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Handbook of Size Exclusion Chromatography and Related ...

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MALDI-MS has been applied to determine the absolute MW <strong>of</strong> many<br />

relatively low MW materials. However, it is limited to samples with narrow<br />

MW distribution <strong>and</strong> with certain polarity (2).<br />

Traditionally, the chromatographic analysis <strong>of</strong> low MW material has been<br />

done by enthalpic interaction chromatography, where the retention mechanism is<br />

based more upon chemical structure than MW. These methods include gas<br />

chromatography, supercritical fluid chromatography, <strong>and</strong> normal-phase <strong>and</strong><br />

reversed phase liquid chromatography (commonly referred to as HPLC, highperformance<br />

liquid chromatography). However, for MW determination for even<br />

low MW material, size exclusion chromatography is by far the most frequently<br />

used method just as for high MW material, because <strong>of</strong> its high speed, automation<br />

capability, <strong>and</strong> rich information (MW distribution <strong>and</strong> its averages <strong>of</strong> various<br />

modes).<br />

The small-pore-size gels for size exclusion chromatography (SEC) are<br />

more difficult to prepare <strong>and</strong> use than large-pore gels for at least two reasons.<br />

(1) Small-pore gels are more fragile than larger-pore gels. In order to achieve a<br />

good resolution, the total pore volume <strong>of</strong> SEC gels needs to be as large as<br />

possible. The volume <strong>of</strong> interstitial voids <strong>of</strong> an SEC column, regardless <strong>of</strong><br />

particle size, is approximately 40% <strong>of</strong> the total column volume. Therefore, the<br />

smaller the pore size the thinner the solid wall, <strong>and</strong> the more fragile. (2) The<br />

applications for low MW separation normally dem<strong>and</strong> high resolution, which<br />

may he achieved by reducing particle size or using several columns in series.<br />

Either method results in a high backpressure, which is detrimental to the fragile<br />

nature <strong>of</strong> a small-pore gel. SEC column technology has recently been improved<br />

significantly. The commercially available small-pore columns have become<br />

more <strong>and</strong> more popular.<br />

This chapter will also discuss other difficult issues for SEC <strong>of</strong> small<br />

molecules, such as MW dependence <strong>of</strong> detection sensitivities, calibration<br />

methods, <strong>and</strong> solvent mismatch interference.<br />

2 RESOLUTION<br />

2.1 Columns<br />

The SEC study <strong>of</strong> small molecules <strong>of</strong>ten dem<strong>and</strong>s a high resolution, especially<br />

when the resolution <strong>of</strong> an individual molecule is needed. The peak resolution,<br />

RS, <strong>of</strong> any chromatographic separation, including SEC, can be calculated<br />

using (3):<br />

© 2004 by Marcel Dekker, Inc.<br />

1<br />

RS ¼= 4(a 1)N 1=2 k0 1 þ k 0<br />

(1)

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