Basic principles and terms

• Gel Permeation Chromatography (GPC) or Size Exclusion Chromatography (SEC) is a specialized chromatography technique for determination of molecular weight distribution in polymers. Sample of polymer solution, containing mixture of molecules differing in size, is separated on column. Under conditions used for this chromatography mode, the molecules are separated according to size, the larger molecules emerging first and the smaller later. With appropriate polymer standards with known molecular weights, calibration curve describing the dependence between the molecular weight and elution volume can be constructed.
• Narrow standard calibration is the most commonly used when polymer standards with narrow molecular weight distribution are available. Calibration curve is constructed from detected peak maximum retention times and known Mp values. Alternatively, (Mw.Mn)^1/2 values can be used, if Mp values are not declared.
• Broad standard calibration is used when only polymer standards with broad molecular weight distribution are available. Clarity GPC Extension uses three types of Broad standard calibration:

• Broad linear - assuming linear calibration curve for broad peak concerned. Linear calibration equation is calculated from the standard peak elution profile and declared Mn and Mw values. Multiple Broad Standards can be used to construct a resulting calibration curve.
• Broad integral - a calibration curve is constructed from the standard peak elution profile and a table of declared cumulative molecular weight distribution values. Multiple Broad Standards can be used to construct a calibration curve.
• Broad on narrow - appropriate K and Alpha values are sought for polymer standard with declared Mn and Mw values, to fit the standard peak elution profile to existing narrow standard calibration curve.

• Flow rate correction - the elution volumes are calculated from retention time and flow rate. Small variations in flow rate have tremendous effect on the correctness of calculated molecular weights. A low molecular weight compound (flow rate marker) can be added to the standards and samples, the retention times in different chromatograms can be than normalized to common base. Flow rate correction can be used with every calibration type using Peak - Flow Marker operation in the Integration Table.
• Universal calibration - the separation of polymer molecules on column is governed by molecular size, not molecular weight. The polymer molecule size is besides its weight dependent on the molecule structure (linear, branched, starlike) and conformation (given by solvent and temperature). A dependence (Mark-Houwink equation) can be used to calculate molecular weights for molecules of the same size, provided the constants K and Alpha of the Mark-Houwink equation are known for both polymers. Universal calibration can be used with every calibration type, except Broad on narrow.