Stability testing in an integrated scheme

ISO Guide 35 deals with RM stability issues and scrutinizes the evaluation of stability testing results under the assumption that either there is no trend at all (a rather rare situation), or any observed deterministic change is insignificant and thus can be neglected. However, market demands for reliable reference materials are obviously not limited to stable or at least seemingly stable materials. In many analytical applications, analytes and measurands under consideration are known, or at least suspected, to be unstable on time scales that may vary widely from measurand to measurand. The Federal Institute for Materials Research and Testing (BAM) has developed (and successfully uses) an integrated approach in its certification practice. The approach is based on an initial stability study and subsequent post-certification monitoring. Data evaluation is model-based and takes advantage of all information collected in the stability testing scheme(s). It thus allows one to deal with any kind of instability observed, to assess limiting time intervals at any stress condition in the range tested, to estimate a final expiry date for materials with detected instabilities or the maximum admissible re-testing interval for seemingly stable materials, and to assess maximum admissible stress loads during delivery of the material to the customer. The article describes (and exemplifies) typical study layout, the model selection, and the integrated data assessment.     

Few-body correlations in the QCD phase diagram

From the viewpoint of statistical physics, nuclear matter is a strongly correlated many-particle system. Several regimes of the QCD phase diagram should exhibit strong correlations. Here I focus on three- and four-body correlations that might be important in the phase diagram.Received: 30 September 2002, Published online: 22 October 2003PACS: 21.65. + f Nuclear matter - 21.45. + v Few-body systems     

Chemical analysis of acidic silicon etch solutions: I. Titrimetric determination of HNO3, HF, and H2SiF6

The chemical etching of silicon using HF-HNO₃ mixtures is a widely used process in the processing of silicon wafers for microelectronic or photovoltaic applications. The control of the etch bath composition is the necessary condition for an effective bath utilization, for the replenishment of the consumed acids, and to maintain a certain etch rate. The present paper describes two methods for the total analysis of the individual etch bath constituents HF, HNO₃, and H₂SiF₆. Both methods start with an aqueous acid-base titration determining the total acid concentration and the concentration of H₂SiF₆. The first method is an acid-base titration using a 0.1 mol L⁻¹ methanolic solution of cyclohexylamine (CHA) as non-aqueous titrant to determine the content of nitric acid. Then, the amount of hydrofluoric acid is calculated from the difference between the total acid and nitric acid content. The second method is based on the determination of the total fluoride concentration using a fluoride ion-selective electrode (F-ISE). The content of hydrofluoric acid is obtained from the difference between the total fluoride content and the amount of fluoride bound as H₂SiF₆. The amount of nitric acid results finally calculated as difference to the total acid content.     

Ab initio studies on polymers. VI. Torsional potential in regular polyethylene chains

Ab initio crystal orbital calculations have been performed on regular polyethylene chains applying basis sets of minimal and double-zeta quality. Relative stabilities of periodic all-trans, all-gauche, and alternating trans–gauche conformers have been evaluated, including extensive geometry optimization. Potential curves for a simultaneous rotation around C? C single bonds from the all-trans to the all-gauche conformation have been computed applying the rigid-rotor approximation, the flexible-rotor approximation, and an additional reoptimization of C? C distances. A rigid-rotor potential curve from the all-trans to the alternating trans-gauche conformation has been computed as well. Results obtained are compared with ab initio calculations on butane and pentane and with semiempirical and empirical force-field studies on polyethylene.     

Solution- and bound-state conformational study of N,N',N"-triacetyl chitotriose and other analogous potential inhibitors of hevamine: application of trNOESY and STD NMR spectroscopy

The solution-state conformations of N,N',N"-triacetyl chitotriose (1) and other potential chitinase inhibitors 2-4 were studied using a combination of NMR spectroscopy (NOESY) and molecular mechanics calculations. Determination solely of the global energy minimum conformation was found to be insufficient for an agreement with the NMR results. An appropriate consistency between the NMR experimental data and theoretical calculations was only reached by assessing the structures as population-weighted average conformers based on Boltzmann distributions derived from the calculated relative energies. Analogies, but also particular differences, between the synthetic compounds 2-4 and the naturally-occurring N,N',N"-triacetyl chitotriose were found. Furthermore, the conformation of compounds 1 and 2 when bound to hevamine was also studied using transferred NOESY experiments and the binding process was found to impart a level of conformational restriction on the ligands. The preferred conformation as determined for 1 in the bound state to hevamine belonged to one of the conformational families found for the compound when free in solution, although full characterisation of the bound-state conformations was impeded due to severe signal overlap. Saturation transfer difference NMR experiments were also employed to analyse the binding epitopes of the bound compounds. We thus determined that it is mainly the acetyl amido groups of the trisaccharide and the heterocyclic moiety which are in close contact with hevamine.     

Metal clips induce folding of a short unstructured peptide into an alpha-helix via turn conformations in water. Kinetic versus thermodynamic products

Short peptides corresponding to two to four alpha-helical turns of proteins are not thermodynamically stable helices in water. Unstructured octapeptide Ac-His1-Ala2-Ala3-His4-His5-Glu6-Leu7-His8-NH(2) (1) reacts with two [Pd((15)NH(2)(CH(2))(2)(15)NH(2))(NO(3))(2)] in water to form a kinetically stable intermediate, [[Pden](2)[(1,4)(5,8)-peptide]](2), in which two 19-membered metallocyclic rings stabilize two peptide turns. Slow subsequent folding to a thermodynamically more stable two-turn alpha-helix drives the equilibrium to [[Pden](2)[(1,5)(4,8)-peptide]] (3), featuring two 22-membered rings. This transformation from unstructured peptide via turns to an alpha-helix suggests that metal clips might be useful probes for investigating peptide folding.     

Automated multiple-layer spotting for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of synthetic polymers utilizing ink-jet printing technology

Recently, a new multiple-layer matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) sample spotting technique for poly(ethylene glycol), offering improved analysis possibilities, was described. In this contribution the application of ink-jet printing to automated, multiple-layer MALDI-TOFMS sample preparation of synthetic polymers is presented, allowing accurate deposition of matrix, additive and analyte solutions. The new sample preparation technique was evaluated for poly(ethylene glycol) as well as poly(methyl methacrylate) standards, and optimized settings for both synthetic polymers have been obtained.     

Cis‐trans Isomerism in Copper(II) Complexes with N‐acyl Thiourea Ligands

The N‐acyl thiourea complexes bis[N,N‐diethyl‐N′‐(p‐nitrobenzoyl)‐thioureato]copper(II) ( 1a,1b ) and bis(N,N‐diphenyl‐N′‐benzoylthioureato)copper(II) ( 2a,2b ) crystallize in each case in two modifications. X‐ray structural analysis shows that 1a and 1b are cis‐trans isomers. This is very unusual for N‐acyl thioureato complexes because with exception of one platinum(II) complex up to now only cis complexes have been found. In contrast X‐ray structural analysis of both forms 2a and 2b of the other complex shows no cis‐trans pair. Both modifications are cis complexes. In solution both isomers of the copper(II) complexes are observable by EPR spectroscopy.     

N,N-Dialkylierte N′-Ferrocenoylthioharnstoffe und Ferrocen-1,1′-dicarbonsäure-di-N,N-dialkyl-thioureide als Liganden für Übergangsmetalle: Darstellung,pH-potentiometrische,röntgenkristallstrukturanalytische und EPR-Einkristalluntersuchungen

N,N-Dialkyl-N′-ferrocenoylthioureas and Ferrocene-1,1′ -dicarbonic-acid-di-N,N-dialkylthioureides as Ligands for Transition Metals: Synthesis, Acid-Dissociation Constants, X-Ray Structure Determination, and EPR-Single Crystal Investigation Reaction of ferrocenoylisothiocyanate or ferrocene-1,1′ -dicarbonic acid -diisothiocyanate with secondary amines gives N,N-dialkyl-N′ -ferrocenoylthioureas or the respective ferrocene-1,1′-dicarbonic acid-di-N,N-dialkyl-thioureides. The former yield neutral complexes with transition metal ions (Ni²⁺, Cu²⁺, Co³⁺, Pt²⁺, Fe³⁺). The acid dissociation constants of the ligands were determined. The EPR spectra of a bis-(N,N-diⁿbutyl-N′-ferrocenoylthioureato)copper(II) single crystal are discussed. The X-ray structure determination of ferrocene-1,1′-dicarbonic acid-di-N,N-diethyl-thioureide and its different behaviour against Ni²⁺ and Cu²⁺ is presented.