Theoretical plates enhancement

When packed into chromatography columns, TRIM beads imprinted with Boc-L-Phe were shown to have column efficiencies and separation abilities superior to ground and sieved bulk material [5]. The theoretical plate number was approximately double that obtained with conventional crushed polymer under the same conditions and the resolution of a racemate was also slightly enhanced. The difference, however, was not that great considering the additional preparation time and effort involved. 

We have developed an LC-based sensor which relies upon the displacement of a drug-dye conjugate from a CAP-imprinted polymer upon application of samples containing the original template molecule (Fig. 20.15). This is essentially a competitive binding assay in a flow injection format. The increased number of theoretical plates available during the separation serves to greatly enhance the selectivity of the assay. 

The number of theoretical plates and, thus, the separation efficiency, is determined by the length of the separator column. If two separator columns are used in series, the resulting enhancement of separation efficiency leads to a better resolution between ions with similar retention characteristics, with a corresponding increase in retention times. The separator column length also determines the exchange capacity. An increase of the exchange capacity via elongation of the separator column is recommended in all cases where the ion to be analyzed is present in an excess of another component. 

Subsequent important contributions to the field of microchip CE were made early on by Jed Harrison and Andreas Manz, who reported a spectacular enhancement of device performance byredesigning the channel layout and increasing the applied electric fields [11], With an imposed field of 1 kV cm , six fluorescence-tagged amino acids were separated with baseline resolution in 15 s, generating 40 000 to 75 000 theoretical plates. In the improved design, the separation channel length was reduced from 13 to 2.2 cm, utilizing the same cross section as in the previous work. 

The smaller particle size used in CEC compared to LC, together with the beneficial effects of EOF on the A and C terms of the van Deemter equation, result in separation efficiencies for CEC which exceed those of LC by a factor of 2-5. Efficiencies of 200 000-500 000 theoretical plates per meter are typical for CEC. The two main beneficial effects of EOF, namely a more uniform flow profile across the column bed, and an enhanced mass transfer, are explained in more detail in the paragraphs below. 

The separation characteristics of a considerable variety of other TLC supports were also tested using different dye mixtures (magnesia, polyamide, silylated silica, octadecyl-bonded silica, carboxymethyl cellulose, zeolite, etc.) however, these supports have not been frequently applied in practical TLC of this class of compounds. Optimization procedures such as the prisma and the simplex methods have also found application in the TLC analysis of synthetic dyes. It was established that six red synthetic dyes (C.I. 15580 C.I. 15585 C.I. 15630 C.I. 15800 C.I. 15880 C.I. 15865) can be fully separated on silica high-performance TLC (HPTLC) layers in a three-solvent system calculated by the optimization models. The theoretical plate number and the consequent separation capacity of traditional TLC can be considerably enhanced by using supports of lower particle size (about 5 fim) and a narrower particle size distribution. The application of these HPTLC layers for the analysis of basic and cationic synthetic dyes has also been reviewed. The advantages of overpressured (or forced flow) TLC include improved separation efficiency, lower detection limit, and lower solvent consumption, and they have also been exploited in the analysis of synthetic dyes. 

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