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Handbook of Size Exclusion Chromatography and Related ...

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5 SEC OF UNDERIVATIZED CELLULOSE<br />

Historically, the two solvents used for SEC <strong>of</strong> underivatized cellulose are cadoxen<br />

<strong>and</strong> LiCl/DMAC. However, during the past decade hardly any reports <strong>of</strong> cadoxen<br />

in conjunction with SEC have appeared. During the same period, LiCl/DMAc has<br />

become the number one choice for various investigations <strong>of</strong> all kinds <strong>of</strong> cellulose<br />

samples.<br />

5.1 Cadoxen<br />

The cadmium–ethylene diamine complex possesses a number <strong>of</strong> desirable<br />

properties for studies <strong>of</strong> cellulose solutions. The solvent is colorless, easy to<br />

h<strong>and</strong>le, <strong>and</strong> dissolves many kinds <strong>of</strong> cellulose samples. The main disadvantages<br />

are that it includes a toxic compound (Cd), it is time-consuming to prepare, <strong>and</strong><br />

that the cellulose solutions have a high viscosity. It has also been reported that<br />

hardwood pulps have a limited solubility in cadoxen (168).<br />

The preparation <strong>of</strong> cadoxen is usually a modification <strong>of</strong> the original<br />

procedure described by Jayme <strong>and</strong> Neuschaffer (169). Ethyleneamine is saturated<br />

with cadmium oxide in the presence <strong>of</strong> sodium hydroxide. The cadmium content<br />

ranges between 4.5 <strong>and</strong> 5.2%, ethyleneamine between 25 <strong>and</strong> 30%, <strong>and</strong> sodium<br />

hydroxide between 0.2 <strong>and</strong> 0.5 M; for a detailed description <strong>of</strong> the preparation <strong>of</strong><br />

cadoxen see Ref. 170. The addition <strong>of</strong> sodium hydroxide increases the dissolving<br />

power but also increases the degradation <strong>of</strong> dissolved cellulose (171,172). The<br />

solvent as well as the cellulose solutions are fairly stable provided they are stored<br />

at 48C in the dark. When the cellulose solution is used within a couple <strong>of</strong> days,<br />

degradation can be neglected (173). It has also been pointed out that watermiscible<br />

organic liquids should not, in general, be added to the cadoxen solution,<br />

since they induce turbidity <strong>and</strong> precipitation (173).<br />

For dissolution <strong>of</strong> cellulose, cadoxen is brought to room temperature <strong>and</strong><br />

added to the sample. The dissolution time for cellulose ranges from a few minutes<br />

up to two hours, depending on type <strong>and</strong> molecular mass <strong>of</strong> the cellulose, the<br />

degree <strong>of</strong> crystallinity, <strong>and</strong> the desired concentration <strong>of</strong> cellulose. Prewetting the<br />

sample with water facilitates the dissolution <strong>of</strong> high molecular mass samples.<br />

Commonly, the solution is diluted with an equal volume <strong>of</strong> water prior to<br />

chromatography. The diluted solution is not capable <strong>of</strong> dissolving additional<br />

cellulose, which makes it possible to use carbohydrate-based packing material in<br />

the subsequent chromatography.<br />

SEC <strong>of</strong> cellulosic samples dissolved in cadoxen solutions was reported<br />

mainly during the late 1960s <strong>and</strong> 1980s (174–184). Various packing materials<br />

have been used such as polyacrylamide gel, agarose gel, vinyl polymer-based gels,<br />

<strong>and</strong> chemically modified silica gels (178) have also been tested. Since crosslinked<br />

© 2004 by Marcel Dekker, Inc.

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