Estimation of the minimum uncertainty of DNA concentration in a genetically modified maize sample candidate certified reference material

Homogeneity testing and the determination of minimum sample mass are an important part of the certification of reference materials. The smallest theoretically achievable uncertainty of certified concentration values is limited by the concentration distribution of analyte in the different particle size fractions of powdered biological samples. This might be of special importance if the reference material is prepared by dry mixing, a dilution technique which is used for the production of the new and third generation of genetically modified (GMO) plant certified reference materials. For the production of dry mixed PMON 810 maize reference material a computer program was developed to calculate the theoretically smallest uncertainty for a selected sample intake. This model was used to compare three differently milled maize samples, and the effect of dilution on the uncertainty of the DNA content of GMO maize was estimated as well. In the case of a 50-mg sample mass the lowest achievable standard deviation was 2% for the sample containing 0.1% GMO and the minimum deviation was less than 0.5% for the sample containing 5% GMO. Received: 5 December 2000 / Revised: 14 March 2001 / Accepted: 19 March 2001     

Adsorption behavior and activity of hexokinase

The study on the adsorption of hexokinase (HK) onto silicon wafers was carried out by means of in situ ellipsometry and atomic force microscopy in the liquid. The thickness values of the adsorbed HK layer determined by both techniques were in excellent agreement and evidenced HK monolayer formation. The adsorption of HK onto Si wafers was favored at low ionic strength, indicating that the adsorption is mainly driven by electrostatic forces, since salt screens not only the segment-segment repulsion but also the segment-surface attraction when the salt concentration increases. The enzymatic activity of free HK and of adsorbed HK was measured as a function of time. Free HK in solution lost activity upon storage. Contrarily, adsorbed HK kept its activity level even after 48 h storage at room temperature. This outstanding behavior was attributed to specific orientation of the HK active site to the solution.     

Effect of pH on the adsorption and activity of creatine phosphokinase

The combination of in situ ellipsometry with atomic force microscopy in the liquid for the study of adsorption of creatine phosphokinase (CPK) onto silicon wafers was shown for the first time. The thickness, adsorbed amount, and topographic information of the adsorbed CPK layers were obtained under different pH conditions. The thickness values of adsorbed CPK layer determined by both techniques were in excellent agreement. At pH 4, CPK monomers present in solution adsorb, forming a very thin (approximately 0.8 nm) layer, indicating CPK unfolding. Upon increasing the pH to 6.8, the adsorbed layer is composed of a mixture of CPK dimers (native structure) and intermediates, increasing the film thickness (approximately 2.4 nm). At pH 9, CPK dimers form monolayers with the highest thickness (approximately 4.0 nm). The nature of interactions between CPK and Si wafers associated with the hydration force seems to control the degree of CPK unfolding upon adsorbing. The enzymatic activity of free CPK and of adsorbed CPK at pH 4, pH 6.8, and pH 9 was measured as a function of pH. In comparison to free CPK in solution, adsorbed CPK presented a shift of the optimal pH from 6.8 toward alkaline pH.     

Estimation of the uncertainty of CRMs in accordance with GUM: application to the certification of four enzyme CRMs

Uncertainties of four enzyme-CRMs that have recently been certified in a co-operation between the IRMM and the International Federation for Clinical Chemistry were estimated. Estimation was based on the sum of the uncertainties of characterization, homogeneity and stability. Data from the certification collaborative study were used to estimate laboratory uncertainties, which form the basis for the uncertainty of characterization. Estimations for the uncertainty of homogeneity were derived from classical homogeneity studies. The estimations of uncertainty of stability caused the most difficulties. Realistic uncertainties fitting the needs of customers while being derived from measurement data based on theoretical considerations were obtained.     

On the influence of interaural differences on temporal perception of noise bursts of different durations

The perception of a composite sound's temporal cues, like synchronous onsets, is considered essential to correct perceptual grouping of its constituent components. The processing of a single sound's spatial cues, already present at its onset, may interact with temporal perception of the onset. The current study investigated the influence of interaural differences on temporal perception of a sound's onset. As a measure of temporal perception, the ability to position the onset of a temporally displaced target sound to the regular meter of diotic reference marker sound onsets was measured for various target sound lateralizations, sensation levels, and target and marker sound durations. For target sounds presented in quiet, no influence of interaural differences on temporal positioning of the onset was found. However, increasing a sound's duration systematically shifted the perceived onset position into its "interior." For target sounds presented at low sensation levels in a noise masker, the precision of temporally positioning the onset generally degraded, though faster for dichotic conditions and for longer durations. The level below which temporal perception precision starts to degrade was found to depend on signal-to-noise ratio rather than on sensation level or duration, and is influenced by the presence of interaural differences.     

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.