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Handbook of Size Exclusion Chromatography and Related ...

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Sincethereduction<strong>of</strong>theinternaldiameterdrasticallyreducestheporevolume<strong>of</strong><br />

the column (squared relationship), asubstantial influence on performance has to<br />

be expected. This is clearly visible in the raw chromatogram. Resolution is much<br />

poorer as compared to the reference chromatogram <strong>of</strong> the conventional column in<br />

Fig. 3. Moreover, the peak shapes are badly affected as can be seen in the relative<br />

change <strong>of</strong> the peak heights <strong>of</strong> the st<strong>and</strong>ards. The higher the molar mass <strong>of</strong> a<br />

st<strong>and</strong>ard the broader the peak gets (<strong>and</strong> the lower the corresponding peak height<br />

will be). This is caused by the high linear flow rate inside the column, which is<br />

necessary to drive the eluent through it; please note that the pump flow rate was<br />

kept constant at 1.0 mL/min. Samples with high molar mass will be affected by<br />

shear degradation under such conditions. The limited efficiency in this setup is<br />

alsoseeninthepoorseparation <strong>of</strong>samplepeaks fromthesolventpeaks attheend<br />

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

3.1.5 Changing Column Aspect Ratio<br />

The simultaneous adaption <strong>of</strong> column length <strong>and</strong> diameter allows the internal<br />

volume <strong>of</strong> the separation system to be kept constant. This is important for SEC,<br />

because the column volume <strong>and</strong> the pore volume <strong>of</strong> the packed bed are directly<br />

related. As pointed out above, the pore volume is one <strong>of</strong> the major factors<br />

influencing peak resolution. Cutting down the column length <strong>and</strong> increasing the<br />

internal dimension <strong>of</strong> the column at the same time can, in theory, reduce the<br />

chromatographic run time while maintaining the efficiency <strong>of</strong> the separation.<br />

Obviouslysuchcolumnshavetobeoperatedathigherflowratestoreducetherun<br />

times. Solvent cannot be saved significantly without interfering with resolution.<br />

Moreover, other effects can influence the separation. In such ascenario,<br />

the accessibility <strong>of</strong> the pores in the packing will be most important. If the<br />

architecture <strong>of</strong> the pores in the column will restrict diffusional migration <strong>of</strong> the<br />

solutes, the column will not <strong>and</strong> cannot perform as expected. Wall effects might<br />

also influence the separation <strong>and</strong> have to be watched closely as column length is<br />

reduced. PSS studied such short wide-bore columns extensively for their use in<br />

fast separations.<br />

Figure8showstheseparationonatestcolumn<strong>of</strong>50 mmlength<strong>and</strong>20 mm<br />

ID run at a flow rate <strong>of</strong> 6 mL/min. The same packing material as in all other<br />

columns was used in this experiment. All other experimental parameters were kept<br />

constant. The first impression <strong>of</strong> the separation is favorable:<br />

. All peaks are present in their correct height (concentration) ratios,<br />

. A good peak symmetry is kept throughout the whole chromatogram,<br />

. No indication <strong>of</strong> sample degradation at high molar masses,<br />

. Separation is carried out in less than 2 min, <strong>and</strong><br />

. Sufficient separation <strong>of</strong> solvent peaks from sample.<br />

© 2004 by Marcel Dekker, Inc.

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