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Handbook of Size Exclusion Chromatography and Related ...

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Figure 11 Data <strong>of</strong> Fig. 10 with spike removal s<strong>of</strong>tware activated.<br />

6 WHY MULTI-ANGLES?<br />

ItshouldbeevidentfromFig.3thatnoisydata presentedwithout ameasure<strong>of</strong> its<br />

statistically expected fluctuations can result in the reported measurements being<br />

both erroneous <strong>and</strong> misleading. In addition, if the data are processed properly,<br />

there should be no need to discard them because <strong>of</strong> the presence <strong>of</strong> such noise;<br />

only the reported precision <strong>of</strong> the results presented will be affected. Towithin the<br />

limitsproscribedbysuchprecisionlimits,thedatawillhaveanassociatedvalidity.<br />

It is those limits <strong>of</strong> precision by which the experimentalist will decide to keep,<br />

discard, or repeat the experimental determinations. Unfortunately,without those<br />

quantitative measures <strong>of</strong> experimental precision, as has been the case historically<br />

with many light-scattering instruments, there is virtually no objective basis for<br />

excluding data known to be flawed.<br />

The use <strong>of</strong> aplurality <strong>of</strong> measurements over abroad range <strong>of</strong> scattering<br />

angles has three benefits. First, <strong>of</strong> course, is the increased precision <strong>of</strong> the<br />

measurement. This is most easily understood if we consider as an example<br />

measurement <strong>of</strong> a sample <strong>of</strong> relatively small size. For such molecules,<br />

the scattering should be the same at all angles. Thus low molar mass samples<br />

are processed as if the measurements at each angle were independent <strong>of</strong> those<br />

madeatotherangles.S<strong>of</strong>orthecase<strong>of</strong>Nangles,wehavemadethedetermination<br />

pffiffiffiffi Ntimes with an expectation <strong>of</strong> a N -fold increase in precision. The same holds<br />

truewhenfittingthemeasuredexcessRayleighratiosasafunction<strong>of</strong>sin 2u=2(the Zimm plot). Each measurement included in the fitting procedure improves the<br />

precision <strong>of</strong> the final result. Here, however, each measurement is weighted<br />

statistically,so that points with high uncertainties have an associated low weight.<br />

© 2004 by Marcel Dekker, Inc.

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