Temperature column oven

A constant-temperature column oven is essential to ensure constant and reproducible elution times and cutting intervals during each HPLC analysis. 

A sub-ambient temperature column oven (5-80 °C) improves your options to optimize selectivity. 

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). 

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.

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... 

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 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... 

Cool on-column Capillary Ramped temperature Track oven Low concentration or thermally labile Minimal sample discrimination and decomposition... 

Most modern HPLC instruments include a column oven that can thermostat the column to at least 100°C. A typical HPLC analysis can be done in half the time by elevating the column temperature from ambient to 50 or 60°C. At temperatures above 100°C, it is not uncommon to decrease analysis time by a factor of 5.26 Also, re-equilibration time for the column is much shorter, so it is possible to achieve ultra-fast gradient analysis with HTLC. 

Salm et al.44 developed a high-throughput analytical method to measure cyclosporine in whole blood. They used a simple SPE procedure, followed by HPLC-MS/MS. An Agilent 1100 liquid chromatograph was coupled with an Agilent Zorbax Bonus C18 reversed-phase column (50 x 2.1 mm, 5 jt/rn particle size). The column temperature was maintained at 70°C in a column oven. The mobile phase consisted of 80% methanol and 20% 40mM ammonium acetate buffer (pH 5.1) delivered isocratically at a flow of 0.4 mL/min. D12 cyclosporine was the IS. 

Lercanidipine — Kalovidouris et al.57 applied UPLC-MS/MS to the determination of lercani-dipine in human plasma after oral administration of lercanidipine. A Waters Acquity UPLC system with cooling autosampler and column oven was coupled with a Waters BEH C18 column (50 x 2.1 mm, 1.7 jum). The mobile phase was composed of 70% acetonitrile in water containing 0.2% v/v formic acid, delivered at a flow of 0.30 mL/min. The column temperature was maintained at 40°C and sample vials at 5°C. 

Whether you realize it or not, the GC column has its own heater—the column oven. If you turn the temperature up, the compounds hotfoot it through the column very quickly. Because they spend less time in the stationary phase, they don t separate as well, and the GC peaks come out very sharp but not well separated. If you turn the temperature down some, the compounds spend so much time in the stationary phase that the peaks broaden and overlap gets very bad. The optimum is, as always, the best separation you need in the shortest amount of time. There are two absolute limits, though. 

Not many LC setups have ovens for temperatures like those for GC. This is because eluents tend to boil at temperatures much lower than the compounds on the column, which are usually solids anyway. And eluent bubbling problems are bad enough, without actually boiling the solvent in the column. This is not to say that LC results are independent of temperature. They re not. But if a column oven for LC is present, its purpose more likely is to keep stray drafts and sudden chills away than to have a hot time. 

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... 

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 multidimensional gas chromatography system (multi-stage column system) is effective for analysis of difficult samples and can be built up by connecting several column ovens, i.e. tandem GC systems, each of which has independent control functions such as for temperature programming. 

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. 

The various separation parameters should be adjusted to provide optimum resolution. These include mobile phase flow rate, stationary phase particle size, gradient elution, and column temperature (using an optional column oven). 

Qualitative and quantitative analyses with HPLC are very similar to those with GC (Sections 12.7 and 12.8). In the absence of diode array, mass spectrometric, and FTIR detectors that give additional identification information, qualitative analysis depends solely on retention time data, tR and C (Remember that tR is the time from when the solvent front is evident to the peak) Under a given set of HPLC conditions, namely, the mobile and stationary phase compositions, mobile phase flow rate, column length, temperature (when the optional column oven is used), and instrument dead volume, the retention time is a particular value for each component. It changes only when one of the above parameters changes. Refer to Section 12.7 for further discussion of qualitative analysis. 

One drawback of high-temperature GC analysis is that sample degradation for the high molecular weight AEs and APEOs might occur. High-temperature capillary columns are coated with a stabilised bonded polysiloxane film, which allows a column oven temperature of up to 400°C. 

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. 

Erba model 4160 (Erba Science (UK) Ltd, Swindon, Wilts) with a sub-ambient attachment was fitted with a 25m BP1 flexible silica capillary column (SGE Ltd, Milton Keynes, Bucks.). Following 10 minutes at 10°C the column oven was temperature programmed at 3°C min-1 up to 150°C. 

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