Topics - Column Packing
There are a wide range of different packings (both in type and particle diameter) that can be used in both GC and LC. However, there are two basic types of packing, supports that carry the stationary phase (e.g. Celite or calcined Celite used mostly in GC) and materials that act as the stationary phase itself (silica gel and bonded phases used mostly in LC). Celite is a very inactive form of almost pure silica that comprise the skeletons of diatoms deposited in seas millions of years past. To reduce its residual activity, which is largely due to surface hydroxyl groups and heavy metal ions, it is washed with acid, calcined and finally treated with hexamethyl disilazane to remove the last remaining surface hydroxyl groups. The material is screened to particle diameters ranging between 150 m and 200 m, the closer the cut of particle size, the easier the column is to pack and the greater the efficiency that will be obtained. The support is coated with the stationary phase by adding a solution of the stationary phase in a volatile solvent to a weighed amount of support and evaporating the solvent in a rotary evaporator. A dry packing process is usually employed for packing GC columns. Stationary phase loading for GC columns range from 3-4 % w/w to about 15 % w/w. Silica gel and bonded phases for use in LC are prepared in a range of particle diameters, particles 3 m in diameter are used for packing short (3 cm) columns, particles 5 m in diameter are employed for packing columns 5 to 10 cm long and particle diameters of 10 m for packing 20 cm long columns. Particle diameters of 20 m can be used to pack columns up to 1 m in length (the packing procedure, however, can be very difficult to pack). LC columns are slurry packed under high pressures.