On the Repeatability of Friction and Wear Tests for Polyimides in a Hertzian Line Contact

Tribological properties for polymers are mainly experimentally determined, while few standards are available and variation in data is often large. Polyimide is slid against steel on a cylinder-on-plate tribotester at 50–200 N and 0.3 m/s. There is a statistical variation of ±7% on dynamic friction, ±10% on static friction, ±8% on wear rates from weight loss and ±12% from wear rates from dimensional measurements, or even higher at high normal loads. Different parameters influencing statistical repeatability are discussed. Friction for polymers shows higher variation and wear rates show lower variation than steel/steel pairs due to visco-elastic deformation that has a contribution to friction but reduces stress concentrations. It is experimentally verified that the visco-elastic deformation of polymers in line contact is higher than calculated from theoretical models. The role of recoverable and permanent deformation is verified and there is a difference in deformation at 50–100 N and 150–200 N revealed from static loading tests, creep tests and wear measurements. The limit between running-in and steady-state coincides with the stabilization in contact pressures after 250 μm diameter reduction. Besides test rig design, a variation in counterface roughness seems the second most important influence.     

Determination of the carbon-13 content of sugars and pulp from fruit juices by isotope-ratio mass spectrometry (internal reference method). A European interlaboratory comparison

An interlaboratory comparison by ¹³C-isotope ratio mass spectrometry was organised by working group No. 1 of the European Commission of Standardisation (CEN/TC 174) in order to define the repeatability (r) and the reproducibility (R) of the ¹³C determination in sugars and pulp, isolated from the same fruit juice. Six unlabelled juices were analysed in 19 laboratories in the European Union, Australia and the USA. Three samples were authentic juices (orange, grapefruit, pineapple) and the remaining samples were the same juices with an addition of sugar at 15 g l⁻¹ (orange, pineapple) or 11.8 g l⁻¹ (grapefruit). The sugar was prepared by mixing equal amounts of beet and cane sucroses. The different laboratories used the same experimental protocol under different conditions (operator, conversion system for CO₂ preparation, mass spectrometer). The results for the sugars (r = 0.27%., R = 0.82%.) were comparable to those from an earlier ring test (r = 0.30%., R = 0.70%.), while the results for the pulp presented higher interlaboratory variability (r = 0.38%., R = 1.89%.) possibly due to experimental difficulties. Nevertheless, the RSD_R for free sugars ranged 0.9–3.2% and for pulp, 0.9–9.3%. On the basis of the Horwitz equation and taking account of the concentration of the isotopes measured, an RSD_R of 7–8% might have been expected. The experimentally determined RSD_R values were less than twice the theoretically expected values, which is a criterion of an acceptable method. The results were therefore considered to be acceptable by specialists in the field of isotopic analysis, and the method applied was found to improve the sensitivity of the ¹³C determination for the detection of sugar addition in fruit juices.     

Uncertainty sources in UV-Vis spectrophotometric measurement

An overview is given of the most important uncertainty sources that affect analytical UV-Vis spectrophotometric measurements. Altogether, eight uncertainty sources are discussed that are expected to have influence in chemical analysis. It is demonstrated that the well-known intrinsic (or “physical”) sources of uncertainty that originate from the instrument itself (repeatability of spectrophotometer reading, spectrophotometer drift, stray light, etc.) often have significantly lower contributions to the combined uncertainty of the result than the “chemical” sources of uncertainty that originate from the object under study (interference from the constituents of the matrix, decomposition of the photometric complex, etc.). Although selectivity of a photometric procedure is often considered more a validation topic than an uncertainty topic, it is very often important to include it also in the uncertainty budget.Usually the most difficult part of uncertainty estimation of a chemical measurement result is to evaluate the magnitude of the actual uncertainty components, especially the chemical ones. For most of the uncertainty sources discussed in this paper, approaches for their evaluation are given. A generic uncertainty budget for absorbance is presented. Electronic Supplementary Material Supplementary material is available for this article at     

用方法精密度检验钛合金光谱分析标准物质的均匀性

采用方差分析方案使用重复性(r)检验钛合金光谱分析标准物质均匀性的方法.结果表明:这种方法能较好地反映出受检样品的不均匀性,较之其他方法简单,方便,但要求比较严格.     

Comparison of microwave-assisted digestion procedures for total trace element content determination in calcareous soils

The aim of the study was to determine total trace (Cd, Co, Cr, Cu, Mn, Pb and Zn) and major (Al and Fe) element concentrations in calcareous soils using microwave-assisted digestion procedures. The literature showing lack of consensus regarding digestion procedures and unsatisfying recoveries for calcareous materials, four procedures using various acid combinations (HCl, HNO₃, H₂O₂, HF) and volumes were tested using a certified reference material (CRM 141R) and natural calcareous soil samples. Digests were analysed by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Repeatability (R.S.D. <5%) and recoveries (82-116%) showed that the procedures were precise and accurate for most elements. Five calcareous soil samples from a Champagne vineyard plot were, then, subjected to these procedures. In calcareous materials, the presence of HF resulted in Al being severely underestimated (recovery <5%) and Co overestimated (recovery >124%) due to complex formation or spectrochemical interferences, respectively. As digestion was not significantly influenced by the addition of H₂O₂, the procedure corresponding to Aqua regia (HCl-HNO₃) appeared as the best compromise and was selected for further multielemental environmental studies on calcareous materials, even if the absence of HF could lead to incomplete digestion of accessory silicate minerals. Results for a vineyard plot showed that the soils were contaminated (3.65 mg kg⁻¹ Cd, 67 mg kg⁻¹ Cr, 278 mg kg⁻¹ Cu, 143 mg kg⁻¹ Pb and 400 mg kg⁻¹ Zn) as a consequence of urban waste and copper-treatment applications.     

Influencing parameters on measurement accuracy in dynamic mechanical analysis of thermoplastic polymers and their composites

Long-term predictions of material properties such as stiffness and creep resistance are important in many engineering applications and require high reliability and accuracy. This is especially true for polymer materials and their composites as their viscoelastic nature results in time-dependent material behaviour and any measurement uncertainties or errors amplify in long-term predictions. To measure this behaviour at smallest loadings, Dynamic Mechanical Analysis (DMA) is frequently declared as an ideal method. However, the measurement accuracy and repeatability of this method is strongly influenced by (i) the testing fixture and corresponding loading mode, (ii) the sample preparation and (iii) the plotting scale to interpret the test results. In this study, relevant experimental parameters were found for DMA and a proper procedure was designed, which was then applied to measure the viscoelastic behaviour of a highly temperature and creep resistant thermoplastic polymer (polyethersulfone) and of a highly graphite filled polypropylene composite. In combination with finite element simulations and in-situ strain measurements by digital image correlation (DIC), the main influences on measurement accuracy of three-point-bending DMA were identified and subsequently used to determine measurement guidelines. Using these guidelines, DMA measurements allow quantitative determination of the viscoelastic response for rigid polymer and composite materials.     

Repeatability: some aspects concerning the evaluation of the measurement uncertainty

Various publications stress the importance of the repeatability (i.e. precision) of the calculation of the measurement of uncertainty. We reveal by detailing an example from production control in the pharmaceutical industry that the effect of other influence quantities should not be neglected, because their magnitude is even larger than the contribution of repeatability. We review the role of repeatability within the calculation of measurement uncertainty for several common validation and day-to-day measurement scenarios. They show that measurement models need to consider the measurement sequences of the various scenarios. Otherwise the size and effect of the repeatability might be overestimated. At the end Monte Carlo simulations were used to investigate the determination of the repeatability under certain restrictions. The simulation uncovered a significant bias toward the common formula for calculating the standard deviation when it is based on a duplicated measurement of a sample. Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher     

A simplified approach to the estimation of analytical measurement uncertainty

The present study summarizes the measurement uncertainty estimations carried out in Nestlé Research Center since 2002. These estimations cover a wide range of analyses of commercial and regulatory interests. In a first part, this study shows that method validation data (repeatability, trueness and intermediate reproducibility) can be used to provide a good estimation of measurement uncertainty.In a second part, measurement uncertainty is compared to collaborative trials data. These data can be used for measurement uncertainty estimation as far as the in-house validation performances are comparable to the method validation performances obtained in the collaborative trial.Based on these two main observations, the aim of this study is to easily estimate the measurement uncertainty using validation data.     

A priori method of evaluating uncertainties in qualitative chromatographic analysis: probabilistic approach

Probabilistic algorithms to evaluate result reliability in qualitative chromatographic analysis are discussed in the paper. The elementary uncertainty (P₀), concerned with a single test (comparison of sample and reference peak positions), is treated as the sum of misidentification and omission probabilities. Both constituents are calculated separately using the simplified model and Laplace functions. In the model, the main reasons for elementary uncertainties are random normally distributed deviations during retention characteristic measurement. Algorithms to calculate both constituents of P₀ have to take into account real measurement precision, supposed composition of the sample, content of the database, chosen coincidence criterion and other factors. At a high selectivity of retention, the 3 value is recommended as the most convenient coincidence criterion. It leads to more reliable and unambiguous attribution of peaks in the chromatogram. For cases that are more complicated, the probabilistic algorithms based upon Bernoulli theorem are proposed to calculate the summary uncertainty of identification, concerned with the multiple test. They take into account P₀ value, the number of repeated single tests (n) in the similar or different conditions, and chosen identification criterion K (minimal number of coincidences). The above-mentioned algorithms lead to a priori optimisation of the mode of operation of any identification software system associated with the chromatograph. They can be useful during a metrological validation of corresponding qualitative analysis methods.Presented at the Second International Conference on Metrology—Trends and Applications in Calibration and Testing Laboratories, November 4–6, 2003, Eilat, Israel.The opinions reflected in this paper are those of the author only. AQUAL does not necessarily endorse them.     

Determination of quinolones by fluorescent excitation emission

In this work the viability of a fluorescent technique for the determination of quinolones is studied. This analytical technique allows one to analyze the effect of the increasing order of the analytical signal from a univariate calibration (zero order data) to partial least squares (PLS) calibration (first order). The comparison has been done through the figures of merit of the analytical procedure (technique and calibration) in accordance with the ISO norm and the 2002/657/EC European Decision about residuals.