Characteristics of electroosmotic flow and migration of neutral solutes under stepwise gradient elution of capillary electrochromatography

A theoretical study on the velocity of electroosmotic flow (EOF) and the retention times of neutral solutes under multiple-step gradient of capillary electrochromatography (CEC) was carried out, focusing on that with three kinds of mobile phases. Through the model computations, the detaining time of the second kind of mobile phase in the column was proved to play an important role in affecting EOF. The variation speed of EOF was shown to be determined by the differences among dead times in different steps. In addition, the prediction of the retention times of 13 aromatic compounds under gradient mode was performed with the deduced equations. A relative error below 3.3% between the calculated and experimental values was obtained, which demonstrated the rationality of the theoretical deduction. Our study could not only improve the comprehension of stepwise gradient elution, but also be of significance for the further optimization of separation conditions in the analysis of complex samples.

Migration of neutral solutes by double stepwise gradient elution in capillary electrochromatography

Characteristics of electroosmotic flow (EOF) and the migration of neutral solutes under double stepwise gradient elution in capillary electrochromatography were studied systematically. EOF velocity proved to be the function of operation time changing with the introduction of the second mobile phase. Accordingly, the retention of components also changed. The migration of neutral solutes was studied under the following three situations; A, components eluted when the column was filled only with the first kind of mobile phase; B, solutes eluted still in the first kind of mobile phase while at that time two kinds of mobile phase coexisted in the column and C, samples eluted in the second kind of mobile phase. Equations to describe the retention times of components under these three kinds of conditions were deduced and applied to predict the retention times of 12 aromatic compounds. Relative errors between experimental and calculated values were below 5.0%, which proved the reliability of the equations. In addition, parameters that might affect the retention time of solutes, such as the transferring time of mobile phase vials, the capacity factors of components and EOF velocities two steps were studied systematically (C) 2001 Elsevier Science B.V. All rights reserved.

Migration of large-scale subaqueous bedforms measured with seagrasses(Cymudocea nodesa) as tracers

The vertical growth of seagrasses in response to burial by migration of bedforms is combined with dating techniques to provide precise and rapid estimates of the migration speed of subaqueous dunes over seagrass patches. Two methods to estimate the time interval between the passage of successive dunes and the motion of single dunes through seagrass patches are described. The second method is more precise. The application of these methods to vegetated (Cymodocea nodosa) subaqueous dunes in the Alfacs Bay (NW Mediterranean) showed that the dunes traveled at an average speed of $13.0 \pm 0.6 m yr^-1$ and demonstrated that the methods can resolve migration speeds from 0.15 to $980 m yr^-1$ with this particular seagrass species. In areas vegetated with different seagrass species, bedform migration can be estimated over different time scales. The strong coupling between seagrass and sediment dynamics resembles the coupling of vegetation and land dunes.

INTRAANNULAR AND INTERANNULAR SUBSTITUENT MIGRATION AND INTRAMOLECULAR IPSO SUBSTITUTION OF BIPHENYL DERIVATIVES IN MASS-SPECTROMETRY

Biphenyl derivatives undergo extensive intraannular substituent migrations and subsequent intramolecular ipso substitutions giving rise to a fluorenyl cation and a biphenylene radical cation as common products in mass spectrometry. For biphenyls bearing an alkyl group, interannular substituent migration resulting in a substituted tropylium ion is also observed. Electron-withdrawing groups are found to be much more favourable to these reactions than the electron-donating ones.

LUMINESCENCE AND ENERGY MIGRATION IN THE OXYAPATITE CA2GD8(SIO4)6O2 DOPED WITH SEVERAL RARE-EARTH AND MERCURY-LIKE IONS

Energy transfer phenomena have been observed by activating the oxyapatite host-lattice Ca2Gd8(SiO4)6O2 with Eu3+, Tb3+, Dy3+, Sm3+. This is based on the energy migration in the Gd3+ sublattice and trapping by the activators. The trapping efficiency for G

CHRONOAMPEROMETRIC STUDIES ON MIGRATION AND DIFFUSION OF COUNTERION IN POLY(3-METHYLTHIOPHENE)

Small amplitude potential step experiments were carried out to study the counterion transfer process in oxidized poly(3-methylthiophene) (PMT) film. The results demonstrate that anion transfer process in PMT film is migration rather than diffusion. A porous metal electrode model-single hole model, which takes into account both the ionic resistance of the film and the uncompensated solution resistance, was found suitable to describe the potential step experiments. According to this model, the ionic resistivity of oxidized PMT film was calculated to be 5.0 x 10(4) OMEGA.cm, and, in turn, the diffusion coefficent of ClO4- ion in PMT film 3.7 x 10(-9) cm2/s.

PHYLOGEOGRAPHIC PATTERNS INDICATE TRANSATLANTIC MIGRATION FROM EUROPE TO NORTH AMERICA IN THE RED SEAWEED CHONDRUS CRISPUS (GIGARTINALES, RHODOPHYTA)1

Inferring how the Pleistocene climate oscillations have repopulated the extant population structure of Chondrus crispus Stackh. in the North Atlantic Ocean is important both for our understanding of the glacial episode promoting diversification and for the conservation and development of marine organisms. C. crispus is an ecologically and commercially important red seaweed with broad distributions in the North Atlantic. Here, we employed both partial mtDNA Cox1 and nrDNA internal transcribed spacer region 2 (ITS2) sequences to explore the genetic structure of 17 C. crispus populations from this area. Twenty-eight and 30 haplotypes were inferred from these two markers, respectively. Analysis of molecular variance (AMOVA) and of the population statistic Theta(ST) not only revealed significant genetic structure within C. crispus populations but also detected significant levels of genetic subdivision among and within populations in the North Atlantic. On the basis of high haplotype diversity and the presence of endemic haplotypes, we postulate that C. crispus had survived in Pleistocene glacial refugia in the northeast Atlantic, such as the English Channel and the northwestern Iberian Peninsula. We also hypothesize that C. crispus from the English Channel refugium repopulated most of northeastern Europe and recolonized northeastern North America in the Late Pleistocene. The observed phylogeographic pattern of C. crispus populations is in agreement with a scenario in which severe Quaternary glaciations influenced the genetic structure of North Atlantic marine organisms with contiguous population expansion and locally restricted gene flow coupled with a transatlantic dispersal in the Late Pleistocene.

River mouth bar formation, riverbed aggradation and channel migration in the modern Huanghe (Yellow) River delta, China

This paper addresses the recent (1970s-1990s) processes of river mouth bar formation, riverbed aggradation and distributary migration in the Huanghe River mouth area, in the light of station-based monitoring, field measurements and remote sensing interpretation. The results show that the morphological changes of the river mouth bar have been closely associated with the largely reduced fluvial discharge and sediment load. Landforrn development such as bar progradation occurred in two phases, i.e. before and after 1989, which correspond to faster and lower bar growth rates, respectively. Fast riverbed aggradation in the mouth channel was strongly related to river mouth bar progradation. During 1976-1996, about 2.8% of the total sediment loads were deposited in the river channel on the upper to middle delta. Therefore, the river water level rose by a few meters from 1984 to 1996. The frequent distributary channel migration, which switched the radial channel pattern into the SE-directed pattern in the mid-1980s, was linked with mouth bar formation. Marine conditions also constrain seaward bar progradation. Furthermore, the history of river mouth bar formation reflects human impacts, such as dredging and dyking in order to stabilize the coastal area. (c) 2005 Elsevier B.V. All rights reserved.

Observation and theoretical analysis for the sand-waves migration in the North Gulf of South China Sea

According to the environmental characteristic of the north gulf of South China Sea, a quasi-3D mechanics model has been built for simulating the small scale sand-waves migration in the seas of southwest of Hainan Island. Based on the submarine micro-geomorphic data induced by multi-beam system and hydrographic survey record, the migrations of the sand-waves in the study area are predicted. The results show that calculation is consistent with the observation data in the groove of sand ridge, but not well in the crest of sand ridge. It is indicated that the mechanics model should be used to predict the migration of the small scale sand-waves which are dominated by bed load in the seas. This paper is very meaningful to project the route of submarine pipeline.

Diel vertical migration of zooplankton following optimal food intake under predation

A model is developed to investigate the trade-offs between benefits and costs involved in zooplanktonic diel vertical migration (DVM) strategies. The 'venturous revenue' (VR) is used as the criterion for optimal trade-offs. It is a function of environmental factors and the age of zooplankter. During vertical migration, animals are assumed to check instantaneously the variations of environmental parameters and thereby select the optimal behavioral strategy to maximize the value of VR, i.e. taking up as much food as possible with a certain risk of mortality. The model is run on a diel time scale (24 h) in four possible scenarios during the animal's life history. The results show that zooplankton can perform normal DVM balancing optimal food intake against predation risk, with the profile of DVM largely modified by the age of zooplankter.

Control mechanisms of diel vertical migration: Theoretical assumptions

We explore control mechanisms underlying the vertical migration of zooplankton in the water column under the predator-avoidance hypothesis. Two groups of assumptions in which the organisms are assumed to migrate vertically in order to minimize realized or effective predation pressure (type-I) and to minimize changes in realized or effective predation pressure (type-II), respectively, are investigated. Realized predation pressure is defined as the product of light intensity and relative predation abundance and the part of realized predation pressure that really affects organisms is termed as effective predation pressure. Although both types of assumptions can lead to the migration of zooplankton to avoid the mortality from predators, only the mechanisms based on type-II assumptions permit zooplankton to undergo a normal diel vertical migration (morning descent and evening ascent). The assumption of minimizing changes in realized predation pressure is based on consideration of DVM induction only by light intensity and predators. The assumption of minimizing changes in effective predation pressure takes into account, apart from light and predators also the effects of food and temperature. The latter assumption results in the same expression of migration velocity as the former one when both food and temperature are constant over water depth. A significant characteristic of the two type-II assumptions is that the relative change in light intensity plays a primary role in determining the migration velocity. The photoresponse is modified by other environmental variables: predation pressure, food and temperature. Both light and predation pressure are necessary for organisms to undertake DVM. We analyse the effect of each single variable. The modification of the phototaxis of migratory organisms depends on the vertical distribution of these variables. (C) 2001 Academic Press.