Sampling Devices

There are two types of sample injection systems employed in gas chromatography, the sample valve and the sample syringe. The GC sample valve is a rotary valve system that is used mostly in gas analysis or vapor sample analysis. On rotation, the valve interposes the sample, contained in a sample loop, in line with the carrier gas flow so that it is swept directly onto the column. Sample valves are not often employed for chiral chromatographic analyses.

Liquid samples, neat, or in the form of a solution, are usually placed on a GC column by means of a injection syringe in conjunction with a silicone septum. Syringe injection systems take two forms, those that are used for packed columns and those that are used for capillary columns. Today about 80% (or possible more) of all GC analyses are carried out on capillary columns, but for the sake of completion, the syringe system that is used with packed columns will also be described.

Injection System for Packed Columns

A diagram of a commonly used injection system for packed columns is shown in figure 56. Calibrated syringes are used for injecting samples and can vary in size from 1ml to 50 or even 100ml depending on the nature of the sample being analyzed. Probably the most commonly used syringe sizes are 1ml and 5ml.

Figure 56 Injection System for Packed Column

In order to place the sample on the column without interrupting the carrier gas flow rate a septum type injector is used. The septum injector comprises of a silicone rubber disc sandwiched between two metal discs, each pierced in the center with guide holes for the syringe needle The metal discs compress the silicone and so after the needle has pierced the septum and the injection made, the needle can be withdrawn, and the compressed silicone rubber disc expands into the hole making a gas type seal. A correctly fabricated silicone rubber injection system will allow many hundreds of injections to be made before a leak can be detected.

After penetrating the septum, the plunger of the syringe is depressed, and the needle discharges the sample into a heated glass liner. In the glass liner the sample is vaporized and swept onto the column by the carrier gas. The glass liner is called a 'flash heater" and, as it operates at a elevated temperature may cause the thermal decomposition of certain samples. Alternatively, the heater can be switched off, and, using a longer needle that can reach and penetrate the column packing, the sample can be discharged directly into the packing, and be volatilized at the column temperature. This procedure is called 'on column' injection. Even under temperature programming conditions, using this technique, the sample is never heated above the temperature at which it elutes from the column.