Automatic Injection Systems

Most modern gas chromatographs include an automatic sampling device for routine analysis. Automatic sampling devices normally involve some form of transport mechanism that may be a form of a carousel or a belt conveyor system. The transporter carries a series of vials, usually of glass, that contain, alternately, sample and washing solvent. The sampling mechanism can be quite complicated, and involve a elaborate sequence of transfer operations which are controlled by a microprocessor. The procedure is as follows; the syringe is washed with solvent, rinsed with sample, reloaded with the sample, moved by the transport mechanism to the column injector and finally the contents are discharged into/onto the column. When the separation is finished, the next cycle commences with the next syringe washing procedure. In some routine analytical laboratories there may also be automatic sample preparation, such as extraction, concentration, derivatization procedures etc. usually carried out by microprocessor-controlled robots

The Column Oven and Temperature Programmer

GC column ovens must be capable of operating over a wide temperature range (e.g. from 5 C to 400 C). However, in practice, the maximum oven temperature that is required is often no more than 250 C. This maximum temperature limitation is particularly true, when chiral stationary phases are used, as many deteriorate at temperatures higher than 250 C. For similar reasons, initial temperatures below 50 C are rarely necessary. The oven must have a very strong circulating fan to ensure an even temperature is maintained throughout the oven. The temperature stability in all parts of the oven should be 0.5 C or less. During isothermal operation, the column temperature should be maintained constant to 0.2 C of the set temperature. For practical purposes the minimum oven volume should not be less than 1-2 cu. ft. This oven volume will allow more than one column to be installed and permit switching valves to provide facilities for multidimensional chromatography.

The hardware and software of the temperature programmer should be able to provide a range of linear temperature gradients that can extend from 0.5 C/min. to about 20 C/min. Nonlinear programs, having logarithmic and exponential forms, are sometimes useful but most GC analyses can be successfully completed using linear temperature programs. The software should allow the program rate to be changed at any time during chromatographic development and, if required, allow intermittent isothermal periods to be inserted when necessary. All column/detector connections that pass though the column oven wall (e.g. to the detector oven) must be supplied with their own heater to prevent any part of the conduit from falling below the column oven temperature, which would cause sample condensation and will result in broad and distorted peaks.