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3. : J. , 145, 371 (1978). 4. R. in Cs. , New York, Vol. 3, in press. 5. : Ger. Pat. 29, 305, 15 (1979). 6. : J. , 106, 249 (1975). 7. A. , Cary, North Carolina, 1979. 8. : J. , 125, 129 (1976). 9. : J. , 185, 99 (1979). 10. : J. , 199, 35 (1980). SPHERICAL AND IRREGULAR SILICA. DOES SHAPE INFLUENCE THE SELECTIVITY? Hartmut Müller, Heinz Engelhardt Angewandte Physikalische Chemie, D-6600 Saarbrücken, Germany Universität des Saarlandes For testing columns for efficiency in HPLC mainly non polar, neutral solutes like aromatic hydrocarbons are used.

1mm Flow : 1 ml/min Samples Tetrachloroethene Benzene Naphthalene Acenaphthene Anthracene Fluoranthene Chrysene 3A Beniofluoranthene LiChrospher Si 100 tlmin) 21* Fig. 3: Demonstration of influence of specific surface area and packing density on solute retention 31 surface pH on solute retention is not as clear as expected from these datas. The influence of physical properties on a chromatographic separation will be demonstrated in Fig. 3. As expected the k" values of the hydrocarbons on LiChrospher Si 500 are by a factor of 5 smaller than on LiChrospher Si 100 which corresponds to the differences in specific surface area and packing density.

FIow: 1 ml/min Samples-. 0 9,10 not eluted tlmin] 9 8 7 6 5 4 3 2 1 0 Fig. 7: Reproducibility of silicas surface properties (compare Fig. 5) Conclusion The pH value of silica caused by impurities on its surface influences the selctivity and the efficiency of columns if non polar and medium polar solutes are separated with non polar eluents. It is not very surprising, that these properties influence also selectivity of bonded phases based on these materials (12). In these cases because polar and aqueous eluents are used this influence is very striking.

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