Packed columns phase loading

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

Flooding and Loading Since flooding or phase inversion normally represents the maximum capacity condition for a packed column, it is desirable to predict its value for new designs. The first generalized correlation of packed-column flood points was developed by Sherwood, Shipley, and Holloway [Ind. Eng. Chem., 30, 768 (1938)] on the basis of laboratory measurements primarily on the air-water system. 

Columns, packed (GC) 36, 107 evaluation 130 glass 129 etal 129 phase loading 129 plastic 130 preparation 127 stationary phases 108 supports 118 Coluans,packed (LC) 63... 

In many industrial applications of packed columns, it is desirable to know the volumetric hold-up of the liquid phase in the column. This information might be needed, for example, if the liquid were involved in a chemical reaction or if a control system for the column were being designed. For gas-liquid systems the hold-up of liquid Hw for conditions below the loading point has been found(48) to vary approximately as the 0.6 power of the liquid rate, and for rings and saddles this is given approximately by ... 

The most popular method used is a dynamic method, the saturation method. In this technique, the non-volatile, heavy solid solute is loaded into a saturator, or a battery of two or more saturators connected in series, and remains there as a stationary phase during the experiment. In most cases the saturator is in the form of a packed column. At constant pressure, a steady stream of supercritical fluid (solvent) passes through a preheater, where it reaches the desired system temperature. Then this fluid is continuously fed to the bottom of the saturator, and the solute is stripped from the stationary heavy phase in the column. The supercritical fluid saturated with the solute leaves the saturator at the top. 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

FIGURE 11-2. Separation of mixture of phthalates by GPC on a 100-A Ultrastyragel column. Analytical loadings are shown in (a) and preparative loadings are shown in (b). Mobile phase THF. Flow rate 1 mL/min. Detector UV at 254 nm, 0.5 AUFS in (a) and 2.0 AUFS in (fe) RI at X8 in (a) and X64 in (b). Sample dioctyl, dibutyl, and dimethyl phthalates with 50 /xL injection in (a) and 100-/xL injection in (b). (Note Actual separation will depend upon the quality of the mobile phase and column packing). 

Packed columns, made of glass or metal, are 1 to 3 m long, with a 2- to 4-mm id. Those used for analysis typically have liquid phase loadings of about 5% (w/w) on a solid support. 

In order to obtain a commercial loading of the near-critical extractant, the extraction is sometimes carried out at enhanced pressures in the droplet regime. In such cases the liquid phase does not flow downwards as a film adhering to the packings of a column as is usually assumed, rather it falls down as a swarm of droplets. On the basis of the separation of a mixture of partial glycerides the behavior of packed columns in the droplet regime (instable flowing films) the efficiency of different column installations are compared. A mixture of 55 wt.% propane and 45 wt.% carbon dioxide is used as an extractant. 

Methodology Prior to experimentation the stationary phase constmct is prepared and poured into the column. Notably, the columns used for affinity chromatography tend to be shorter than those used for other column chromatography methods. Once the stationary phase construct has settled (packed) in the column, a loading buffer is passed through to equilibrate the system before addition of loading buffer... 

Another problem, that of adsorption at any of the several interfaces (such as gas-Uquid and gas-solid), may arise with minute samples. Martin found gas-chromatographic retention volumes to be markedly affected by such adsorptions and in some cases to be the cause of a larger contribution to retention volume than solution in the bulk stationary phase. He both measured and calculated the extent of such adsorptions for a variety of solutes. Adsorption at a gas-liquid interface is serious for polar stationary phases when surface areas are relatively high, when the ratio of stationary phase to solid support is low, and when the temperature is low. The effects of adsorption are minimized by the use of relatively highly loaded columns and nonpolar stationary phases and by avoiding solute-stationary phase pairs in which the infinite-dilution activity coefficients deviate markedly from unity. Ottenstein indicated that adsorption on a liquid surface can be considered negligible in packed columns when liquid loadings exceed 5% and activity coefficients are less than 10. 

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