Oven temperature control isothermal operation

The column is enclosed in a thermostatically controlled oven so that its temperature is held constant to within 0.5 °C, thus ensuring reproducible conditions. The operating temperature may range from ambient to over 400 °C and for isothermal operation is kept constant during the separation process. 

To ensure operation under reproducible conditions, the column is enclosed in a thermostatically controlled oven whose temperature can be held constant to within 0.1°C. Operating temperatures range from ambient to over 400°C and may remain constant during a separation - isothermal operation - or automatically increased at a predetermined rate to speed the elution process - temperature programming (p. 106). The latter is a form of gradient elution. Rapid temperature equili-... 

The PTGC technique involves increasing the column temperature at a preset rate during the elution process. This rate may be constant throughout the run, or periods of isothermal operation may be automatically programmed at set times between temperature increases. Generally, the electronically controlled ovens are designed to increase temperature at rates from 0.5-30°C per minute. The initial temperature should be chosen to minimize the retention time for the least retained solute, while the final temperature must be sufficient to elute the least volatile compound in a reasonable time. The instrument then automatically resets the temperature to the initial value in preparation for the next sample. 

In a very simple GC analysis of a group of similar analytes which would elute close together, the analyst chooses an appropriate stationary phase, optimizes its film thickness, and selects the necessary column diameter and length, all as outlined in Section 12.5. Then the oven is set to a column temperature where k for these analytes is between 2 and 10 [cf. Eq. (11.3)]. The highest value of temperature T at which adequate resolution of the closest eluting pair is observed, will yield the fastest analysis. Such operation is called isothermal GC. It is the simplest for the temperature controllers to maintain, and as soon as the last component has eluted, the instrument is ready to accept injection of the next sample. 

Independent Temperature Control—Haas test method requires the temperature control of five columns, column switching valves and sample lines. The columns consist of polar, non-polar, Tenax, platinum, and molecular sieve columns. The specifications for these columns are listed in Table 1. The polar column, non-polar column, column switching valves, and sample lines require isothermal operation at a temperature equivalent to e temperature of the gas chromatograph oven. These components may be located in the gas chromatograph oven. The Tenax column, platinum column, and molecular sieve column require operation at temperatures other than the gas chromatograph oven temperature. These columns may be temperature controlled by any means that will meet the following specifications ... 

Automatic operation of the metering pump allows repetitive injections with unattended operation. Precise control of the carrier-gas flow ensures stable chromatograms and reproducible timings for collection of samples. Independent column oven and vaporizer temperatures are available up to 300°C and these can be operated with temperature programming or isothermally. The latter option is the most common. 

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