Transfer of fractions

According to the treatment of Hinze et al. (1963), the charge transfer obeys the equality principle outlined in equation 1.31 (section 1.7). The transfer of fractional charges a and for two atoms A and B results in a stable bond with a fractional ionic character ... 

Most of the frequently used comprehensive 2D LC systems employ a microbore HPLC column in the first dimension, operated at low flow rate, both under isocratic and gradient conditions. This enables the transfer of fractions of small volume via the multiport valve equipped with two identical... 

On the other hand, the ability of a semiconductor to donate or accept electrons is uniquely related to the energy of the electron in its Fermi level EF. The transfer of fractional charge 8e can be viewed as a redox equilibrium between the dopant and the matrix in which the role of the electron donor and of the electron acceptor is relative and governed by the difference between % and EF, respectively. 

Setting up a 2D chromatographic separation system is actually not as difficult as one might first think. As long as well-known separation methods exist for each dimension [8], the experimental aspects can be handled quite easily in most cases. Off-line systems just require a fraction collection device and something or someone who reinjects the fractions into the next chromatographic dimension. In on-line 2D systems, the transfer of fractions is preferentially done by automated injection valves, as was proposed by Kilz et al. [9]. 

The importance of transfer operations in multiple chromatographic steps has been emphasized above. They are very costly in time directly, and also indirectly because of the lai e losses that are usually entailed, particularly in isotopic-labeling methods (see, e.g., Bl, Rl, R2). In this connection one would emphasize the value of simple elution and running-up methods for the transfer of fractions or extracts to and from chromatograms (B27, B29, B33a, G7). 

As discussed in previous chapters, the hardware for GCxGC is nowadays well developed (e.g., see Chapter 4), and reliable modulators are commercially available from various manufacturers (Chapter 2). For automated transfer of fractions from the LC to the GCxGC, we have recently described two approaches, one based on a six-port switching valve and a second one based on a syringe [16]. The two interfaces are shown schematically in Figure 3. 

Repeated cycle 3 ready down moving ready hold Transfer of fraction into injector... 

FIGURE 21 Schematic representation of 2D-LC principle. P 1 and P 2 are pumps, D 1 and D 2 are detectors. Id- and 2d-columns employ distinct retention mechanisms. TF stands for procedures and devices that ensure transfer of fractions among columns. For detailed e q)lanation, see the text. 

The important issue of 2D-LC represents the abovementioned transfer of column effluent between the Id and the 2d columns, which can be done either off-line or online. In the off-hne approach, the fractions from the Id column are collected and successively re-injected into the 2d column. In this case, the unit TF is just a fraction collector. The macromolecules within particular fractions from the Id column are immixed so that resulting overall separation selectivity may be challenged. Moreover, entire procedure is laborious and slow. Various approaches were elaborated for the online transfer of fractions from the Id column into the 2d column. Often, the fractions from the Id column are cut into small parts that are one-by-one gradually transported into the 2d SEC column for independent characterization. This is the method of choice if the first-dimension separation produces broader peaks, such as it does liquid chromatography under critical conditions of enthalpic interactions, LC CC (see section 11.8.3). The operation principle of such chop-and-reinject method is evident from Figure 22. In this case, the TF unit from Figure 21 is a switching valve. 

On-line multicolumn preffactionation is an efficient but complex method of isolating fractions for HPLC analysis [I]. Switching devices can implement the transfer of fractions to other extraction columns or to an analytical column. Each solvent used for extraction must be compatible with the subsequent step. Such online procedures minimize sample loss from manual manipulations. On-line multi-column techniques are most effective for high-volume repetitive procedures where the time involved in method development is inconsequential due to throughput. If cross-contamination must be avoided, columns or cartridges must be discarded after each use In other cases, regeneration is possible. 

The transfer of fractions from the first to the second dimension can be performed either in the off- or on-line mode both techniques are widely employed and both present distinct pros and cons. In off-line methods (LC—LC), fractions from the first chromatographic step are collected via a fraction collector, separated from the solvent by evaporation, redissolved, then reinjected onto the second column. The most striking advantage of such an approach derives from the great flexibility in coupling any separation modes in terms of mobile phases and buffer compatibility, time, and duration. The separation power can be maximized by independent optimization of each dimension, while sensitivity can be tuned by regulating the sample concentration injected in both dimensions. 

Figure 1.4. Temperature dependence of the change in Gihhs energy, enthalpy and entropy upon transfer of ethane and butane from the gas phase to water. The data refer to transfer from the vapour phase at 0.101 MPa to a hypothetical solution of unit mole fraction and are taken from ref. 125.

The physical process of Hquid—Hquid extraction separates a dissolved component from its solvent by transfer to a second solvent, immiscible with the first but having a higher affinity for the transferred component. The latter is sometimes called the consolute component. Liquid—Hquid extraction can purify a consolute component with respect to dissolved components which are not soluble in the second solvent, and often the extract solution contains a higher concentration of the consolute component than the initial solution. In the process of fractional extraction, two or more consolute components can be extracted and also separated if these have different distribution ratios between the two solvents. 

In hydrological studies, the transfer of water between reservoirs is of primary interest. The magnitudes of the main reservoirs and fluxes (volume per time) are given in Figure 7. The oceans hold ca 76% of all the earth s water. Most of the remainder, ie, ca 21%, is contained in pores of sediments and in sedimentary rocks. A Httle more than 1% (or 73% of freshwater) is locked up in ice. The other freshwater reservoir of significant size is groundwater. Lakes, rivers, and the atmosphere hold a surprisingly small fraction of the earth s water. 

Equations (13-111) to (13-114), (13-118) and (13-119), contain terms, Njj, for rates of mass transfer of components from the vapor phase to the liquid phase (rates are negative if transfer is from the liquid phase to the vapor phase). These rates are estimated from diffusive and bulk-flow contributions, where the former are based on interfacial area, average mole-fraction driving forces, and mass-... 

Stripping or desorption is the transfer of gas, dissolved in a liquid, into a gas stream. The term is also applied to that section of a Fractionating column below the feed plate. 

To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

Consider a lean phase, j, which is in intimate contact with a rich phase, i, in a closed vessel in order to transfer a certain solute. The solute diffuses from the rich phase to the lean phase. Meanwhile, a fraction of the diffused solute back-transfers to the rich phase. Initially, die rate of rich-to-lean solute transfer surpasses that of lean to rich leading to a net transfer of the solute from the rich phase to the lean phase. However, as the concentration of the solute in the rich phase increases. 

A A multistage extraction column uses gas oil for the preliminary removal of phenol from wastewater. The flowrate of wastewater is 2.0 kg/s and its inlet mass fraction of phenol is 0.0358. The mass fraction of phenol in the wastewater exiting the column is 0.0168. Five kg/s of gas oil are used for extraction. The inlet mass fraction of phenol in gas oil is 0.0074. The equilibrium relation for the transfer of phenol from wastewater to gas oil is given by... 

Transfer of an LC fraction of 300 jl1 volume occurred by the conventional retention gap technique. In fact, Figure 2.3fb) shows the GC chromatogram obtained after the transfer of the LC fraction. 

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