( al artefact. Thedetermined Mn values...)Practical Aspects of Evaluation of Chromatographic Data in Size Exclusion Chromatography

Calibration
Without absolute MW detector the GPC/SEC system must be calibrated with appropriate polymer standards. The elution time is related to molecular size, not to the desired molecular weight. The molecular size is dependent also on polymer structure, solvent and temperature. For calibration, standards of the same polymer should have to be used in ideal case, however this is not always possible. Chromatographic software usually supports several calibration options. The most commonly used is narrow standard calibration.

Well defined polymer standards with narrow MW distribution are commercially available for wide range of polymers. The declared Mp values can be directly used for construction of calibration curve. However, they are not available for all polymers, and with the exeption of the most common ones are usually quite expensive. Most commonly the readily available polystyrenes are used for calibration. Resulting molar mass should be then designated polystyrene equivalent molar mass.

The selection of appropriate curve fit for the calibration data is of prime importance. The prerequisite for precise data is use of high resolution columns and enough points to characterise the column set properties. The unknown polymer should be within the calibration curve range, the extrapolation should be avoided. For so called linear columns the linear regression is appropriate in their working range, the cubic curve may fit better at the extremes. For higher polynomial fits, (4 or more) there must be enough points with high reproducibility. The software should check for curve monotonicity in the valid range and should not allow the use of curve showing local maxima or minima.

Universal calibration
Dependence between molecular size and molecular weight can b
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