28.02.2013 Views

Handbook of Size Exclusion Chromatography and Related ...

Handbook of Size Exclusion Chromatography and Related ...

Handbook of Size Exclusion Chromatography and Related ...

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

appropriate for the early fractions is too large to narrow the distribution in late<br />

fractions.<br />

Itisimportantnottohaveastrongadsorption.Whenpolymerisinjectedinto<br />

acolumn filled with strongly adsorbing media, polymer <strong>of</strong> any MW will be<br />

adsorbed.Selectivepartitioning<strong>of</strong>low-MWcomponentsintothestationaryphase<br />

willnotoccur.Strongadsorptioncanbeprohibitedbyusingsmallpores,sincehigh-<br />

MW components will find it difficult to enter the pore to be adsorbed.<br />

Using athicker column or cascading the columns increases the processing<br />

capacity.The results obtained (21) are promising. By carefully choosing the right<br />

combination <strong>of</strong> columns in the right order, an even higher resolution with an<br />

increased capacity was demonstrated (21).<br />

Good separation with aweakly adsorbing medium in the theta condition<br />

raises ahope that aweakly adsorbing medium may also <strong>of</strong>fer abetter separation<br />

than nonadsorbing medium in a good solvent. It now appears that good<br />

resolutionenjoyedintheseparation<strong>of</strong>PVPinwater(5)(goodsolvent)isascribed<br />

to theweak adsorption. The smaller trailing edge compared with the leading edge<br />

in early fractions in Fig. 4b indicates adsorption. Enriching low-MW components<br />

in late fractions, which is uncommon in HOPC in good solvent, was helped by the<br />

adsorption <strong>and</strong> concomitant narrowing <strong>of</strong> the pores. Another evidence <strong>of</strong> the<br />

adsorption is gradual degradation in separation performance as separation is<br />

repeated on the same column (5). In fact, the first-time use <strong>of</strong> the column resulted<br />

in a higher resolution from early to late fractions than those shown in Fig. 4b. In<br />

that study, the total mass <strong>of</strong> the polymer recovered was not measured, however. We<br />

expect it will be difficult to find a solvent that removes all <strong>of</strong> the adsorbed polymer<br />

when the adsorption occurs in a solvent that solvates the polymer well.<br />

To utilize adsorption, a theta solvent in HOPC has an advantage that a good<br />

solvent does not have: when washed in a good solvent after separation in the<br />

theta solvent, the adsorbed polymer will be released, <strong>and</strong> the column will return to<br />

the state before the run <strong>and</strong> be ready for the next batch <strong>of</strong> separation. Another<br />

batch <strong>of</strong> separation conducted under the same condition produced identical<br />

results (9). Adsorption occurs as a result <strong>of</strong> a precarious balance <strong>of</strong> polymer–<br />

polymer interactions <strong>and</strong> polymer–pore surface interactions. A slight change in<br />

the surface such as octyl modification is sufficient to suppress the adsorption. The<br />

surface modification is not limited to TMS, C8, <strong>and</strong> C18. Another surface may<br />

give an even better separation.<br />

10 SUMMARY<br />

The advantage <strong>of</strong> HOPC in a theta solvent was demonstrated, especially with a<br />

column that weakly adsorbs the polymer. The same method was applied to a<br />

higher-MW sample <strong>of</strong> PCL. Again, the resolution was better when separated in<br />

© 2004 by Marcel Dekker, Inc.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!