When considering Normal Phase chromatography techniques, plain silica gel is the optimal stationary phase due to its high surface area (500-600 m²/g). Additionally, a 70Å pore diameter works well for most analytes, especially those having a molecular weight of less than 1,000 Daltons. Higher molecular weight analytes require larger pore diameters (>100Å) for entrance into pores as well as optimal interaction with the surface chemistry.
Water also plays an important role in that an increase of the water on the surface will shorten elution times by deadening the silica surface.
Unmodified silica gel offers the following advantages:
- Acidic surface pH
- High loading capacity
- Ease of solvent evaporation from products
Difficulty arises however when certain organic additives (such as triethylamine (TEA)) are added to the mobile phase.
- These additives are hard (virtually impossible) to completely remove
- Note that the use of strong acids or bases may be harmful to some target analytes
In cases like this, a modified silica gel would serve well. These modified silica gels will increase selectivity for acid, neutral, and basic compound separations.