Oven temperature control separations

As practiced by the UL, the procedure for selecting an RTI from Arrhenius plots usually involves making comparisons to a control standard material and other such steps to correct for random variations, oven temperature variations, condition of the specimens, and others. The stress-strain and impact and electrical properties frequently do not degrade at the same rate, each having their own separate RTIs. Also, since thicker specimens usually take longer to fail, each thickness will require a separate RTI. 

Figure 7.4 Schematic diagram of a gas chromatography (GC) system. The carrier gas enters from the left, and the sample is injected into the gas flow and is carried through the capillary column inside a temperature-controlled oven where the components are separated. Detection here is by flame ionization, where the eluent increases the conductivity of the flame.

A precise control of the column temperature is not only a must but also a requisite, whether it is intended to maintain an invariant-temperature or to provide a programmed-temperature. Importantly, the temperature of the column oven must be controlled by a system that is sensitive enough to changes of 0.01°C and that maintains an accurate control to 0.1 °C. In normal practice, an air-bath chamber surrounds the column and air is circulated by a blower through the thermal compartment. However, separate temperature controls are very much desirable for the vaporizer block as well as the detector-oven. 

The injection temperature can be a signiflcant issne for thermally unstable samples or where samples are stored for hours in an antosampler prior to injection. For this reason, most manufacturers sell autosamplers with optional thermostated sample compartments. This can be done either by placing the sample tray in an air bath oven or by a condnctive temperature control of the sample rack. The need to keep samples cool prior to injection when conpled with elevated temperature separation increases the complexity of the flow system reqnired. For such application, a separate mobile phase pre-heater with a low volnme placed between the injector and the column is a good choice. Alternatively, the injector valve wonld need to be monnted ontside the antosampler or in the column oven to insure preheating of the mobile phase before the colnmn. 

The inlet system, detector, and any heated accessories should have separate temperature-controlled ovens. They should not be in the column oven since it is not desirable that the temperature of these parts change during the analysis. These separate ovens should also be well isolated from the column oven since many detectors will have response changes that will result in baseline drift and quantification errors. The connecting lines between oven and these heated parts, especially the detector, should be short and should not have any "cold spots"... 

Once the sample reaches the chromatographic column, the separation process starts. The time necessary for a component injected into the chromatographic column to elute is called the absoiute retention time tR. The separation is based on different retention times of the components of the mixture. These retention times are different because the partition of each analyte between the two phases, the gas phase in motion and the stationary phase, are different. Hydrogen, helium, and nitrogen are common gases used as mobile phase. Two basic types of columns are known packed columns containing solid support particles coated with the stationary phase, and open-tubular columns with the stationary phase as a film on the inner wall (capillary columns). Because the retention time tpfi) of the analyte i is temperature dependent, the chromatographic column of any (GC) is put in an oven with temperature control capability. 

Gas chromatography (GC) is the most common and successful method of soil-gas analysis. The detection limits are about 1-10 ppb by volume. The basic components of a gas chromatographic system are a carrier gas and a flow control system, a column packed with a gas-separating material, an oven for temperature control of the column, a sample introduction device, a detector and a recording system (Fig. 8-8). 

A modem HPLC system is shown schematically in Figure 2. The equipment consists of a high-pressure solvent delivery system, a sample auto injector, a separation column, a detector (often an UV or a DAD) a computer to control the system and display results. Many systems include an oven for temperature control of the column and a pre-column that protects the analytical column from impurities. The actual separation takes place in the column, which is packed with chemically modified 3.5-10 pm (often silica) particles. A mobile phase is pumped through the column with the high-pressure pump and the analytes in the injected sample are separated depending on their degree of interaction with the particles. A proper choice of stationary and mobile phase is essential to reach a desired separation. 

The pendulum is housed in the cabinet at the left the oven is separated from the optical transducer by an insulated 3/4 inch horizontal aluminum plate. The temperature controller, digital voltmeter, scanner, and computer are in the rack at the right. The atmosphere control panel and liquid nitrogen container are shown in the background. 

Incidentally, the injector may have a separate injector oven, and the detector may have a separate detector oven. Set them both 10 to 20°C higher than the column temperature. You can even set these above the boiling points of your compounds, since you do not want them to condense in the injection port or the detector, ever, For those with only one temperature control, sorry. The injection port, column, and detector are all in the same place, all in. the same oven, and all at the same temperature. The maximum temperature, then, is limited by the decomposition temperature of the column. Fortunately, because of that dew point phenomenon, you really don t have to work at the boiling points of the compounds either. 

The sample solution may contain over 100 chemically similar compounds, so an accurate control of the experimental parameter is crucial for a good separation. In addition to the selection of the most suitable stationary phase and column dimension and optimization of the oven temperature program, other experimental parameters are decisive for the band broadening such as a proper injection procedure and a proper carrier gas flow rate. Instead of helium, hydrogen can be used as a carrier gas, thus allowing an increase in the gas flow rate without a loss in column efficiency. ° ° 2° i49 i50... 

The reaction tube for the Na atom experiments consisted of a low-carbon stainless steel tube, enclosed by a pair of sapphire windows, with a side-arm reservoir for Na. It was in the center of an aluminum-block oven enclosed in high-porosity firebricks, and was maintained at 528 0.2 K. The reservoir temperature was controlled separately and was maintained 37 °C cooler than the reaction tube during experiments. The Na atoms were excited by the dye laser tuned to one of the Na doublet Hues at 589.1 or 589.7 nm. Typical signal averaged absorption curves obtained are shown in Fig. 2. 

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