Towards the certification of the purity of calibrant reference materials for thyroid hormones: a chicken and egg dilemma

The certification of the purity of CRMs intended for calibration, where no other certified material already exists for comparison, raises principle questions on how to determine the purity of a "first" calibrant in the calibration hierarchy. We developed and certified two calibration CRMs for their purity in thyroid hormones taking into consideration inorganic residues, residual solvents and organic impurities detectable by HPLC-UV and HPLC-MS. IRMM-468 was certified for a thyroxine (T(4)) mass fraction of 98.6+/-0.7% and IRMM-469 was certified for a 3,3',5-triiodothyronine (T(3)) mass fraction of 97.1+/-0.7%. The approach we used aims to determine the purity of these two CRMs to the best of our knowledge and taking all scientific aspects properly into account for the estimation of an uncertainty related to the stated purity.     

The mechanism of cavitation-induced scission of single-walled carbon nanotubes

Aqueous suspensions of length selected single-walled carbon nanotubes were studied by atomic force microscopy (AFM) in order to probe the influence of sonication on nanotube scission. The maximum of the tube length distribution, lM, initially exhibits a power law dependence on the sonication time, t - roughly as lM approximately t(-0.5). This and the limiting behavior observed at longer times can be rationalized to first order in terms of a continuum model deriving from polymer physics. In this picture, the strain force associated with cavitation scales with the square of the nanotube length. Scission stops when the strain force falls below the critical value for nanotube disruption.     

Determination of tributyltin (TBT) in marine sediment using pressurised liquid extraction–gas chromatography–isotope dilution mass spectrometry (PLE–GC–IDMS) with a hexane–tropolone mixture

Extraction conditions for the determination of tributyltin (TBT) in sediment samples have been developed further. The analytical procedure is based on spiking with isotopically labelled analyte, pressurised liquid extraction (PLE) with a hexane/tropolone mixture, Grignard derivatization and quantification by GC–MS. It was applied to two unknown sediment samples as part of an intercomparison exercise of the Comité Consultatif pour la Quantité de Matière (CCQM). The detection limit was approximately 1.5 ng/g TBT as Sn, while the repeatability and intermediate precision (as the coefficient of variation) were 1.9% and 3.2%, respectively. The expanded uncertainty was 6.2% (coverage factor k = 2), and the accuracy was confirmed by measurement of a certified reference material.     

Evaluation of uncertainty of reference materials

 Certification of reference materials is far more than just characterisation of a selected homogeneous batch of material. From the perspective of the ISO Guide on the Expression of Uncertainty in Measurement (GUM) all uncertainty sources relevant to the user of an individual certified reference material (CRM) sample at a moment in time should be part of the CRM uncertainty. This not only includes the full uncertainty of the batch characterisation (rather than the statistical variation), but also all uncertainties related to possible between-bottle variation, instability upon long-term storage and instability during transport to the customer. Received: 21 April 1999 · Accepted: 24 September 1999     

Surface modification of thin polystyrene films

 The sulfonation of polystyrene (PS) films with 50 and 96% sulfuric acid as a function of time is presented. In contrast to previous literature reports, we showed that the treatment of PS films even with dilute sulfuric acid yields sulfonated surfaces after reaction times of 30 s–1 h. The hydrophilicity of the modified PS increased considerably in comparison to the unreacted PS films. X-ray photoelectron spectroscopy yielded evidence for the sulfonation of PS at the surface. Unreacted spin-coated PS films were very smooth, while modified PS showed some clumps dispersed on a flat surface, as analyzed by atomic force microscopy. The surface morphology was identified as a phase-separated system composed of domains of unreacted PS and a matrix of sulfonated PS by fluorescence microscopy using the positively charged dye rhodamine B. The adsorption of the polycation diallyldimethylammonium chloride on the sulfonated PS surface could be detected. The thickness of the adsorbed polycation was 2.2 nm. Received: 3 November 1998 Accepted in revised form: 23 February 1999     

Vanillin

Ohne Zusammenfassung     

Cassia?l

Ohne Zusammenfassung