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PubMed1996

Patient retention in mobile and fixed-site methadone maintenance treatment.

The retention of patients (n = 399) enrolled in mobile health services (MHS), a Baltimore outpatient mobile methadone treatment program, was compared to patient retention (n = 1588) in six Baltimore fixed-site programs. Mobile program patients were retained for a median of 15.53 months in treatment in comparison to 3.90 for fixed-site patients (n = 664) from the MHS served zip codes (MHSZIPS) and 6.27 for fixed-site patients (n = 924) from zip codes other than those served by MHS (OTHERZIPS), (P < 0.001). Using Cox regression, the characteristics of patients associated with earlier discharge were (1) higher number of arrests, (2) more frequent cocaine use and (3) lower family income. These predictors of shorter retention were generally more prevalent among patients from the MHS served zip codes. Therefore, the longer retention in treatment of MHS patients as compared to OTHERZIPS fixed-site program patients is even more striking. Consistent with these differences in retention, were finding in a prior study suggesting that the mobile program provided greater access to services in reducing patient transportation cost and travel time. Thus, mobile methadone maintenance treatment appears to be a useful means of providing services.

Adult↗
PubMed1991

Reversed-phase high-performance liquid chromatography of amphoteric beta-lactam antibiotics: effects of columns, ion-pairing reagents and mobile phase pH on their retention times.

The separation of five amino beta-lactam antibiotics by reversed-phase high-performance liquid chromatography was studied as an insight into their retention behaviour. These five amphoteric compounds are cephradine, cephalexine, cefaclor, ampicillin and amoxicillin. Both octadecylsilane-bonded silica (C18) columns and phenyl-bonded silica (phenyl) columns were used, with mobile phase pH values between 2.5 and 7.4. In the absence of ion-pairing reagents the retention times for all the five compounds were the shortest at pH 4-6. The phenyl column was found to improve the separation between cephradine and ampicillin at pH values lower than 3, when these two compounds appeared as fused peaks on the C18 on C18 columns, with mobile phases both with and without ion-pairing reagents, were compared. The addition of 0.005 or 0.02 M tetraethylammonium acetate to the mobile phase did not result in significant ion-pair formation, except at pH values higher than 5.5. A strong ion-pairing effect was obtained at pH values higher than 6 with 0.005 or 0.02 M tetrabutylammonium phosphate, and the retention was decreased at pH values lower than 4. On the other hand, 0.005 M heptanesulphonic acid exhibited an ion-pair retention effect at pH values lower than 5. The molecular structures and pK(a) values were used to account for the retention behaviour of these antibiotics in the various mobile phases.

Amoxicillin↗
PubMed1996

Effect of beta-cyclodextrin derivatives on the retention of steroidal drugs.

The retention of eighteen steroid drugs was determined on a beta-cyclodextrin polymer (beta CDP)-coated silica column using methanol-water mixtures as eluents. The relative strength of inclusion formation between the drugs and hydroxypropyl-beta CD (HP beta CD) and dimethyl-beta CD was determined by charge transfer chromatography carried out on reversed-phase thin-layer chromatography plates. The retention characteristics of drugs were correlated with their physicochemical parameters and with their inclusion complex-forming capacity. Calculations indicated that the inclusion complex-forming capacity of the drugs has little impact on the retention that is due to the HP beta CD and water-insoluble beta CD polymers exposed to different retention characteristics. The hydrophilic molecular parameters of drugs significantly influenced their retention. This result suggests that the selectivity of the beta CDP-coated column may be different from that of the traditional reversed-phase columns.

Chromatography, High Pressure Liquid↗
PubMedDate not supplied

Overpressured layer chromatographic study of retention behaviour of various benzodiazepine derivatives on layers impregnated with tricaprylmethylammonium chloride.

The retention behaviour of various benzodiazepine derivatives was investigated on silica gel layers impregnated with tricaprylmethylammonium chloride (TCMA). The chromatograms were developed by means of overpressured layer chromatography (OPLC). As for the case of amino- and nitrosalicylic acids, pyrimidine derivatives, barbiturates, penicillins, cephalosporins and tetracyclines, the retention of benzodiazepine derivatives increased with increasing layer TCMA concentration with eluents containing methanol and water, but not TCMA. On increase of the methanol content of the eluent, a retention-decreasing effect was observed. On layers impregnated with TCMA, a linear relationship existed between the RM values of the benzodiazepines and the methanol content of the eluent. A similar relationship held for silica gel layers impregnated with paraffin oil (traditional reversed-phase). There was no correlation between the results obtained on layers treated with TCMA or with paraffin oil. On TCMA-impregnated layers, the retention of compounds having different chemical structures showed no dependence on the pH of the eluent. There were two reasons for this. Firstly, as it was established, above a certain RF value, the pH of the layer in the presence of TCMA was almost identical irrespective of the original pH of the buffer. Secondly, below this RF value, the actual pH of the layer did not have a strong enough effect to cause appreciable differences between the retentions of the dissociated and undissociated species of the analytes. The conditions for optimum separation are given.

Anti-Anxiety Agents↗
PubMed2006

Platelet-derived growth factor B retention is essential for development of normal structure and function of conduit vessels and capillaries.

OBJECTIVE: Extracellular retention of PDGF-B has been proposed to play an important role in PDGF-B signalling. We used the PDGF-B retention motif knockout mouse (RetKO) to study the effects of retention motif deletion on development of micro- and macrovascular structure and function. METHODS: Passive and active properties of conduit vessels were studied using myograph techniques and histological examination. Capillary structure and function was studied using measurements of capillary density in skeletal muscle and by assessing aerobic physical performance in a treadmill setup. Cardiac function was assessed using echocardiography. RESULTS: Myograph experiments revealed an increased diameter and stiffness of the aorta in RetKO. Histological examination showed increased media collagen content and a decreased number of aortic wall layers, however with a similar number of vascular smooth muscle cells. This outward eutrophic remodelling of the aorta was accompanied by endothelial dysfunction. RetKO showed decreased capillary density in skeletal muscle and signs of a defective delivery of capillary oxygen to skeletal muscle, as shown by a decreased physical performance. In RetKO mice, echocardiography revealed an adaptive eccentric cardiac hypertrophy. CONCLUSION: We conclude that retention of PDGF-B during development is essential for a normal conduit vessel function in the adult mouse. Furthermore, PDGF-B retention is also necessary for the development of an adequate capillary density, and thereby for a normal oxygen delivery to skeletal muscle. The lack of primary effects on cardiac function supports the redundant role of PDGF-B in cardiac development.

Animals↗
PubMed2004

Prediction of gas chromatographic retention indices of a diverse set of toxicologically relevant compounds.

For a set of 846 organic compounds, relevant in forensic analytical chemistry, with highly diverse chemical structures, the gas chromatographic Kovats retention indices have been quantitatively modeled by using a large set of molecular descriptors generated by software Dragon. Best and very similar performances for prediction have been obtained by a partial least squares regression (PLS) model using all considered 529 descriptors, and a multiple linear regression (MLR) model using only 15 descriptors obtained by a stepwise feature selection. The standard deviations of the prediction errors (SEP), were estimated in four experiments with differently distributed training and prediction sets. For the best models SEP is about 80 retention index units, corresponding to 2.1-7.2% of the covered retention index interval of 1110-3870. The molecular properties known to be relevant for GC retention data, such as molecular size, branching and polar functional groups are well covered by the selected 15 descriptors. The developed models support the identification of substances in forensic analytical work by GC-MS in cases the retention data for candidate structures are not available.

Calibration↗
PubMed2004

Reversed-phase retention thermodynamics of pure-water mobile phases at ambient and elevated temperature.

The use of pure water at superheated temperatures, between 100 and 200 degrees C, as a mobile phase for reversed-phase separations is explored. The thermodynamics of the retention process at low temperature (15-55 degrees C) are compared to the thermodynamics at elevated temperature (125-175 degrees C). Significant differences in the enthalpy of the retention process are observed between the two temperature ranges. This is possibly due to changes in the hydrogen-bond network of the pure-water mobile phase, which would change the solvation, and therefore retention, of non-polar solutes. The change in thermodynamic values between the two temperature regions invalidates extrapolation of retention as a function of temperature between the two temperature regions for the prediction of room-temperature pure-water retention factors. The thermodynamic changes observed as the temperature is increased are similar to those seen when mobile phase composition is changed (by adding organic modifier) at constant temperature.

Hydrogen Bonding↗
PubMed2004

Effect of surface hydrophobicity distribution on retention of ribonucleases in hydrophobic interaction chromatography.

The effect of surface hydrophobicity distribution of proteins on retention in hydrophobic interaction chromatography (HIC) was investigated. Average surface hydrophobicity as well as hydrophobic contact area between protein and matrix were estimated using a classical thermodynamic model. The applicability of the model to predict protein retention in HIC was investigated on ribonucleases with similar average surface hydrophobicity but different surface hydrophobicity distribution. It was shown experimentally that surface hydrophobicity distribution could have an important effect on protein retention in HIC. The parameter "hydrophobic contact area," which comes from the thermodynamic model, was able to represent well the protein retention in HIC with salt gradient elution. Location and size of the hydrophobic patches can therefore have an important effect on protein retention in HIC, and the hydrophobic contact area adequately describes this.

Chromatography, Liquid↗
PubMed2004

Retention characteristics of porous graphitic carbon in subcritical fluid chromatography with carbon dioxide-methanol mobile phases.

Numerous relationships usually used in high-performance liquid chromatography (HPLC) for describing the retention on porous graphitic carbon (PGC) have been applied in subcritical fluid chromatography, with CO2-methanol mobile phases. As reported in HPLC, octanol-water partition coefficient failed to fit the retention, whereas satisfactory results were obtained with the sum of partial negative charges. A better fit was reached by using the solvation parameter model, allowing a better understanding of the interactions developed between the solute, the stationary and the mobile phases. Results show that the dominant contribution to retention was given by the polarizability (E) and the volume (V), while the hydrogen-bond basicity (B) was not selected in the retention model, whatever the methanol content. The increase in methanol percentage favours the retention decrease, mainly through the volume for hydrophobic compounds, and through the hydrogen-bond acidity for polar compounds.

Carbon Dioxide↗
PubMed2004

Prediction of retention times and efficiency in linear gradient programmed pressure analysis on capillary columns.

A method for the prediction of the retention time and the resolution of chromatographic peaks in different experimental conditions by starting from few experimental data measured in isothermal and isobaric analyses was published previously. In this paper, the same mathematical model was implemented for calculating the retention times and the column efficiency in programmed pressure runs. Some models originated from the Golay equation and reported in the literature are compared, and a new modified equation for the calculation of the peak width at half height is proposed. The procedure for the prediction of the retention time and the peak width at half height at programmed pressure of the carrier gas and different column temperature and linear gradient by using retention data of different compounds obtained in few isobaric runs is described. The prediction of the retention time and the separation efficiency of compounds with different polarity gave good results for the programmed pressure runs with linear gradient. The effect of the variation of the initial parameters of the experimental analyses and of the mathematical model on the accuracy of the prediction has been evaluated.

Chromatography, Gas↗
PubMed2005

Comparative characteristics of HPLC columns based on quantitative structure-retention relationships (QSRR) and hydrophobic-subtraction model.

The study was aimed at quantitative comparison of retention properties of modern stationary phases for reversed-phase HPLC. Three approaches, the calculated logarithm of octanol/water partition coefficient (clogP)-based model, the molecular modeling descriptors-based model and the hydrophobic-subtraction model, were compared and discussed. Gradient retention time, tR, of a series of test analytes was a dependent variable in the quantitative structure-retention relationship (QSRR) equations describing retention in terms of analytes' structure descriptors. The QSRRs derived were used to characterize in quantitative manner the specific retention properties of nine representative reversed-phase HPLC. Either the theoretically calculated logarithm of octanol/water partition coefficient, or the structural descriptors from molecular modeling were employed to quantitatively characterize the structure of the analytes. The three molecular modeling-derived structural descriptors considered were: the total dipole moment, the electron excess charge of the most negatively charged atom and the water-accessible molecular surface area. In addition to the above standard QSRR approaches, a recently developed parameterization of reversed-phase column selectivity based on the hydrophobic-subtraction model of Snyder et al. [L.R. Snyder, J.W. Dolan, J.W. Carr, The hydrophobic-subtraction model of reversed-phase column selectivity, J. Chromatogr. A 1060 (2004) 77] was considered. According to the hydrophobic-subtraction model, reversed-phase columns are characterized by five selectivity parameters derived from the linear solvation energy relationships (LSER) theory. Values of these parameters are available for more than 300 different columns. It has been demonstrated that the clogP-based model, the molecular modeling descriptors-based model and the hydrophobic-subtraction model provide generally similar classification of the HPLC columns studied. Some differences in column classification by the three approaches considered are discussed in terms of specific properties of individual stationary phases. All the approaches allow a quantitative, although multidimensional, characteristic of HPLC columns, however, the nonempirical QSRR-based approach is simpler and require less labor.

Chromatography, High Pressure Liquid↗
PubMed2006

Effect of temperature on the retention of amino acids and carbohydrates in high-performance anion-exchange chromatography.

Effect of temperature on the retention of amino acids and carbohydrates in high-performance anion-exchange chromatography (HPAEC) was investigated. Amino acids retention exhibited both exothermic and endothermic behavior. Significant selectivity variations of some weakly retained amino acids were achieved through changing temperature. However, temperature was almost ineffective at changing the selectivity of strongly retained amino acids. Sugars retention exhibited only exothermic behavior. However, individual carbohydrates showed different relative changes in retention time with increased temperature. There is the general regularity, which is the variation of retention time increases as the sequence of monosaccharide, disaccharide, trisaccharide and tetrasaccharide when the column temperature is increased. Selectivity changes of carbohydrates, including elution order reversal, were also achieved by varying temperature. Temperature was effective at changing the selectivity of between amino acids and carbohydrates. In achieving optimal separation of amino acids and carbohydrates, temperature is a valuable tool.

Amino Acids↗
PubMed2006

Speciation of alkylated dibenzothiophenes through correlation of structure and gas chromatographic retention indexes.

The gas chromatographic (GC) speciation of polycyclic aromatic sulfur heterocycles (PASH) in deeply desulfurized fuels is difficult without recourse to authentic standard compounds. Here we investigate the GC retention indexes for 30 alkylated dibenzothiophenes on two methylphenylsiloxane stationary phases of different selectivity (5% and 50% phenyl groups) with a view to avoid the synthesis of all congeners. The influence of the substitution pattern on the retention indexes is discussed. With the measured data it is possible to carry out a multiple linear regression (MLR) to calculate parameters for predicting the retention indexes of unknown polymethylated isomers based only on their structural features. Nine trimethyldibenzothiophene in a synthesis product are identified through their retention indexes. The retention indexes for a total of 43 alkyldibenzothiophenes are listed.

Alkylation↗
PubMed2006

Retention behaviour of volatile C(1)-C(3) fluoroalkanes upon selected preconcentration adsorbents I. Carbon molecular sieves and activated charcoals.

The retention behaviour of several gaseous fluorinated greenhouse gases on carbon-based adsorbents is presented. Retention, calculated on the basis of compound breakthrough volume (BTV), is dependent on the molecular composition of the adsorbate, with compounds possessing chlorine or polarizable hydrogens being better retained than those possessing higher fluorine content. Of the adsorbents tested the carbon molecular sieves (CMSs) of highest surface area show greater retention than those with lower area. Retention of fluorocarbons is generally higher on activated charcoals but this adsorbent type can cause irreversible retention, possible degradation and is more difficult to use practically due to its heterogeneous composition. These breakthrough volume results can be used to determine the best combination and quantities of each adsorbent that can be used within a preconcentration device with a view to developing an analytical system for the determination of fluorocarbon gases in low concentration air samples.

Adsorption↗
PubMed2006

Temperature dependence of the Kováts retention index. The entropy index.

According to a novel equation, the temperature dependence of the Kováts retention index, dI/dT is proportional to the difference of the Kováts retention index, I, and the new entropy index, I(S), defined similarly as the retention index, but based on solvation entropy instead of the free energy of solvations. The new relationship was tested with the experimental retention and thermodynamic data published by Kováts and coworkers for 32 compounds on 6 different stationary phases. Very good correlations (r>0.99) were observed for dI/dT versus (I-I(S)) and dI/dT versus deltaDeltaC(p), the molar heat capacity difference of the solute and the hypothetical n-alkane, which has the same retention index as the solute. Deviations in the dI/dT versus deltaDeltaC(p) relationship were observed only for alcohols, suggesting a different solvation mechanism for alcohols as compared with other compounds.

Chromatography, Gas↗
PubMed2006

Solvation parameter models for retention on perfluorinated and fluorinated low temperature glassy carbon stationary phases in reversed-phase liquid chromatography.

The retention of solutes on two fluorinated low temperature glassy carbon (F-LTGC) stationary phases under reversed-phase liquid chromatographic conditions was studied by employing the solvation parameter model. The two fluorinated glassy carbon stationary phases were produced by slowly heating zirconia particles that were encapsulated with oligo[1,3-dibutadiyne-1,3-(tetrafluoro)phenylene] precursor polymer to two different final temperatures (200 and 400 degrees C). The resulting carbon particles had different amounts of fluorine after thermal processing. The solvation parameter models indicated that different intermolecular interactions are important in describing retention on the two stationary phases. The interactions that are important for describing retention on the 200 degrees C processed F-LTGC stationary phase are hydrogen bond basicity> or =dispersion>hydrogen bond acidity>dipolarity/polarizability. The interactions that describe the retention on the 400 degrees C processed F-LTGC are hydrogen bond basicity>dispersion>excess molar refraction> or =hydrogen bond acidity. The solvation parameter model for the 200 degrees C processed F-LTGC showed similar trends in the relative importance of intermolecular interactions as previously found for octadecyl-polysiloxane stationary phases, while the 400 degrees C processed F-LTGC had similar intermolecular interactions with solutes as found with porous glassy carbon in that pi-pi interactions with the carbon surface contribute more so to the retention.

Carbon↗
PubMed2005

Predictors of retention in methadone programs: a signal detection analysis.

Retention in Opioid Agonist Therapy (OAT) is associated with reductions in substance use, HIV risk behavior, and criminal activities in opioid dependent patients. To improve the effectiveness of treatment for opioid dependence, it is important to identify predisposing characteristics and provider-related variables that predict retention in OAT. Participants include 258 veterans enrolled in 8 outpatient methadone/l-alpha-acetylmethadol (LAAM) treatment programs. Signal detection analysis was utilized to identify variables predictive of 1-year retention and to identify the optimal cut-offs for significant predictors. Provider-related variables play a vital role in predicting retention in OAT programs, as higher methadone dose (> or =59 mg/day) and greater treatment satisfaction were among the strongest predictors of retention at 1-year follow-up.

Adult↗
PubMed2004

Shade and flow effects on ammonia retention in macrophyte-rich streams: implications for water quality.

Controlled releases of NH4-N and conservative tracers (Br- and Cl-) to five reaches of four streams with contrasting macrophyte communities have shown differing retentions, largely as a result of the way plants interact with stream flow and velocity. First-order constants (k) were 1.0-4.8 d(-1) and retention of NH4-N was 6-71% of amounts added to each reach. Distance travelled before a 50% reduction in concentration was achieved were 40-450 m in three streams under low-flow conditions, and 2400-3800 m at higher flows. Retention (%) of NH4-N can be approximated by a simple function of travel time and k, highlighting the importance of the relationship between macrophytes and stream velocity on nutrient processing. This finding has significant management implications, particularly with respect to restoration of riparian shade. Small streams with predominantly marginal emergent plants are likely to have improved retention of NH4-N as a result of shading or other means of reducing plant biomass. Streams dominated by submerged macrophytes will have impaired NH4-N retention if plant biomass is reduced because of reduced contact times between NH4-N molecules and reactive sites. In these conditions water resource managers should utilise riparian shading in concert with unshaded vegetated reaches to achieve a balance between enhanced in-stream habitat and nutrient processing capacity.

Ammonia↗