Solvent presaturation

The number of crosscurrent stages N that are required to reach a specified raffinate composition, in Bancroft coordinates can be calculated direc tly if K is constant, the ratio of extraction solvent to feed solvent S /F is kept constant, and fresh extraction solvent Y, = 0 (presaturated with feed solvent) is used in each stage [Eq. (15-9)]. 

This equation, although originating from the plate theory, must again be considered as largely empirical when employed for TLC. This is because, in its derivation, the distribution coefficient of the solute between the two phases is considered constant throughout the development process. In practice, due to the nature of the development as already discussed for TLC, the distribution coefficient does not remain constant and, thus, the expression for column efficiency must be considered, at best, only approximate. The same errors would be involved if the equation was used to calculate the efficiency of a GC column when the solute was eluted by temperature programming or in LC where the solute was eluted by gradient elution. If the solute could be eluted by a pure solvent such as n-heptane on a plate that had been presaturated with the solvent vapor, then the distribution coefficient would remain sensibly constant over the development process. Under such circumstances the efficiency value would be more accurate and more likely to represent a true plate efficiency. 

The reaction starts spontaneously and is mildly exothermic. Moderating the temperature by use of a water bath diminishes the amount of bromine and product carried off by the carbon dioxide evolved. The reaction can be followed by use of a tetra-chloroethane bubbler, and at the end of the reaction the solvent in the bubbler can be used to wash the mercuric bromide. The checkers followed the reaction with a wet test meter presaturated with carbon dioxide 52-60% of the theoretical amount of carbon dioxide was evolved. 

Presaturation Selective irradiation of a nucleus prior to application of a nonselective pulse causes it to be saturated, so its resonance is suppressed. This technique may be employed for suppressing solvent signals. 

MHz H NMR spectra were measured on solutions of ca 25 mg pectin/0.5 ml D2O on a Bruker AMX600 NMR spectrometer. The temperature was 350 K to diminish the viscosity of these solutions and 32 scans were measured. Solvent suppression was performed using presaturation during the recycle delay. 

Pre-saturation In this technique prior to data acquisition, a highly selective low-power rf pulse irradiates the solvent signals for 0.5 to 2 s to saturate them. No irradiation should occur during the data acquisition. This method relies on the phenomenon that nuclei which have equal populations in the ground and excited states are unable to relax and do not contribute to the FID after pulse irradiation. This is an effective pulse sequence of NOESY-type pre-saturation that consists of three 900 pulses RD - 900 - tx - 900 - tm - 90° - FID, where RD is the relaxation delay and t and tm are the presaturation times. 

Fig. 7a-e Proton spectra of 1 a Dissolved in CDC13 b in D20 c in D20/H20 d with presaturation of the water signal e with presaturation using a digital filter. Signals marked with are due to an impurity (solvent from recrystallization of 1)... 

Quininone, the most readily available member of the series, was used for the autoxidation studies. The Doering autoxidation procedure,4 that employs only tert-BuOH, was modified to include a THF tert-BuOH (4 1) mixture as the solvent. Likewise, the pressurized Parr bottle setup as described4 was replaced with a simple subsurface gas addition the solvent was presaturated with O2 gas, (compressed air could also be used as the O2 source) followed by t-BuOK addition and continued O2 gas purge. The autoxidations could likewise be conducted in the presence of ethanol or methanol, thereby producing the corresponding ethyl or methyl esters. Formation of these esters could occur via the reactive intermediate bicyclic lactam.4... 

Standard onedimensional NMR One-dimensional NMR — simple one-pulse experiment, typically with presaturation of solvent during the recycle delay with a weak RF field To quantify small molecules To identify some simple small molecules... 

Figure 6.36 500 MHz H NMR spectra obtained during a stop-flow LC-NMR experiment on a 1 mg injection of a crude sample of a drug compound, (a) LC chromatogram, (b) spectram corresponding to the parent bulk drug compound acquhed for 64 transients and (c) to the impurity peak RRT 0.87 (—3% by area), acquhed for 1024 transients. NOESY-type presaturation was used to suppress the solvent resonances. Bruker DRX500 H/ C 4-mm z-gradient probe with a 120 pi active cell volume.

No reaction was observed in the anhydrous solvents the solvents were presaturated with distilled water at room temperature. 

For difficult separations, multiple extractions are frequently carried out, although in many cases the background is also coextracted. Using multiple extractions, polar interferences may sometimes be transferred from the aqueous into organic solvents that can dissolve minute amounts of water. This problem cannot be eliminated by simple presaturation of the extraction solvent but only by washing the extract with small amounts of water (58). Another relevant issue to be considered in trace residue analysis concerns the purity of the organic solvents, since they can introduce solvent impurities into the sample extract. Therefore, the need for high solvent purification should not be overlooked in some applications. 

The y, -double bond of a linear dienone is selectively reduced over palladium in the presence of a trace of base.15,62,85,86,137 Presaturation of the catalyst with hydrogen enhances the selectivity in the reaction.137 In the absence of base, the use of strontium carbonate or barium sulfate as a support for the palladium is preferred.56,57 A benzene solvent is also beneficial.78... 

The principle of presaturation relies on the phenomenon that nuclei which are unable to relax, because their population in the ground state a and the excited state (3 is the same, do not contribute to the free induction decay after pulse irradiation. Prior to data acquisition, a highly selective low-power pulse irradiates the desired solvent signals for 0.5 to 2 s, thus leading to saturation of the solvent signal frequency. During data acquisition, no irradiation should occur. NOESY-type presaturation is an effective pulse sequence of presaturation. The pulse sequence consits of three 90° pulses (similar to the first increment of a NOESY experiment) ... 

Here, presaturation is performed with the use of shaped pulses, which have a broader excitation profile. This method is therefore better suitable for the suppression of multiplets. The advantages of this technique are that it is easy to apply, easy to implement within most NMR experiments, and multiple presaturation is possible, and that it is very effective. The disadvantages are that transfer of saturation can occur (in aqueous solutions) to slowly exchanging protons that would be detectable without saturation. Another drawback is that spins with resonances close to the solvent frequency will also be saturated and 2D cross peaks will be absent. 

The choice of the suppression method to be used depends on both the solvent and sample characteristics. Therefore, HPLC-NMR suppression via presaturation... 

The most commonly used solvent suppression technique is presaturation. Modern NMR spectrometers are normally equipped with at least two independent frequency channels. Using these, there are usually no problems in conducting experiments where two solvents are presatureated at the same time. However, the number of solvents which can be suppressed is limited by the available hardware. 

It would be impossible to handle ternary solvent systems such as acetonitrile/ water/methanol or mixtures which contain further additives such as triethyla-mine. Therefore most techniques nowadays use modulated shaped pulses for multiple presaturation or pulse frequency generation (PFG) techniques. With basic hardware, it is then possible to suppress an arbitrary number of solvents. 

This main difficulty in coupling HPLC to NMR spectroscopy is faced by methods known as solvent suppression techniques, where the large solvent signals are reduced by special pulse sequences, switched prior to the information-selecting and acquisition pulses. Therefore, many efforts have been made to develop effective and minor-disturbing pulse sequences, such as presaturation, zero excitation and PFG-pulse sequences (WET) (see Chapter 1 and the following chapters). Despite the possibility of also suppressing several of the... 

H and 19F NMR spectra are recorded with a normal one-pulse sequence or, alternatively, the XH spectra are recorded with a sequence that allows simultaneous solvent suppression with presaturation (31) or a sequence that includes some other method of suppression 13C 1H and 1P 1H spectra are recorded with proton broadband (composite pulse) decoupling (32), and 31P spectra with gated proton decoupling (33). 

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