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Defining an SEC calibration

Most commercially available data handling systems which have SEC software written for them assume that the calibration will be carried out using a series of narrow MMD calibrants. The molecular masses are always used in the logarithmic form, and the calibration is usually established either as a linear relationship between the log molecular mass and the elution time, or by using a polynomial relationship. Different systems permit the use of various orders of polynomial, but in practice a linear fit or a third-order polynomial are the most commonly encountered. [Pg.47]

If a linear plot of elution time against the log molecular mass is a good description of the elution behaviour of the calibrants, then this is a very satisfactory manner of defining the calibration. The manufacturers of SEC columns now put a significant emphasis on producing columns, or combinations of columns, which can effectively be defined by such a linear plot. However, if the linear definition does not accurately reflect the observed elution behaviour of the calibrants, serious errors in the calculation can occur. [Pg.47]

A third-order polynomial, of the log molecular mass against the elution time, frequently appears to be an accurate definition of the calibration. Unfortunately, a third-order polynomial is not ideal for defining the calibration in practice it is very difficult to reproduce such a calibration, with quite small differences at the low-molecular-mass end causing large changes at the high-molecular mass end of the calibration. When these discrepancies are observed as the actual numbers, rather than their log value, the differences can appear to be very large indeed. [Pg.47]

Alternatively, the calibration can be described by a multi-straight-line plot, a series of linear calibrations between each point. This manner of describing the calibration can be quite accurate and reproducible providing that a large number of points are used. Unfortunately, if the calculation is carried out [Pg.47]


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