Oven temperature control chromatography

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. 

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.

Gas Chromatography. The basic components of a gas chromatograph are a carrier gas system, a column, a column oven, a sample injector, and a detector. Very pure helium is the near-universal carrier gas for environmental and many other analyses. Open tubular GC columns are constructed of fused silica with low-bleed stationary phases of varying polarity chemically bonded to the silica surface. Columns are typically 30-75 m in length and have inside diameters (ID) in the range of about 0.25-0.75 mm. The column oven is capable of precise temperature control and temperature programming at variable rates for variable times. 

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). 

Chromatography. Gas chromatographic analysis (Tracor, Model 220) was performed with a 1.8 m 4 mm i.d. "U" column packed with 1.5% SP-2250/1.95% SP-2401 coated on Supelcoport 100/120 support. The oven temperature was 220°C, and the inlet and detector temperatures were 250 and 350°C, respectively. The gas flow was 60 mL/min with 95 5 argon methane. Detection was accomplished with a Ni° electron capture detector controlled by a linearized electrometer (Tracor). Output signals were processed electronically (Varian CDS-401). 

SEC was performed according to the method of Chin et al. (1994). A Shodex KW802.5 SEC column (Waters Corp., Milford, MA., USA) was used and a Waters liquid chromatography system consisting of the following components was used for the analysis Waters 501 high pressure pump. Waters 717 autosampler, InterAction column temperature control oven, Waters 484 UV/VIS detector, and Waters Millenium 2.0 computer software package. 

Accurate pressure- and temperature-control facilities are required for supercritical chromatography. The mobile phase must be heated to the correct temperature in a spiral before the injection valve. The spiral, valve, colunrn and detector should all be placed in an oven. A restrictor must be placed behind the detector so that the whole system can be maintained at a sufficiently high pressure. As columns it is possible to use either open capillaries, which allows us to obtain very high plate numbers, as well as packed columns, as was the case in the separation of Fig. 22.6. 

Separations in gas chromatography are carried out within the temperature limits from about — 100°C to 450°C. Purpose-built instruments are usually required for high-temperature operation between 375°C and 450°C. Subambient temperature operation using the boil over vapors from Hquid nitrogen or carbon dioxide for cooHng is available as an option for standard instruments. The oven temperature is adjusted using an electrically controlled solenoid valve to pass coolant into the oven where it is mixed with air and then circulated at high velocity. 

A schematic drawing of the main parts of the SFC system is shown in Fig. 2. It consists of a high-pressure pump for pressurizing and delivering the solvent, usually CO2 connected to an oven, generally a modified gas chromatography used as the temperature controller for the SFC column. The injector should introduce small sample volumes into the column and a restrictor is placed between the end of the column and the detector to maintain the mobile phase in the supercritical state. A detailed description of each part of the system follows. 

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-... 

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