Migration and Retention

In a chromatographic system, the sample is introduced in a small volume at the inlet of a column or another carrier of the stationary phase. The mobile phase transports the sample components in contact with the stationary phase throughout the column. 

Due to different interactions between the sample components and the stationary phase, the sample components migrate through the system at different speeds and elute from the column with different retention times. 

The retention time is defined as the time between the sample introduction and the elution from the column. 

At the end of the column, a detector provides a signal for all eluting components (universal detection) or for a limited number (selective detection). 

In a sample with many components, some compounds will coelute, partly or completely, depending on the complexity of the sample and the peak capacity of the column. With mass spectrometric detection, even coeluting components can be identified. 

---

Solution Casting. The production of unsupported film and sheet by solution casting has generally passed from favor and is used only for special polymers not amenable to melt processes. The use of solvents was generally very hazardous because of their flammabiUty or toxic nature. The cost of recovery and disposal of solvents became prohibitive for many lower price film appHcations. The nature of the drying operations leads to problems with solvent migration and retention that are not problems with melt-processed polymers. 

Beyond roles of chemokine receptors in hematopoiesis and innate immunity, roles for chemokines in adaptive immunity emerged. Moreover, other nonleukocyte migration properties of chemokine receptors have been identified. These include roles in the biology of endothelial cells (Chapter 15), cancer (Chapter 16), smooth muscle (Chapter 11), fibroblasts (Chapter 14), stem cells (Chapter 8), and all cell types associated with nervous system tissues (Chapter 17). In many instances, broad functional overlap is evident as chemokines can direct the migration of these cells just as they do with leukocytes. In certain instances, the ability of chemokines to retain cell populations within a specific microenvironment is as important as their migration-promoting properties. However, it is also clear that migration and retention are not the sole end points. 

Liquid phase migration and retention — NAPL held suspended by the water table or capillary fringe or perched above low permeability zones (water wet soil) in the unsaturated zone. 

The above approach could also be applied to evaluate the orthogonality between CE systems and CSs using migration and retention parameters, respectively. 

Pourcelot, L. Gauthier-Lafaye, F. 1999. Hydrothermal and supergen clays of the Oklo natural reactors conditions of radionuclide release, migration and retention. Chemical Geology, 157, 155-174. 

NADH-dependent reductase, thus allowing the biopterin cofactor to function catalytically (72JBC(247)6082). That the conversion of phenylalanine to tyrosine involves an arene oxide intermediate is suggested by the observation of the so-called NIH shift phenomenon (i.e. migration and retention of the para substituents such as deuterium, tritium, methyl and bromine when these para-substituted phenylalanines are enzymatically hydroxylated) <66BBR(24)720, 67MI11000). 

In mammalian liver microsomes, cytochrome P-450 is not specific and catalyzes a wide variety of oxidative transformations, such as (i) aliphatic C—H hydroxylation occurring at the most nucleophilic C—H bonds (tertiary > secondary > primary) (ii) aromatic hydroxylation at the most nucleophilic positions with a characteristic intramolecular migration and retention of substituents of the aromatic ring, called an NIH shift,74 which indicates the intermediate formation of arene oxides (iii) epoxidation of alkenes and (iv) dealkylation (O, N, S) or oxidation (N, S) of heteroatoms. In mammalian liver these processes are of considerable importance in the elimination of xenobiotics and the metabolism of drugs, and also in the transformation of innocuous molecules into toxic or carcinogenic substances.75 77... 

The formation of salicylic acid from 218 and the D migration and retentions observed in the conversion of 220 to salicylic acid are in agreement with the suggestion that 218 is an intermediate in the ortho hydroxylation of benzoic acid in higher plants, notwithstanding the fact that a 2,3-oxide is an equally attractive intermediate. 

Evidence for the direct oxidation of benzene (or substituted benzenes) to benzene oxide (or substituted benzene oxides) by enzymatic (Refs. 5 and 35 and references therein) and chemical (Refs, 35 and 36 and references therein) methods is available both from the observation of the migration and retention of ring substituents during aromatic hydroxylation (NIH shift),and from the nature of the isolated products (phenols, rans-dihydrodiols). As a direct consequence of its thermal instability and high reactivity, benzene oxide 1 has not yet been isolated as an oxidation product of benzene. 

Similar results were found in Texas daffodils for the biosynthesis of norpluviine (108) the site of labelling is indicated. Further, tritium retention in norpluviine derived from [3- H,/3- C]cinnamic acid was 28%. This agrees well with a predicted value of 25% resulting from para-hydroxylation with hydrogen migration and retention, where half the tritium would be lost in the first hydroxylation and half the remainder in the second. 

Reduction.—Lithium aluminium hydride reduction of cw-bromohydrins e.g. 124) in the 13a-series involves the semipinacolic 17a—> 16a-hydride shift illustrated (Scheme 5), as revealed by the migration and retention of a deuterium label. 

A third amino acid hydroxylase, tyrosine hydroxylase, causes the NIH Shift with phenylalanine (21) but not with its normal substrate, tyrosine (Figure 3). Thus, the hydroxylation of 4-tritiophenylalanine with either tyrosine hydroxylase or phenylalanine hydroxylase leads to migration and retention of tritium in the product, tyrosine. However, with 3,5-ditritiotyrosine as substrate for tyrosine hydroxylase, complete loss of one tritium occurs in the conversion to 3,4-dihydroxy-5-tritio-phenylalanine. 

Table I. Migration and Retention of Deuterium or Tritium During Enzymatic Hydroxylation of Specifically Labeled Substrates...

Fenton (4), Udenfriend (3), Hamilton (16), or Viscontini ( ) which have been considered as possible models for enzymatic hydroxylation do not lead to significant migration and retention of tritium during the formation of 4-hydroxyacetanilide (Table II). Thus, only model hydrox-ylating systems proceeding via cationic intermediates produce the migration of substituents characteristic of enzymatic hydroxylation. It appears... 

This rearrangement proceeded with 85 percent migration and retention of D and thus was unlikely to have involved a cyclohexanone intermediate. The second broad application of D-labelled compounds in mechanistic studies consists of experiments in which the rate of a reaction involving the breaking of a C—H bond is compared to the rate of the parallel reaction for the C—D compound. If a kinetic isotope effect is found, then the breaking of the C—H bond is more or less implicated in the rate-determining step of the reaction. 

Narcissus Texas a tritium retention in norpluviine (12) of 28% was observed, which is very close to the predicted value resulting from para-hydroxylation with hydrogen migration and retention. 

The extensive studies on the migration and retention of deuterium or tritium in aromatic substrates which have been carried out indicate that the magnitude of the retention is considerably influenced by the nature of substituents in the aromatic ring. Several inconsistencies which have been observed are only adequately resolved by assuming that decomposition of the arene oxide (Figure 4.13) can take place by both pathways a and b Thus in some cases such as chlorobenzene and anisole (61 and 62) hydroxylation gives the 4-hydroxy derivatives but the retention of deuterium in this product is dependent on whether the deuterium was initially present at the position of substitution or in the adjacent position. In these cases deuterium is preferentially retained when it is originally present in the substrate in the adjacent position to substitution (62) , and it must be assumed that both pathways a and 6 are operative in the decomposition of the intermediate arene oxides. 

Dilution and dispersion characteristics, including the migration and retention characteristics of soils, radionuclide movement in hydrogeological formations, the reconcentration ability of sediments and biota, and other effects which may be needed to determine radionuclide movement and exposure pathways ... 

---