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Handbook of Size Exclusion Chromatography and Related ...

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mass <strong>and</strong> biological activity, for metal binding proteins. Table 1shows that<br />

the concentrations <strong>of</strong> sodium, calcium, iron, <strong>and</strong> aluminum vary greatly in<br />

commercial silicas. Note that the metal ion levels when measured by<br />

spectroscopic techniques represent bulk properties, not the levels present at the<br />

accessible silica surface. Deactivation procedures, such as the treatment <strong>of</strong> silica<br />

with strong acids or bases (28) or chelating agents (29), effectively remove metal<br />

ion impurities from the silica surface. The effect <strong>of</strong> surface treatments on the<br />

concentration <strong>of</strong> metal ion impurities is shown for Supelcosil LC-18-DB in<br />

comparison with that <strong>of</strong> untreated Supelcosil LC-Si. Metal ions present in<br />

Zorbax PSM-60 were removed by EDTAtreatment (29). The reproducibility for<br />

the measurement <strong>of</strong> metal ions in silica by ICP-AES is excellent as demonstrated<br />

by the data from duplicate blind measurements for Lot 180 <strong>of</strong> 5mm 120 A ˚<br />

Hypersil silica. The reproducibility <strong>of</strong> the manufacturing process is given for two<br />

lots <strong>of</strong> Hypersil (Lot 180 <strong>and</strong> Lot 195). Of course, the level <strong>of</strong> metal ions in a<br />

silica depends on that <strong>of</strong> the raw materials. For example, Table 1also contains<br />

data for Nyacol 2040, acommercial silica sol <strong>of</strong> 20 nm nominal particle size,<br />

used in the manufacturing <strong>of</strong> HPLC-grade silicas.<br />

2.2 Chromatographic Characteristics<br />

The attributes <strong>of</strong> an SEC column packing material are listed in Table 2. As<br />

indicated, the support must be optimized with respect to specific resolution,<br />

efficiency, column pressure, <strong>and</strong> mechanical, chemical, <strong>and</strong> thermal stability.<br />

Recovery <strong>of</strong> mass <strong>and</strong> activity is particularly important in the analysis <strong>and</strong><br />

purification <strong>of</strong> biopolymers. It also plays a role in the analysis <strong>of</strong> nonbiochemical<br />

synthetic polymers on silica-based SEC columns. In addition to recovery losses by<br />

adsorption, the recovery for both groups <strong>of</strong> polymers can also be reduced by<br />

polymer degradation as a result <strong>of</strong>, for instance, mechanical shear.<br />

As explained elsewhere in this book, resolution in SEC can be expressed in<br />

terms <strong>of</strong> the peak st<strong>and</strong>ard deviation <strong>and</strong> the slope <strong>of</strong> the calibration curve. As in<br />

other HPLC modes, the efficiency <strong>of</strong> SEC columns can be improved by decreasing<br />

particle size. The relationship between column efficiency (or plate number N) <strong>and</strong><br />

velocity can be expressed in dimensionless (reduced) parameters. The reduced<br />

plate height h is equal to the ratio <strong>of</strong> the height <strong>of</strong> a theoretical plate <strong>and</strong> the particle<br />

size as shown in Eq. (1). The reduced velocity v is equal to the product <strong>of</strong> the linear<br />

velocity kvl <strong>and</strong> particle size dp divided by the solute diffusion coefficient Dm, as<br />

shown in Eq. (2).<br />

h ¼ H<br />

(1)<br />

© 2004 by Marcel Dekker, Inc.<br />

v ¼<br />

dp<br />

kvl dp<br />

Dm<br />

(2)

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