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At least 883 records · Page 49Linked to original sources
PubMed2004

Air/polymer distribution coefficients for polycyclic aromatic hydrocarbons by solid-phase microextraction sampling.

Two methods to estimate distribution coefficients (K) between air and poly(dimethylsiloxane) (PDMS) coating of solid-phase microextraction (SPME) fibers for eight low molecular polycyclic aromatic hydrocarbons (PAHs) there are presented. The PDMS phases were used for determination of the coefficients according to equilibrium theory with help of a developed static calibration system (SCS). Another way to estimate the coefficients is based on the use of a linear relationship between the logarithm of the coefficients (log K) and linear temperature-programmed retention indexes (LTPRI) of the compounds without necessity to calibrate. The log K values for both of methods ranged from 5.2 (naphthalene) to 8.9 (pyrene) at 22 degrees C. Relative standard deviation (R.S.D.) of log K for each compound determined by static calibration was no more than 5.3%. R.S.D. of retention times for LTPRI indices did not exceed 0.28% for repeated injection. All experiments were implemented on a GC-MS system.

Air↗
PubMed2006

Application of the chromatographic retention index system for the estimation of the calibration constants of permeation passive samplers with polydimethylsiloxane membranes.

The paper presents the results of research on the calibration of permeation passive samplers equipped with polydimethylsiloxane (PDMS) membranes using the physico-chemical properties of the analytes. Strong correlations were found between the calibration constants of the samplers and the linear temperature-programmed retention indices of the analytes determined on columns coated with pure PDMS (r2 = 0.914). These correlations make it possible to estimate the calibration constants for unidentified analytes, which is impossible when using conventional procedures. This, in turn, enables the deployment of permeation passive samplers in the same way in which active samplers are deployed. The reproducibility of the calibration constants determined in different laboratories and retention indices determined using different chromatographic systems was very good, indicating that the calibration constants estimated using this approach should be reproducible as well. The approach proposed should lead to more widespread use of permeation passive samplers.

Calibration↗
PubMed2007

Bacterial flora and risk of infection of the ovine teat duct and mammary gland throughout lactation.

We collected samples of teat duct material and mammary secretion from ewes in three farms (flock A, polyparous n=7; flock B, polyparous n=6, primiparous n=4; flock C, polyparous n=4): 14 samples immediately after lambing (before sucking of lambs), 244 samples during the suckling period and 156 samples during the milking period. Conventional bacteriological techniques were used. The results were modeled using survival analysis, initially by the Kaplan-Meier method and then by the Cox Proportional Hazards method. Then, we calculated the minimum true risk of an "at-risk" teat or mammary gland being infected and analyzed these data with STATA using the GLLAMM program for Generalised Linear Latent and Mixed Models. During the suckling period, bacteria were isolated from 52 (21%) duct material and 19 (8%) secretion samples; respective results for the milking period were 20 (13%) and 9 (6%). There was an increased risk of duct rather than secretion samples being infected (P<0.001). There was a significant difference among flocks in isolating bacteria from duct (P<0.01) or secretion (P<0.001) samples during suckling period, but not during hand-milking period (P>0.4 and 0.1, respectively). There were no differences between isolation of bacteria from duct (P>0.5) or secretion (P>0.7) samples among primiparous and polyparous animals. Most bacterial isolates were staphylococci. Persistent isolation of the same bacterial species from duct material samples obtained from a particular ewe was recorded with five Staphylococcus spp. and two Mannheimia haemolytica isolates. The results indicate that infections of the teat duct can take place easily; however, not all infections result to infection of the mammary gland. The results support experimental evidence that defence mechanisms of the healthy teat are able to limit the infection. Maintenance of healthy teats contributes to effective defence mechanisms, and coupled with minimal infections of the teat duct, would contribute to the prevention of mastitis in ewes.

Animals↗
PubMed2006

The use of a segmented transducer for rib sparing in HIFU treatments.

The use of focused ultrasound as a minimally invasive treatment for tumours is rapidly expanding. Target organs include the liver and kidneys. Both single element and phased array transducers may be used in the clinic. The presence of the rib cage presents a problem in high intensity focused ultrasound (HIFU) treatment planning, due to its high attenuation of the HIFU beam resulting in a loss of power at the focus as well as an increase in the risk of damage at the rib and to overlying tissues, including the skin. In this paper, a linearly segmented transducer, in which all active elements are driven in phase, has been investigated. The aim of the study was to investigate how a beam with a clinically useful profile could be achieved by removing the contribution of edge segments from one side of the transducer to the field. We have considered the case in which the HIFU beam approaches the rib cage during a treatment and investigated configurations of the transducer for which up to three segments on the edge are switched off. This problem has been studied initially using a linear acoustic field program to model the segmented transducer's acoustic beam profile. Experimental measurements of the transducer's acoustic field were performed using an automated beam plotting system. Temperature measurements were made on a rib surface for two transducer configurations using a fine wire thermocouple. A thermochromic liquid crystal material was used to assess qualitatively the heating pattern generated by the ultrasound beam. We show the rib sparing potential of the segmented transducer during HIFU treatment by demonstrating a reduction in the prefocal width of the ultrasound beam when edge segments are switched off. This has been predicted with the acoustic field model and demonstrated experimentally by acoustic field measurements and observations of the heating pattern generated by the ultrasound beam. A significant decrease in the temperature rise on a rib was observed in the case for which three edge segments were switched off compared with when all segments were active. We conclude that a segmented transducer extends the potential for treating liver tumours. In the case where the tumour lies behind, but close to the edge of, the ribs, energy loss at the focus and excessive heating in the rib and overlying tissue can be avoided by switching off edge segments.

Acoustics↗
PubMed2001

Effect of clinical teaching on student performance during a medicine clerkship.

PURPOSE: To measure what proportion of student clerkship performance can be attributed to teachers' educational skills as reported by students. SUBJECTS AND METHODS: From August 1992 to June 1994, we collected critiques of teacher skills from 314 third-year students at the end of a 12-week medicine clerkship. Interns, residents, attending physicians, and student preceptors were rated (on a 1 to 5 scale) on teaching behaviors from the 7 categories of the Stanford Faculty Development Program framework. A linear regression model was used to determine the relative contributions of the rated teaching behaviors in predicting final student performance and improvement across the clerkship ("student growth"), measured using end-of-clerkship variables (clinical grades, National Board of Medical Examiners medicine shelf examination, practical laboratory examination, and an analytical essay examination) and preclerkship variables (pre-third-year grade point average [GPA], United States Medical Licensing Examination, Step I, and clerkship pretest). RESULTS: Data were available for 293 (93%) of 314 students, who completed a total of 2,817 critiques. The students' preclerkship GPA accounted for the greatest percentage of variance in student performance (28%, P < 0.0001). Clinical teaching behaviors accounted for an additional 6% (P < 0.0001) of the variance. For student growth across the clerkship, teaching accounted for 10% of the variance (P < 0.0001). Among the 7 Stanford educational categories, teaching behaviors promoting control of session (r2 = 5%, P = 0.0002) and fostering understanding and retention (r2 = 4%, P = 0.001) had the greatest effect. The resident had the most effect on student growth (r2 = 6%, P = 0.0001) when compared with other teaching levels. Teaching had a greater effect on growth for students with preclerkship GPA above the mean (16% versus 6%), for older students (24% versus 7%), and for students with a nonscience undergraduate degree (33% versus 9%). CONCLUSION: The preclerkship GPA, reflecting 2 years of work, was the most important predictor of student performance. Teaching behavior, as measured by student assessments, also affected student performance.

Adult↗
PubMed1997

GTP regulation of (-)-stepholidine binding to R(H) of D1 dopamine receptors in calf striatum.

(-)-Stepholidine (SPD) exhibits antagonist effects on normosensitive dopamine (DA) receptors, but it has an agonist action on rotation in unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. The present work endeavors to further elucidate the mechanism of its agonist action on D1 receptors. [3H]R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-be nzazepine ([3H]SCH-23390) and [3H]spiperone were used, respectively, as radioligands in D1 and D2 DA receptor binding assays in calf striatal membranes. Experimental data were analyzed by a non-linear regression computer program, GraphPAD InPlot 3.15. The competition curves were fitted first by a single-site equation and then by a two-site equation. The results showed that both apomorphine (APO) and SPD competitively inhibited [3H]SCH-23390 binding. Their competition curves fitted best to the two-site equation (P < 0.05) with a high-affinity site (R(H)) and a low-affinity site (R(L)) to DA receptors. The K(H) and K(L) values (nM) were 2.7 +/- 0.45 and 378 +/- 62 for APO, and 3.9 +/- 2.2 and 126 +/- 25 for SPD, respectively. In contrast, the competition curve of SCH-23390, a selective D1 DA receptor antagonist, fitted best to a single-site model with a Ki value of 1.7 +/- 0.5 nM. The R(H) of APO or SPD could be decreased by the addition of 450 microM GTP. In the [3H]spiperone binding test, the APO curve was modeled best by the two-site equation, while the SPD curve fitted best to a single-site model. In the rotational behavior test, APO induced 441 +/- 20 turns/30 min in the 6-OHDA-lesioned rats, and SPD induced 310 +/- 42 turns/30 min, while SCH-23390 antagonized the SPD-induced rotation but did not induce rotational behavior. These results suggest that SPD possesses agonist actions on D1 but antagonist effects on D2 DA receptors.

Adenylyl Cyclases↗
PubMed2000

Authentic standards for the reductive-cleavage method. The positional isomers of partially methylated and acetylated or benzoylated 1,5-anhydroribitol.

The eight positional isomers of methylated and benzoylated 1,5-anhydroribitol were generated simultaneously from 1,5-anhydroribitol by sequential partial methylation and benzoylation, and the four meso isomers and two enantiomeric pairs of isomers so-formed were isolated in chemically pure form by high-performance liquid chromatography. The corresponding acetates were obtained by debenzoylation of the pure isomers and acetylation. Reported herein are the 1H NMR spectra of the benzoates and the electron-ionization mass spectra of the acetates and the tri-O-methyl derivative. Also reported for the acetates and the tri-O-methyl derivative are their linear temperature-programmed gas-liquid chromatography retention indices on three different capillary columns.

Acetylation↗
PubMed2000

Authentic standards for the reductive-cleavage method. The positional isomers of partially methylated and acetylated or benzoylated methyl 2-(acetylmethylamino)-2-deoxy-beta-D-glucopyranoside.

Described herein is the synthesis of eight positional isomers of methylated and acetylated or benzoylated methyl 2-(acetylmethylamino)-2-deoxy-beta-D-glucopyranoside. The compounds were generated simultaneously from methyl 2-(acetylmethylamino)-2-deoxy-beta-D-glucopyranoside by sequential partial methylation and benzoylation and isolated in pure form by high-performance liquid chromatography (HPLC). The desired acetates were obtained by debenzoylation and acetylation of the pure isomers. Reported herein are the 1H NMR spectra of the benzoates and the electron-ionization mass spectra of the acetates and the tri-O-methyl derivative. Also reported for the acetates and the tri-O-methyl derivative are their linear temperature-programmed gas-liquid chromatography (GLC) retention indices on three different capillary columns.

Acetates↗
PubMed1998

Authentic standards for the reductive-cleavage method: the positional isomers of partially methylated and acetylated or benzoylated 1,5-anhydro-L-arabinitol.

Described herein is the synthesis of the eight positional isomers of methylated and acetylated or benzoylated 1,5-anhydro-L-arabinitol. The compounds were generated simultaneously from 1,5-anhydro-L-arabinitol by sequential partial methylation and benzoylation and isolated in pure from by high-performance liquid chromatography. The desired acetates were obtained by debenzoylation and acetylation of the pure isomers. Reported herein are the 1H NMR spectra of the benzoates and the electron-ionization mass spectra of the acetates and the tri-O-methyl derivative. Also reported for the acetates and the tri-O-methyl derivative are their linear temperature-programmed gas-liquid chromatography retention indices on three different capillary columns.

Acetylation↗
PubMed2000

Optimized combinations of abatement strategies for urban mobile sources.

The maximum incremental reactivity (MIR) scale was chosen as a practical index for quantifying ozone-forming impacts. The integer linear and nonlinear programming techniques were employed as the optimization method to maximize MIR and volatile organic compound (VOC) reductions, and minimize ozone's marginal cost with varied control costs. Mobile vehicles were divided into nine categories according to the demands of decision makers and the distinctive features of local circumstance in metro-Taipei. The emission factor (EF) and vehicle kilometers traveled (VKT) of each kind of vehicle were estimated by MOBILE5B model via native parameters and questionnaires. Compressed natural gas (CNG) and inspection and maintenance (I/M) were the alternative control programs for buses and touring buses; liquefied petroleum gas (LPG), I/M, methanol, electrical vehicle (EV) were for taxis and low duty gasoline vehicles. EV, methanol, and I/M were the possible control methods for two-stroke and four-stroke engine motorcycles; I/M programs for low-duty diesel trucks, heavy-duty diesel trucks, and low-duty gasoline trucks. The results include the emission ratios of specific vehicle to all vehicles, the best combination of abated measures based on different objectives, and the marginal cost for ozone and VOC with varied control costs.

City Planning↗
PubMed1997

Kinetics of selenium incorporation into tissues of female mallard ducks.

Selenium is essential for both mammalian and avian species, although its metabolism in birds has been less thoroughly studied. Little information has been available on the kinetics of selenium in birds, especially as it relates to the teratogenicity seen in waterfowl consuming excessive amounts. This study describes the pharmacokinetics of small amounts of 75Se as selenious acid injected into female mallard ducks. Labeled selenium was injected into a wing vein of restrained animals and tissues taken at five different time points up to 24 h post-injection. Selenium levels as percent of injected dose were determined in liver, kidney, heart, lung, adrenals, thyroid, spleen, pancreas, ovaries, intestine, muscle and plasma. Estimates of kinetic parameters (uptake and elimination rates, time of maximum concentration and maximum concentration) were obtained with a non-linear kinetics computer program (PCNONLIN, SCI Software, Lexington, KY). Results showed four basic patterns of distribution, uptake and elimination. Visceral tissues exhibited a triphasic pattern with a rapid rise, a decline followed by a distinctive increase in levels between the last two time points. Brain uptake was continuous over the 24 h. Plasma rose rapidly and then declined to a constant level. The ovaries as a tissue of interest relating to the teratogenic effects of selenium showed the greatest relative increase at 24 h, suggesting kinetic mechanisms consistent with a pathway that might lead to accumulation of toxic levels and teratogenic effects during embryo development.

Animals↗
PubMed2001

Development of a high-performance liquid chromatographic method for bioequivalence study of flavoxate tablets.

An improved HPLC method was developed for the concentration determination of the metabolite of flavoxate, 3-methyl-flavone-8-carboxylic acid (MFCA), in plasma in an attempt to compare two flavoxate tablet formulations. This HPLC method was validated by examining the precision and the accuracy for inter-day and intra-day runs in a linear concentration range of 0.1-24 microg/ml. The coefficients of variation (C.V.) of inter-day and intra-day assays were 0.24-7.18% and 0.06-5.70%, respectively. The standard errors of mean (S.E.M.) were -0.004-8.68% and -2.52-4.86% for inter-day and intra-day assays, respectively. Bioequivalence of the two formulations was determined on 12 normal healthy male volunteers in a single-dose, two-period, two-sequence, two-treatment crossover study. MFCA plasma concentrations were analyzed with this validated HPLC method. The normal pivotal parameters, AUC(0-last), AUC(0-inf) and Cmax, were calculated and compared using the SAS General Linear Model computer program. The two one-sided t distribution test was also performed, as well as the 90% confidence-interval method, for the mean difference of the three pivotal parameters. The results suggest that these two flavoxate tablet formulations are non-bioequivalent when orally administered in a 400-mg dose of two tablets. This result was consistent with the in vitro dissolution of these two formulations.

Adult↗
PubMed1990

Prediction of gas chromatographic retention indexes for polychlorinated dibenzofurans.

A model has been developed by using molecular connectivity to describe the relationship between the molecular structure of polychlorinated dibenzofurans (PCDFs) and their gas chromatographic linear temperature-programmed retention characteristics on a 30-m fused silica column coated with a DB-5 stationary phase. Model variables account for the number of chlorine atoms present, the position and the relationship on each aromatic ring, the chlorine atom interaction between the two rings, and the skeletal structure of each PCDF isomer.

Benzofurans↗
PubMed2003

Calibration of permeation passive samplers with silicone membranes based on physicochemical properties of the analytes.

Passive sampling is a very attractive alternative to active sampling due to its simplicity and low cost. Among the passive samplers used in air analysis, permeation passive samplers are the least affected by ambient conditions, including humidity, air currents, and temperature changes. The biggest drawback of permeation passive samplers is the need to calibrate them experimentally for each individual target analyte. The paper presents the results of research on the calibration of permeation passive samplers based on physicochemical properties of the analytes. Strong correlations were found between the calibration constants of the samplers and the number of carbon atoms among families of compounds (R2 ranging from 0.8507 for alcohols to 0.9995 for aromatic hydrocarbons), the molecular weights of the compounds (R2 = 0.8742), their boiling points (R2 = 0.8911), and linear temperature-programmed retention indexes (R2 = 0.9225). The last correlation makes it possible to estimate the calibration constants for unidentified analytes, which is impossible when the conventional procedure is used. This makes it possible to deploy permeation passive samplers in the same way in which active sampling is deployed.

Journal Article↗
PubMed2006

Line-walking method for predicting the inhibition of P450 drug metabolism.

A new method, called line-walking recursive partitioning (LWRP), for partitioning diverse structures on the basis of chemical properties that uses only nine descriptors of the shape, polarizability, and charge of the molecule is described. We use a training set of over 600 compounds and a validation set of 100 compounds for the cytochrome P450 enzymes 2C9, 2D6, and 3A4. The LWRP algorithm itself incorporates elements from support vector machines (SVMs) and recursive partitioning, while circumventing the need for the linear or quadratic programming methods required in SVMs. We compare LWRP with a many-descriptor SVM model, using the same dataset as that described in the literature.(1) The line-walking method, using nine descriptors, predicted the validation set with about 84-90% accuracy, a success rate comparable to that of the SVM method. Furthermore, line-walking was able to find errors in the assignment of inhibitor values within the validation set for the 2C9 inhibitors. When these errors are corrected, the model predicts with an even higher level of accuracy. Although this method has been applied to P450 enzymes, it should be of general use in partitioning molecules on the basis of function.

Algorithms↗
PubMed2004

Structure of the Na(x)Cl(x+1) (-) (x=1-4) clusters via ab initio genetic algorithm and photoelectron spectroscopy.

The application of the ab initio genetic algorithm with an embedded gradient has been carried out for the elucidation of global minimum structures of a series of anionic sodium chloride clusters, Na(x)Cl(x+1) (-) (x=1-4), produced in the gas phase using electrospray ionization and studied by photoelectron spectroscopy. These are all superhalogen species with extremely high electron binding energies. The vertical electron detachment energies for Na(x)Cl(x+1) (-) were measured to be 5.6, 6.46, 6.3, and 7.0 eV, for x=1-4, respectively. Our ab initio gradient embedded genetic algorithm program detected the linear global minima for NaCl(2) (-) and Na(2)Cl(3) (-) and three-dimensional structures for the larger species. Na(3)Cl(4) (-) was found to have C(3v) symmetry, which can be viewed as a Na(4)Cl(4) cube missing a corner Na(+) cation, whereas Na(4)Cl(5) (-) was found to have C(4v) symmetry, close to a 3x3 planar structure. Excellent agreement between the theoretically calculated and the experimental spectra was observed, confirming the obtained structures and demonstrating the power of the developed genetic algorithm technique.

Computer Simulation↗
PubMed2006

Evaluation of cholecystokinin (CCK-8) peptide thermal stability for use as radiopharmaceutical by means isothermal and nonisothermal approaches.

The purpose of this research was to study the thermal stability of cholecystokinin octapeptide (CCK-8) in aqueous solution at pH 12 and ionic strength 0.01 M, which were kept as constants, by using isothermal and nonisothermal methods. The isothermal decomposition of CCK-8 was investigated as a function of temperature (40 degrees C to 70 degrees C). Nonisothermal stability studies were performed using a linear increasing temperature program. Two different nonisothermal studies were carried out at 0.25 degrees K and 0.5 degrees K per hour, and the temperature interval varied from 40 degrees C to 82 degrees C. The degradation of CCK-8 followed first-order kinetics, obeying the Arrhenius equation in the experimental temperature range. This indicated that the degradation mechanism of CCK-8 could be the equal within the temperature range studied. The nonisothermal approach resulted in activation energy (Ea) and shelf-life (t90%) values that agree well with those obtained by the isothermal method. The level of uncertainty in the estimates of t90% and Ea values is determined mainly by the extent of drug degradation and temperature change during the experiment. Therefore, nonisothermal experiments save time, labor and materials (i.e. the amount of drugs necessary to conduct the experiment) compared to the classic isothermal experiments, if they are performed using a suitable experimental design and a precise analytical method.

Chromatography, High Pressure Liquid↗
PubMed1976

Localization of implanted radioactive wires and computation of the dose distribution.

A method is described for the spatial localization of implanted curved radioactive wires. The input data are obtained from two orthogonal radiographs taken on a simulator. A specially constructed double light-box is used for the determination of corresponding points on the projections of the wire in the AP and lateral radiographs. The wires are localized as a series of straight segments and formulae are derived for the computation of the segment activity in order to calculate the dose distribution for uniformly and non-uniformly activated wires. All calculations can be included as additional subprograms in a dose computation program for straight linear sources.

Radiotherapy↗