Chromatography column, residence time

Fig. 9. Reversed-phase separations of cytochrome c digests obtained with trypsin-modified beads (left) and trypsin-modified monolithic reactor (right) in a tandem with a chromatographic column (Reprinted with permission from [90]. Copyright 1996 Wiley-VCH). Conditions digestion (left curve) trypsin-modified beads reactor, 50 mm x 8 mm i.d., 0.2 mg of cytochrome c, digestion buffer, flow rate 0.2 ml/min, 25 °C, residence time, 15 min (right curve) trypsin immobilized onto molded monolith other conditions the same as with trypsin-modified beads. Reversed-phase chromatography column, Nova-Pak C18,150 mm x 3.9 mm i.d., mobile phase gradient 0-70% acetonitrile in 0.1% aqueous trifluoroacetic acid in 15 min, flow rate, 1 ml/min, injection volume 20 pi, UV detection at 254 nm...

Catalytic tests of n-pentane oxidation were carried out in a laboratory glass flow-reactor, operating at atmospheric pressure, and loading 3 g of catalyst diluted with inert material. Feed composition was 1 mol% n-pentane in air residence time was 2 g s/ml. The temperature of reaction was varied from 340 to 420°C. The products were collected and analyzed by means of gas chromatography. A FlP-l column (FID) was used for the separation of C5 hydrocarbons, MA and PA. A Carbosieve Sll column (TCD) was used for the separation of oxygen, carbon monoxide and carbon dioxide. 

Because of the relative facility of thermal rearrangement to phenols, melting points of arene oxides are not an entirely reliable index of purity. The checkers found variation from 119 to 135° (dec.). Purification by chromatography on activity IV alumina is also possible, but residence time on the column should be held to a minimum. 

The essential components of a gas chromatography system are shown in Figure 3.4. The mobile phase (called the carrier gas) is inert, usually helium, nitrogen, or argon. The gas is directed past an injection port, the entry point of the sample. The sample, dissolved in a solvent, is injected with a syringe through a rubber septum into the injection port. The column, injection port, and detector are in individual ovens maintained at elevated temperatures so that the sample components remain vaporized throughout their residence time in the system. 

In NP chromatography non-polar solvents (e.g. alkanes) are used as eluents in cases of insufficient eluting strength the polarity of the eluent can be raised by addition of more polar solvent. More polar sample components have longer residence times in NP chromatography, i.e. they leave the column later because they are retained longer. 

Because the electroosmotic flow affects the amount of time a solute resides in the capillary, both the separation efficiency and resolution are related to the direction and flow of the EOF. The EOF flow profile, as shown in Figure 4.7, is comparatively pluglike. Unlike the laminar flow that is characteristic of pressure-driven fluids,5 the EOF has minimal effect on resistance to mass transfer. As a result, the plate count in a capillary is far larger than that of a chromatography column of comparable length. 

A more modern method to determine the MMD is GPC, gel permeation chromatography, also named size-exclusion chromatography, SEC. A polymer solution is passed over a column with a porous structure. The residence time of the chains on the column depends on the diameter of the coiled chain smaller chains can migrate through more pores (they can also enter into the smaller ones), and it takes a longer time for them to pass along the column. The bigger ones cannot enter into any of the side-pores and pass in the shortest time. 

The technique involves creating within a column a stationary phase of the solid material of interest. The stationary phase may be a thin polymeric coating on an inert substrate, a finely divided solid, or a thin polymeric coating on the column wall. A volatile probe of known characteristics is passed through the column via an inert mobile phase and the output is monitored. The residence time of the probe and the shape of the chromatogram indicate the characteristics of the stationary phase and its interaction with the probe. Thus, IGC is a variation of conventional gas chromatography. 

The catalytic oxidation of ethane at 573-648 K was carried out at atmospheric pressure in a fixed bed flow reactor. Mbftures of ethane (4 mol%), oxygen (4-12 mol%), and helium (balance) were fed to the reactor with a residence time of 38 g. h/mol C2H6, using a catalyst load of 0.36 g, (particle size 0.25-0.42 mm) mixed with SiC bits (dilution 1 4 vA) to reduce the heat release per imit volume. Reactants and products were analysed by gas chromatography on a Vaiian 3400, equ ped with a thennal conductivity detector, using Porapak QS (3 m) and molecular sieve 13X (1 m) columns. In all reaction conditions, the mass and carbon balances were within 10012 %. 

The proportionate-pattern case is a classical one in the theory of chromatography, and was treated by DeVault (D2), Walter (Wl), Wilson (W7), and Weiss (W3). It is assumed that equilibrium is maintained everywhere in the column, that is, that N approaches infinity, due to high mass-transfer rates or to long residence times. 

Rp values are generally calculated to two decimal places. Some authors prefer to tabulate values as whole numbers, as hRp values equivalent to 100 Rp. The Rp value is not linearly related to the distribution properties of the separation system. The Rm value is used in studies that attempt to correlate migration properties to solute structure. The Rm value is equivalent to the ratio of the residence time of the solute in the stationary and mobile phases, and is formally equivalent to the retention factor (log k) in column liquid chromatography. It is calculated from the Rp value by Rm (or log k) = log [(1 -... 

Gas chromatography is a separation technique based on the fact that different components in the mixture exhibit different average residence times due to interactions with the porons packing material. These interactions can be classified as intrapellet diffusion and the column operates similar to a packed catalytic tubular reactor. The important mass transfer mechanisms are convection and diffusion. Hence, it is important to calculate the mass transfer Peclet number that represents an order-of-magnitude ratio of these two mass transfer rate processes. Intrapellet diffusion governs residence times, and interpellet axial dispersion affects the degree to which the output curve is broadened. For axial dispersion in packed columns and packed catalytic tubular reactors. 

From reaction engineering it is known that in the case of an infinite number of vessels with a continuous feed stream, all fluid elements have the same residence time in the total system. This corresponds to a plug flow of an ideal tube reactor without any dispersiom Here the real chromatography column is replaced by a vessel cascade to quantify the separation efficiency. 

Throughout the discussion of chromatography, we have focused on pairs of analytes that were difficult to separate and the need for columns of high efficiency. It is important to remember that in multicomponent mixtures, some analytes are not so difficult to separate. In such cases, another principle must be considered, namely, the time required to move all the components through the column. Further, the longer the residence time, the more diffusion effects will spread the band out In a mixture in which two adjacent components have reasonably different k values, then the eluent might be altered when the first component clears the column to reduce the k of the second. This can be done by continuous or concrete step gradients. 

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