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Distribution Parameters (distribution + parameter)
Selected AbstractsDielectric relaxation and crystallization of ultraviscous melt and glassy states of aspirin, ibuprofen, progesterone, and quinidineJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 5 2007G.P. Johari Abstract Molecular relaxation in ultraviscous melt and glassy states of aspirin, ibuprofen, progesterone, and quinidine has been studied by dielectric spectroscopy. The asymmetric relaxation spectra is characterized by the Kohlrausch distribution parameter of 0.46,±,0.02 for aspirin to 0.67,±,0.02 for progesterone. The dielectric relaxation time varies with the temperature, T, according to the Vogel,Fulcher,Tammann Equation, log10(,0),=,AVFT,+,[BVFT/(T,,,T0)], where AVFT, BVFT, and T0 are empirical constants. The extrapolated ,0 at calorimetric glass-softening temperature is close to the value expected. The equilibrium permittivity, ,0, is lowest for ibuprofen which indicates an antiparallel orientation of dipoles in its liquid's hydrogen-bonded structure. A decrease in ,0 with time shows that ultraviscous aspirin, progesterone, and quinidine begin to cold-crystallize at a relatively lower temperature than ibuprofen. ,0 of the cold-crystallized phases are, 4.7 for aspirin at 290 K, 2.55 for ibuprofen at 287 K, 2.6 for progesterone at 320 K, and 3.2 for quinidine at 375 K. It is argued that hydrogen-bonding, the Kohlrausch parameter, extent of localized motions and the long-range diffusion times all determine the physical and chemical stability of an amorphous pharmaceutical during storage. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 1159,1175, 2007 [source] Destabilisation of homogeneous bubbly flow in an annular gap bubble columnTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010Fahd M. Al-Oufi Abstract Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in an open tube and a annular gap bubble column, when operated at the same gas superficial velocity, using a porous sparger. Measurements were carried out in a vertical 0.102,m internal diameter column, with a range of concentric inner tubes to form an annular gap, giving diameter ratios from 0.25 to 0.69; gas superficial velocities in the range 0.014,0.200,m/s were investigated. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (1) the presence of the inner tube causes large bubbles to form near the sparger, which destabilise the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles, and (2) the shape of the void fraction profiles changes with gap geometry and this affects the distribution parameter in the drift-flux model. Both of these effects serve to reduce the mean gas void fraction in an annular gap bubble column compared to an open tube at the same gas superficial velocity. Des résultats expérimentaux sont présentés pour montrer qu'il existe de très grandes différences dans les fractions de vide gazeux moyennes mesurées dans un tube ouvert et une colonne à bulles à espace annulaire, lorsqu'ils sont utilisés à la même vitesse superficielle de gaz, au moyen d'un aérateur poreux. On a effectué des mesures dans une colonne verticale avec un diamètre interne de 0.102,m, avec une portée de tubes internes concentriques pour former un espace annulaire, procurant des rapports de diamètre de 0.25 à 0.69; des vitesses superficielles de gaz de 0.014 à 0.200,m/s ont été étudiées. La fraction de vide gazeux moyenne diminue avec le rapport croissant du diamètre interne à externe de la colonne à espace annulaire et la transition à la circulation hétérogène se produit à des vitesses superficielles de gaz et fractions de vide plus basses. Deux raisons sont proposées et validés par les vérifications expérimentales: (1) la présence du tube interne provoque la formation de grandes bulles près de l'aérateur, ce qui déstabilize l'écoulement à bulles homogène et réduit la fraction de vide moyenne; cet état a été confirmé en injectant délibérément de grandes bulles dans une dispersion homogène de plus petites bulles et, (2) la forme des profils de fraction de vide change avec la géométrie de l'espace qui les sépare, ce qui a des conséquences sur le paramètre de distribution du modèle à flux de dérive. Ces deux effets servent à réduire la fraction de vide gazeux moyenne dans une colonne à bulles à espace annulaire, en comparaison avec un tube ouvert à la même vitesse superficielle de gaz. [source] Effect of degree of fluid saturation on transport coefficients in disturbed soilsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2004A. Tuli Summary To improve the predictive capability of transport models in soils we need experimental data that improve their understanding of properties at the scale of pores, including the effect of degree of fluid saturation. All transport occurs in the same soil pore space, so that one may intuitively expect a link between the different transport coefficients and key geometrical characteristics of the pores such as tortuosity and connectivity, and pore-size distribution. To understand the combined effects of pore geometry and pore-size distribution better, we measured the effect of degree of water saturation on hydraulic conductivity and bulk soil electrical conductivity, and of degree of air saturation on air conductivity and gaseous diffusion for a fine sand and a sandy loam soil. To all measured data were fitted a general transport model that includes both pore geometry and pore-size distribution parameters. The results show that both pore geometry and pore-size distribution determine the functional relations between degree of saturation, hydraulic conductivity and air conductivity. The control of pore size on convective transport is more for soils with a wider pore-size distribution. However, the relative contribution of pore-size distribution is much larger for the unsaturated hydraulic conductivity than for gaseous phase transport. For the other transport coefficients, their saturation dependency could be described solely by the pore-geometry term. The contribution of the latter to transport was much larger for transport in the air phase than in the water phase, supporting the view that connectivity dominates gaseous transport. Although the relation between effective fluid saturation and all four relative transport coefficients for the sand could be described by a single functional relation, the presence of a universal relationship between fluid saturation and transport for all soils is doubtful. [source] A regional frequency analysis of United Kingdom extreme rainfall from 1961 to 2000INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2003H. J. Fowler Abstract Multi-day rainfall events are an important cause of recent severe flooding in the UK, and any change in the magnitude of such events may have severe impacts upon urban structures such as dams, urban drainage systems and flood defences and cause failures to occur. Regional pooling of 1-, 2-, 5- and 10-day annual maxima for 1961 to 2000 from 204 sites across the UK is used in a standard regional frequency analysis to produce generalized extreme value growth curves for long return-period rainfall events for each of nine defined climatological regions. Temporal changes in 1-, 2-, 5- and 10-day annual maxima are examined with L-moments using both a 10 year moving window and the fixed decades of 1961,70, 1971,80, 1981,90 and 1991,2000. A bootstrap technique is then used to assess uncertainty in the fitted decadal growth curves and to identify significant trends in both distribution parameters and quantile estimates. There has been a two-part change in extreme rainfall event occurrence across the UK from 1961 to 2000. Little change is observed at 1 and 2 days duration, but significant decadal-level changes are seen in 5- and 10-day events in many regions. In the south of the UK, growth curves have flattened and 5- and 10-day annual maxima have decreased during the 1990s. However, in the north, the 10-day growth curve has steepened and annual maxima have risen during the 1990s. This is particularly evident in Scotland. The 50 year event in Scotland during 1961,90 has become an 8-year, 11-year and 25-year event in the East, South and North Scotland pooling regions respectively during the 1990s. In northern England the average recurrence interval has also halved. This may have severe implications for design and planning practices in flood control. Copyright © 2003 Royal Meteorological Society [source] Determination of molecular weight distribution parameters of polymers and additives by field desorption mass spectrometryLUBRICATION SCIENCE, Issue 2 2002K. Vivekanandan Abstract The applicability of the field desorption mass spectrometry (FD-MS) technique to the determination of the molecular weight distribution parameters of various commercially available petroleum sulphonates is demonstrated. The technique has been substantiated using standards such as polyethylene glycols, polystyrenes, and sodium petroleum sulphonates. This study of molecular weight averages compares the results from gel permeation chromatography and FD-MS. [source] Maternal serum free-,-chorionic gonadotrophin, pregnancy-associated plasma protein-A and fetal nuchal translucency thickness at 10,13+6 weeks in relation to co-variables in pregnant Saudi womenPRENATAL DIAGNOSIS, Issue 4 2007Mohammed-Salleh M. Ardawi Abstract Objective To establish normative values and distribution parameters of first-trimester screening markers, namely, fetal nuchal translucency (NT), maternal serum free ,-human chorionic gonadotrophin (,-hCG) and pregnancy-associated plasma protein-A (PAPP-A), at 10 to 13+6 weeks of gestation in Saudi women and to evaluate the effect of co-variables including maternal body weight, gravidity, parity, fetal gender, twin pregnancy, smoking and ethnicity on these markers. Methods A cohort of Saudi women (first cohort n = 1616) with singleton pregnancies prospectively participated in the present study, and fetal NT together with maternal serum free ,-hCG and PAPP-A were determined at 10 to 13+6 weeks of gestation. The distribution of gestational age-independent multiples of the median (MoM) of the parameters was defined and normative values were established, and correction for maternal body weight was made accordingly. The influence of various co-variables was examined using the data collected from the first and the second (n = 1849) cohorts of women and 62 twin pregnancies, and compared with other studies. Results All markers exhibited log-normally distributed MoMs. Gestational age-independent normative values were established. Maternal body weight was corrected, particularly for maternal free ,-hCG and PAPP-A using standard methods. Fetal NT showed a negative relationship with increasing gravidity (r = ,0.296) or parity (r = ,0.311), whereas both free ,-hCG and PAPP-A exhibited a significant positive relationship. There was a significant increase in the MoM of free ,-hCG in female fetuses. Smoking decreased MoM values of free ,-hCG (by 14.6%; P < 0.01) and PAPP-A (by 18.8%; P < 0.001). Twin pregnancy showed significant increases in MoM values of free ,-hCG (by 1.87-fold) and PAPP-A (by 2.24-fold), with no significant changes in fetal NT MoM values. Fetal NT MoM values were lower in Africans and Asians but higher in Orientals, as compared to Saudi women (P < 0.05; in each case). MoM values (body weight-corrected) of free ,-hCG were 25.2% higher in Africans and 19.4% higher in Orientals but 6.8% lower in other Arabian and Asian (by 5.8%) women as compared to Saudi women (P < 0.05; in each case). Conclusions The normative values and distribution parameters for fetal NT, maternal serum free ,-hCG and PAPP-A were established in Saudi singleton pregnancies, the maternal body weight together with smoking, twin pregnancy and ethnicity being important first-trimester screening co-variables. Gravidity, parity and fetal gender are also considered to influence one or more of the first-trimester markers examined. Copyright © 2007 John Wiley & Sons, Ltd. [source] ADAM 12 as a first-trimester maternal serum marker in screening for Down syndromePRENATAL DIAGNOSIS, Issue 10 2006Jennie Laigaard Abstract Background A Disintegrin And Metalloprotease 12 (ADAM 12) is a glycoprotein synthesised by placenta and it has been shown to be a potential first-trimester maternal serum marker for Down syndrome (DS) in two small series. Here we analyse further, the potential of ADAM 12 as a marker for DS in a large collection of first-trimester serum samples. Materials and Methods The concentration of ADAM 12 was determined in 10,14-week pregnancy sera from 218 DS pregnancies and 389 gestational age-matched control pregnancies, which had been collected as part of routine prospective first-trimester screening programs (DS = 105) or as part of previous research studies (DS = 113). ADAM 12 was measured using a semi-automated time resolved immunofluorometric assay and median values for normal pregnancies were established by polynomial regression. These medians were then used to determine population distribution parameters for DS and normal pregnancy groups. Correlation with previously established PAPP-A and free ,-hCG multiple of the medians (MoMs) and delta nuchal translucency (NT) were determined and used to model the performance of first-trimester screening with ADAM 12 in combination with other first-trimester markers at various time periods across the first trimester. The benefits of a contingent testing model incorporating early measurement of PAPP-A and ADAM 12 were also explored. Results The maternal serum concentration of ADAM 12 was significantly reduced (p = 0.0049) with an overall median MoM of 0.79 in the DS cases and a log10 MoM SD of 0.3734 in the DS cases and 0.3353 in the controls. There was a significant correlation of ADAM 12 MoM in DS cases with gestational age (r = 0.375) and the median MoM increased from 0.50 at 10,11 weeks to 1.38 at 13 weeks. ADAM 12 was correlated with maternal weight (r(controls) = 0.283), PAPP-A (r(controls) = 0.324, r(DS) = 0.251) but less so with free ,-hCG (r(controls) = 0.062, r(DS) = 0.049) and delta NT (r(controls) = 0.110, r(DS) = 0.151). ADAM 12 was significantly (p = 0.026) lower in smokers (0.87 vs 1.00) and elevated in Afro-Caribbean women compared to Caucasian women (1.34 vs 1.00). Population modelling using parameters from this and an earlier study showed that a combination of ADAM 12 and PAPP-A measured at 8,9 weeks and combined with NT and free ,-hCG measured at 12 weeks could achieve a detection rate of 97% at a 5% false-positive rate or 89% at a 1% false-positive rate. PAPP-A and ADAM 12 alone at 8,9 weeks could identify 91% of cases at a 5% false-positive rate. Using this as part of a contingent-screening model to select an intermediate risk group of women for NT and free ,-hCG at 11,12 weeks would enable the detection of 92% of cases with a 1% false-positive rate at a cost of providing NT and free ,-hCG for 6% of women with 94% of women having completed screening by the 10th week of pregnancy. Conclusion ADAM 12 in early first trimester is a very efficient marker of DS. In combination with existing markers, it offers enhanced screening efficiency in a two-stage sequential first-trimester screening program or in a contingent-screening model, which may have benefits in health economies where universal access to high quality ultrasound is difficult. More data on early first-trimester cases with DS are required to establish more secure population parameters by which to assess further the validity of these models. Copyright © 2006 John Wiley & Sons, Ltd. [source] The evolution of the z distribution of normal neutron stars in the GalaxyASTRONOMISCHE NACHRICHTEN, Issue 8 2010Y.-C. Wei Abstract Under the two initial 1-D one parameter velocity distribution forms (one is normal, the other is exponential), the z direction scale height evolution of normal neutron stars in the Galaxy is studied by numerical simulation. We do statistics for the cases at different time segments, also do statistics for the cumulative cases made of each time segment. The results show in the cumulative cases the evolution curves of the scale heights are smoother than in the each time segment, i.e., the cumulation improve the signal-to-noise ratio. Certainly the evolution cases are different at different Galactic disk locations, which also have very large difference from the average cases in the whole disk. In the initial stages of z evolution of normal neutron stars, after the beginning transient states, the cumulative scale heights increase linearly with time, and the cumulative scale height increasing rates have linear relationship with the initial velocity distribution parameters, which have larger fluctuation in the vicinity of the Sun than in the whole disk. We utilize the linear relationship of the cumulative scale height increasing rates vs. the initial velocity distribution parameters in the vicinity of the Sun to make comparison with the observation near the Sun. The results show if there is no magnetic decay, then the deserved initial velocity parameters are obvious lower than the present well known results from some authors; whereas if introducing magnetic decay, for the 1-D normal case we can make consistence among concerning results using magnetic decay time values which are supported by some authors, while for the 1-D exponential case the results show the lackness of young pulsar samples in the larger z in the vicinity of the Sun (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Mean Reversion and the Distribution of United Kingdom Stock Index ReturnsJOURNAL OF BUSINESS FINANCE & ACCOUNTING, Issue 9-10 2006David Ashton Abstract:, Our purpose here is to develop the Pearson Type IV distribution as a candidate for modelling the evolution of short period stock index returns. Here, early work by Praetz (1972 and 1978) and Blattberg and Gonedes (1974) has shown that the scaled ,t' distribution, which is a particular (symmetric) interpretation of the Pearson Type IV, provides a reasonable description of the way stock index returns evolve over time. Our analysis shows this is certainly not the case for the daily stock index returns on which our empirical analysis is based. There is significant skewness in the data and this cannot be captured by symmetric distributions like the scaled ,t' and normal distributions. However, the Pearson Type IV, which is a skewed generalisation of the scaled ,t', is capable of modelling the skewness inherent in our data and in such a way that it satisfies asymptotically efficient goodness of fit criteria. Furthermore, the Pearson Type IV can be derived from a stochastic differential equation with standard Markov properties. This enables one to integrate the distributional and time series properties of the returns process and thereby, facilitates both the interpretation and understanding of the role played by the distribution's parameters in the generation of the underlying stock index returns. [source] | |||||||||||||