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, silica gels can be synthesized with a particular range of pore diameters to suit a specific application. Alternatively, standard silica gels can be blended together to give a mixed gel with definitive exclusion properties for specific separations. The exclusion properties of a silica gel cannot be obtained with sufficient accuracy for chromatographic use from nitrogen adsorption data or mercury porosity tests. It is necessary to determine the range of pore diameters and pore volume of a silica gel by a special experimental procedure that is designed to obtain accurate retention volume measurements for solutes eluted in relatively small elution volumes. In exclusion chromatography, all the peaks will be contained in a mobile phase volume equivalent to that of the total pore volume of the column. Consequently, the column volume itself must be large and a column 25 cm long and 4.6 mm I.D. is a practical size to obtain results having adequate accuracy. The sample volume used should

Desty et al. (4), tried to eliminate the activity of the open tubular column surface by developing the first silica-based columns and invented an extremely clever device for drawing soft glass capillaries. Desty produced both circular rigid soft glass and circular rigid Pyrex capillary columns, but their permanent circular shape, made them difficult to fit to unions connecting columns to injector and column to detector. By careful surface treatment the rigid glass tubes could be coated with polar stationary phases such as CARBOWAX�. Dandenau (5) introduced flexible fused silica capillary columns using the quartz fiber drawing technique. The solid quartz rod used in quartz fiber drawing was replaced by a quartz tube and the drawing rates adjusted appropriately. The quartz tubes had to be coated on the outside with polyimide to prevent moisture attacking the surface and producing stress

, the tetrabutyl ammonium salt would be completely desorbed and the interactions of the solutes with the stationary phase would become almost exclusively dispersive. This is an example where a complex phase system was necessary because there was limited column efficiency available. It is likely that a column with intrinsically more efficiency might achieve the separation with a much simpler solvent system and a more straightforward solvent program. An example of the use of native silica is given by for the analysis of Darvocet� and its generic equivalent formulation. Darvocet� is an acetaminophen product in which the active ingredient (and other material in the medicine) are weakly polar and, consequently, lend themselves to separation on a strongly polar stationary phase such as silica gel. The analysis is depicted in figure 59. The analysis is completed in less than 4 minutes using a short column 3.3 cm long and 4.6 mm in diameter. The silica packing had a particle

3. 2-ethylpyridine 8. toluene 4.phenol 9. phenylacetyl acetone 5. butyl benzoate 10. naphthalene   Column length 15 cm, diameter 2 mm, mobile phase 50% (v/v) aqueous acetonitrile, flow rate 0.19 ml/min., temperature 40˚C, detector UV at 254 nm.   Figure 56 The Relative Performance of a Column Packed with Polymer Based Material And that from a Column Packed with Silica Based Material It is seen that although the polymer columns exhibits are a large number of theoretical plates and uracil and caffeine, and toluene and phenylacetyl acetone are better separated, 2-ethyl pyridine and phenol coelute whereas on the silica based column they are reasonably well resolved

6 mm Column Packing C18 Reverse Phase, (cyanopropile chain) Column Temperature 30oC Mobile Phase 25%0.01M potassium phosphate (adjusted to pH 7 with 85% phosphoric acid)60% acetonitrile and 15% methanol Flow-Rate 2 ml/min. Detector UV adsorption at 215 nm Sample Volume 100ml     An example of the use of native silica is in the analysis of Darvocet� and its generic equivalent formulation. The separation obtained is shown in figure 44. Darvocet� is an acetaminophen product of which acetaminophen itself is the active ingredient together with other weakly polar substances present. Consequently, the mixture lends itself to separation on silica gel. The analysis was completed in less than 4 minutes using a short column 3.3 cm long and 4.6 mm in diameter. The silica packing had a particle size of 3 m

nbsp;                  11.8%w/w Total Column Volume                                                         4.15 ml Total Volume of Silica in the Column                                    0.96 ml Total Volume of Stationary Phase in the Column                    0.40 ml Volume of Chromatographically Available Stationary Phase    0.49 ml &