Enrichment selection techniques

If the micro-organism has been obtained from a culture collection or from a researcher in another laboratory, a recommended medium to promote good growth will normally be suggested. If, on the other hand, the micro-organism has been obtained by an enrichment selection technique, then the enrichment medium itself will self-evidently support growth. 

In vitro selection is a combinatorial approach in which functional molecules are selected from large libraries of randomized RNAs or DNAs by selection techniques that are suitable for the enrichment of a particular property such as the binding to a target molecule or a particular catalytic activity (Fig. 5). 

Because homologous recombination is a rare event, it is extremely difficult and forbidding to identify and isolate cells which undergo such events by standard screening methods. A number of vectors and selection techniques have been developed for the enrichment and identification of the cell containing homologous recombination events. ES cells into which vector DNA is introduced generally by electroporation are selected for the presence of the vector by virtue of its ability to confer resistance to antibiotics such as... 

In addition to selecting naturally occurring microbial cultures that possess a desired metabolic trait, it is also possible to use enrichment culture techniques to develop microbial cultures with unique biochemical traits. The substrate range of enzymes catalyzing a certain reaction can be expanded through the use of enrichment culture techniques. 

A commonly used system in environmental analysis is the heart-cutting technique which uses the separation power of the first column to obtain a higher selectivity than with the previously described precolumn enrichment. The two columns are coupled via a switching valve, as shown in Figure 13.5. 

When a first column of a very short length (and therefore a low selectivity) is used (this is especially suitable for multiresidue methods), we talk about an on-line precolumn (PC) switching technique coupled to LC (PC-LC or solid-phase extraction (SPE)-LC). This is particulary useful for the enrichment of analytes, and enables a higher sample volume to be injected into the analytical column and a higher sensitivity to be reached. The sample is passed through the precolumn and analytes are retained, while water is eliminated then, by switching the valve, the analytes retained in the precolumn are transferred to the analytical column by the mobile phase, and with not just a fraction, as in the previous cases. 

Since natural Au consists solely of Au, the interface-selective enrichment technique cannot be applied in Au studies. The absorber thickness for Au is required to be large and therefore multilayered samples of Au layers/3r/ metal layers have to be prepared. The spectra for Au/Fe with varying Au-layer thickness are shown in Fig. 7.83 [437]. The results were interpreted as follows large magnetic hyperfine fields at Au sites exist only within two monolayers at the interface region, which are supposed to be induced by direct coupling with anti-ferromagnetically oriented Fe 3d atoms. 

In summary, the development of materials for the extraction of pesticides from water samples has progressed from simple liquid-liquid extraction for the principal active compound to sophisticated SPE media capable of exclusively trapping the target pesticide and metabolites selectively. The development of alkyl bonded phase silica cartridges and extraction disks combined with on-line extraction techniques is currently the principal means used for the extraction and trace enrichment of pesticides and metabolites from water. 

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