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Handbook of Size Exclusion Chromatography and Related ...

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st<strong>and</strong>ard deviations <strong>of</strong> the measured quantities, systematic errors are more easily<br />

identified. Such might include selecting the wrong wavelength for the light source<br />

used, entering the wrong value <strong>of</strong> the refractive index increment, erroneous<br />

calibration<strong>of</strong>therefractiveindexorotherconcentrationsensitivedetectors,errorsin<br />

calibrating the MALS photometer, <strong>and</strong> so on.<br />

The importance <strong>of</strong> the Zimm plot procedure to yield the second virial<br />

coefficient, weight-average molar mass, <strong>and</strong> average mean-square radius for<br />

unfractionated samples requires that MALS s<strong>of</strong>tware be capable <strong>of</strong> performing<br />

such determination, as well. This is easily implemented by preparing aset <strong>of</strong><br />

aliquots spanning abroad range <strong>of</strong> unfractionated sample concentrations. Using a<br />

syringe pump or large injection loop plus chromatographic pump, individual<br />

aliquots are injected sequentially during a “collection” event. The resulting<br />

chromatograph at each scattering angle appears as aseries <strong>of</strong> plateaus such as<br />

shown in Fig. 4for the scattering at 908 from aliquots <strong>of</strong> starch sample in 90%<br />

DMSO. Apeak region is selected from each plateau as indicated by the vertical<br />

lines. The s<strong>of</strong>tware then combines the corresponding data from all scattering<br />

anglesaveragedovereachpeakregionwiththeuser-enteredconcentration<strong>of</strong>each<br />

preparedaliquot toyieldaZimmplotsuchasthatshowninFig.5.Fromthisplot,<br />

the s<strong>of</strong>tware calculates the molar mass (7:46 + 0:09) 10 6 , the r.m.s. radius<br />

85:7 + 1:7 nm, <strong>and</strong> the second virial coefficient (1:46 + 0:19) 10 5 molmL/<br />

g 2 . All s<strong>of</strong>tware should be able to calculate <strong>and</strong> plot the important distributions <strong>of</strong><br />

mass <strong>and</strong> r.m.s. radius (for MALS, the latter has a lower limit <strong>of</strong> about 8–10nm).<br />

Figure 4 Series <strong>of</strong> plateaus at 908, each corresponding to a different starch concentration<br />

injected into a flow cell using a syringe pump.<br />

© 2004 by Marcel Dekker, Inc.

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