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

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

[Eq. (6)] on copolymer composition is needed. Various procedures have been<br />

proposed which interpolate these constants from those for homopolymers (89).<br />

Montaudoetal.(1)havedemonstratedtheirlimitedvalidityusingaMALDImass<br />

spectrometry. Viscometric detectors <strong>and</strong> hyphenations <strong>of</strong> polymer HPLC with<br />

mass spectrometry help in mitigating these problems while application <strong>of</strong> lightscattering<br />

detectors for copolymers is somewhat limited.<br />

In afew practical cases, the usual order <strong>of</strong> separation systems in 2D-HPLC<br />

that was described in Sec. 2 can be changed. Atypical example represents<br />

stereoregular polymers. For example, limiting viscosity numbers <strong>of</strong> poly(methyl<br />

methacrylate) in good eluents does not depend on their tacticity (M Bohdanecky,<br />

personal communication). As a result, universal calibration dependences for<br />

polymers <strong>of</strong> the same nature but differing in their stereoregularities should<br />

coincide <strong>and</strong> SEC can be used as the first dimension separation system to<br />

discriminatemacromolecularanalytes almost exclusivelyaccordingtotheir molar<br />

mass. SEC fractions can be forwarded into the second dimension column, for<br />

example into an LC CC system for separation <strong>of</strong> polymer species according to<br />

stereoregularity (61,62). However, this approach cannot be used for 1,2- <strong>and</strong> 3,4polyisoprenes<br />

because their calibration dependences are mutually shifted (89).<br />

<strong>Size</strong> exclusion chromatographic methodology is treated in detail in several<br />

chapters <strong>of</strong> this book <strong>and</strong> does not need to be elucidated here. It is, however,<br />

necessary to again mention the danger <strong>of</strong> enthalpic interactivity <strong>of</strong> many SEC<br />

columns. The latter may be augmented by some mobile phases used in the first<br />

dimension column. Therefore, eluent <strong>and</strong> consequently also sample matrix must<br />

sometimes be changed between the first <strong>and</strong> second dimension columns (Sec. 9).<br />

The problems connected with sample storage <strong>and</strong> reconcentration between both<br />

column systems will also be discussed in Sec. 9.<br />

If the first dimension separation system only partially suppresses <strong>and</strong> does<br />

notfullyeliminatetheeffect<strong>of</strong>onecharacteristic,thecalculation<strong>of</strong>corresponding<br />

distributions is complicated (Sec. 1). However, the contour representation <strong>of</strong><br />

results allows estimation at least <strong>of</strong> the distribution limits (4,5).<br />

Aspecific problem <strong>of</strong> 2D-HPLC <strong>of</strong> complex polymer systems consists in<br />

determination<strong>of</strong>theentirepolymerconcentration<strong>and</strong>/orrelativeconcentration<strong>of</strong><br />

complex polymer constituents in the column effluent (Sec. 10).<br />

7 HPLC-LIKE PROCEDURES<br />

Separations <strong>of</strong> numerous complex polymer systems can be achieved using HPLClike<br />

procedures, which apply the same instrumentation <strong>and</strong> retention mechanisms<br />

as the true HPLC methods. The multitude <strong>of</strong> retention–elution steps are<br />

responsible for chromatographic separations. However, if retention–elution<br />

processes are selective enough, one single (full) retention <strong>and</strong> subsequent (full)<br />

© 2004 by Marcel Dekker, Inc.

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

Saved successfully!

Ooh no, something went wrong!