Silicas Fractosil

Synthesis of modified silica 56 Two different types of low surface area silica were employed. The silica Fractosil 2500 (grain size = 80 pm) from Merck AG has a macroporous structure with relatively large pores (>250 nm) and a surface area (BET) of 8.3m g . The monosphere silica from Merck exhibits a spherical non-porous structure with a surface area (BET) of only 1.7 m gThe silica species were functionalized with 3-aminopropyl groups (-0.5 mmol per g Si02) on the surface as follows. The silica was dried in vacuum (<1 Pa at 200 °C). A total of 20 g of silica was charged in a 250-mL round-bottomed flask with 150 mL dried w-xylene. 2.2 g (10 mmol) 3-amino-propyltriethoxysilane was added, and the mixture was refluxed for 8 h. The filtered product was washed thoroughly with acetone. The solid 56 was dried under vacuum at 80 °C. 

The following chromatograms were obtained with the Fractosil 200 silica gel and the Pye moving wire detector as shown in Figure 2. They were plotted by the computer. The axes are the response in millivolts versus time in seconds. When the computer reconstructed the chromatogram, the Y-axis could be magnified from 0 to 520 millivolts. 

Experience has shown that the Fractosil 200 silica gel is not very effective for removing fat interferences when lipophilic compounds are to be analyzed. However, it s very efficient when polar compounds are to be isolated. 

The silica-based supports are supplied by Bio-Rad Labs (Richmond, CA, U.S.A., under the trademark Bio-Glass) by Electro-Nucleonics, Inc. (Fairfield, NJ, U.S.A.) and Pierce Chem. Co. (Rockford, IL, U.S.A. under the name CPG (controlled-Pore Glass)) by Waters Associates (Milford, MA, U.S.A. under the name Porasil) by E. Merck (Darmstadt, G.F.R., under the names Fractosil or LiChrospher) by DuPont Instrument Products Div. (Wilmington, DE, U.S.A., under the name SE Series or Zorbax), or by Rhone Poulenc (France, under the name Spherosil). The modified silica-based supports are supplied by Electro-Nucleonics, Inc. (Fairfield, NJ, U.S.A.) or Pierce Chem. Comp. (Rockford, IL, U.S.A., under the trademark Glycophase-... 

Deoxynucleosides were attached to the support using the following general procedure. HPLC grade silica gel (12 g, Fractosil 200, Merck) was exposed to a 15% relative humidity (saturated LiCl) for at least 24 h. The silica was then treated with 3-triethoxysilylpropylamine (13.8 g, 0.01 Min dry toluene) for 12 h at 20 C and 18 h at reflux. It was isolated by centrifugation, washed successively (3 times each) with toluene, methanol and 50% aqueous methanol. The silica was then shaken with 50% aqueous methanol (200 ml) at 20 C for 18 h. 

Long-chain alkylamine controlled-pore glass (CPG), 125 to 177-pm sized beads with 500-A pores, is available from the Pierce Chemical Co., Rockport, IL. A similar aminopropyl CPG support with either 1000- or 2000-A pores is available from Penninsula Laboratories, Inc., Belmont, CA, Fractosil 500 silica gel (300- 400-A pores, 70-230 mesh) is available from E. Merck, Darmstadt, Germany. 

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