Gas Chromatography - Injection Devices > Open Tubular Column Injection Systems > Page 15

The glass liner can be fitted with a separate heater and the volatalization temperature can, thus, be controlled. This "flash heater" system is available in most chromatographs. By using a syringe with a long needle, the tip can be made to penetrate past the liner and discharge its contents directly into the column packing. This procedure is called 'on-column injection' and, as it reduces peak dispersion on injection and thus, provides higher column efficiencies, is often the preferred procedure.

 

Open Tubular Column Injection Systems

Due to the very small sample size that must be placed on narrow bore capillary columns, a split injection system is necessary, a diagram of which is shown in figure 8.

Figure 8 The Split Injection System

The basic difference between the two types of injection systems is that the capillary column now projects into the glass liner and a portion of the carrier gas sweeps past the column inlet to waste. As the sample passes the column opening, a small fraction is split off and flows directly into the capillary column, ipso facto this device is called a split injector. The split ratio is changed by regulating the portion of the carrier gas that flows to waste which is achieved by an adjustable flow resistance in the waste flow line. This device is only used for small diameter capillary columns where the charge size is critical.