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Handbook of Size Exclusion Chromatography and Related ...

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oad range <strong>of</strong> scattering angles. Such measurements have permitted the deduction<br />

<strong>of</strong> molar mass, molecular mean square radius, <strong>and</strong> the second virial coefficient.<br />

Because measurement <strong>of</strong> scattered light at many angles seemed a difficult <strong>and</strong><br />

time-consuming task, instrumentation was introduced to make measurements at<br />

fewer angles than theoretically desirable. With them came the need to retain<br />

the absolute concept <strong>of</strong> the terminology “light scattering” while at the same time<br />

differentiating their measurements from those capable <strong>of</strong> making the<br />

determinations over a full “range <strong>of</strong> angles.” The term “multi-angle light<br />

scattering,” or simply MALS, is <strong>of</strong>ten used to describe this full light scattering<br />

concept. To comply with this more recent designation, the term MALS will be<br />

used throughout this chapter to refer to general light scattering measurements.<br />

The term “absolute” is frequently seen in reference not only to results derived<br />

from MALS measurements, but also, inappropriately, to methods requiring<br />

calibration against st<strong>and</strong>ards <strong>of</strong> known molar mass. Just what is meant by the term<br />

“absolute”? A measurement <strong>of</strong> molar mass is said to be absolute if, <strong>and</strong> only if:<br />

. The measurement requires no reference to any mass st<strong>and</strong>ards.<br />

. All parameters <strong>of</strong> the measurement are determined directly. These<br />

include<br />

– refractive indices <strong>of</strong> all cells <strong>and</strong> fluids;<br />

– geometries <strong>of</strong> the sample cells <strong>and</strong> detectors (distances, shape,<br />

composition, <strong>and</strong> solid angles subtended at the sample by the<br />

scattered light detectors);<br />

– wavelength <strong>of</strong> the light source;<br />

– concentrations <strong>of</strong> the solutes;<br />

– response <strong>of</strong> the detectors (for example, for a DRI detector, the<br />

relation between the output voltage change <strong>and</strong> the corresponding<br />

change <strong>of</strong> fluid refractive index);<br />

– temperature <strong>and</strong> its effects on the physical parameters <strong>of</strong> the<br />

experiment.<br />

. There is no a priori assumption <strong>of</strong> molecular conformation <strong>and</strong>/or<br />

structure.<br />

Some types <strong>of</strong> instruments use light scattering for their determinations, but<br />

require calibration, as the solvent refractive index is changed, with mass st<strong>and</strong>ards<br />

for each such solvent. They are not absolute as they become totally dependent on<br />

the stability <strong>and</strong> reproducibility <strong>of</strong> the st<strong>and</strong>ards employed.<br />

This chapter focuses on many <strong>of</strong> the elements <strong>of</strong> the MALS measurement<br />

technique that can affect the final results. It lists some <strong>of</strong> the causes <strong>of</strong> erroneous<br />

results <strong>and</strong> (hopefully) provides helpful guidance to various features <strong>of</strong> the<br />

instrumentation that are <strong>of</strong>ten overlooked. A major objective <strong>of</strong> the chapter,<br />

© 2004 by Marcel Dekker, Inc.

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