Dominant Retention Mechanism Research Articles

Basic Chromatographic Properties of Polyethylene Glycol-type, Polymer-based Monolithic Columns

We have prepared polyethylene glycol (PEG)-type, polymer-based monolithic columns (PEG-type columns) to study the basic properties of PEG-type monolithic polymers. Chromatographic characteristics of the PEG-type columns were obtained using a semi-micro HPLC system while polycyclic aromatic hydrocarbons and benzene derivatives having various functional groups were utilized as solutes. Results were compared with those of commercially available polymer-based packed columns. PEG-type columns showed greater recognition ability for planar solutes based on separation factors, alpha (k triphenylene/k o-terphenyl) and alpha (k pyrene/k naphthalene). Benzene derivatives having a carboxyl group or a phenolic hydroxyl group, i.e., benzoic acid and phenol, tended to be retained more preferentially on the PEG-type column than other solutes without a carboxyl group or a phenolic hydroxyl group. These recognition abilities towards carboxyl group and/or phenolic hydroxyl group were found to be greater on the PEG-type columns having the longer ethylene oxide chains. However, unexpectedly HILIC mode did not work, even in 95% AN mobile phase, for nucleic-acid bases, benzoic acid, or phenol that careful studies suggested hydrophobic interaction was the dominant retention mechanism, while relatively weak hydrogen bonding between proton on carboxyl group or phenolic hydroxyl group of the retained solutes and oxygen in ethylene oxide chain on the PEG-based polymer prolonged the hydrophobic-based retentions.

Recent Advances on Hydrogen Retention in ITER’s Plasma-Facing Materials: Beryllium, Carbon, and Tungsten

Management of tritium inventory remains one of the grand challenges in the development of fusion energy, and the choice of plasma-facing materials is a key factor for in-vessel tritium retention. The Atomic and Molecular Data Unit of the International Atomic Energy Agency organized a Coordinated Research Project (CRP) on the overall topic of tritium inventory in fusion reactors during the period 2001-2006. This dealt with hydrogenic retention in ITER’s plasma-facing materials – Be, C, and W – and in compounds (mixed materials) of these elements as well as tritium removal techniques. The results of the CRP are summarized in this paper together with recommendations for ITER. Basic parameters of diffusivity, solubility, and trapping in Be, C, and W are reviewed. For Be, the development of open porosity can account for transient hydrogenic pumping, but long-term retention will be dominated by codeposition. Codeposition is also the dominant retention mechanism for carbon and remains a serious concern for both Be- and C-containing layers. Hydrogenic trapping in unirradiated tungsten is low but will increase with ion and neutron damage. Mixed materials will be formed in a tokamak, and these can also retain significant amounts of hydrogen isotopes. Oxidative and photon-based techniques for detritiation of plasma-facing components are described.

The mobility and bioavailability of trace metals in tropical soils: a review

In recent years, initiatives on critical loads of metals have resulted in the development of models to predict critical, i.e. mobile or bioavailable, concentrations of potentially toxic elements in soils. These models have been based on data from temperate soils and, considering the fundamentally different soil chemistry of the major tropical soil types, it is unlikely that such models could be applied successfully in the tropics. In particular, the major tropical soil types, particularly oxisols and ultisols, are distinguished from most temperate soils by low activity clays, low organic matter contents, low pH values and high levels of Fe oxides. The aim of this review paper is to collate existing information on the behaviour of trace metals in tropical soils, particularly in relation to metal mobility and bioavailability, and note differences, where these exist, with temperate soils. The main findings are as follows. The acidic nature of many tropical soils may not, of itself, lead to greater metal solubility. The relatively low organic matter content of tropical soils suggests that it cannot be relied upon to retain metals. However, when it is present, it appears to be important in the retention of copper, lead and zinc. The clay fractions of many tropical soils are dominated by kaolinite which is less able than 2: 1 layer silicate clays to adsorb metals. However, oxisols and ultisols in particular have relatively high Fe oxides contents and these may be effective in metal sorption. Overall though, soils such as oxisols and ultisols appear to sorb relatively low amounts of added metals, most likely explained by the low activity clay minerals they contain and the rapid decomposition of organic matter incorporated into them. Dominant retention mechanisms, where they occur, are inner and outer sphere adsorption but not precipitation. Cadmium, Cu, Zn and Ni appear to be mobile in tropical soils. Chromium may be mobile in the A horizon but retained in the B horizon. The evidence for Pb points to surface retention, as is often observed for this metal in temperate soils. Metals in contaminated tropical soils appear to be rather bioavailable, judging by the few studies on plant uptake. It should be noted that the above statements are based on a relatively small number of studies. Further work is required if we are to gain an improved understanding of the mobility and bioavailability of trace metals in tropical soils.

Retention process in reversed phase TLC systems with polar bonded stationary phases

The retention of a solute in RP chromatography is a very complex process which depends on many factors. Therefore, the study of the influence of a mobile phase modifier concentration on the retention in different reversed phase chromatographic systems is very important for understanding the rules governing retention and mechanisms of substance separation in a chromatographic process. Composition changes and the nature of mobile phases enable tuning of the separated analytes' retention over a wide range of retention parameters and optimization of the chromatographic process as well. Optimization of the chromatographic process can be achieved by several different methods; one of them is the so-called interpretative strategy. The key approach adopted in this strategy is the implementation of adequate retention models that couple the retention of solute with the composition of a mixed mobile phase. The use of chemically bonded stationary phases composed of partially non-bonded silica matrix and organic ligands bonded to its surface in everyday chromatography practice leads to questions of the correct definition of the retention model and the dominant retention mechanism in such chromatographic systems. The retention model for an accurate prediction of retention factor as a function of modifier concentration and the heterogeneity of the adsorbent surface should be taken into consideration. In this work the influence of mobile-phase composition on the retention of sixteen model substances such as phenols, quinolines, and anilines used as test analytes in different RP-TLC systems with CN-, NH2-, and Diol-silica polar bonded stationary phases has been studied. The aim of this study is to compare the performance of three valuable retention models assumed as the partition, adsorption/partition, and adsorption mechanism of retention. All the models were verified for different RP-TLC systems by three statistical criteria. The results of investigations presented in this work demonstrate that the best agreement between the experimental and calculated Rf values was obtained by the use of new-generation retention models, which assume heterogeneity of adsorbent surface. The results reported here show that heterogeneity of the adsorbent surface may be important in analysis of the elution process in liquid chromatography. Consideration of the goodness of fit for the experimental data to the examined retention models is in conformity with the adsorption mechanism of retention on all polar bonded stationary phases in most eluent systems for most investigated compounds.

Tritium recycling and retention in JET

JET's 1997 Deuterium Tritium Experiment (DTE1) allows a detailed study of hydrogenic isotope recycling and retention in a pumped divertor configuration relevant to ITER. There appear to be two distinct forms of retained tritium. (1) A dynamic inventory which controls the fueling behaviour of a single discharge, and in particular determines the isotopic composition. This is shown to be consistent with neutral particle implantation over the whole vessel surface area. (2) A continually growing inventory, which plays a small role in the particle balance of a single discharge, but ultimately dominates the hydrogenic inventory for an experimental campaign comprising thousands of pulses. This will be the dominant retention mechanism in long-pulse devices like ITER. The JET retention scaled-up to ITER proportions suggests that ITER may reach its tritium inventory limit in less than 100 pulses.

Influence of iron/aluminium ratio on the retention of lead and copper by amorphous iron–aluminium oxides

Results are presented from a study on the influence of pH on the retention of Pb and Cu by amorphous iron, aluminium and mixed iron–aluminium oxides, plus kaolinite clay. The composition (Fe/Al molar ratio) of the oxides significantly affects their surface properties. Increasing the Fe/Al ratio generally enhances the surface area although the maximum surface area is found at Fe/Al=1. The cation exchange capacity (CEC) was dependent upon pH and Fe/Al ratio, decreasing in order Fe>3FeAl>Al>Fe3Al>FeAl≫kaolinite at pH 6.3–7. The oxide points of zero charge could be divided into two groups: Fe/Al≤1 (Al, Fe3Al, FeAl; pH ZPC=6.2–6.4) and Fe/Al>1 (3FeAl, Fe; pH ZPC=5.3–5.8). Batch equilibrium suspension tests were conducted with Pb and Cu (single metal solutions) to assess contaminant retention capability of each oxide and kaolinite. Pb retention by mixed oxides at a particular pH increases with Fe/Al ratio in the order Fe>3FeAl>FeAl>Fe3Al>Al. The same order is observed at low to medium Cu concentrations (0.5, 2.5, 5 mmol/l) but at higher metal loading (25 mmol/l), only Fe oxide has a clearly greater retention capacity. Oxides with Fe/Al<1 (Al, Fe3Al) retain more Cu than Pb in single contaminant tests at equal metal concentration. Bulk precipitation tests (no oxides or kaolinite) combined with surface charge measurements suggest that adsorption is the dominant retention mechanism, particularly at less than pH 6. Amorphous oxides demonstrate considerably greater heavy metal accumulation capacity, specific surface area, and surface charge than kaolinite.

Retention mechanisms of polycylic aromatic nitrogen heterocyclics on bonded amino phases in normal-phase liquid chromatography

Two aminoalkyl bonded phases, aminopropylsilica and dimenthylaminopropylsilica, operated in the normal-phase mode were investigated regarding the retention mechanisms of polycyclic aromatic nitrogen heterocyclics (PANHs). By blocking active interaction sites with methyl substituents, the retention mechanisms could be ascertained. The bonded functional groups of the aminoalkyl stationary phases were found to be the primary adsorption sites. Hydrogen bonding was found to be the dominant retention mechanism for carbazole-type PANHs on both stationary phases and for acridine-type PANHs on the aminopropyl phase. For acridine-type PANHs on dimethylaminopropylsilica, retention was found to be mainly due to electron donor-acceptor interaction. Residual silanols on the stationary phase support material on these non-end-capped bonded phases were found to contribute only slightly to the retention of both acridine- and carbazole-type PANHs. A strong dependence of retention on steric hindrance of the n-electrons on the nitrogen of acridine-type solutes was shown. Further, the necessity to consider not only the polarity and solvent strength but also the selectivity of the mobile phase is demonstrated.

Influence of temperature on the mechanism by which compounds are retained in gas-liquid chromatography

The influence of temperature on the retention mechanism and solvation interactions of 46 varied solutes in 10 representative stationary phases of different polarity within the temperature range of 60 to 140°C is discussed. Gas-liquid partition is shown to the dominant retention mechanism for most solutes with inrterfacial adsorption of increasing importance at low phase loadings, low temperatures and for solutes of different polarity to that of the stationary phase. Guidelines are presented for predicting those conditions for which interfacial adsorption is likely to be a significant retention mechanism. A cavity model is used to characterize the solvation process in terms of the free energy contributions to solvation from the cavity-dispersion interactions and the sum of the remaining polar interactions. As a function of temperature it is shown that the contribution from polar interactions are only weakly temperature dependent over the temperature range studied while the cavity-dispersion interactions term shows a much more significant variation becoming less favorable for solute transfer at higher temperatures. In all cases, the contribution of the cavity-dispersion interactions term is favorable for solute transfer from the gas phase to the liquid phase. Principal component analysis is used to identify the factors contributing to the solvation process and their individual temperature dependence. In the case of the cavity-dispersion interactions term one factor accounts for more than 99.7% of the total variance. Three factors are identified as contributing to the polar interactions term. The first principle component accounts for more than 95% of the total variance at all temperatures and by correlation with other independent scales of dipolarity/polarizability is identified as representing the contribution from orientation and induction interactions. The two remaining principal components are shown to represent hydrogen-bond formation and charge-transfer complexation involving systems with π-electrons. The temperature dependence of the principal components provides insights into the general role of polar intermolecular interactions on the solvation process and their temperature variation.

PH influence on selectivity and retention of heavy metals in some clay soils

The problem of varying species of heavy metals existing as individual species or in combination with each other in simulated contaminant leachates (solutions) is studied in relation to selectivity and retention in four different kinds of soil. The influence of pH on the retention characteristics is also examined. The results obtained from the heavy metals single-species and composite-species adsorption tests indicate that in evaluating the attenuation potential of the soil substrate (i.e., contaminant attenuation), account needs to be given to the sensitivity of the soil – heavy metals contaminant system to the pH of the leachate and the competitive effect between the heavy metals present in the leachate. A change in the soil solution pH results in a corresponding change of the dominant retention mechanism of heavy metals in the soils. At high pH values, precipitation mechanisms (e.g., precipitating as hydroxides and (or) as carbonates) dominate. As pH decreases, precipitation becomes less important, and cation exchange becomes dominant. High amounts of heavy metals are retained in the soils if the soil buffer capacity remains high enough to resist a change in pH. The selectivity order of heavy-metal retention in soils depends on the pH of soil solution. At soil solution pH values above 4 or 5, when precipitation prevails, the selectivity order obtained is given as Pb &gt; Cu &gt; Zn ≥ Cd, as demonstrated for the illite, montmorillonite, and natural clay soils. At lower soil solution pH values, the selectivity order obtained is given as Pb &gt; Cd &gt; Zn &gt; Cu, as can be seen in the case of kaolinite and montmorillonite. Key words : heavy metals, retention, selectivity, preferential adsorption, precipitation.

Solute—solvent interactions in tetra- n-butylphosphonium salts studied by gas chromatography

The gas chromatographic properties of nine low-melting tetra- n-butylphosphonium (QBP) salts are described and compared to the properties of the analogous tetra- n-butylammonium (QBA) salts. The QBP salts, particularly those containing strong nucleophilic anions (such as chloride, bromide, iodide and nitrate) provide efficient column packings with wider liquid ranges than those observed for the QBA salts. The dominant retention mechanism for a wide range of organic solutes on the QBP salts is established as gas—liquid partitioning with only small contributions from interfacial adsorption. Based on the measurement of the partial molar Gibbs free energy of solution per methylene group and the molar retention volumes for McReynolds test probes and a series of aromatic polarity probes it is established that the principal difference in retention properties between the QBP and QBA salts is due to the greater solubility of hydrocarbons in the QBP salts. The polar selectivity probes show similar trends for the QBA and QBP salts but quantitative differences exist that depend on both the nature of the cation and anion. The retention characteristics of the QBA salts can only be used as a qualitative guide to the properties of the QBP salts.

Stationary phase effects in reversed-phase liquid chromatography of acids and ion pairs

Summary Reversed-phase partition chromatography has been performed with 1-pentanol as the stationary phase, applied on to the hydrophobic support by adsorption from an aqueous mobile phase containing a low concentration of 1-pentanol (1.9–2.5%). By ion-pair chromatography of car☐ylates with tetrapropylammonium and tetrabutylammonium as counter ions, ion pairs are retained by partition to the adsorbed layer of liquid stationary phase. Also for hydrophilic car☐ylic acids, which are retained in uncharged form as the acids, the dominant retention mechanism is partition to the liquid stationary phase, while hydrophobic car☐ylic acids are retained by an interaction with the hydrophobic support. The interaction of the hydrophobic acids with the support can be regulated by varying the amount of 1-pentanol stationary phase, which is effected by changing the concentration of 1-pentanol in the mobile aqueous phase.

Retention and Flow Characteristics of Polymer Solutions in Porous Media

Abstract Retention and flow characteristics of a solution containing Pusher 700, a high-molecular-weight, partially hydrolyzed polyacrylamide, were studied partially hydrolyzed polyacrylamide, were studied in an 86-md core made by compacting Teflon powder. The quantity, of polymer retained during linear displacement experiments ranged from 10 to 21 mu gm/gm for polymer concentrations of 100 to 500 ppm in 2-percent NaCl solutions. Nearly all retention ppm in 2-percent NaCl solutions. Nearly all retention was attributed to mechanical entrapment because of low polymer adsorption on the Teflon surface. Flow rate affected polymer retention. In increase in velocity was accompanied by polymer retention. Polymer was expelled when the flow rate was Polymer was expelled when the flow rate was reduced. Inaccessible pore volume was about 19 percent of the total pore volume. percent of the total pore volume.Resistance factors in different sections of the core ranged Pam 2 to 10 /or solutions of 100 to 500 ppm polymer concentration in 2-percent NaCl. ppm polymer concentration in 2-percent NaCl. Permeability reduction resulting from polymer Permeability reduction resulting from polymer retention produces the resistance factor in most of the core at a velocity of 3.2 ft/D. Resistance factors in the Teflon cores were two to three times lower than those reported for natural porous media where polymer is also retained by adsorption. Introduction The search for a low-cost, effective mobility control agent is currently focused on dilute aqueous solutions containing partially hydrolyzed polyacrylamides or polysaccharides. Rheological polyacrylamides or polysaccharides. Rheological properties have been studied, including the properties have been studied, including the effects of polymer concentration, shear rate, electrolyte concentration, and type of electrolyte. Correlation of rheological data and models with the flow behavior of polymer solutions in porous media has been complicated by the many interactions that occur between the complex porous matrix and the polymer solutions. Some data have been correlated using non-Newtonian rheological models to describe the variation of fluid viscosity with the apparent shear rate that the fluid experiences as it flows through the tortuous paths in porous media. These correlations have adjustable parameters determined from the particular set of parameters determined from the particular set of data used to develop the correlation. Investigators studying partially hydrolyzed polyacrylamide solutions observed apparent polyacrylamide solutions observed apparent viscosities 5 to 20 times the values measured in a conventional viscometer at the shear rates believed to exist in the porous media. These viscosity increases were not anticipated from the rheological behavior of the fluids. Pye introduced the concept of the resistance factor to quantify this effect. Burcik observed a decrease in the mobility of brine in a Berea sandstone disk that had been previously contacted with partially hydrolyzed previously contacted with partially hydrolyzed polyacrylamide. The mobility reduction persisted polyacrylamide. The mobility reduction persisted even after 100 PV of brine had been flushed through the disk. Burcik concluded that polymer molecules retained in the pore structure by adsorption or mechanical entrapment were hydrophillic and restricted the flow of water. Gogarty made an extensive experimental study of partially hydrolyzed polyacrylamide solutions in porous media and concluded that these polymer porous media and concluded that these polymer solutions reduced the permeability of the porous media. He noosed that polymer retention in natural cores occurred by mechanical entrapment and adsorption. Both mechanisms contributed to the resistance and residual or flushed resistance factors observed with polyacrylamide solutions. Other evidence of interactions between the polymer solution and the porous matrix was found. polymer solution and the porous matrix was found. Adsorption of polymer molecules on the surface of materials present in the porous matrix has been demonstrated in batch adsorption experiments. Material-balance calculations made on the streams entering and leaving porous media following step changes in concentrations show retention of polymer molecules in the porous media. polymer molecules in the porous media. A dependence of polymer retention on flow rate has been reported. Szabo devised a set of static and flow experiments in which polymer adsorption was held to a low level by using silica sand with a small surface area. Mechanical entrapment was found to be the dominant retention mechanism in short sand packs. packs. SPEJ P. 111