Capillary Chromatography - Automatic Sampling Systems
Automatic Sampling Systems.
Most capillary gas chromatographs can be fitted with an automatic sampling device. Automatic samples usually have a transport mechanism that in the form of a carousel or some type of conveyor system. The transport system contains a series of vials that usually contain, alternately, sample and washing solvent. The sampling mechanism is quite intricate, comprising a complex sequence of operations that are usually controlled by a computer. A sequence of procedures such as the following usually takes place. The syringe is washed with solvent, rinsed with the sample, reloaded with sample and the contents discharged into the column. After the analysis the next cycle starts with another syringe washing procedure. The operation of the automatic sampler is simplified by arranging the injection system to be vertical with the septum at the top. The syringe to be lifted to the correct height, rotated so that it is placed over the septum of the injection device and lowered to allow the needle to penetrate the septum and enter the vaporizer or column. The syringe is then mechanically discharged and the syringe reverses its three dimensional program of movement until it is again over the next sample vial.
The Column Oven
The capillary column oven is almost identical to that used for packed columns. It should be capable of operating from about 5ûC to 350ûC. Temperatures above 350ûC are rarely needed for capillary GC separations, although there are stationary phases (cf., Dexsil) that are stable at 350ûC and above, but applications that require such operating temperatures are relatively few. The oven should be programmable, with an adequate practical range of heating rates and, in addition have cooling facilities to allow sub ambient temperature operation if so required. In practice, non-linear program are very rarely needed. The oven requires suitable facilities for capillary column assemble and attachment so the columns are rigidly supported and have capacity for a wide range of column lengths.