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Handbook of Size Exclusion Chromatography and Related ...

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allow removal <strong>of</strong> mobile phases <strong>and</strong> creation <strong>of</strong> acontinuous film <strong>of</strong> polymer<br />

eluted from the column (LC transform instruments). The inhomogeneities <strong>of</strong> the<br />

polymerfilmthatisdepositedontoagermaniumdiscarepartiallycompensatedfor<br />

by measurements at two appropriatewavelengths. Further progress in IR polymer<br />

detectiondependsonimprovementinbothsamplefilmdepositiontechnology<strong>and</strong><br />

sensitivity <strong>of</strong> IR measurement itself.<br />

Inspite<strong>of</strong>their relativelylowsensitivity,differentialrefractometersarevery<br />

popular in SEC. Their response is affected by the chemical composition <strong>of</strong><br />

the polymer sample. Evidently, refractive index (RI) detectors can hardly be<br />

appliedingradientprocedures.Inthecase<strong>of</strong>mixedmobilephasestheRIdetector<br />

response is affected also by preferential solvation <strong>of</strong> the sample. Pasch <strong>and</strong><br />

Trathnigg(4)proposedcorrectionsfor thislatereffectbyapplyinghyphenation<strong>of</strong><br />

refractometric <strong>and</strong> densitometric detectors. Unfortunately,densitometric detectors<br />

are even less sensitive than RI detectors <strong>and</strong>, therefore, they afford sample<br />

reconcentration. This is feasible practically only with oligomers. Refractometers<br />

alsodetectsystempeaksthatarecausedbypreferentialevaporation,displacement,<br />

<strong>and</strong> preferential solvation effects typical for mixed mobile phases (Sec. 4.2).<br />

Dependences between detector response <strong>and</strong> sample concentration are usually<br />

rather nonlinear for evaporative light-scattering detectors (ELSD) <strong>and</strong> their slopes<br />

depend on the polymer chemical structure <strong>and</strong> to some extent also on sample molar<br />

mass. The response <strong>of</strong> present ELSD instruments depends also on the eluent nature/<br />

composition (72). This latter feature represents an important limitation <strong>of</strong> ELSD for<br />

all types <strong>of</strong> barrier procedures <strong>and</strong>, especially, for eluent gradient methods.<br />

Absolute detectors such as (solution) light-scattering photometers <strong>and</strong><br />

viscometers continuously monitor molar mass <strong>of</strong> macromolecules in the column<br />

effluent. They are discussed in several chapters <strong>of</strong> this book. Molar mass detectors<br />

render important services in SEC <strong>of</strong> homopolymers <strong>and</strong> also polymers with<br />

complex architectures, such as branched species. Unfortunately both types <strong>of</strong><br />

detectors suffer from serious drawbacks for most complex polymer systems with<br />

changing chemical structure. They are practically incompatible with procedures<br />

that utilize mobile phases with varying composition. On the other h<strong>and</strong>, if SEC is<br />

used as the second dimension separation system, both above detectors can produce<br />

valuable information on macromolecules in the effluent.<br />

A very important group <strong>of</strong> detectors for HPLC <strong>of</strong> complex polymers <strong>and</strong> a<br />

good hope for future developments includes nuclear magnetic resonance <strong>and</strong> mass<br />

spectrometry devices. These are discussed in detail in other chapters <strong>of</strong> this book.<br />

In spite <strong>of</strong> both their high acquisition price <strong>and</strong> operational cost, these instruments<br />

will certainly find application in many 2D-HPLC procedures.<br />

Without solving important detection problems, many 2D-HPLC separations<br />

<strong>of</strong> complex polymers produce only semiquantitative data on their molecular<br />

characteristics. This holds especially for high polymer systems because oligomer<br />

detection is <strong>of</strong>ten less problematic. However, even semiquantitative data on binary<br />

© 2004 by Marcel Dekker, Inc.

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