Column Oven Temperature Control

It is seen that in order to measure retention volumes with a precision of 0.1%, the temperature control must be +/- 0.04°C. This level of temperature control on a thermostat bath is not difficult to achieve but it is extremely difficult, if not impossible, to return to a specific temperature to within +/- 0.04°C after prior change. To achieve a precision of retention volume measurement of 1%, the temperature control must be +/- 0.4°C. This is far more practical as most column oven temperature can be set to a given temperature to within +/-0.25°C. Although the data was obtained for three specific solutes, the results can be taken as reasonably representative for all solutes and phase systems. In most practical analyses, the precision limits of retention volume measurement will be about 1% but this will not include the reproducibility of the flow rate given by the pump. As... 

The column oven temperature will now rise as rapidly as the controller can supply the heat (Figure 6.17A). This type of programming is usually used for "cooking-out" or conditioning columns after they have been in use for some time. 

The FID uses a continuous quartz braid to transport the column effluent (sprayed onto the braid through a 0.1-mm orifice) through a solvent removal zone and into the analytical and cleaning flames. The FID flows were as follows 140 ml/min H2 and 400 ml/min air for the analytical flame 300 ml/min H2 and 150 ml/min 02 for the cleaning flame. The oven temperature control was set to slightly less than mid-range, which yields a block temperature of approximately 140°C. 

Column oven thermostatically controlled enclosure for the chromatographic column, maintained to 0.1 °C, may be isothermal or temperature programmed. 

The direct fluid interface consists of a probe housing a portion of the column or transfer line with a restrictor at the tip. The functions of the restrictor are to maintain the sample in solution throughout the interface, to control the flow rate through the column, and to provide a mechanism to release the fluid into the ion source as a gas. The critical features of the interface are that it must maintain the column oven temperature... 

A capillary column, 30 m or longer, with a thick film of stationary phase, offers an alternative to cryogenic oven temperature control for solute-focusing purposes, which is especially attractive with auxiliary sample introduction techniques of purge and trap and thermal desorption. 

As noted earlier, control of the column s temperature is critical to attaining a good separation in gas chromatography. For this reason the column is located inside a thermostated oven. In an isothermal separation the column is maintained at a constant temperature, the choice of which is dictated by the solutes. Normally, the tern-... 

For LC, temperature is not as important as in GC because volatility is not important. The columns are usually metal, and they are operated at or near ambient temperatures, so the temperature-controlled oven used for GC is unnecessary. An LC mobile phase is a solvent such as water, methanol, or acetonitrile, and, if only a single solvent is used for analysis, the chromatography is said to be isocratic. Alternatively, mixtures of solvents can be employed. In fact, chromatography may start with one single solvent or mixture of solvents and gradually change to a different mix of solvents as analysis proceeds (gradient elution). 

Many HPLC instruments are already furnished with temperature controls for the column. Unified chromatography requires a much wider temperature range than is currently practiced in HPLC. Until better defined by experience, a temperature range from about —60 to about 350°C seems reasonable as a specification. Since this is well in the range of a GC oven with subambient temperature capability, no new technology is required. 

It follows that, at least for SEC, column temperature control can be important. An example of a commercially available column oven is shown in figure 17. The available temperature range varies a little from instrument to instrument but the model shown above has an operational range from 10°C to 99°C. One of the problems associated with the temperature control of ovens is the high thermal capacity of... 

Other thermal zones which should be thermostated separately from the column oven include the Injector and detector ovens. These are generally insulted metal blocks heated by cartridge heaters controlled by sensors located in a feedback loop with the power supply. Detector blocks are usually maintained at a temperature selected to minimize detector contamination from condensation of column bleed or sample components and to optimize the response of the detector to the sample. The requirements for i injectors may be different depending on the injector design and may include provision for temperature program operation. 

Cool on-column >250 pm column (i.d.) 1 ppm (FID) Reduced thermal degradation and discrimination Wide range of analyte concentrations High sample capacity (LVI) Autosamplers Direct quantification Excellent precision Control of operational conditions (initial oven temperature) Optimisation required Not applicable for polar solvents Column contamination by dirty matrices Poor long term stability... 

Today s gas chromatograph is a modern, computer-controlled instrument, consisting of an integrated inlet, column oven and detector, with electronically controlled pneumatics and temperature zones. It has an inlet capable of both the split and splitless-injection techniques and it has a highly sensitive (detection limit in the pictogram range) detector... 

---