Phase addition

Two mechanisms for chiral separations using chiral mobile-phase additives, analogous to models developed for ion-pair chromatography, have been... 

An alternative model has been proposed in which the chiral mobile-phase additive is thought to modify the conventional, achiral stationary phase in situ thus, dynamically generating a chiral stationary phase. In this case, the enantioseparation is governed by the differences in the association between the enantiomers and the chiral selector in the stationary phase. 

High Performance Liquid Chromatography. Although chiral mobile phase additives have been used in high performance Hquid chromatography (hplc), the large amounts of solvent, thus chiral mobile phase additive, required to pre-equiUbrate the stationary phase renders this approach much less attractive than for dc and is not discussed here. 

TaF has been characterized by ir, Raman, x-ray diffraction, and mass spectrometry (3,11,12). TaF has been used as a superacid catalyst for the conversion of CH to gasoline-range hydrocarbons (qv) (12) in the manufacture of fluoride glass and fluoride glass optical fiber preforms (13), and incorporated in semiconductor devices (14). TaF is also a catalyst for the Hquid-phase addition of HF to polychlorinated ethenes (15). The chemistry of TaF has been reviewed (1,16—19). Total commercial production for TaF is thought to be no more than a few hundred kilograms aimuaHy. 

Fluidized-Bed Vinegar Reactors. Intimate contact of air A.cetohacter is achieved in fluidized-bed or tower-type systems. Air introduced through perforations in the bottom of each unit suspends the mixture of Hquid and microorganisms within the unit. Air bubbles penetrating the bottom plate keep Jicetobacter m. suspension and active for the ethanol oxidation in the Hquid phase. Addition of a carrier for the bacterial ceUs to the Hquid suspension is reported to improve the performance (58—60). 

Immobilization. The abiUty of cyclodextrins to form inclusion complexes selectively with a wide variety of guest molecules or ions is well known (1,2) (see INCLUSION COMPOUNDS). Cyclodextrins immobilized on appropriate supports are used in high performance Hquid chromatography (hplc) to separate optical isomers. Immobilization of cyclodextrin on a soHd support offers several advantages over use as a mobile-phase modifier. For example, as a mobile-phase additive, P-cyclodextrin has a relatively low solubiUty. The cost of y- or a-cyclodextrin is high. Furthermore, when employed in thin-layer chromatography (tic) and hplc, cyclodextrin mobile phases usually produce relatively poor efficiencies. 

Isopropyl acetate is produced by the catalytic vapor-phase addition of acetic acid to propylene. A high yield of the ester can be realized (about 99%) ... 

The first step is the liquid phase addition of acetic acid to butadiene. The acetoxylation reaction occurs at approximately 80°C and 27 atmospheres over a Pd-Te catalyst system. The reaction favors the 1,4-addition product (l,4-diacetoxy-2-butene). Hydrogenation of diacetoxybutene at 80°C and 60 atmospheres over a Ni/Zn catalyst yields 1,4-diacetoxybu-tane. The latter compound is hydrolyzed to 1,4-butanediol and acetic acid ... 

Dirubidium phthalocyanine (PcRb2) and dicesium phthalocyanine (PcCs2) can be prepared by chemical-vapor deposition of benzene-1,2,4,5-tctracarbonitrile and the metal chloride.135 In the solid phase, additional rubidium atoms are complexed between peripheral cyano groups. 

The Effect of Mobile-Phase Additives and Cone-Voltage 147... 

Involatile inorganic buffers, when used as mobile-phase additives, are the prime canse of blocking of the pinhole. The situation can be alleviated either by replacing them by a more volatile alternative, such as ammonium acetate, or by nsing post-colnmn extraction to separate the analytes from the buffer, with the analytes, dissolved in an appropriate organic solvent, being introduced into the mass spectrometer. 

There are two notable features of the quantitative performance of this type of interface. It has been found that non-linear responses are often obtained at low analyte concentrations. This has been attributed to the formation of smaller particles than at higher concentrations and their more easy removal by the jet separator. Signal enhancement has been observed due to the presence of (a) coeluting compounds (including any isotopically labelled internal standard that may be used), and (b) mobile-phase additives such as ammonium acetate. It has been suggested that ion-molecule aggregates are formed and these cause larger particles to be produced in the desolvation chamber. Such particles are transferred to the mass spectrometer more efficiently. It was found, however, that the particle size distribution after addition of ammonium acetate, when enhancement was observed, was little different to that in the absence of ammonium acetate when no enhancement was observed. 

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

Table 5.16 LC-MS-MS signal responses" obtained from wheat forage matrix samples using various mobile-phase additives (injection volumes of 50 p,l). From Choi, B. K., Hercules, D. M. and Gusev, A. I., LC-MS/MS signal suppression effects in the analysis of pesticides in complex environmental matrices , Fresenius J. Anal. Chem., 369, 370-377, Table 2, 2001. Springer-Verlag GmbH Co. KG. Reproduced with permission...

Volume of Monomer and Particle Phases. Additional time dependent differential equations were written for the volumes of the particle and aqueous phases. The volume of the monomer phase was calculated from the total emulsion volume and the volumes of the aqueous and polymer phases. This procedure allowed the determination of whether monomer droplets are present (Interval II) or absent (Interval III). 

Figure 7. Slow inactivation of Na channels is potentiated by STX. The graph shows the time required for the recovery of Na channels to an activatable state after a long (1 sec, +50 mV) inactivating depolarization. When tested by a brief test pulse, control currents (A) recovered in a fast (r = 233 msec) phase. Addition of STX (q, 2 nM, which approximately halved the currents with no inactivating pulse) approximately doubled the fraction of currents recovering in the slow phase and also increased the time constant of slow recovery. The fast recovery rate was unaffected. (Reproduced with permission from Ref. 47. Copyright 1986 The New York Academy of Sciences).

Cyclodextrins as chemically banded layers [102] or mobile phase additives [103-105] have been used successfully to resolve a wide variety of alkaloids, steroids and dansyl- and naphthylamide-amino acid derivatives. The low solubility in aqueous solution and f high cost of cyclodextrins restricted the use of these additives > initially. These limitations were overcome by the availability of ... 

There is a wide variety of commercially available chiral stationary phases and mobile phase additives.32 34 Preparative scale separations have been performed on the gram scale.32 Many stationary phases are based on chiral polymers such as cellulose or methacrylate, proteins such as human serum albumin or acid glycoprotein, Pirkle-type phases (often based on amino acids), or cyclodextrins. A typical application of a Pirkle phase column was the use of a N-(3,5-dinitrobenzyl)-a-amino phosphonate to synthesize several functionalized chiral stationary phases to separate enantiomers of... 

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