Page 32 - Tandem Techniques

Page 14

Injection systems for packed columns are simple, reliable and can be designed to give both accurate and
precise quantitative results. A diagram of a septum injector for a packed column is shown in Figure 1.2.

The injector is situated in a small oven, often a simple metal block that is maintained at a temperature
about 20°C above the maximum the column will attain during the development of the chromatogram.
This is necessary to ensure that all the sample is volatilized and is called flash heating; the oven is
sometimes referred to as the 'flash heater'. The injector itself consists of a tube with a septum cap that
maintains the system pressure tight when the hypodermic syringe enters the injector. The carrier gas
enters the side of the injector and the column is connected to the end by a suitable union. The injector
usually contains a glass liner to prevent the sample coming in contact with the catalytically active metal
surface of the injector body. The injection procedure involves drawing the sample, as a fluid, into a
specially designed hypodermic syringe, inserting it through the septum via a hole in the metal cap and
discharging the contents onto the glass liner. The sample instantly volatilizes and is swept onto the
column. An alternative procedure that is, in fact, to be preferred as it eliminates any possibility of
thermal decomposition is on-column injection. The same injector can be used, but the injection oven is
turned off and a syringe with a long needle is used so that it penetrates to the column packing and the
sample is discharged onto the column itself. Both methods, if carried out with due care, can provide
precise and accurate quantitative results.

The capillary column was invented by Golay [3] and takes the form of an open tube with the stationary
phase coated on the inside surface as a thin film. Originally the tube was constructed of metal but, due
to surface activity of the metal, it was soon replaced by the soft glass capillary tubes invented by Desty
[4]. The soft glass tubes were rigid and thus somewhat difficult to fit into the chromatograph and so
they, in turn, were replaced by flexible fused silica tubes developed by Dandenau [5]. The flexible silica
tubes were fabricated by coating the tube immediately after drawing with a polyimide resin. This
prevented stress corrosion by sealing the external surface of the tube from contact with air and, in
particular, from