques or by fractionation of very broad standards. All narrow standards have small polydispersities with sharp and slim chromatogram profile. The Mp value is well defined and independent of the column resolution. Tip: Mp (molar mass at the peak maximum) is the only well defined point in the chromatogram and so far dedicated in order to construct a calibration curve. Narrow standards are defined by average molar mass values such as Mn (Number average molar mass) and Mw (Weight average molar mass). Mn and Mw are well established values to define the molar mass distribution of a polymer. The construction of the calibration curve in GPC is based on the Mp values of the polymers in a calibration kit, e.g., an ensemble of polymers of the same type but different molar masses.
Broad standards
Broad reference materials are made by radical polymerization, polycondensation or coordinative polymerization (polyolefins). The PDI of broad standards is usually >1.5. For broad standards the Mp value is a function of the column resolution and therefore not defined. Broad standards are characterized by Mw and Mn. Modern GPC/SEC software packages allow the construction of a calibration curve by the use of Mw, Mn or the intrinsic viscosity [η] of broad standards. Up to 8 different broad standards can be used and only one of the Mn, Mw, or [η] values is nescessary.
Broad standards are used:
• to validate a chromatographic system
• to determine physical constants such as the Mark-Houwink constants K and a.
• to construct a calibration curve
• to proof a column missmatch.