Net retention

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

When the mobile phase is a gas, a compressibility factor j must be applied to the adjusted retention volume to give the net retention volume ... 

Stereochemical analysis can add detail to the mechanistic picture of the Sj l substitution reaction. The ionization mechanism results in foimation of a caibocation intermediate which is planar because of its hybridization. If the caibocation is sufficiently long-lived under the reaction conditions to diffirse away from the leaving group, it becomes symmetrically solvated and gives racemic product. If this condition is not met, the solvation is dissymmetric, and product with net retention or inversion of configuration may be obtained, even though an achiral caibocation is formed. The extent of inversion or retention depends upon the details of the system. Examples of this effect will be discussed in later sections of the chapter. 

In such a process, the water molecule fonned in the elimination step is captured primarily fiom the fixmt side, leading to net retention of configuration for the alcohol. For the ester, the extent of retention and inversion is more balanced, although it vari among individual systems. It is clear om die data in Table 5.18 that the two pairs of stereoisomeric amines do not form the same intermediate, even though a simple mechanistic interpretation would sugg that both would fmm the 2-decalyl cation. The coUap of the ions to product is pvidoitly so rapid that diere is not time for relaxation of the initially formed intermediates to reach a common stnicture. 

When X=CN, the isotopic exchange occurs with 99% retention of configuration, but when X=CPh, only about 30% net retention is observed. Explain this result. 

When the relationship between the distribution coefficient of a solute and solvent composition, or the corrected retention volume and solvent composition, was evaluated for aqueous solvent mixtures, it was found that the simple relationship identified by Purnell and Laub and Katz et al. no longer applied. The suspected cause for the failure was the strong association between the solvent and water. As a consequence, the mixture was not binary in nature but, in fact, a ternary system. An aqueous solution of methanol, for example, contained methanol, water and methanol associated with water. It follows that the prediction of the net distribution coefficient or net retention volume for a ternary system would require the use of three distribution coefficients one representing the distribution of the solute between the stationary phase and water, one representing that between the stationary phase and methanol and one between the stationary phase and the methanol/water associate. Unfortunately, as the relative amount of association varies with the initial... 

In this aforementioned Heine reaction the initial ring opening takes place by iodide ions. Subsequent ring closure by S 2 displacement of iodide by reaction with the negative oxygen center then leads to the products. This process proceeds with double inversion at the same carbon atom, thus with net retention. Hydrolysis of these oxazolines gives j9-hydroxy-a-amino acids (Scheme 31) [1,38]. The stereochemical course of ring expansion is the same as that observed in Scheme 29. 

For trisubstituted olefins, the nucleophile attacks predominantly at the less substituted end of the allyl moiety, e.g. to afford a 78 22 mixture of 13 and 14 (equation 7). Both the oxidative addition of palladium(O) and the subsequent nucleophilic attack occur with inversion of configuration to give the product of net retention. The synthesis of the sex pheromone 15 of the Monarch butterfly has been accomplished by using bis[bis(l,2-diphenylphosphinoethane)]palladium as a catalyst as outlined in equation 8 . A substitution of an allyl sulfone 16 by a stabilized carbon nucleophile, such as an alkynyl or vinyl system, proceeds regioselectively in the presence of a Lewis acid (equation 9) . The... 

The net retention volume and the specific retention volume, defined in Table 1.1, are important parameters for determining physicochemical constants from gas chromatographic data [9,10,32]. The free energy, enthalpy, and. entropy of nixing or solution, and the infinite dilution solute activity coefficients can be determined from retention measurements. Measurements are usually made at infinite dilution (Henry s law region) in which the value of the activity coefficient (also the gas-liquid partition coefficient) can be assumed to have a constant value. At infinite dilution the solute molecules are not sufficiently close to exert any mutual attractions, and the environment of each may be considered to consist entirely of solvent molecules. The activity... 

Net retention time (volume) 5 HMR, bonded phases (LC) 336 NMR, interface (LC) 998 Noise (detecto 86... 

Retention 4 adjusted 4 dead time 4 holdup time 4 net retention 5 relative pressure (GC) 4 series connected columns (GC) 59... 

Specific Retention Volume V, Net retention volUM at 0 C for unit weight of stationary phase V, 273 W, T. 

As indicated by the mechanism, carboalumination is a syn addition. The resulting vinylalanes react with electrophiles with net retention of configuration. The electrophiles that have been used successfully include iodine, epoxides, formaldehyde, and ethyl chloroformate.229 We will also see in Chapter 8 that the vinylalanes can undergo exchange reactions with transition metals, opening routes for formation of carbon-carbon bonds. 

The transformation proceeds with excellent stereoselectivity by kinetic formation of the 2,5-trans-disubstituted pyrrolidine 2-328 [182]. The tertiary amine can now initiate a nucleophilic backside displacement of the vicinal iodide in 2-328, leading to an aziridinium salt 2-329 [183]. This event ensures a net retention of the stereochemistry at C-13 in the following attack of the ester carbonyl in the butyrolactone ring closure to give 2-330. 

A concept of anion mobility may be considered a useful paradigm for explaining the net retention and loss of cations from soils, and thus exposure pathways. This paradigm relies on the simple fact that total cations must balance total anions in soil solution (or any other solution), and, therefore, total cation leaching can be thought of as a function of total anion leaching. The net production of anions within the soil (e.g., by oxidation or hydrolysis reactions) must result in the net production of cations (normally H+), whereas the net retention of anions (by either absorption or biological uptake) must result in the net retention of cations. 

The retention of the band or peak beyond what V0 predicts depends on the magnitude of the equilibrium constant and logically on the volume Vs or area As of the stationary phase. The equation of importance is Vr — V0+KVS and the net retention V/ = KVS. Two main factors influence the value of the equilibrium constant and these are the chemical nature of the mobile and stationary phases. Chemistry is molecules and while true thermodynamics knows no molecules or forces between molecules, chemists think in terms of molecular properties. Among those properties, there is a consideration of the kinds of forces that exist between molecules. Granted that thermodynamics are energy not force considerations but it is useful to understand the main forces involved in the interaction between molecules. Put another way,... 

It is a great deal of work to actually determine a true equilibrium constant and most chemical separation methods speak in terms of values which are proportional to the actual equilibrium constant. At constant flow, the time that a given type of molecule is retained is related to the time for the void volume to pass after the sample is placed in a column or on a plate with the addition of the time for the net retention volume. If the flow remains constant, the temperature of the separation remains constant and no stationary phase is gained or lost, one can attempt qualitative identification using retention times. It is more reasonable to calculate the ratio of net retention volume to the void volume and call the result partition factor or capacity factor, k. ... 

It is desirable that the equilibrium constant for a solute be not zero or very large lest there be no net retention or near infinite retention. The catch comes in the fact that liquids, which are relatively good solvents for a given type of molecule are also solvents for each other. This means the risk involved is by washing off the stationary phase with the mobile phase. Yet liquid-liquid methods offer much promise for relatively nonvolatile but soluble molecules and their separation of one from the other. The discovery of liquid-liquid chromatography earned Martin and Synge the Nobel Prize when they applied it to amino acids with water mobile phases and organic liquid stationary phases. 

Does solubility in the mobile phase influence net retention Polyethylene glycols are more soluble in cold solvents than warm. What is the noted effect if the temperature of the liquid chromatography separation of poly ethers is raised ... 

Retention volumes of monosubstituted benzenes, benzoic acid, phenols, and anilines have been measured in RPLC [76]. Buffered acetonitrile/water and tetrahydrofuran/water eluents were used with an octadecylsilica adsorbent. From the net retention volumes, a substituent interaction effect was calculated and described with the linear free energy relationship developed by Taft. The data was interpreted in terms of hydrogen bonding between the solutes and the eluent. 

High phosphate diets cause decreased Ca absorption, secondary hyperparathyroidism, accelerated bone resorption and soft tissue calcification in some animals, but not in normal humans. Although phosphates may decrease Ca absorption in man at very high (> 2000 mg/day) Ca intakes, they do not do so at more moderate Ca levels and enhance Ca absorption at very low levels (< 500 mg/day). Phosphates increase renal tubular reabsorption and net retention of Ca. At low Ca intakes, phosphates stimulate parathyroid hormone (PTH) secretion without causing net bone resorption. 

The partial racemization of isolated 2-octanol suggests that the hydrolysis may proceed via ionization of optically active substrates as in the Sjjl hydrolysis in homogeneous solution. The hydrolysis via ionization may be suppressed in media with low dielectric constant like micelles (Okamoto and Kinoshita, 1972), resulting in net retention. The ineffectiveness of the stereochemical influence of the CTAB micelle may be interpreted as a consequence of the mutual repulsion of the positively charged head groups of [46] and CTAB, so there is need for molecules of solvent to be incorporated between surfactant head groups (Sukenik et al., 1975). An appreciable increase in retention was also observed in a reversed micellar system (Kinoshita and Okamoto, 1977). 

Ethyl methyl ethylphosphonothionate (94 X = S) and the 4-methyl-l,3,2-dioxaphosphorinans (95 X = S R = OMe or NMe2) are oxidized with 90% H202 to the phosphoryl analogues with net retention of configuration phosphine sulphides, in contrast, are oxidized with inversion and racemization.64... 

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