Quantitative analysis of DHEA and androsterone in female urine: investigating the effects of menstrual cycle, oral contraception and training on exercise-induced changes in young women

Dehydroepiandrosterone (DHEA) and its metabolite androsterone (A) are natural steroids secreted in high quantities in human body. To assess the influence of oral contraceptives, menstrual cycle phase, and also physical exercise (acute and chronic such as training) on these metabolites excretions, a collection of 28 female urine specimens was organized. A three-extraction-step method was developed, and the analyses were performed by gas chromatography–mass spectrometry using deuterated 19-noretiocholanolone as the internal standard. Sample hydration state was found to be of great importance for kinetic studies, as it directly influenced the concentrations. No influence of menstrual cycle and training was found for androsterone and DHEA. However, oral contraceptive intake lowered DHEA excretion in urine and A seems to be slightly affected by exercise.     

Application of direct analysis in real time to the study of chemical vapor generation mechanisms: identification of intermediate hydrolysis products of amine-boranes

Analytical and Bioanalytical Chemistry - In order to elucidate controversial results emerging in chemical vapor generation (CVG) for trace element determination, we conducted a series of...     

Letter: Gödel-Type Space-Time Metrics

A simple group theoretic derivation is given of the family of space-time metrics with isometry group SO(2,1) × SO(2) × first described by Gödel, of which the Gödel stationary cosmological solution is the member with a perfect-fluid stress-energy tensor. Other members of the family are shown to be interpretable as cosmological solutions with an electrically charged perfect fluid and a magnetic field.     

Étude thermodynamique de la complexation de l''argent par quelques pyrazines substituées

Stability constants of silver complexes with the substituted ligands 2-methyl-2,5-dimethyl-, 2,6-dimethyl-, 2-chloro- and 2-amino-pyrazines are determined by means of a potentiomeric method, using metal complex electrodes at 25°C and 0.1M KNO₃ as an ionic strength in aqueous medium.

Standard enthalpies of the two steps of complexation observed are obtained by a direct calorimetric technique (except for the Ag-2-aminopyrazine system whose complexes are not sufficiently soluble) under the same experimental conditions. The corresponding standard entropies are calculated.

The discussion of the thermodynamic functions takes into account the nature and the position of the substitutents to interprete the coordination ability of each amine.     

Complexation metallique des imides. II. Grandeurs thermodynamiques de formation des complexes de Ag(I), Cd(II), Co(II), Ni(II), Cu(II), Zn(II) avec l''hydantoine

Thermodynamic values for the formation of complexes of Ag⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ ions with hydantoïn were determined at 25°C in aqueous medium 0.5 M KNO₃. Stability constants were calculated from pH metric measurements while corresponding standard enthalpies were deduced from direct calorimetric determinations. The results are interpreted and compared to those of succinimide complexes previously studied.     

Low-redshift formula for the luminosity distance in a LTB model with cosmological constant

We calculate the low-redshift Taylor expansion for the luminosity distance for an observer at the center of a spherically symmetric matter inhomogeneity with a non-vanishing cosmological constant. We then test the accuracy of the formulas comparing them to the numerical calculation for different cases for both the luminosity distance and the radial coordinate. The formulas can be used as a starting point to understand the general non-linear effects of a local inhomogeneity in the presence of a cosmological constant, without making any special assumption as regards the inhomogeneity profile.     

Quantification of nitrite and nitrate in seawater by triethyloxonium tetrafluoroborate derivatization-headspace SPME GC-MS

Triethyloxonium tetrafluoroborate derivatization combined with direct headspace (HS) or SPME-gas chromatography-mass spectrometry (GC-MS) is proposed here for the simultaneous determination of nitrite and nitrate in seawater at micromolar level after conversion to their corresponding volatile ethyl-esters (EtO-NO and EtO-NO₂). Isotopically enriched nitrite [¹⁵N] and nitrate [¹⁵N] are employed as internal standards and for quantification purposes. HS-GC-MS provided instrumental detection limits of 0.07 μM NO₂⁻ and 2 μM NO₃⁻. Validation of the methodology was achieved by determination of nitrite and nitrate in MOOS-1 (Seawater Certified Reference Material for Nutrients, NRC Canada), yielding results in excellent agreement with certified values. All critical aspects connected with the potential inter-conversion between nitrite and nitrate (less than 10%) were evaluated and corrected for by the use of the isotopically enriched internal standard.