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PubMed2001

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography. V. Study of retention mechanism using linear solvation energy relationships.

A linear solvation energy relationship model was used to characterize the retention behavior of a stationary phase based upon a nematic side-on liquid crystalline polymer (SOLCP) in reversed-phase liquid chromatography. The set of solutes was constituted of a high variety of compounds whose molecular sizes were considerably smaller than the mesogenic unit size. The results showed good statistical fits for these retention data in 65:35, 75:25 and 85:15 (v/v) methanol-water mobile phases. Both the cavity term and excess molar refraction are the most important favorable retention-governing parameters, whereas the solute hydrogen bond acceptor basicity is the most unfavorable retention parameter. Hydrophobicity and pi-pi interactions decrease strongly when the percentage of methanol increases, leading to an important retention decrease despite the fact that the hydrogen bond interaction weakens as the organic solvent is added. The shape recognition ability of this side-on liquid crystalline stationary phase on polycyclic aromatic hydrocarbon solutes is partly explained by the solutes' high polarizability due to the presence of pi-electrons. However, the solute polarizability is not sufficient and a stationary phase's "structure effect" must to be taken into account for the shape discrimination observed. The strong interaction between liquid crystal molecules caused likely a adsorption retention mechanism rather than a partition mechanism.

Chromatography, High Pressure Liquid↗
PubMed1986

Correlation between the molecular structure of cardiac glycosides, steroid hormones and carbohydrates and their retention in high-performance liquid chromatography.

Cardiac glycoside molecules and other molecules can be thought as consisting of a number of functional groups that make individual average contributions to the overall retention. To evaluate these contributions it was necessary to solve a system of linear equations by the last-squares method for a number of cardiac glycosides and steroid hormones, the retention volumes of which were determined under standard conditions. Experimental values of the logarithm of retention volumes are compared with calculated values, ln VR' = sigma niai, where ai = ln (VR')i is the contribution to the retention of group i and ni is the number of functional groups in a molecule. Such a comparison shows that these values are in good agreement and the method may be used to evaluate the retentions of cardiac glycosides, steroid hormones and carbohydrates. The correlation of biological activity of cardiac glycosides and the logarithm of retention volumes is expressed by a linear equation, the coefficients of which have been determined. This equation shows that biological activity is related to the hydrophilic-hydrophobic properties of cardiac glycosides and consequently with the transport of these compounds to the receptor.

Carbohydrates↗
PubMed1989

Interplay of hydrophobic and electrostatic interactions in biopolymer chromatography. Effect of salts on the retention of proteins.

The effect of salt on the retention behavior of proteins in electrostatic and hydrophobic interaction chromatography is described by a three-parameter equation, log k' = A - B log ms + Cms, where k' is the retention factor and ms is the molality of the salt in the eluent. Parameter B, termed the electrostatic interaction parameter, depends on the characteristic charge of the protein and the salt counterion and governs the change of retention with the salt concentration in ion-exchange chromatography. According to the model the magnitude of the hydrophobic interaction parameter C is determined by the hydrophobic contact area upon protein binding at the stationary phase surface and the properties of the salt as measured by its molal surface tension increment. Retention data measured at different salt concentrations in the eluent on a variety of ion exchangers can be fitted to the above equation which yields U-shaped plots of log k' against log ms. The limiting slopes of the appropriate plots at sufficiently low and high salt concentrations can be used to evaluate the electrostatic and hydrophobic interaction parameters, respectively. The approach, which is based on a combination of established treatments of electrostatic and hydrophobic interactions offers a convenient framework for analyzing retention data in biopolymer high-performance liquid chromatography and for the characterization of stationary phases. Furthermore, it may facilitate some characterization of protein molecules on the basis of their retention behavior as a function of the concentration and nature of the salt in the eluent. In the treatment of electrostatic interactions use is made of the counterion condensation theory that is believed to make possible a more comprehensive analysis than the traditional stoichiometric ion-exchange model which assumes binding of the proteins by coulombic interactions at discrete sites. The treatment of hydrophobic interactions is based on an adaptation of the solvophobic theory which predicts that the hydrophobic portion of the free energy of binding is proportional to the hydrophobic contact area and the microthermodynamic surface tension of the aqueous salt solution. Despite its simplicity the theory was successful in explaining the observed effect of the nature and concentration of salt in the eluent, the pH and the effect of the density of fixed charges at the surface of the stationary phase in the absence of specific salt effects.

Biopolymers↗
PubMed2002

Prediction of the retention in reversed-phase liquid chromatography using solute-mobile phase-stationary phase polarity parameters.

A previously reported algorithm, based on the equation: log k = (log k)o + p(PN(m) - PN(s)), that relates the retention in reversed-phase liquid chromatography with solute (p), mobile phase (PN(m)) and stationary phase (PN(s)) relative polarity parameters, is improved. The retention data reported by several authors for different sets of compounds, eluted with acetonitrile-water and methanol-water mixtures, are used to test the algorithm and elaborate a database of p values. The methodology is successfully applied to predict the retention using PN(m), values calculated as PN(m) = 1.00 - (2.13phi)/(1+1.4phi) for acetonitrile-water and PN(m) = 1.00 - (1.33phi)/(l1 + 0.47phi) for methanol-water, phi being the organic solvent volumetric fraction. The polarity parameters are demonstrated to be useful to transfer retention data between solvent systems and between columns. Accordingly, the retention in a solvent system is predicted by characterising the working column with a small training set of compounds having diverse polarities, and using the p values known for another solvent system or column. The p polarity parameter is found to be a good descriptor of the retention, allowing the prediction of the expected elution order and peak overlaps.

Algorithms↗
PubMed2002

Effect of molecular interactions on retention and selectivity in reversed-phase liquid chromatography.

The linear solvation energy relationships (LSERs) have been applied in the last years for description and prediction of retention and selectivity in reversed-phase liquid chromatography with good results. Widely different stationary phases have been compared and characterized by LSERs. In recent publications the influence of the type of the organic moderator and the composition of the mobile phase have also been described. However, the influence of the molecular properties of the solutes to be separated has never been discussed. According to the LSER model variation in retention factors (log k) with solute structure can be related to their potential for various intermolecular interactions. The retention factor is given as the sum of the terms of the LSER equation representing various types of molecular interactions. For this reason the influence of the structure and molecular properties of the solutes to be separated can also be investigated using the LSER equation. In this study we shall demonstrate how the specific molecular interactions influence chromatographic retention and selectivity. We intend to show that retention and selectivity depend on all participants of the system. In addition to the structure and properties of the stationary phase and the type and composition of the mobile phase the molecular properties of the solutes, characterized by the solvation parameters, will also influence the type and extent of the various molecular interactions governing retention and selectivity.

Chromatography, Liquid↗
PubMed2002

Modeling of retention behavior in capillary electrochromatography from chromatographic and electrophoretic data.

A phenomenological approach was presented to describe the retention behaviors of solutes in capillary electrochromatography (CEC). Equations for calculation of the retention time and actual chromatographic retention factor for ionic solutes, weak monoprotic acid and weak monoprotic base were derived, which can be described by two general expressions regardless the charge status of the solute. The general expressions enable calculation of the retention time and retention factor in CEC from chromatographic and electrophoretic data, which were experimentally verified with a variety of compounds and a variety of CEC modes. Based on this approach, the chromatographic retention and the electrophoretic migration in the CEC systems studied were found to be two independent processes. The validity of this approach was also discussed.

Chromatography, Micellar Electrokinetic Capillary↗
PubMed2003

Predictive approaches to gradient retention based on analyte structural descriptors from calculation chemistry.

Quantitative structure retention relationships (QSRRs) were applied to predict reversed-phase HPLC gradient retention. The performance of the recently recommended QSRR models was compared. One tested model is based on structural descriptors from molecular modeling. To quantitatively characterize the structure of analytes the following three structural descriptors are employed: total dipole moment, electron excess charge of the most negatively charged atom and water-accessible molecular surface area. Reliability of the resulting gradient retention time predictions was compared to that provided by the models relating retention to the theoretically calculated logarithm of n-octanol-water partition coefficient, log P. The requested values of log P were obtained using three commercially available softwares. The predicted retention parameters were compared for a series of structurally diversified small molecular mass analytes. It has been demonstrated that the retention predictions from both the molecular modeling descriptors-based and the log P-based QSRR are characterized by similar errors. It has been hypothesized that the optimization of separation based on QSRRs and the linear solvent strength theory might be of practical analytical value.

Chromatography, High Pressure Liquid↗
PubMed2003

Application of the reversed-phase liquid chromatographic model to describe the retention behaviour of polydisperse macromolecules in gradient and isocratic liquid chromatography.

This paper illustrates how conventional models of chromatographic behaviour can be used to predict the separation behaviour of polydisperse macromolecules. Using polystyrene and polymethylmethacrylate homo- and co-polymeric standards, the models were validated by comparing experimental retention behaviour with that predicted by the chromatographic model. The experimental retention time of each of the samples was entered into a spreadsheet application, which calculated the parameters that best described retention (for a given model). When a correlation between the relevant parameters and molecular mass was established, that correlation was used to predict the change in retention behaviour over the molecular-mass range. An expression introduced in a previous paper, to calculate the critical mobile-phase composition of a homopolymer was validated using polystyrene homopolymers. A second expression, which can predict the elution behaviour of copolymers, was also validated. This expression can predict the retention of a copolymer, based solely onthe retention of the homopolymeric units that make up the copolymer.

Calibration↗
PubMed2003

Mosaic increments for predicting the gas chromatographic retention data of the chlorobenzenes.

The chlorinated organic compounds are very important from the point of view of the chemical industry and environmental protection, and therefore the gas chromatographic analysis of these compounds is very interesting for analytical chemists. In this paper we studied the relationship between the molecular structure and gas chromatographic retention on several stationary phases having different polarity and at several temperatures of benzene and 12 chlorobenzene compounds as model compounds. A coding system involving primary (mosaic increments) and secondary (bond increments)calculation methods was developed. The retention indices of benzene and the chlorobenzenes calculated on HP-5 at 120 degrees C shows a better performance of the mosaic increments (average absolute deviation delta of 1.7 retention index units) compared with the bond increments (delta = 11.7 retention index units). Retention factors, k, calculated with mosaic increments for chlorobenzenes on SPB-1 and WAX-10, at 140 degrees C, yield average relative errors of epsilon = 0.9 and 3.5%, respectively. Therefore, the presented paper provides a new possibility for precalculation of the retention data.

Chlorobenzenes↗
PubMed1998

Calculation of peptide retention coefficients in normal-phase liquid chromatography.

The retention of 121 peptides was studied on a TSK Amide-80 column using solutions containing 0.1% trifluoroacetic acid and an increasing linear gradient of water in acetonitrile. The contribution of each residue to retention was calculated by linear multiple regression analysis. This paper described the contribution values 'hydrophilicity retention coefficients'. The result is an index of hydrophilicity retention coefficients for normal-phase liquid chromatography, analogous to the hydrophobicity indices calculated for the reversed-phase liquid chromatography. The order of residues in the index of one mode was substantially the inverse of the others'. Using the new hydrophilicity retention coefficients, retention times could be predicted for peptides of known amino acid content and sequence.

Amino Acid Sequence↗
PubMed1999

Retention modelling of electrostatic and adsorption effects of aliphatic and aromatic carboxylic acids in ion-exclusion chromatography.

The retention mechanism of aliphatic and aromatic carboxylic acids in ion-exclusion chromatography has been investigated with consideration of simultaneous electrostatic repulsion effects and hydrophobic adsorption effects. A mathematical relationship between the retention factor of the analyte and the mobile-phase composition (sulfuric acid concentration and percentage of methanol), the type of analyte (pKa and hydrophobicity) and some physical characteristics of the stationary phase has been derived. Thirteen carboxylic acids (comprising mono- and divalent, aliphatic and aromatic acids) were chosen and used to acquire retention data on three different cation-exchange stationary phases (in which the sulfonate functional groups are bound to polystyrene-divinylbenzene, polymethacrylate or silica) using 14 mobile-phase compositions of varying pH and percent methanol. These retention data were used to derive the parameters necessary to solve the retention model using non-linear regression. In this way, a quantitative measure of the effects of adsorption phenomena on analyte retention were obtained. The model was then used to optimise the separation of nine carboxylic acids.

Carboxylic Acids↗
PubMed2003

Free amino acids are important for the retention of protein and non-protein meals by the midgut of Aedes aegypti females.

There is a relationship between the normal progress of digestion and the retention or elimination of the proteins ingested with the meal by Aedes aegyti females. The addition of soybean trypsin inhibitor (STI) to a protein meal prevented digestion and resulted in a rapid elimination of the undigested proteins. The addition of a mix of free amino acids to a protein meal together with STI resulted in a significant increase in the retention of the undigested proteins during the first 10-15 hrs after feeding. The effect of the free amino acids on the retention of the proteins was concentration-dependent between 250 microg/ml and 5 mg/ml. Free amino acids were also important for the retention of non-protein meals. When females were fed a meal containing FITC-dextran (20 kD), most of this compound was eliminated into the feces by 10 hrs; the addition of free amino acid resulted in a significant increase in the retention of the FITC-dextran by the midgut during the first 15 hrs after feeding. The presence of free amino acids in the midgut lumen seems to be an important signal used by the mosquito to regulate the retention of the meal.

Aedes↗
PubMed1997

Preliminary investigation of a novel retentive system for hydrofluoric acid etch-resistant dental ceramics.

STATEMENT OF PROBLEM: A potential limitation to clinical use of In-Ceram and In-Ceram Spinell dental ceramics has been the inability to etch fit surfaces. PURPOSE: This study investigated a novel retentive system for In-Ceram and In-Ceram Spinell ceramics. MATERIAL AND METHODS: The system, Bateman etch retention system, relies on incorporation of plastic chips on the surface of a specimen. The plastic chips were subsequently burnt-out to leave pits on the fitting surface of the ceramic restoration. The effect of these surface pits on flexural strength of test specimens was compared with unetched and sandblasted samples. Shear bond strengths between clean and saliva contaminated samples and Panavia TC resinous cement were also determined. RESULTS: The Bateman etch retention system significantly reduced the flexural strength of both In-Ceram and Spinell ceramic specimens compared with unetched and sandblasted samples (p < 0.05). No significant differences were discovered between shear bond strengths of uncontaminated Bateman etch retention system and sandblasted samples. However, mean shear bond strengths of saliva-contaminated Bateman etch retention system In-Ceram samples were substantially greater than those of saliva contaminated sandblasted samples (p < 0.05). CONCLUSION: The Bateman etch retention system method should be the subject of additional investigation.

Aluminum Oxide↗
PubMed1989

Acute urinary retention associated with prostatic carcinoma.

We describe the prospective evaluation of 90 patients seen at 2 medical centers who presented with acute urinary retention. All patients had a pre-retention history obtained, as well as careful prostate examination, perineal prostate biopsy and followup treatment or monitoring. Prostate cancer was found in 12 of the 90 patients (13.3 per cent), while 1 had metastatic leukemia to the prostate. Of the 90 patients 69 (76.7 per cent) had a palpably benign prostate and 2 malignancies (2.9 per cent) were diagnosed, while 11 malignancies occurred in 21 patients (52.4 per cent) with a suspicious examination. A total of 46 patients (51.1 per cent) underwent further prostate resection and no malignancy was found in any of these specimens: 43 underwent transurethral resection (24.9 gm. average), while 3 underwent open prostatectomy (97 gm. average). Other etiologies of acute retention included benign hyperplasia, other underlying illness, medical procedures, medications, prostatitis and prostatic infarction. Among the 44 patients who did not undergo prostatectomy 13 had treatment of the diagnosed cancer, 9 had resolution of retention and symptoms without intervention, 9 remained catheterized due to severe medical problems, 8 were treated for prostatitis, 2 had discontinuation of sympathomimetic drugs and 1 each underwent urethrotomy, bladder neck incision and resolution of prostatic infarction. In contrast to the older literature in which approximately 25 per cent of the patients presenting with acute urinary retention had prostate cancer, our data suggest a lower incidence. Prostatic biopsy for patients who present with acute urinary retention and a benign examination does not appear to be justified.

Acute Disease↗
PubMed1998

Age-related changes in septal serotonergic, GABAergic and glutamatergic facilitation of retention in SAMP8 mice.

SAMP8/TaJf(P8) mouse strain has an inherited age-related impairment of learning and memory with its onset relatively early in its lifespan. Previously, it was reported that cholinergic and glutamatergic drugs injected into the hippocampus after behavioral training showed considerable shifts in the dose that improved retention in mice at 12 compared to 4 months of age. Cholinergic neurons in the septum supply most of the acetylcholine released in the hippocampus. In the present study, we determined if altered functional status of neurotransmission in the septum might account for the decrease in cholinergic and glutamatergic activity in the hippocampus of older SAMP8 mice. After training on footshock avoidance, P8 mice received a drug injection into the septum. Retention was tested 1 week later. The results indicate that bicuculline, GABA-A, and saclofen, GABA-B, receptor antagonist had to be injected at a higher dose in 12- than in 4-month-old mice to improve retention. The serotonergic antagonists, ketanserin and methiothepin, both showed dose response shifts such that less drug was needed to improve retention in 12- as compared to 4-month-old mice. It required four times more L-glutamate to improve retention in 12- than in 4-month-old mice. Agonists for acetylcholine, dopamine and norepinephrine receptors or an opiate antagonist required little or no change in the dose needed to improve retention in older P8 mice. SAMP8 mice may show an age-related impairment of septohippocampal functioning.

Aging↗
PubMed2002

Bacteriological and epidemiological findings during examination of the uterine content of ewes with retention of fetal membranes.

We included 92 pairs of ewes with or without retention of fetal membranes in a cohort study of 25 flocks in Southern Greece. We obtained two uterine content samples under aseptic conditions, by introducing a swab into the uterus of these ewes, on the 2nd-4th and the 5th-9th day after lambing. We used conventional bacteriological techniques to isolate and identify bacteria and to carry out antimicrobial agents susceptibility testing. The prevalence of bacterial intrauterine contamination among ewes with retention was 24% on the first and 46% on the second sampling (P < 0.0001) and that among ewes without retention was 8 and 2% (P > 0.05), respectively. Clinical signs accompanying the retention of fetal membranes were more frequently observed among ewes with intrauterine contamination than among those without (P = 0.0007). The odds of an ewe having an intrauterine contamination increased multiplicatively by 1.06 when the median duration of retention in the flock increased by 6 h. The principal bacteria isolated from the ewes with retention were Arcanobacterium pyogenes and Escherichia coli; 21% of 73 isolates tested were found resistant to at least one antimicrobial agent.

Actinomycetaceae↗
PubMed2003

Multi-level determinants of retention in a home-visiting child abuse prevention program.

OBJECTIVE: This exploratory study investigated how attributes from multiple domains influence retention in a voluntary home-visiting child abuse prevention program. METHOD: A sample of 1093 "at-risk" families participated in a home-visiting child abuse prevention program. A total of 71 Family Support Workers (FSWs) provided home visitation services in 12 different communities. Hierarchical general linear modeling (HGLM) was used to examine the community, home visitor, and maternal attributes that predicted retention in the program beyond 1 year. RESULTS: Multi-level analyses (HGLM) revealed significant community, home visitor, and maternal level effects. Families were less likely to remain in a home-visiting child abuse prevention program for at least 1 year if they lived in an area with high community violence. Families were more likely to remain when the home visitor received more hours of direct supervision. Older mothers were more likely to remain for at least 1 year than were younger mothers. Hispanic mothers were more likely to remain than were White non-Hispanic mothers. CONCLUSIONS: These findings demonstrate the utility of looking across multiple levels of influence when examining retention in home-visiting child abuse prevention programs. To increase retention rates home visitors will need to be adaptable to fit the needs of families in violent communities. Supervisors can influence retention rates by providing more hours of direct supervision. Other strategies that may contribute to program retention include meeting the scheduling needs of younger mothers and involving the families of younger mothers more effectively.

Adult↗
PubMed2001

Facilitative effects of an adenosine A1/A2 receptor blockade on spatial memory performance of rats: selective enhancement of reference memory retention during the light period.

The present experiment was designed to examine the role of adenosine in spatial working and reference memory in rats using an 8-arm radial maze task which requires the integrity of the hippocampal formation. We investigated the effects of the unselective adenosine A1/A2 receptor antagonist theophylline on acquisition and retention of spatial working and reference memory. As there is evidence that brain extracellular adenosine levels vary significantly during the light-dark cycle, we tested the effects of theophylline both during the light and the dark period. Acquisition of the task was investigated for 10 consecutive days after rats received daily injections of vehicle or theophylline (15 mg/kg, intraperitoneally). Retention was tested in two nondrug sessions 7 and 14 days after completion of acquisition. The results demonstrate that in saline-treated control rats acquisition and retention of reference memory and, to a lesser extent, working memory was superior in the dark period. The results further revealed that daily administration of theophylline interacted with days to selectively enhance reference memory acquisition in the light, but not in the dark, period. In addition, reference memory retention was significantly enhanced in those rats who learned the task under theophylline treatment during the light period. Overall, the results show that in saline-treated control rats the effectiveness of acquisition and retention of spatial information in a radial maze strongly depends on the time of day. The higher levels of maze performance in the dark period might be related to a better functioning of involved brain systems in the active period of the rat. Furthermore, theophylline-induced blockade of adenosine A1/A2 receptors in the light, but not in the dark, period selectively enhanced reference memory acquisition and retention. Variations of brain extracellular adenosine levels during the light-dark cycle might account for the restriction of reference memory enhancing effects of theophylline to the light period.

Animals↗