Of column oven

Such a long column is wound into a coil and fits nicely into a small oven, perhaps 1 to 3 ft3 in size. This oven probably constitutes about half of the total size of the instrument. See Figure 12.3. Connections are made through the oven wall to the injection port and the detector. The temperatures of column ovens typically vary from 50 to 150°C, with higher temperatures possible in procedures that require them. A more thorough discussion of this subject is found in Section 12.5. 

FIGURE 7 Documentation example of column oven temperature verification within OQ or holistic PQ protocol. Courtesy of PerkinElmer LAS, Shelton, CT. 

Several theoretical and empirical studies have been made of the impact of column ovens on separations [84]. The two most important factors that must be considered are the preheating of the mobile phase before it enters the column and the extra-column volume between the injector and the column. Work by Djordevic et al. [76] provides some of the most dramatic evidence for the impact of pre-heating the mobile phase. Their measurements of the axial temperature gradient in 4.6 mm columns at typical flow rates are shown in Table 9.2. 

The introduction of forced air circulation in an oven allowed the control up to 300 - 400°C and the air bath is the main type of column oven available today for laboratory gas chromatographs. Several types of temperature controllers are used to control the temperature of these ovens. They basically differ in terms of cost, accuracy, and flexibility. They are usually advertised as isothermal or temperature-programming controllers but all, in general, use the following basic types of control ... 

Individual temperalure control and indication of column, ovens, detectors, and sample inlets... 

If HPLC analyses are carried out at ambient temperatures, even slight temperature variations will lead to retention time shifts making automated peak detection difficult if not impossible. The use of column ovens therefore is highly recommended. 

To aid in the determination of the retention indices of the photolysis products an iron Constantin thermocouple device is used which automatically indicates column oven temperature on the recorder chart during temperature programming. With proper calibration, the column oven temperature is indicated on the chart of a 1 mV recorder and can be read directly in degrees centigrade. A mixture of n-hydrocarbons from n-pentane to /2-pentacosane is chromatographed daily from which a calibration curve of column oven temperature vs Kovats retention index is prepared. The retention indices of eluted compoimds are obtained by graphical comparison of the retention temperature with the calibration curve. 

For GC, the injector is most frequently a small heated space attached to the start of the column. A sample of the mixture to be analyzed is injected into this space by use of a syringe, which pierces a rubber septum. The injector needs to be hot enough to immediately vaporize the sample, which is then swept onto the head of the column by the mobile gas phase. Generally, the injector is kept at a temperature 50 C higher than is the column oven. Variants on this principle are in use, in particular the split/splitless injector. This injector can be used in a splitless mode, in which the entire injected sample goes onto the column, or in a split mode, in which only part of the sample goes onto the column, the remainder vented to atmosphere. For other less usual forms of injector, a specialist book on GC should be consulted. 

Figure 1.2 Chromatogram of coal-tar oil obtained by using the following conditions column, Waters Spherisorb PAH 5 mm in 250 p.m id X 30 cm fused silica column oven temperature, 100°C UV detector wavelength to 254 nm mobile phase, 100 to 300 bar CO2 and 0.10 to 1.00 p.L min methanol over 30 minutes.

Figure 4.1 Schematic diagram of a coupled column system. The first column (ID) is connected to the second column (2D) tlirough the interface or valve system. The interface can be a diiect coupling, a live T-union, a complex multiport valve, or a thermal or cryogenic modulation system. The stimulus can be the switching of the valve, abalancing pressure to divert flow towards 2D, an added flow that is used in pressure tuning, or the drive mechanism for the modulator. The line to detector 1 will normally be a non-retaining section of column. In a two-oven system, ID and 2D will be in different ovens the dotted line indicates separately heated zones.

The basis of chromatography is in the differential migration of chemicals injected into a column. The carrier fluid takes the solutes through the bed used for elution (mobile phase). The bed is the stationary phase. Based on mobility, the retention-time detectors identify the fast and slow-moving molecules. Based on internal or external standards with defined concentration, all unknown molecules are calculated in a developed method by software. GC columns are installed in an oven which operates at a specified temperature. A diagram of an oven with GC column is shown in Figure 7.16. 

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... 

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... 

Weigh 50 g of sample into the headspace flask with a magnetic stirrer bar. Add immediately 50 mL of 1.5% stannous chloride solution in hydrochloric acid, and close the vessel with the silicone-rubber septum and the screw-cap. Transfer the head-space flasks into the electric oven (or water-bath) heated at 80 °C for 1 h. After 15 min, take the flasks out of the oven and mix the contents with the magnetic stirrer, heated at 50 °C, for 1 min. Return the flasks to the electric oven and repeat the mixing of the samples every 15 min. After 1 h, take the flasks out of the oven, mix the contents of each flask for 1 min, and inject an aliquot of headspace (100-1000 xL) on to the GC column with a gas-tight syringe. 

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. 

Figure 3.8 Alternative designs for cold on-column injectors, h, Injector with a duck bill valve (Hewlett-Packard), B, an injector with provision for secondary cooling of the column inlet (Carlo-Erba), and C, a temperature- programmable on-column injector with its own oven isolated from the column oven (Varian Associates).

The only disadvantage to the use of hydrogen as a carrier gas Is the real or perceived explosion hazard from leaks within the column oven. Experience has shoim that the conditions required for a catastrophic explosion may never be achieved in practice.. However, commercially available gas sensors will automatically switch off the column oven and carrier gas flow at air-hydrogen mixtures well below the explosion threshold limit [143]. 

Copper, aluminum, stainless steel, nickel, or glass tubes bent into various shapes to fit the dimensions of the column oven provide the container for column packings [126]. Neither copper mor aluminum tubing is recommended as both metals are readily SKlditsd active, oxide-coated films formed on the inner walls promote decomposition or tailing of labile and polar solutes. Ptalnless steel is adequate for nonpolar samples but its catalytic activity precludes the analysis of labile solutes. Nickel, after acid passivation, and glass are the most inert column materials. 

The column oven is generally a forced circulation air thermostat of sufficient size to allow comfortable installation of the longest columns normally used. In the design of a column oven it is important to ensure a uniform temperature throughout the column coil region. The temperature uniformity depends on the geometry of the oven, the Ideation of the heater and sensor, and the pattern of mixing and circulation of air. A temperature... 

A schematic diagram of a chromatograph for SFC is shown in Figure 6.10. In general, the instrument components are a hybrid of components developed for gas and liquid chromatography that have been subsequently modified for use with supercritical fluids. Thus, the. fluid delivery system is a pump modified for pressure control and the injection system a rotary valve similar to components used in liquid chromatography. The column oven and... 

---