( e described by Mark Houwink equation. T...)Practical Aspects of Evaluation of Chromatographic Data in Size Exclusion Chromatography

e described by Mark Houwink equation. The parameters K and Alpha can be found in literature or determined by independent methods. This is used in the universal calibration method. We can calculate molecular mass of unknown polymer by using a calibration for another (more common) polymer. We must know the appropriate K and Alpha constants for both polymers at the measurement conditions. The problem with this method is, that the found values can vary widely and the determination is laborious and also prone to errors.

Broad standard calibration
Another approach uses so called broad standard calibration. The characterized broad MW distribution standards are available for wider range of polymers. Alternatively, typical samples/products can be characterized by classical methods or by GPC/SEC with absolute detectors and used as broad standards. With this method, a calibration curve is constructed to fit the measured peak shape to the declared Mw/Mn values or Mw distribution table. Because the declared or determined Mn values are usually prone to significant errros, the use of single broad standard calibration can lead to a great uncertainity in the results. Software supporting multiple broad standard calibration allows a use of several broad standards and thus eliminates this common problem.

Flow rate correction
The calculations of elution volume are based on flow rate the drop counters and other direct volume measuring devices are no more in common use. The calculated values are extremely dependent on the flow rate 1% change in flow rate can cause 10-20ˇ% change in measured Mw values. The flow rate correction method is commonly supported in the chromatography software. Marker (internal standard) is added to standards and samples. Correction is calculated on the basis of difference in marker RT, it usually scales the time axis. It thus can compensa
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