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PubMed2004

Effect of spacer arm length on protein retention on a strong cation exchange adsorbent.

The retention of five proteins was compared on a set of three strong cation exchange adsorbents that differed in spacer arm chemical structure and length. The adsorbents included a commercial product, Amersham Biosciences SP Sepharose Fast Flow, containing a six-carbon spacer between the agarose matrix and the anionic ligand, and two custom-prepared materials. One of the custom adsorbents contained a spacer of about half the length of the SP Sepharose Fast Flow, and the other contained no spacer arm. The adsorbent with no spacer arm was found to be significantly more retentive for all of the test proteins examined, in both isocratic and gradient elution tests. Reducing the spacer arm length by half resulted in increased retention for four of the five proteins, but this increase was less than what was observed when the spacer arm was eliminated. Retention increases were obtained without increasing the density of the anionic charge groups and appear to result from an enhancement of electrostatic or secondary nonelectrostatic interactions, or both. The results indicate that spacer arm length may be a useful variable in manipulating stationary-phase retention properties.

Adsorption↗
PubMed2005

Thermodynamic prediction of polymer retention in temperature-programmed HPLC.

Temperature programming has been used increasingly in liquid chromatography in recent years. In particular, temperature gradient elution has shown great potential in the analysis of complex polymers. In this study, the polymer retention behavior in temperature gradient interaction chromatography is investigated based on thermodynamic consideration of the retention factor. The polymer retention predicted by the model calculation is in good agreement with the experimental results, and the model allows devising a temperature program for designed retention behaviors such as a linear dependence of retention volume on log(molecular weight) of polymers. In addition, the migration behavior of polymeric solute along the separation column can be simulated, which shows strong molecular weight dependence. The migration behavior is also confirmed experimentally by employing different length columns or delayed injection.

Journal Article↗
PubMed2005

A comprehensive two-dimensional retention time alignment algorithm to enhance chemometric analysis of comprehensive two-dimensional separation data.

A comprehensive two-dimensional (2D) retention time alignment algorithm was developed using a novel indexing scheme. The algorithm is termed comprehensive because it functions to correct the entire chromatogram in both dimensions and it preserves the separation information in both dimensions. Although the algorithm is demonstrated by correcting comprehensive two-dimensional gas chromatography (GC x GC) data, the algorithm is designed to correct shifting in all forms of 2D separations, such as LC x LC, LC x CE, CE x CE, and LC x GC. This 2D alignment algorithm was applied to three different data sets composed of replicate GC x GC separations of (1) three 22-component control mixtures, (2) three gasoline samples, and (3) three diesel samples. The three data sets were collected using slightly different temperature or pressure programs to engender significant retention time shifting in the raw data and then demonstrate subsequent corrections of that shifting upon comprehensive 2D alignment of the data sets. Thirty 12-min GC x GC separations from three 22-component control mixtures were used to evaluate the 2D alignment performance (10 runs/mixture). The average standard deviation of first column retention time improved 5-fold from 0.020 min (before alignment) to 0.004 min (after alignment). Concurrently, the average standard deviation of second column retention time improved 4-fold from 3.5 ms (before alignment) to 0.8 ms (after alignment). Alignment of the 30 control mixture chromatograms took 20 min. The quantitative integrity of the GC x GC data following 2D alignment was also investigated. The mean integrated signal was determined for all components in the three 22-component mixtures for all 30 replicates. The average percent difference in the integrated signal for each component before and after alignment was 2.6%. Singular value decomposition (SVD) was applied to the 22-component control mixture data before and after alignment to show the restoration of trilinearity to the data, since trilinearity benefits chemometric analysis. By applying comprehensive 2D retention time alignment to all three data sets (control mixtures, gasoline samples, and diesel samples), classification by principal component analysis (PCA) substantially improved, resulting in 100% accurate scores clustering.

Journal Article↗
PubMed2006

Key to analyte migration and retention in electrochromatography.

This work identifies electrical field-induced concentration polarization (CP) as a key physical mechanism influencing the retention behavior of charged analytes in electrochromatography with fixed beds of porous adsorbent particles. Due to an insufficient screening of intraparticle surface charge, under most general conditions the porous (permeable) particles become charge-selective. CP is caused by coupled mass and charge transport normal to the charge-selective external surface of the permeable particles, which leads to concentration gradients of ionic species in the adjoining interparticle electrolyte solution. Cation-exchange (cation-selective) particles were employed to investigate the influence of applied voltage on the retention factor of counterionic, i.e., positively charged, analytes. It is demonstrated by macroscopic retention data and microscopic studies resolving the CP phenomenon on a particle scale that the dependence of CP on electrical field and mobile-phase ionic strengths is directly reflected in concomitant changes of analyte retention. The CP zones that develop at the interface between interparticle and intraparticle pore space are recognized by charged, but not electroneutral analytes while entering or leaving the particles. The intensity of these convective-diffusion boundary layers (CP zones) depends on the applied field strength and charge selectivity of a particle. Thus, it is the charge-selective transport between the interparticle and intraparticle pore space in packed beds that prevails under typical experimental conditions in electrochromatography and that forms the physical basis for a general electrical field dependence of the retention factor of charged analytes.

Journal Article↗
PubMed1996

Use of moment of inertia in comparative molecular field analysis to model chromatographic retention of nonpolar solutes.

A quantitative structure-retention relationship (QSRR) was developed from chromatographic data on 31 unsubstituted 3-6 ring polycyclic aromatic hydrocarbons (PAHs) using the 3D-QSAR method known as comparative molecular field analysis (CoMFA). The resulting CoMFA model gave an excellent correlation to high-performance liquid chromatography retention data for these PAHs yielding r2 values of 0.947 (conventional) and 0.865 (cross-validated). The steric and electrostatic contributions to the CoMFA model were 100% and 0%, respectively. A unique feature of this study was the use of moment of inertia, I, as a basis for CoMFA alignment of the PAH molecules. The moment of inertia also provided an alternative method for calculating the solute length-to-breadth ratio (L/B), which has been applied in previous QSRR studies as a molecular descriptor for PAH retention. By virtue of its mathematical simplicity and lack of ambiguity, the present derivation of L/B from I offers several advantages over other geometry-based schemes. Finally, Ix was evaluated for use as a molecular descriptor in QSRR regression analysis to predict the log of the retention index (log I) for these PAHs. The correlation with PAH retention was weak when the moment of inertia was considered alone but improved dramatically (r2 = 0.928) when the moment of inertia and connectivity index chi were used in combination as descriptors.

Chromatography, High Pressure Liquid↗
PubMed1996

Retention characteristics of polybutadiene-coated zirconia and comparison to conventional bonded phases.

This paper presents a detailed study of retention on a reversed-phase material made by coating polybutadiene (PBD) on porous zirconia. PBD-coated zirconia particles with six different carbon loads (0.25-5.6% carbon by weight) were prepared by evaporatively depositing and cross-linking PBD on microparticulate porous zirconia. Retention data of a homologous series of alkylbenzenes were obtained on the six PBD phases as a function of mobile phase composition in methanol-water and acetonitrile-water mixtures from 20 to 50% (v/v). The results obtained for the phase were compared to those for conventional octadecylsilane (ODS) bonded phases, and the effect of the amount of PBD on retention was studied in detail. We find that, per amount of bonded phase, the PBD phase is less retentive than is the ODS phase, but it has comparable hydrophobic selectivity. Furthermore, the PBD phase has about the same sensitivity toward changes in mobile phase composition as does the ODS phase, and its solute shape selectivity is similar to that of a monomeric ODS phase. Finally, we conclude that retention arises primarily from a partition-like process.

Butadienes↗
PubMed1998

Comparison of the retention characteristics of aromatic and aliphatic reversed phases for HPLC using linear solvation energy relationships.

The similarities and differences in retention characteristics of aromatic and aliphatic phases have been elucidated by the use of linear solvation energy relationships (LSERs). Three aromatic phases and three aliphatic phases were investigated in a series of mobile phases. The results of LSERs on a polymer-based aromatic phase, poly(styrene-divinylbenzene) resin (PRP-1) are very different from those on either silica- or zirconia-based aromatic and aliphatic phases. Retention on all aromatic and aliphatic phases except PRP-1 is dominated by two factors: the solute size and hydrogen bond acceptor basicity. On the other hand, in addition to these two major contributions, retention on PRP-1 is markedly influenced by the solute hydrogen bond donor acidity. We believe that PRP-1 exhibits a more adsorption-like retention mechanism than do the other phases. With the inorganic oxide-based phases, the aromatic phases are less retentive than the aliphatic phases but show a larger dependence on molecular polarizability. The enhanced polarizability of aromatic phases is the likely cause of some differences in their chromatographic selectivity relative to the aliphatic phases.

Chemical Phenomena↗
PubMedDate not supplied

Immobilized artificial membrane chromatography: quantitative structure-retention relationships of structurally diverse drugs.

The chromatographic capacity factors (log k') for 32 structurally diverse drugs were determined by high performance liquid chromatography (HPLC) on a stationary phase composed of phospholipids, the so-called immobilized artificial membrane (IAM). In addition, quantitative structure-retention relationships (QSRR) were developed in order to explain the dependence of retention on the chemical structure of the neutral, acidic, and basic drugs considered in this study. The obtained retention data were modeled by means of multiple regression analysis (MLR) and partial least squares (PLS) techniques. The structures of the compounds under study were characterized by means of calculated physicochemical properties and several nonempirical descriptors. For the carboxylic compounds included in the analysis, the obtained results suggest that the IAM-retention is governed by hydrophobicity factors followed by electronic effects due to polarizability in second place. Further, from the analysis of the results obtained of two developed quantitative structure-permeability studies for 20 miscellaneous carboxylic compounds, it may be concluded that the balance between polarizability and hydrophobic effects is not the same toward IAM phases and biological membranes. These results suggest that the IAM phases could not be a suitable model in assessing the acid-membrane interactions. However, it is not possible to generalize this observation, and further work in this area needs to be done to obtain a full understanding of the partitioning of carboxylic compounds in biological membranes. For the non-carboxylic compounds included in the analysis, this work shows that the hydrophobic factors are of prime importance for the IAM-retention of these compounds, while the specific polar interactions, such as electron pair donor-acceptor interactions and electrostatic interactions, are also involved, but they are not dominant.

Algorithms↗
PubMed2004

Retention of arsenic and selenium compounds using limestone in a coal gasification flue gas.

Volatile arsenic and selenium compounds present in coals may cause environmental problems during coal combustion and gasification. A possible way to avoid such problems may be the use of solid sorbents capable of retaining these elements from flue gases in gas cleaning systems. Lime and limestone are materials that are extensively employed for the capture of sulfur during coal processing. Moreover, they have also proven to have good retention characteristics for arsenic and selenium during combustion. The aim of this work was to ascertain whether this sorbent is also useful for retaining arsenic and selenium species in gases produced in coal gasification. The study was carried out in a laboratory-scale reactor in which the sorbent was employed as a fixed bed, using synthetic gas mixtures. In these conditions, retention capacities for arsenic may reach 17 mg g(-1) in a gasification atmosphere free of H2S, whereas the presence of H2S implies a significant decrease in arsenic retention. In the case of selenium, H2S does not influence retention which may reach 65 mg g(-1). Post-retention sorbent characterization, thermal stability, and water solubility tests have shown that chemical reaction is one of the mechanisms responsible for the capture of arsenic and selenium, with Ca(AsO2)2 and CaSe being the main compounds formed.

Air Pollution↗
PubMed2006

Virus retention and transport in chemically heterogeneous porous media under saturated and unsaturated flow conditions.

Retention and transport of colloids and microorganisms are complex processes, especially in the vadose zone due to the more complicated water flow regime and additional interfacial reactions involved. In this study, we examined the retention and transport behavior of two bacteriophages, MS-2 and phiX174, in homogeneous and chemically heterogeneous media under variably saturated conditions. Column experiments with glass beads (treated to have either hydrophilic or hydrophobic surface properties) were conducted using a phosphate-buffered saline solution at different pore water ionic strengths ranging from 0.025 to 0.163 M. In columns packed with 100% hydrophilic glass beads, retention of the viruses increased with decreasing water content and increasing ionic strength, a result similar to those reported in the literature. However, greater retention of both MS-2 and phiX174 was observed in saturated columns than in unsaturated columns packed with a 1:1 mixture of hydrophilic and hydrophobic glass beads, especially at high ionic strengths. This result contradicts the common belief that viruses (and colloids in general) are subject to greater removal in unsaturated media. Our study suggests that while the mechanisms controlling colloid interfacial interactions (i.e., attachment on solid-water and air-water interfaces and film straining) on the pore scale are relevant, nonuniform wetting conditions due to heterogeneous grain surface hydrophobicity can strongly influence water flow and phase interconnection. Under these conditions, hydrodynamic effects on the mesopore scale will dominate pore-scale interfacial reactions in controlling the extent of colloid retention and movement in unsaturated media.

Bacteriophage phi X 174↗
PubMed2006

Saturation to improve pollutant retention in a rain garden.

Rain gardens have been recommended as a best management practice to treat stormwater runoff. Replicate rain gardens were constructed in Haddam, CT, to treat roof runoff. The objective of this study was to assess whether the creation of a saturated zone in a rain garden improved retention of pollutants. The gardens were sized to store 2.54 cm (1 in) of runoff. Results show high retention of flow; only 0.8% overflowed. Overall, concentrations of nitrite+ nitrate-N, ammonia-N, and total-N (TN) in roof runoff were reduced significantly by the rain gardens. Total-P concentrations were significantly increased by both rain gardens. ANCOVA results show significant reductions in TN (18%) due to saturation. Redox potential also decreased in the saturated garden. Rain garden mulch was found to be a sink for metals, nitrogen, and phosphorus, but rain garden soils were a source for these pollutants. The design used for these rain gardens was effective for flow retention, but did not reduce concentrations of all pollutants even when modified. These findings suggest that high flow and pollutant retention could be achieved with the 2.54 cm design method, but the use of an underdrain could reduce overall pollutant retention.

Construction Materials↗
PubMed2002

Retention of aroma compounds in starch matrices: competitions between aroma compounds toward amylose and amylopectin.

The retention of three aroma compounds-isoamyl acetate, ethyl hexanoate, and linalool--from starch-containing model food matrices was measured by headspace analysis, under equilibrium conditions. We studied systems containing standard or waxy corn starch with one or two aroma compounds. The three studied aroma compounds interact differently: ethyl hexanoate and linalool form complexes with amylose, and isoamyl acetate cannot. However, in systems containing one aroma compound, we observed with both starches a significant retention of the three molecules. These results indicate that amylopectin could play a role in the retention of aroma. In systems containing two aroma compounds in a blend, the retentions measured for isoamyl acetate and for linalool were either equal to or less than those in systems where they were added alone. This phenomenon was attributed to competition between aroma compounds to bind with starch. The retention of aroma compounds blended in starch-based systems gave us additional information which confirmed that interactions occur not only with amylose but also with amylopectin.

Acyclic Monoterpenes↗
PubMed2004

Aroma retention in sol-gel-made silica particles.

The retention performance of aroma molecules from different chemical classes (e.g., alcohols, esters, aldehydes, and terpenes) by silica particles made by hydrolysis of tetraethyl orthosilicate is investigated. Since particle morphology, porosity, and pore size distribution can be controlled by the sol-gel preparation method, the influence of the nanoconfinement in the microporous matrix on aroma retention is studied as well as the effect of the initial aroma load of the particles. As the porosity is decreased, aroma molecules are entrapped more efficiently in the silica particles. The retention performance decreased from alcohols > aldehydes >/= esters > terpenes as with polar organic matrices. Open sol-gel-made silica particles show an increased retention with increasing aroma load, while denser silica matrices show a maximum retention with increasing load.

Alcohols↗
PubMed2005

Lactic acid decreases Fe(II) and Fe(III) retention but increases Fe(III) transepithelial transfer by Caco-2 cells.

Lactic acid (LA) has been proposed to be an enhancer for dietary iron absorption, but contradictory results have also been reported. In the present study, fully differentiated Caco-2 cell monolayers were used to evaluate the effects of LA (1-50 mmol/L) on the cellular retention and transepithelial transport of soluble non-heme iron (as ferric nitrilotriacetate). Our data revealed a linear decline in Fe(III) retention with respect to the concentration of LA added. In the presence of 50 mmol/L LA, retention of Fe(III) and Fe(II) decreased 57% and 58%, respectively. In contrast, transfer of Fe(III) across the cell monolayer was doubled, while Fe(II) transfer across the cell monolayer decreased 35%. We conclude that LA reduces cellular retention and transepithelial transport of Fe(II) by Caco-2 cells in a dose-dependent manner. However, while LA also reduces retention of Fe(III) by Caco-2 cells, the transfer of Fe(III) across cell monolayers is enhanced, possibly due to effects on paracellular transport.

Biological Transport↗
PubMed2000

Effect of pH on retention of aroma compounds by beta-lactoglobulin.

Interactions between volatile compounds and BLG in aqueous solution were studied using static and dynamic headspace techniques (exponential dilution). The intensity of interactions between methyl ketones (C7-C9), ethyl esters (C6-C9), limonene, myrcene, and beta-lactoglobulin (BLG) were estimated by determination of the relative infinite dilution activity coefficients (gamma(r)). For a constant pH value, the methyl ketones retention by BLG increased significantly with the hydrophobicity of the volatiles, whereas the retention reached a maximum for ethyl octanoate in the ester series, indicating a possible steric hindrance. For limonene and myrcene an unexpected increase in headspace concentration or "salting out" effect was noticed for acid pH. The variations of the retention according to the pH increase of the medium from pH 3 to pH 11 could be related to structural modifications of the BLG. The retention increase observed between pH 3 and pH 9 resulted from the flexibility modification of the protein, allowing better accessibility to the primary or the secondary hydrophobic sites, whereas the dramatic decrease observed at pH 11 was the consequence of the alkaline denaturation of BLG. Electrostatic interactions occurring at pH 7.5 could also explain the observed retention increase.

Acyclic Monoterpenes↗
PubMed2006

Retention prediction of peptides based on uninformative variable elimination by partial least squares.

A quantitative structure-retention relationship analysis was performed on the chromatographic retention data of 90 peptides, measured by gradient elution reversed-phase liquid chromatography, and a large set of molecular descriptors computed for each peptide. Such approach may be useful in proteomics research in order to improve the correct identification of peptides. A principal component analysis on the set of 1726 molecular descriptors reveals a high information overlap in the descriptor space. Since variable selection is advisable, the retention of the peptides is modeled with uninformative variable elimination partial least squares, besides classic partial least squares regression. The Kennard and Stone algorithm was used to select a calibration set (63 peptides) from the available samples. This set was used to build the quantitative structure-retention relationship models. The remaining 27 peptides were used as independent external test set to evaluate the predictive power of the constructed models. The UVE-PLS model consists of 5 components only (compared to 7 components in the best PLS model), and has the best predictive properties, i.e., the average error on the retention time is less than 30 s. When compared also to stepwise regression and an empirical model, the obtained UVE-PLS model leads to better and much better predictions, respectively.

Algorithms↗
PubMed2006

Prediction of liquid chromatographic retention times of peptides generated by protease digestion of the Escherichia coli proteome using artificial neural networks.

We developed a computational method to predict the retention times of peptides in HPLC using artificial neural networks (ANN). We performed stepwise multiple linear regressions and selected for ANN input amino acids that significantly affected the LC retention time. Unlike conventional linear models, the trained ANN accurately predicted the retention time of peptides containing up to 50 amino acid residues. In 834 peptides, there was a strong correlation (R2 = 0.928) between measured and predicted retention times. We demonstrated the utility of our method by the prediction of the retention time of 121,273 peptides resulting from LysC-digestion of the Escherichia coli proteome. Our approach is useful for the proteome-wide characterization of peptides and the identification of unknown peptide peaks obtained in proteome analysis.

Chromatography, High Pressure Liquid↗
PubMed2000

Uptake and retention of selenite and selenomethionine in cultured K-562 cells.

The selenium uptake and retention have been studied in K-562 cells exposed to selenite or selenomethionine. In the uptake experiments the cells were exposed to two doses of selenite (5 or 50 microM) or selenomethionine (10 or 50 microM). In the retention study the cells were treated for 2 h with the above mentioned doses of the selenocompounds before being observed at different times. The selenium uptake in cells exposed to selenite 5 microM began to saturate at 8 h, but increased again between 48 and 96 h. In cells exposed to selenite 50 microM the selenium uptake never reached a maximum, however, at 48 and 96 h the cell viability decreased strongly. The two doses of selenite showed different retention patterns, with a relatively small cellular decrease of selenium after treatment with selenite 5 microM compared to treatment with 50 microM of selenite. The selenium uptake in cells exposed to selenomethionine 10 microM or selenomethionine 50 microM began to saturate at 24 h and 48 h, respectively. The retention patterns were similar for both selenomethionine doses with a continuous decrease of the selenium concentration during the whole observation period. The results indicated a more controlled uptake and retention pattern of selenomethionine compared to selenite.

Humans↗