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.

wheren<strong>and</strong>n0 aretheRIs<strong>of</strong>thesample <strong>and</strong>solvent,respectively,kisaconstant,<br />

dn=dc is the specific refractive index (the increment <strong>of</strong> refractive index to the<br />

concentration <strong>of</strong> asolute), <strong>and</strong> cis the concentration <strong>of</strong> the solution. The dn=dc<br />

approximatelyequalsthedifference<strong>of</strong>refractiveindexes<strong>of</strong>solute<strong>and</strong>solvent(7):<br />

dn<br />

dc<br />

nsolute nsolvent (4)<br />

Therefractiveindex<strong>of</strong>anoligomericmaterialhasalinearrelationtothereciprocal<br />

<strong>of</strong> its MW,as demonstrated with hydrocarbons in Fig. 4. Therefore, dn=dc is<br />

proportional to the reciprocal <strong>of</strong> solute MW (8):<br />

dn dn<br />

¼ þ<br />

dc dc 1<br />

k0<br />

Mn<br />

where k 0 is aconstant. As shown in Fig. 4, k 0 is <strong>of</strong>ten anegative number.<br />

However, it can be positive if the chain ends with a high refractive index<br />

functional group, such as phenyl, chloride, <strong>and</strong> bromide. For apolymer with<br />

high Mn, the k 0 =Mn term is insignificant, <strong>and</strong> dn=dc reaches aconstant value,<br />

(dn=dc) 1.For the low MW material, dn=dc varies according to k 0 =Mn. When<br />

dn=dc is relatively large, the variation due to MW may be not obvious.<br />

However, when dn=dc is small, the variation will become significant. Figure 5<br />

shows that the signal <strong>of</strong> hydrocarbons in THF gradually diminishes <strong>and</strong><br />

changes to negative as the MW is reduced. Solvent selection may exaggerate<br />

or minimize the dn=dc effects on MW.Therefore, it is important to choose a<br />

mobile phase that has arefractive index as far from the samples as possible<br />

for SEC <strong>of</strong> low MW samples.<br />

Figure6AshowsthattheSECcurve<strong>of</strong>asiliconecopolymersampleinTHF<br />

starts with anegative signal (around 4.8min), becomes positive around 5.6min,<br />

<strong>and</strong> becomes negative again around 6.6min. The variation <strong>of</strong> polymer refractive<br />

index, <strong>and</strong> thus dn=dc <strong>and</strong> detector sensitivity,may be due to acombination <strong>of</strong><br />

changes in MW <strong>and</strong> chemical structure. The peaks after 8.8min are solvent<br />

mismatches. The solvent mismatches show the concentration differences <strong>of</strong> small<br />

molecules, such as H2O, N2, O2, <strong>and</strong> other additives in the mobile phase,<br />

introduced during the sample preparation procedure. Owing to the low sensitivity<br />

<strong>of</strong> this sample, the solvent mismatches appear to be exaggerated. When it is<br />

analyzed in toluene, the entire chromatogram is negative <strong>and</strong> the mismatches<br />

becamenegligible,asshowninFig.6B,inwhichcasetheMWdistributioncanbe<br />

calculated easily.<br />

ItisconvenienttocreateachartlikeFig.7,whichliststherefractiveindex<strong>of</strong><br />

commonly used solvents on one side <strong>and</strong> commonly analyzed polymers on the<br />

other. Using this chart, the selection <strong>of</strong> a good polymer/solvent pair for SEC<br />

analyses becomes much easier.<br />

© 2004 by Marcel Dekker, Inc.<br />

(5)

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

Saved successfully!

Ooh no, something went wrong!