Stationary port technique

This technique is known as the stationary port technique since the feed solution and the desorbent solution are always added at the same port and the product streams and the recycle stream are always removed from another port. Technichem and Finn Sugar manufacture chromatography systems which utilize the stationary port technique. 

Different sections of the column serve as a specific zone during the cycle operation. Unlike the stationary port technique, the liquid flow is not uniform throughout the column. Because of the variations in the additions and withdrawals of the different fluid streams, the liquid flow rate in each of the zones will be different. 

The flow rate and the pressure drop per unit length of the chromatographic column are much lower for the stationary port compared to the moving port system. Also, the moving port system is much less capital intensive. The moving port technique, however, is calculated to require only one-third of the column volume and ion exchange volume and two-thirds of the desorbent volume compared to the stationary port technique. 

Alain Berthod received his PhD in 1979 from the University of Lyon. He took an assistant professor s position at the French National Center for Scientific Research (CNRS) working in electrochemistry. In 1983 he was promoted as associate professor and in 1993 as research director. He focused on the developing and the use of micellar solutions and microemulsions in chromatography. His interests lie in the separation of chiral molecules and enantiorecognition mechanisms. He has contributed to the development of the countercurrent chromatography technique that uses a sup-port-free liquid stationary phase. He was member of the editorial board of major analytical chemistry and chromatography journals. He is editor-in-chief of Separation Purification Reviews (Taylor Francis, Philadelphia, Pennsylvania). 

Among the above-mentioned methods, SPME is relatively new. It combines sample preparation and injection of the sample into one step. Analytes are adsorbed on a polymeric fiber coated with a stationary phase such as polydimethylsiloxane, which is thermally desorbed in the injection port at 250 °C. The successful use of the technique for the GC/MS analysis of the nerve agents in water has been described (42). Levels of less than... 

Injection ports for packed columns are aligned so that the sample can be deposited on a heated surface just before the column or directly on the end of the column. In the first instance, the injection port is heated above the boiling point of the sample in order to get rapid volatilization, but for on-column use, the injection port is kept at column temperature. On-column injection is usually preferred because there is less chance of decomposition and the sample is not exposed to a high injection port temperature. Remember also that a typical GC analyte will have a retention ratio of 0.25 or much less. This means that 75% or more of it is sorbed in the stationary phase, and that is where the on-column technique puts it—in the stationary phase. It is probably for this reason that on-column injection is so efficient. The use of large volumes of solvents will wash... 

Gas chromatography (GC) is a chromatographic technique that is used to separate volatile organic compounds. A gas chromatograph consists of a mobile (gas) phase, an injection port, a separation column containing the stationary phase, and a detector. The... 

Another useful technique is solid phase microextraction. A fused silica fibre is attached to the base of a syringe with a fixed metal needle. The fibre is coated with a thin layer of stationary phase that is selective for the analytes of interest. The fibre is dipped into the liquid sample or into the headspace above the liquid for a period of time, allowing a fraction of the analyte to be extracted into the fibre. The fibre is then retracted into the syringe and the syringe injected into the injection port where the analyte is thermally desorbed from the fibre into the GC. 

Exactly the same process takes place as that in the Hurrel system but, in effect, the valving makes the columns appear to move instead of the packing. Part of the feed moves with the mobile phase and is collected by a small take-off flow in front of the feed port (B + solvent). The other, more retained portion of the sample, accumulates in a column on the other side of the feed port and is collected by a another small take-off flow behind the feed port (A + solvent). This particular system ideally, produces two products and thus lends itself specifically to the separation of enantiomeric pairs. However, for effective separation with high purity yields, the stationary phase capacity for the two enantiomers must be fairly large and thus the phase system must be carefully selected. The technique has been successfully used to isolate single enantiomer drugs [15-17]. 

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