Receiver phase

Another possibility of constructing a chiral membrane system is to prepare a solution of the chiral selector which is retained between two porous membranes, acting as an enantioselective liquid carrier for the transport of one of the enantiomers from the feed solution of the racemate to the receiving side (Fig. 1-5). This system is often referred to as membrane-assisted separation. The selector should not be soluble in the solvent used for the elution of the enantiomers, whose transport is driven by a gradient in concentration or pH between the feed and receiving phases. As a drawback common to all these systems, it should be mentioned that the transport of one enantiomer usually decreases when the enantiomer ratio in the permeate diminishes. Nevertheless, this can be overcome by designing a system where two opposite selectors are used to transport the two enantiomers of a racemic solution simultaneously, as it was already applied in W-tube experiments [171]. 

In the classical set-up of bulk liquid membranes, the membrane phase is a well-mixed bulk phase instead of an immobilized phase within a pore or film. The principle comprises enantioselective extraction from the feed phase to the carrier phase, and subsequently the carrier releases the enantiomer into the receiving phase. As formation and dissociation of the chiral complex occur at different locations, suitable conditions for absorption and desorption can be established. In order to allow for effective mass transport between the different liquid phases involved, hollow fiber... 

Table 3. Effect of acid in receiving phase on the K+ transport...

Scan Pulse Sign A Sign B Receiver phase code Receiver mode Pulse phase A B... 

Figure 13 Timing diagram for the clean HMBC experiment with an initial second-order and terminal adiabatic low-pass 7-filter.42,43 The recommended delays for the filters are the same than for a third-order low-pass J filter. <5 and 8 are gradient delays, where 8 — <5 + accounts for the delay of the first point in the 13C dimension. The integral over each gradient pulse G, is H/2yc times the integral over gradient G2 in order to achieve coherence selection. The recommended phase cycle is c/)n = x, x, x, x 3 — 4(x), 4(y), 4( x), 4(—y) with the receiver phase c/)REC = x, x.

Figure 22 Pulse sequence of the HMBC-RELAY experiment. Filled and open bars represent 90° and 180° pulses, respectively. All other phases are set as x, excepted otherwise stated. A two-phase cycle x, —x is used for the pulse phases (j>, and

Typically, transport experiments have been performed using a U-tube apparatus in which a solvent such as chloroform, containing the macrocyc-lic carrier, is placed in the tube so that it separates two aqueous phases the source phase containing the metal ion(s) to be transported and the receiving phase into which the transported ions are deposited. A diagrammatic representation of a liquid membrane system is shown in Figure 9.4. 

Other factors influencing the rate of metal-ion transport across artificial membranes have been identified. As might be expected, such transport is dependent on the interplay of several factors. For example, as briefly mentioned already in Chapter 4, it is clear that the strength of complex-ation of the cation by the carrier must be neither too high nor too low if efficient transport is to be achieved. If the stability is low, then uptake of the metal ion from the source phase will be inhibited. Conversely, for those cases where highly stable complexes are formed, there will be a reluctance by the carrier to release the cation into the receiving phase. 

In practice, a particular phase cycle is defined by means of an array of RF pulse settings (to be used cyclically during consecutive scans) and an associated array of receiver phases . The receiver phase , however, does not correspond to any hardware device setting. Rather, it is an interlocution for the various modes of how each single-scan signal should be handled by the data accumulation procedure (add, subtract, quad add, quad subtract, etc.). 

Improved control of experimental conditions. Phase detection prevents the operator from straying from resonance, makes possible reliable offset estimates and receiver phase estimates and even completely automated maintenance of optimal offset settings. 

There are various approaches to the data-reduction task. An often used one consists of computing the modulus of the complex phase-detector signal. This removes all offset imperfections as well as any receiver phase misad-justment, bringing us theoretically to what we would have by summing the outputs of two independent, ideal diode detectors. In this case, however, the original signals are still available and can be used to check various aspects of data quality, carry out additional corrections (such as removal of noise-rectification artifacts), or submitted to alternative evaluation algorithms. 

A minimum two-step phase cycle was used by inverting the selective 90° pulse and the receiver phases on alternate scans. Exorcycle could also be applied on all 180° pulses. 

Liquid-phase adsorption using GAC is one of the most widely used remediation technologies. The major disadvantage of this approach is that it simply transfers contaminants from one phase to another, and further treatment or disposal of the receiving phase is typically required. Biological treatment has the potential to completely destroy contaminants, and it is generally... 

F-Feed phase, M-Membrane phase R-Receiving phase F/M, M/R Interfaces... 

In the early 1970s Li [13] proposed a method that is now called Emulsion (surfactant) Liquid Membrane (ELM) or Double Emulsion Membrane (DEM) (Fig. 3). The name reveals that the three liquid system is stabilized by an emulsifier, the amount of which reaches as much as 5 % or more with respect to the membrane liquid. The receiving phase R, which usually has a smaller volume than the donor solution, F of similar nature, is finally dispersed in the intermediate phase, M. In the next step the donor solution F is contacted with the emulsion. For this purpose, the emulsion is dispersed in the donor solution F by gentle mixing typically in a mixer-settler device. After this step, the emulsion is separated and broken. The enriched acceptor solution is further processed and the membrane liquid M is fed back for reuse. 

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