Determination of organic acids in urine by solid-phase microextraction and gas chromatography-ion trap tandem mass spectrometry previous 'in sample' derivatization with trimethyloxonium tetrafluoroborate

A method for the determination of the organic acids directly in the urine employing derivatization with trimethyloxonium tetrafluoroborate as a methylating agent and sequential extraction by head space and direct immersion/solid phase microextraction is reported. Furoic acid, hippuric acid, methylhippuric acid, mandelic acid, phenylglyoxylic acid and trans, trans muconic acid contained in urine and proposed by the American Conference of Governmental Industrial Hygienists as biological exposure indices were determined after a fast and economically convenient preparation step and sensitive gas chromatography-ion trap-mass spectrometry/tandem mass spectrometry analysis. Urine is rather a complex sample and hence the acquisition method required specific GC-MS instrumentation capable of supporting the changeover, fully automated during a single chromatographic separation, from mass to tandem mass spectrometry and both chemical and electron ionization modes. The automation of the analytical method provides a number of advantages, including reduced analysis time for both routine analysis and method development, and greater reproducibility. The equilibrium and kinetics of this substances vs head space/direct immersion-solid phase microextraction were investigated and evaluated theoretically.     

Free-boundary models of sulphation

Air pollution is the most serious cause of degradation of carbonate stones, which constitute, as marble, travertine or other limestones, some of the most important monuments and artifacts in the world. Mathematicians have now begun to model the degradation problem to assist people working in protection and restoration of these stones. Here, a free boundary model is presented, which describes the growth of a gypsum crust on the surface of marble monuments under SO₂ aggression, and quantifies the influence of environmental factors such as pollutant concentration, humidity and temperature. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Asymptotic behavior of the number of distinct values in a sample from the geometric stick-breaking process

Discrete random probability measures are a key ingredient of Bayesian nonparametric inference. A sample generates ties with positive probability and a fundamental object of both theoretical and applied interest is the corresponding number of distinct values. The growth rate can be determined from the rate of decay of the small frequencies implying that, when the decreasingly ordered frequencies admit a tractable form, the asymptotics of the number of distinct values can be conveniently assessed. We focus on the geometric stick-breaking process and we investigate the effect of the distribution for the success probability on the asymptotic behavior of the number of distinct values. A whole range of logarithmic behaviors are obtained by appropriately tuning the prior. A two-term expansion is also derived and illustrated in a comparison with a larger family of discrete random probability measures having an additional parameter given by the scale of the negative binomial distribution.

     

ACCDB: A collection of chemistry databases for broad computational purposes

The importance of databases of reliable and accurate data in chemistry has substantially increased in the past two decades. Their main usage is to parametrize electronic structure theory methods, and to assess their capabilities and accuracy for a broad set of chemical problems. The collection we present here—ACCDB—includes data from 16 different research groups, for a total of 44,931 unique reference data points, all at a level of theory significantly higher than density functional theory, and covering most of the periodic table. It is composed of five databases taken from literature (GMTKN, MGCDB84, Minnesota2015, DP284, and W4-17), two newly developed reaction energy databases (W4-17-RE and MN-RE), and a new collection of databases containing transition metals. A set of expandable software tools for its manipulation is also presented here for the first time, as well as a case study where ACCDB is used for benchmarking commercial CPUs for chemistry calculations. © 2018 Wiley Periodicals, Inc.     

A classification result for the quasi-linear Liouville equation

Entire solutions of the n-Laplace Liouville equation in ${\mathbb{R}}^n$ with finite mass are completely classified.     

How to measure single-name credit risk concentrations

Credit risk concentration is one of the leading topics in modern finance, as the bank regulation has made increasing use of external and internal credit ratings. Concentration risk in credit portfolios comes into being through an uneven distribution of bank loans to individual borrowers (single-name concentration) or in a hierarchical dimension such as in industry and services sectors and geographical regions (sectorial concentration).     

A Study on the Stability and Enzymatic Activity of Yeast Alcohol Dehydrogenase in Presence of the Self-Assembling Block Copolymer Poloxamer 407

Yeast alcohol dehydrogenase (ADH) is an enzyme widely studied for biotechnological applications due to its involvement in fermentation industry, and various attempts to improve its catalytic properties and its thermal stability have been carried out. In this paper, the influence of a block copolymer (Poloxamer 407) on ADH enzymatic activity and thermal behaviour has been studied in order to get new insights about the use of poloxamers in formulation of sustained release systems for therapeutic proteins. Poloxamer 407 has the ability to form micelles and gel due to its self-assembling and thermoresponsive properties. The effect of the copolymer towards thermal stress and pH changes, which often reduce enzymes activity it has been investigated by means of enzymatic assays and differential scanning calorimetry. Results showed that at pH?9.1 and 7.3, the Poloxamer in the form of unimeric, micellar and gel state is able to effectively preserve the enzyme from thermoinactivation. In addition by calorimetric data Poloxamer 407 has showed an effect in preserving ADH from aggregation at pH?7.3. In conclusion, Poloxamer 407 seems to be very effective in protecting ADH from stress related events, like alkaline inactivation and aggregation.     

Mountain pass critical points for Paneitz-Branson operators

Given a smooth compact Riemannian manifold of dimension , we study fourth order equations involving Paneitz-Branson type operators and the critical Sobolev exponent. Received: 19 June 2001 / Accepted: 27 September 2001 / Published online: 28 February 2002 Esposito's research is supported by M.U.R.S.T. under the national project Variational methods and nonlinear differential equations.