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Q: Calibration for SEC analysis


A: SEC separations are often calibrated using pure, narrow molecular weight standards of the sample components that are being analyzed.
Prepare a calibration plots of various molecular weights of the sample component as a function of elution volume/time. 
Make sure that the SEC pump delivery is constant . The apparent molecular weight of the sample can vary significantly with a small flow fluctuation, since the relationship between Molar mass and elution volume is logarithmic
Because the actual component is not always available, Various standards can be used instead. Narrow polystyrene standards are commonly used for calibrations that run in organic solvents.  Because SEC separations are based on Molecular size (weight) the polystyrene calibration plots allow to estimate the molecular weight of the sample, by interpolation.

There are various methods to calibrate the detectors used and find the plot's best polynomial fit.  These methods are well covered by the software used to acquire the data.  For example, WinGPC UniChrom provides a wide selection of calibration methods computed with proprietary calibration fit routines that yield high accuracy and precision: 

• Calibration with narrow standards
• Universal calibration with Mark-Houwink constants
• Broad standards calibration and cumulative match calibration are provided with step-by-step guides that assure integrity of process
• More calibration variety with software modules
  

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