Mixed-phase packed columns

Maximum gas and liquid throughput rates of packed columns depend on the type of packing and packing geometry, the relative free void fraction and the physical properties of the mixed phases. The column becomes flooded beyond a certain volumetric flux of the vapor at a given flux of the reflux. A controlled counterflow of the phases then does not exist the separation efficiency of the column is reduced drastically. 

Packed columns Porous and non-porous silica Lipophilic modified silica gels, e.g., C18 Mixed mode phases, e.g., C8/SCX ... 

Elimination of coextracted materials and concentration of tetracyclines have also been accomplished using mixed-phase extraction membranes with both re-versed-phase and cation-exchange properties (294,295), or solid-phase extraction columns packed with cation-exchange materials such as CM-Sephadex C-25 (301), aromatic sulfonic acid (310), and carboxylic acid (283, 300). For the same purpose, metal chelate affinity chromatography has also been employed. In this technique, the tetracyclines are specifically absorbed on the column sorbent by chelation with copper ions bound to small chelating Sepharose fast flow column (278-281, 294-296). 

The increased ability of capillary GC to resolve organic species caused an unanticipated drawback for broad spectrum analysis. The amount of stationary phase on a capillary GC column is much less than on a packed column. This condition increases the likelihood of observing a decrease in chromatographic performance caused by the sample matrix. The altered stationary phase may cause reduced precision of retention times and peak areas. The changes in the chromatographic performance of the stationary phase are measured by the Grob general purpose test mix (9). 

The use of mixed phase columns in the gas chromatographic determination of estriol is based on earlier observations in our laboratory that detector response was greater from mixed phase columns than it was from single phase packings. This is illustrated in Figure 10.3. 

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. 

Fig. 4.6. Dependence of EOF linear velocity on pH of the mobile phase using a CEC column packed with a Mixed Mode stationary phase. Column 25 cm Hypersil Mixed Mode packed capillary. Mobile phase 75Q McCN-.50 mM Na2HP04. Voltage 20 kV. Marker thiourea.

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