Polyanions on siliceous packings

Among the first well-characterized narrow-MWD standards for aqueous SEC were sodium salts of poly(styrenesulfonic acid) (NaPSS), prepared by the method of Carroll and Eisenberg (44), and subsequently made commercially available by Pressure Chemical Co. A number of reports deal with the chromatographic behavior of NaPSS on CPG, with particular attention to the effect of ionic strength. 

The elution of NaPSS from Spherosil, a porous silica, was studied in detail 

In view of the role one may presume for simple electrolyte - namely, its effect on the Debye-Huckel screening length - a dependence of Ksec on 1-1/2 was anticipated (25). However, the ionic strength was adjusted not by eluant modification, but by varying sample concentration, so the failure of the data for NaPSS eluted in pure H2O to conform to the aforementioned linear dependence is difficult to interpret. 

Small porous silica particles in the 10 pm size range, such as LiChrospher (Merck), provide columns with much higher efficiency than CPG or Spherosil. The elution volumes of carboxymethylcellulose (CMC), po1y(methacrylic acid) (PMA), and heparin - a sulfonated mucopolysaccharide - were examined as a function of sample concentration and ionic strength on high-resolution (30,000 plates m l) LiChrospher columns (50). Ve for heparin was found to increase smoothly with I, approaching a constant value at 0.7 I 1. In 0.5 M NaAc Ve increased linearly with sample mass. Chromatograms for CMC and PMA in 0.5 M NaAc were also reported essentially without discussion. 

The ionic-strength dependence of the elution of CMC was studied by Barth and Regnier (51) on glycerylpropylsilyl derivatized LiChrospher. No explicit attempt was made to separate polyion expansion effects from polyion-substrate interaction. However, one could note that the salt-dependence of the intrinsic viscosity essentially vanished at I 0.1, whereas Ksec continued to decrease in the range 0.1 I 0.5. This rather qualitative observation suggests that polyion-substrate repulsion persists even for derivatized silica. 

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