SEARCH · PubMed Health

Results for “Retention”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 451 records · Page 25Linked to original sources

PubMed2006

NMDA receptor blockers prevents the facilitatory effects of post-training intra-dorsal hippocampal NMDA and physostigmine on memory retention of passive avoidance learning in rats.

In the present study, the effects of post-training intra-dorsal hippocampal (intra-CA1) injection of an N-methyl-D-aspartate (NMDA) receptor agonist and competitive or noncompetitive antagonists, on memory retention of passive avoidance learning was measured in the presence and absence of physostigmine in rats. Intra-CA1 administration of lower doses of the NMDA receptor agonist NMDA (10(-5) and 10(-4) microg/rat) did not affect memory retention, although the higher doses of the drug (10(-3), 10(-2) and 10(-1) microg/rat) increased memory retention. The greatest response was obtained with 10(-1) microg/rat of the drug. The different doses of the competitive NMDA receptor antagonist DL-AP5 (1, 3.2 and 10 microg/rat) and noncompetitive NMDA receptor antagonist MK-801 (0.5, 1 and 2 microg/rat) decreased memory retention in rats dose dependently. Both competitive and noncompetitive NMDA receptor antagonists reduced the effect of NMDA (10(-2) microg/rat). In another series of experiments, intra-CA1 injection of physostigmine (2, 3 and 4 microg/rat) improved memory retention. Post-training co-administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) and physostigmine (1 microg/rat), doses which were ineffective when given alone, significantly improved the retention latency. The competitive and noncompetitive NMDA receptor antagonists, DL-AP5 and MK-801, decreased the effect of physostigmine (2 microg/rat). Atropine decreased memory retention by itself and potentiated the response to DL-AP5 and MK-801. In conclusion, it seems that both NMDA and cholinergic systems not only play a part in the modulation of memory in the dorsal hippocampus of rats but also have demonstrated a complex interaction as well.

Analysis of Variance↗

PubMed2004

Relative retention of the fibroblast growth factors FGF-1 and FGF-2 on strong cation-exchange sorbents.

The isocratic retention of two heparin-binding fibroblast growth factors, FGF-1 (acidic FGF) and FGF-2 (basic FGF), was compared on a set of six preparative strong cation-exchange adsorbents. The FGFs comprise a solute pair that are structurally equivalent, yet differ in protein parameters of potential importance in cation-exchange chromatography, such as isoelectric point, net charge, and the number and distribution of basic amino acids. The cation-exchange adsorbents comprise a diverse set of materials in common use for protein purification, with physical and chemical properties that have been characterized and described previously. Isocratic k' values for the two proteins obtained on each adsorbent at several different [NaCl] are compared with one another and with corresponding data for hen egg lysozyme, which is also strongly retained on cation-exchangers. Of the six adsorbents examined, three showed strong retention of both FGFs, with equivalent k' values for FGF-1 and FGF-2. Three others, which showed weaker overall retention for the FGF pair, showed much larger retention differences between FGF-1 and FGF-2. The trends in retention order among the stationary phases are very similar to those seen previously with other unrelated proteins. However, retention differences between the two FGFs, and between the FGFs and lysozyme, do not correlate well with simple charge properties such as net charge, indicating, as in some previous studies, the importance of local regions on the protein surface in determining retention. These observations are interpreted in terms of the structural features of the proteins and the physicochemical properties of the adsorbents.

Adsorption↗

PubMed2004

Accurate prediction of the retention behaviour of polydisperse macromolecules based on a minimum number of experiments.

This study illustrates how retention models can be used to accurately predict the retention behaviour of polydisperse macromolecules in LC separations. It highlights that the number of experiments required can be drastically reduced when the relationship between the model parameters and molecular structure parameters (e.g. molar mass) can be incorporated into one global model. A practical implication of this work is that an appropriate model can then be used for the determination of molar-mass distributions for polydisperse samples. The globalised model can predict retention time as a function of molar mass and gradient slope. Both the original and globalised versions of the model were rigorously validated in terms of the difference between the predicted and experimental retention times. The original model had very low residuals and there was no apparent dependence of the errors on the applied gradient, the molar mass or the retention times. Confidence intervals for the model parameters (S and ln k0) were determined using a bootstrapping analysis of the residual errors in the predicted retention times. Confidence intervals were seen to broaden significantly as the mass of the polymer increased. The parameters were also seen to be highly correlated. For the global model, retention-time residuals remained quite low, even when the number of experiments used to determine the model parameters was reduced from approximately 100 to 10.

Chromatography, High Pressure Liquid↗

PubMed2005

Development of an inorganic cations retention model in ion chromatography by means of artificial neural networks with different two-phase training algorithms.

This paper describes development of artificial neural network (ANN) retention model, which can be used for method development in variety of ion chromatographic applications. By using developed retention model it is possible both to improve performance characteristic of developed method and to speed up new method development by reducing unnecessary experimentation. Multilayered feed forward neural network has been used to model retention behaviour of void peak, lithium, sodium, ammonium, potassium, magnesium, calcium, strontium and barium in relation with the eluent flow rate and concentration of methasulphonic acid (MSA) in eluent. The probability of finding the global minimum and fast convergence at the same time were enhanced by applying a two-phase training procedure. The developed two-phase training procedure consists of both first and second order training. Several training algorithms were applied and compared, namely: back propagation (BP), delta-bar-delta, quick propagation, conjugate gradient, quasi Newton and Levenberg-Marquardt. It is shown that the optimized two-phase training procedure enables fast convergence and avoids problems arisen from the fact that every new weight initialization can be regarded as a new starting position and yield irreproducible neural network if only second order training is applied. Activation function, number of hidden layer neurons and number of experimental data points used for training set were optimized in order to insure good predictive ability with respect to speeding up retention modelling procedure by reducing unnecessary experimental work. The predictive ability of optimized neural networks retention model was tested by using several statistical tests. This study shows that developed artificial neural network are very accurate and fast retention modelling tool applied to model varied inherent non-linear relationship of retention behaviour with respect to mobile phase parameters.

Algorithms↗

PubMed2005

Revised solute descriptors for characterizing retention properties of open-tubular columns in gas chromatography and their application to a carborane-siloxane copolymer stationary phase.

An iteration procedure is used to calculate revised solute descriptors for 103 varied compounds suitable for characterizing the retention properties of stationary phases for gas chromatography using the solvation parameter model. The iteration procedure utilizes a database of retention factors obtained on up to 39 open-tubular columns and up to five temperatures in the range 60-140 degrees C for the 103 solutes. The average of the standard deviation [Sigma(logk(exp)-logk(calc))(2)/(n(c)-1)](0.5) where logk(exp) is the experimental retention factor, logk(calc) the model predicted retention factor, and n(c) the total number of retention factors) on all columns is 0.018 for the revised solute descriptors compared with 0.045 for the original values. When used to characterize the retention properties of six open-tubular columns selected to represent different selectivity groups the revised solute descriptors afford improved values for the multiple correlation coefficient and standard deviations of the system constants, and about a three-fold improvement in the standard error of the estimate compared with the original solute descriptors. The revised solute descriptors were used to model retention on the carborane-siloxane copolymer stationary phase Stx-500. This phase has low cohesion, is weakly electron lone pair repulsive, weakly dipolar/polarizable, and weakly hydrogen-bond basic. It has no hydrogen-bond acidity. Its separation properties are similar to those of the poly(diphenyldimethylsiloxane) stationary phases containing 5% diphenylsiloxane monomer, but it is not selectivity equivalent to these phases, being more dipolar/polarizable and a weaker hydrogen-bond base.

Chromatography, Gas↗

PubMed2005

Retention profiles and mechanism of anion separation on latex-based pellicular ion exchanger in ion chromatography.

A retention model based on stoichiometric approach has been developed in order to describe analyte retention of anions on latex-based pellicular ion exchanger. The chromatographic process entails two stepwise and complex equilibria, first is ion-pair forming of analyte or eluent ion with ion-exchange sites under the effect of electrostatic forces due to the sulfonic layer behind the aminated functional groups of stationary phase. Second component is the ion-exchange between the analyte and eluent ions. As a new parameter of the fractional electrostatic coefficient of the ion exchange capacity was introduced to develop retention profiles of anions. Analysis of the dependence of the capacity factors on the eluent concentrations at different values of fractional coefficient shed light on the possible complex mechanism. Extensive experimental retention data were obtained for 14 anions (formate, acetate, propionate, pyruvate, lactate, chloride, nitrate, oxalate, malonate, succinate, tartarate, fumarate, maleate, sulphate) using hydroxide eluents of varying concentration. The ion-pair formation and ion-exchange selectivity constants for analyte and eluent species are determined using derived retention equation from experimental data by nonlinear iterative calculation. The model was utilized to predict retention data under elution conditions of practical importance. The predicted and obtained retention factors are in good agreement, which confirms the predictive power of the model.

Anion Exchange Resins↗

PubMed2006

Quantitative structure-retention relationship studies using immobilized artificial membrane chromatography I: amended linear solvation energy relationships with the introduction of a molecular electronic factor.

Linear solvation energy relationships (LSERs) amended by the introduction of a molecular electronic factor were employed to establish quantitative structure-retention relationships using immobilized artificial membrane (IAM) chromatography, in particular ionizable solutes. The chromatographic indices, log k(IAM), were determined by HPLC on an IAM.PC.DD2 column for 53 structurally diverse compounds, including neutral, acidic and basic compounds. Unlike neutral compounds, the IAM chromatographic retention of ionizable compounds was affected by their molecular charge state. When the mean net charge per molecule (delta) was introduced into the amended LSER as the sixth variable, the LSER regression coefficient was significantly improved for the test set including ionizable solutes. The delta coefficients of acidic and basic compounds were quite different indicating that the molecular electronic factor had a markedly different impact on the retention of acidic and basic compounds on IAM column. Ionization of acidic compounds containing a carboxylic group tended to impair their retention on IAM, while the ionization of basic compounds did not have such a marked effect. In addition, the extra-interaction with the polar head of phospholipids might cause a certain change in the retention of basic compounds. A comparison of calculated and experimental retention indices suggested that the semi-empirical LSER amended by the addition of a molecular electronic factor was able to reproduce adequately the experimental retention factors of the structurally diverse solutes investigated.

Algorithms↗

PubMed2007

Hydrophobic interaction chromatography of proteins. II. Solution thermodynamic properties as a determinant of retention.

A general thermodynamic relation was derived to correlate protein solubility to retention in hydrophobic interaction chromatography (HIC). This relation is built on a thermodynamic formulation presented previously by Melander, Horváth and co-workers in the context of the solvophobic theory, but the final result is independent of this model framework. The relation reflects an increase in protein retention in HIC under conditions that promote precipitation or crystallization, consistent with early descriptions of HIC. To examine the contribution of protein solubility to retention in HIC, isocratic elution experiments were performed with four different commercially available agarose media and four model proteins (ribonuclease A (RNA), lysozyme (LYS), myoglobin (MYO), and ovalbumin (OVA)). A wide variety of retention trends were observed as a function of protein, adsorbent type, salt type and concentration, and pH. In general, however, the results show that solubility, or its surrogate, the second osmotic virial coefficient, which reflects solution thermodynamic properties, correlates well with HIC retention in many cases; this includes correctly predicting reverse Hofmeister effects, which cannot be explained by retention models based on the solvophobic theory and preferential interaction theory. However, solution properties could not explain retention behavior under some conditions. In those cases, effects such as protein-surface interactions or conformational change could be important determinants of protein adsorption.

Chromatography, High Pressure Liquid↗

PubMed2006

Retention of quinolones on human serum albumin and alpha1-acid glycoprotein HPLC columns: relationships with different scales of lipophilicity.

The retention of 10 quinolone antibacterial agents on HPLC stationary phases supporting human serum albumin (HSA) or alpha(1)-acid glycoprotein (AGP) was investigated. Among ofloxacine and flumequine, the two chiral compounds in the selected set, only the latter showed a split chromatographic peak and only on HSA but not on AGP, indicating that enantioselective specific sites play only a minor role in the retention. The retention of quinolones, which included four acidic and six zwitterionic congeners, was correlated with various lipophilicity scales: (i) theoretically calculated values, clogP, (ii) values measured at pH 7.4 by the shake-flask method, logD(7.4), and (iii) values extrapolated by retention data measured by ion-pair reversed-phase high performance liquid chromatography (RP-HPLC). We assumed that the latter values, logP(i.p.), were close to the lipophilicity of the neutral forms, logP(N), for both acidic and zwitterionic congeners. No relationship was found between retention on serum proteins and clogP values, whereas a reasonable relationship was found with logD(7.4) values, but only when the two subclasses, acidic and zwitterionic congeners, were considered separately. The relationship between retention data on serum proteins and logP(i.p.) values indicated that the affinity for serum proteins depends on the lipophilicity of the neutral forms only for logP values up to 1.5. Above this value, protein retention does not further increase, becoming almost constant. Based on both the observations above reported and the small values of the slopes of regression equations, we conclude that the interaction of the more lipophilic quinolones, mainly the zwitterions, with serum proteins is not governed uniquely by lipophilicity but also by other mechanisms, probably of electrostatic nature.

Blood Proteins↗

PubMed2005

Heritage retention and bean intake correlates to dietary fiber intakes in Hispanic mothers--Qué Sabrosa Vida.

OBJECTIVE: To determine the correlation between fruit/vegetable and bean stage of change and heritage retention with fiber intake in Hispanic mothers. DESIGN: Cross-sectional study design used baseline survey data from the Qué Sabrosa Vida nutrition intervention developed by the University of Texas School of Public Health, Human Nutrition Center (Houston, TX). Participants were recruited and data gathered from telephone surveys using random digit dial methodology ensuring proportional representation of demographic subgroups. SUBJECTS: Hispanic mothers (N=293) responded to the baseline survey in El Paso, TX. ANALYSES: Using SPSS version 11.0 (SPSS Inc, Chicago, IL), descriptive statistics, chi2 , correlations, and analyses of variance were used to determine the relationship of variables to fiber intake. Potential confounding variables included were grain stage of change, age, education level, and marital status. RESULTS: Mean fiber intake for mothers was 21.9 g/day and mean age was 35.8 years. Chi2 of fruit/vegetable stage was P =.784. Chi 2 of bean stage, heritage retention score, age, and education to fiber intake were significant, P =.008, P =.004, P =.005, and P =.001, respectively. Bean stage, heritage retention score, age, and education level were significantly correlated. One-way analysis of variance of bean stage, heritage retention score, age, and education level by fiber intake were significant, P =.006, P =.001, P =.004, and P =.002, respectively. CONCLUSIONS: Older Hispanic mothers in the action/maintenance bean stage with strong heritage retention score had greater fiber intakes than younger mothers in pre-action bean stage with weak heritage retention score. As education level increased fiber intake decreased. Encouragement of bean consumption and heritage retention may increase fiber intake in this Hispanic mother population.

Acculturation↗

PubMed2005

Liposome-encapsulated vincristine, vinblastine and vinorelbine: a comparative study of drug loading and retention.

A comparative study of the loading and retention properties of three structurally very closely related vinca alkaloids (vincristine, vinorelbine and vinblastine) in liposomal formulations has been performed. All three vinca alkaloids showed high levels of encapsulation when accumulated into egg sphingomyelin/cholesterol vesicles in response to a transmembrane pH gradient generated by the use of the ionophore A23187 and encapsulated MgSO4. However, despite the close similarities of their structures the different vinca drugs exhibited very different release behavior, with vinblastine and vinorelbine being released faster than vincristine both in vitro and in vivo. The differences in loading and retention can be related to the lipophilicity of the drugs tested, where the more hydrophobic drugs are released more rapidly. It was also found that increasing the drug-to-lipid ratio significantly enhanced the retention of vinca alkaloids when the ionophore-based method was used for drug loading. In contrast, drug retention was not dependent on the initial drug-to-lipid ratio for vinca drugs loaded into liposomes containing an acidic citrate buffer. The differences in retention can be explained on the basis of differences in the physical state of the drug inside the liposomes. The drug-to-lipid ratio dependence of retention observed for liposomes loaded with the ionophore technique may provide a way to improve the retention characteristics of liposomal formulations of vinca drugs.

Animals↗

PubMed2006

Nitrogen in river basins: sources, retention in the surface waters and peatlands, and fluxes to estuaries in Finland.

Nitrogen export from diffuse and point sources and its retention in the major river basins of Finland is quantified and discussed. The estimated total export from river-basins in Finland was 119,000 tonnes N a(-1) for the period 1993 to 1998 based on N export from different land use types defined in a GIS-based assessment model, incorporated with estimates of N inputs from atmospheric deposition and point sources. Agriculture contributes 38% of the total export, varying in the range 35-85% in the south-western basins and 0-25% in the northern basins. This estimate of N export from agriculture was based on regional N balances together with data from small agricultural research catchments. Forestry contributes on average 9%, with increasing dominance towards eastern and northern parts of the country: from 2% to 15% in the southern-mid-western Finland basins to 10-30% in the large northern basins. 'Background' N export from forests on both mineral and organic soils contributes 27% on average; in the northern basins it may contribute from 40% up to 90% of the total load. The estimate was calculated based on practically all data available from 42 small, experimental catchments in Finland. Of the total N input to Finnish river-systems, 0% to 68% was retained in surface waters and/or peatlands, with a mean retention of 22%. The highest retention of N (36-61%) was observed in the basins with the highest lake percentages. The lowest retention (0-10%) of N was in the coastal basins with practically no lakes. In the national N mass balance, 38,000 tonnes N a(-1) (32%) was estimated as lake retention and 4,000 tonnes N a(-1) (3%) as retention in peatlands. On the basis of mass balances and sensitivity analysis, retention was in most cases estimated to be in the range of 7.5-12.5 kg ha(-1)a(-1) in lakes and 0-1.5 kg ha(-1)a(-1) in peatlands. The model results were tested using the split-sample technique and uncertainty estimates for different data sources are provided and discussed.

Agriculture↗

PubMed2000

Oral glucose retention, saliva viscosity and flow rate in 5-year-old children.

There are significant differences of glucose retention in site-specificity and individuals. Sixty-two 5-year-old nursery schoolchildren participated in this study on the relation between the viscosity of saliva and flow rate and glucose retention. Each child was instructed to rinse his/her mouth with a glucose solution (0.5 M, 5 ml) and then to spit out. Three minutes after rinsing, glucose retention was determined. Resting saliva was collected by a natural outflow method, then the flow rate was determined. A rotational viscometer was used to determine the viscosity. Glucose retention and flow rate were correlated at the left maxillary primary molars, and glucose retention and viscosity were correlated at the maxillary central primary incisors. It was concluded that glucose retention after glucose mouth rinsing was site-specific, and that glucose retention and the index of decayed, missing and filled primary teeth (dmft) were slightly correlated with the salivary viscosity and flow rate.

Analysis of Variance↗

PubMed1986

Sexual differences in the distribution and retention of organic and inorganic mercury in methyl mercury-treated rats.

At 56 days of age, male and female Long-Evans rats received 1 mumole of 203Hg-labeled methyl mercuric chloride per kilogram sc and total, organic, and inorganic mercury contents and concentrations in tissues were determined for up to 98 days postdosing. Whole body clearance of mercury was faster in females than in males, and females attained higher peak percentages of the methyl mercury dose in kidney and brain than did males. Females had significantly higher mean percentages of the mercury dose present in the kidney and brain as organic or total mercury and in brain as inorganic mercury than did males. Males had significantly higher mean percentages of the dose present as organic or total mercury in pelt and whole body than did females. When expressed on a concentration basis, the only significant sexual difference was in the higher average concentration of organic mercury in the kidneys of females. When expressed on a tissue content basis, significant male-female differences in the kinetics (sex X time interactions) of organic mercury retention were found in kidney, brain, skeletal muscle, pelt, and whole body. Significant sex X time interactions in the concentrations of organic mercury were found in kidney, skeletal muscle, and whole body. Kinetics of retention and concentration of inorganic Hg in the pelt differed significantly for males and females. Discordance in degree of statistical significance of differences in mercury contents and concentrations reflected in part differences in relative body composition of males and females. Integrated exposures of tissues of males and females to organic or inorganic mercury were determined by fitting multiexponential retention functions to retention data. Differences in integrated exposure were estimated by the female-to-male ratio of areas under retention curves. Reconstruction of whole body organic and inorganic mercury burdens from constituent tissues indicated that integrated exposures of males and females to inorganic mercury were equal but females had a lower integrated exposure to organic mercury. Integrated exposure of liver to either form of mercury was about equal in males and females. However, the integrated exposure of the brain of females to inorganic mercury was 2.19 times that of males suggesting a sexual difference in accumulation or retention of inorganic mercury in the nervous system. These sexual differences in distribution and retention of organic and inorganic mercury after methyl mercury exposure may underlie reported sexual differences in sensitivity to the toxic effects of methyl mercury.

Animals↗

PubMed2001

Baclofen-impairment of memory retention in rats: possible interaction with adrenoceptor mechanism(s).

This study concerned the influence of adrenoceptor agonists and antagonists on baclofen-induced impairment of memory retention. Intracerebroventricular injection of baclofen (0.25--2 microg/rat) reduced memory retention in rats. The combination of different doses of baclofen with a low dose of clonidine (0.5 microg/rat) elicited a greater decrease in memory retention. Yohimbine (1 microg/rat) potentiated the response to a low dose, but decreased the response to higher doses of baclofen. Single administration of clonidine (0.5--2 microg/rat) but not yohimbine (1--4 microg/rat) itself decreased memory retention. The combination of clonidine with yohimbine did not show any interaction. The low dose of phenylephrine (0.5 microg/rat) or prazosin (0.5 microg/rat) also potentiated the inhibition of memory retention by baclofen. Phenylephrine (0.5--3 microg/rat) increased, while prazosin (0.5--2 microg/rat) decreased memory retention. The combination of the two drugs showed an interaction. It may be concluded that an adrenoceptor mechanism may interact with the memory retention impairment induced by baclofen.

Adrenergic alpha-Agonists↗

PubMed2001

Comparison of selected retention models in reversed-phase liquid chromatography.

Retention models are usually compared by how well the model equation fits retention data for one solute taken over a range of mobile phase compositions. Even when retention data for multiple solutes are used, the quality of the fit is often judged by the statistical goodness-of-fit alone. This study compared four different RPLC retention models, encompassing three distinct mathematical forms. Each model was fit to the retention data of multiple solutes and the sets of best-fit parameters were examined in terms of the underlying physico-chemical assumptions of the models. Next, for the linear and quadratic models, some of the model parameters were calculated a priori and the rest of the model parameters were then obtained in subsequent fittings. The sets of best-fit parameters obtained in this manner were more consistent with the underlying assumptions of these models than were the sets of parameters obtained entirely through regressions to the experimental data. Thus, the extraction of parameters by fitting a model to the retention data of a single solute may result in unreliable values for those parameters, even in the case of a fit that would be considered good when judged by conventional statistical criteria. That is, although parameters extracted in such a fashion may be suitable for optimization or similar uses, they may not be suitable for determining the appropriateness of the underlying assumptions of retention models.

Chromatography, Liquid↗

PubMed2002

Comparative study of the linear solvation energy relationship, linear solvent strength theory, and typical-conditions model for retention prediction in reversed-phase liquid chromatography.

This paper describes two new retention models for predicting retention under different reversed-phase liquid chromatography (RPLC) conditions. The first one is a global linear solvation energy relationship (LSER) that expresses retention as a function of both solute LSER descriptors and mobile phase composition. The second is a so-called "typical-conditions model" that expresses retention under a given chromatographic condition as a linear function of retention under different so-called "typical" conditions. The global LSER was derived by combining the local LSER model and the linear solvent strength theory (LSST) of RPLC. Compared to local LSER and the LSST models, the global LSER model requires far fewer retention measurements for calibrating the model when different solutes and different mobile phase compositions are involved. Its fitting performance is equal to the local LSER model but worse than that of LSST. The poor fit of the global LSER results primarily from the local LSER model and not from the LSST model. The typical-conditions model (TCM) was developed based on a concept of multivariate space that is conceptually compatible with LSER. However, no LSER descriptors are used in the TCM approach. The number of input conditions needed in the typical-conditions model is determined by the chemical diversity of the solutes and the conditions involved. Principal component analysis (PCA) and iterative key set factor analysis (IKSFA) were used to find the number of typical conditions needed for a given data set. Compared to LSER, LSST, and global LSER, the typical-conditions model is more precise and requires fewer retention measurements for calibrating the model when different solutes and different stationary and/or mobile phases are involved.

Chromatography, Liquid↗

PubMed2002

Temperature effects on the retention of n-alkanes and arenes in helium-squalane gas-liquid chromatography. Experiment and molecular simulation.

Experiments and molecular simulations were carried out to study temperature effects (in the range of 323 to 383 K) on the absolute and relative retention of n-hexane, n-heptane, n-octane, benzene, toluene and the three xylene isomers in gas-liquid chromatography. Helium and squalane were used as the carrier gas and retentive phase, respectively. Both the experiments and the simulations show a markedly different temperature dependence of the retention for the n-alkanes compared to the arenes. For example, over the 60 K temperature range studied, the Kovats retention index of benzene is found to increase by about 16 or 18+/-10 retention index units determined from the experiments or simulations, respectively. For toluene and the xylenes, the experimentally measured increases are similar in magnitude and range from 14 to 17 retention index units for m-xylene to o-xylene. The molecular simulation data provide an independent method of obtaining the transfer enthalpies and entropies. The change in retention indices is shown to be the result of the larger entropic penalty and the larger heat capacity for the transfer of the alkane molecules.

Alkanes↗