Column dimensions high-speed

Column dimensions depend on the intended use and the most frequent commercial configurations are listed in Table LI. Generally, the column plate number, the pressure drop across the column and the separation time at a constant flow rate are directly proportional to the column length. The allowed sample amount which can be separated without column overloading increases with the second power of the column diameter as does the flow rate and the consumption of the mobile phase at a constant flow rate. Most separations are performed on conventional analytical columns, 10-25 cm long, 3-4.6 mm in diameter, packed with 5 pm (less frequently 3, 7 or 10 pm) particles. With so-called high-speed columns of the same diameter, but 3-6 cm long, simple separations can be accomplished in 1-3 min so that the productivity of the laboratory is considerably increased and solvent consumption per analysis reduced. 

There have been several approaches to overcome the traditionally slow SEC separations, which are caused by the diffusion processes in SEC columns. Most of them are column-related (see High-Speed SEC Columns, Small Particle Technology, and Smaller SEC Column Dimensions ) one utilizes the column void volume (cf. Overlaid Injections ), while another replaces separation with simplified sample preparation (see Flow Injection Analysis ). Cloning existing methods and instrumentation is also reviewed with respect to the potential time gain (see Cloning of SEC Systems ). Benefits and limitations of each method are summarized in Table 1. 

Figure 32-7 High-speed isocratic separation. Column dimensions 4-cm length, 0.4 cm inside diameter packing 3-p.m spherisorb mobile pha.se 4.1% ethyl acetate in n-hexane. Compounds ...

The high speed of separation and detection of the whole-column cIEF makes it attractive as a second separation dimension in a two-dimension (2-D) separation scheme. For example, using a 1-cm-long separation column, the lEF separation, and detection of the separated zones can be performed in 30 s [51]. As the second separation dimension, the whole-column detection cIEF has been successfully coupled to gel filtration chromatography [52]. The whole-column detection cIEF can also be coupled with other CE separation dimensions [53]. 

FIGURE 5.21 High-speed isothermal separation of a 20-component mixture using a pressure-tunable column ensemble. The plots of band position versus time were obtained from the retention factors for the mixture components on the two separate colimms along with the column dimensions and the inlet, outlet and junction point pressures. A window diagram was used to determine the junction point pressure for the complete separation of the first 14 components. Compounds are 1, n-pentane 2, methyl alcohol 3, 2,2-dimethylbntane 4, 1,1,1-trichloroethane 5, cyclopentane 6, w-hexane 7, w-propyl alcohol 8, cyclohexane 9, benzene 10, n-heptane 11, l,2-dichloropropane 12, toluene 13, n-bntyl alcohol 14, w-octane 15, 2-hexyl alcohol 16, n-pentyl alcohol 17, ethylbenzene 18, m-xylene 19, w-nonane 20, o-xylene. 

The operation of the interface between the both columns, called the modulator, is key to GCxGC. The modulator operates in an alternating trap and inject mode with a modulation frequency related to the fast second-dimension separation. Short elution sections from the first column are integrated by cold trapping and injected for the second-dimension separation. Each injection pulse generates a high speed chromatogram (Bertsch, 1999 Beens et al., 2001). 

Because HPLC and HPCE are based on different physico-chemical principles, HPCE may be expected to address areas in which HPLC has shortcomings [884]. One such area is time of separation. In terms of speed of analysis, selectivity, quantitation, methods to control separation mechanism, orthogonality, CE performs better than conventional electrophoresis and varies from HPLC (Table 4.49). CE has very high efficiency compared to HPLC (up to two orders of magnitude) or GC. For typical capillary dimensions 105—106 theoretical plates are common in CE compared to 20 000 for a conventional HPLC column and... 

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