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Handbook of Size Exclusion Chromatography and Related ...

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Figure 5 Time-sliced peak output from a concentration detector (DRI).<br />

simplest method it is generally restricted in its utility owing to the lack <strong>of</strong><br />

availability <strong>of</strong> many different polymer st<strong>and</strong>ard types. It also requires a sufficient<br />

number <strong>of</strong> st<strong>and</strong>ards <strong>of</strong> different molecular weights so as to completely cover the<br />

entire dynamic range <strong>of</strong> the column set or, at least, the range <strong>of</strong> molecular weights<br />

spanned by the samples’ molecular weight distributions. Narrow st<strong>and</strong>ards<br />

currently available include polystyrene, poly(methyl methacrylate), poly<br />

(ethylene), (used for nonaqueous GPC), <strong>and</strong> poly(ethylene oxide) or poly(ethylene<br />

glycol), poly(acrylic acid), <strong>and</strong> polysaccharides (used in aqueous GPC) are<br />

common commercially available st<strong>and</strong>ards. It is instructive to study the<br />

mechanism <strong>of</strong> narrow st<strong>and</strong>ard calibration since all <strong>of</strong> the other methods are<br />

based upon it. A thorough review <strong>of</strong> this subject has been provided by Cazes (11).<br />

In this approach, the raw chromatogram obtained as output from the<br />

concentration detector is divided into a number <strong>of</strong> time slices <strong>of</strong> equal width as<br />

depicted in Fig. 5. For a polydisperse sample the number <strong>of</strong> time slices must be<br />

greater than 25 for the computed molecular weight averages to be unaffected by<br />

the number <strong>of</strong> time slices used. (Most commonly available SEC data programs<br />

utilize a minimum <strong>of</strong> several hundred time slices routinely for each analysis.) An<br />

average molecular weight is assigned to each time slice based upon the calibration<br />

curve <strong>and</strong> it is further assumed for computational purposes that each time slice is<br />

monodisperse in molecular weight. A table is constructed with one row assigned to<br />

each time slice. The following columns are created for this table: retention volume,<br />

area (Ai), cumulative area, cumulative area percent, molecular weight (Mi), Ai<br />

divided by Mi, <strong>and</strong> Ai times Mi. The area column <strong>and</strong> the last two factors are also<br />

summed for the entire table.<br />

Once this data table has been completed it is possible to compute the<br />

molecular weight averages or moments <strong>of</strong> the distribution. The most common<br />

© 2004 by Marcel Dekker, Inc.

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