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Handbook of Size Exclusion Chromatography and Related ...

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chromatogram at 4mL/min, which is finished after about 3.5 min instead <strong>of</strong><br />

13 min. The price to pay for this time gain is clearly visible in the chromatogram:<br />

. Peak separation is dramatically reduced compared to Fig. 3,<br />

. High molar mass peaks show tailing (degradation).<br />

The number <strong>of</strong> theoretical plates is reduced by more than afactor <strong>of</strong> 2<br />

(43,000 instead <strong>of</strong> 92,500) <strong>and</strong> the asymmetry factor <strong>of</strong> 0.7 shows peak skewing.<br />

Thespecificresolution<strong>and</strong>otherperformancecriteriaareaffectedinthesameway.<br />

Running columns far beyond their flow rate rating will also reduce the effective<br />

lifetime <strong>of</strong> the column <strong>and</strong> lead to higher costs.<br />

3.1.3 Reducing Column Length<br />

Column length <strong>and</strong> chromatographic run times are directly proportionally related,<br />

whilecolumnefficiencychangeswith thesquare root <strong>of</strong>column length only.This<br />

means acolumncutinhalf will generateresults twice asfast,whiletheresolution<br />

will be reduced by afactor <strong>of</strong> 1.4 only.The bad news is that cutting run times by<br />

larger factors is very limited. In order to reduce the run time by afactor <strong>of</strong> 10 the<br />

column length has to be reduced from 30 cm to 3cm. Obviously, this is not<br />

possiblewithoutsacrificingtoomuch performance.Additionally,theporevolume<br />

will also be reduced proportionally with column length. Therefore it is very<br />

important to check experimentally how much the resolution <strong>of</strong> such columns will<br />

be affected by their reduction in length.<br />

An identical experiment using a column <strong>of</strong> only 5cm length (while<br />

maintaining the internaldiameter <strong>of</strong> 8mm) was performed in order to relate these<br />

results to conventional separation. Figure 6shows the chromatogram <strong>of</strong> the short<br />

column (length cut by a factor <strong>of</strong> 6). As expected the run time in this experiment is<br />

significantly reduced (to about 2.5 min). However, the resolution <strong>of</strong> peaks is much<br />

lower again <strong>and</strong> peaks show significant tailing. The fine structure <strong>of</strong> the solvent<br />

peaks is no longer visible <strong>and</strong> all components are merged into a single peak at the<br />

end <strong>of</strong> the chromatogram.<br />

Short columns like this one cannot be used, if tests have to carried out<br />

according to ISO 13885 or DIN 55672 SEC st<strong>and</strong>ards. They require that peak<br />

positions (as a direct measure <strong>of</strong> resolution) have to be at least 6 cm apart in the<br />

column (9).<br />

3.1.4 Reducing Column Diameter<br />

So far, limitations from chromatographic theory have been found to be effective in<br />

real-life scenarios for polymers too. Reducing internal column diameters has long<br />

been used in GC <strong>and</strong> HPLC to speed up separations <strong>and</strong> overcome sensitivity<br />

issues. PSS checked out this approach for macromolecules in their investigation.<br />

© 2004 by Marcel Dekker, Inc.

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