Comparability of measurement results for pesticide residues in foodstuffs: an open issue?

The dispersion of results from proficiency tests for the analysis of pesticide residues in foodstuffs suggests that improvements in the compatibility of measurement results are needed. Currently observed divergences can make the evaluation conclusion on foodstuffs compliance with certain legislation dependent on the consulted laboratory. This work discusses the origin and impact of this lack of compatibility, following the metrological concepts presented at the latest version of the “International Vocabulary of Metrology” (VIM3), thus allowing for a clear diagnostic of the problem. The reporting of results from different measurement methods uncorrected for the observed analyte recovery makes them traceable to different “operationally defined measurement procedures” (VIM3) and, therefore, not comparable. When results from different measurement methods are reported corrected for analyte recovery, R, and R is different for spiked and incurred residues, measurement results may be not compatible if this effect is not considered on the uncertainty budget. This discussion is illustrated with metrological models for any possible combination of “measurement performance” and “maximum residue level”. These models are complemented with experimental data of the analysis of pesticide residues in a sample of ginseng powder from a proficiency test. The adopted experimental design allowed the identification of additional threats to metrological compatibility in this field. Solutions to the faced problem are discussed for practicability and impact on regulatory issues. The use of a universal “reference measurement procedure” proves to be the most feasible way of ensuring comparability of measurements in this field.     

Reviewing extrapolation procedures of the electronic properties on the π-conjugated polymer limit

In this article, the extrapolation procedures of π-π* electronic transition energy on π-conjugated oligomers are reexamined. Different models, including the simplest coupled oscillator, the free electron, the Hückel approach, the molecular exciton model, and some specific fitting-functions, are compared using the transition energies derived from theoretical calculations on three thiophene-based oligomer series. Specifically, oligomers of up to 30 repeating units have been considered to include the saturation effects as a function of chain length. The coupled oscillator model of W. Kuhn and the fitting-function of Hirayama are the models that present the better suit on the transition energy interpolation as a function of chain length. Using only the first four oligomers of the series (n = 2 up to 8) yields an estimation of the transition energy on the polymer limit with an average error of ～1.5%. The vertical and adiabatic ionization potential present a better fit with the Hückel model approach. Finally, implications of the environmental polarity on the electronic properties, molecular geometry, charge distribution, and aromaticity are shortly discussed.     

Is the problem of molecular structure just the quantum measurement problem?

In a recent article entitled “The problem of molecular structure just is the measurement problem”, Alexander Franklin and Vanessa Seifert argue that insofar as the quantum measurement problem is solved, the problems of molecular structure are resolved as well. The purpose of the present article is to show that such a claim is too optimistic. Although the solution of the quantum measurement problem is relevant to how the problem of molecular structure is faced, such a solution is not sufficient to account for the structure of molecules as understood in the field of chemistry.

     

Where is the limit of highly fluorinated high-oxidation-state osmium species?

The structures and stabilities of various osmium fluorides and oxyfluorides in high oxidation states have been studied by quantum-chemical calculations at DFT (B3LYP), MP2, CCSD, and CCSD(T) levels. The calculations indicate that the homoleptic fluorides all the way up to OsF8 may exist, even though OsF8 will be difficult to prepare. The last missing osmium oxyfluoride, OsOF6, is computed to be thermochemically stable against mononuclear gas-phase elimination reactions. The problem with the nonexistence of such highly fluorinated complexes appears thus to be mainly in difficult synthetic access under typical condensed-phase conditions. Matrix-isolation techniques might provide a means to characterize the highly fluorinated OsVIII and OsVII species.     

Illuminating the flesh of bone identification – An application of near infrared spectroscopy

Near infrared (NIR) reflectance spectroscopy coupled with chemometric analysis was evaluated as a non-destructive tool to discriminate skull bone samples from different animal species. In total 70 skull bones from animals of three classes (mammalians, avian and reptiles) were scanned in the wavelength range between 950 to 1650 nm. Principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) were used to analyse the NIR spectra of the skull samples. Correct classification rates of 96% and 81% were obtained for the classification of skull bone samples according to avian and mammalian classes, respectively. Overall, a 91% correct classification rate was obtained for the classification of skull samples according to the class (mammalian and avian). This study demonstrates the potential of NIR spectroscopy coupled with chemometric as data processing, as a means of a rapid, non-destructive classification technique for skull bone samples.     

On the use of the ‘uncertainty budget’ to detect dominant terms in the evaluation of measurement uncertainty

In the evaluation of measurement uncertainty, the uncertainty budget is usually used to identify dominant terms that contribute to the uncertainty of the output estimate. Although a feature of the GUF method, it is also recommended as a qualitative tool in MCM by using ‘nonlinear’ equivalents of uncertainty contributions and sensitivity coefficients. In this paper, the use of ‘linear’ and ‘nonlinear’ parameters is discussed. It is shown that when and only when the standard uncertainty of the output estimate is nearly equal to the square root of the sum of the squares of the individual uncertainty contributions, will the latter be a reliable tool to detect the degree of contribution of each input quantity to the measurand uncertainty.     

Systematization of the results of the chromatography–mass spectrometry identification of the products of quercetin oxidation by atmospheric oxygen in aqueous solutions

The coordination of the results of different works for the systematization of the structures of products previously detected in the complex samples is an important stage in the interpretation of the results of the identification of the components of complex samples of natural origin by chromatography–mass spectrometry with the low reproducibility of their mass spectra under the conditions of electrospray ionization and limited reference information. The data processing of this kind was carried out for the products of the oxidation of the most common natural flavonoid quercetin (3,5,7,3',4'-pentahydroxyflavone) by atmospheric oxygen in weakly alkaline aqueous alcohol solutions. For the correlation of peaks in chromatograms with the structures of oxidation products, their reversed-phase HPLC retention indices in the scale of reference n-alkyl phenyl ketones were determined for the first time. It was confirmed that not all of the oxidation products are stable in solution; some of them can accumulate or disappear during the storage of the samples.     

Comparison of the results of thermal denaturation of β-lactoglobulin obtained by DSC and UV-spectroscopy

The thermal denaturation of β-lactoglobulin in the presence of urea and alkylurea solutions were measured. In the presence of a high concentration of urea this protein shows not only heat but also cold denaturation. For studying the effect of temperature two methods were used, differential scanning calorimetry (DSC) and UV-spectroscopy. DSC provides direct model-independent determination of the transition enthalpy in comparison with UV-spectroscopy, which gives only apparent or van't Hoff enthalpy of transition. The UV-melting curves were analyzed on the basis of a two-state approximation. The apparent standard enthalpies of thermal denaturation, ΔH _app. ^o , were compared with calorimetric ones.     

Estimation of the uncertainty components associated with the measurement of radionuclides in air filters using γ-ray spectrometry

This paper discusses and assesses the major sources of uncertainty arising in measurement of radionuclide activity concentrations in air. Besides counting statistics, the main contributors to the overall uncertainty of the activity concentration of long-lived radionuclides are the heterogeneities of the calibration source and the test samples (approximately 4%). In addition, in the case of radionuclides with half-lives of the order of days or shorter, a significant uncertainty results from the variability of the source term because the peak concentration of the radionuclide might occur either at the beginning or at the end of the sampling period. For¹³¹I (T_1/28 days), for instance, the relative standard uncertainty was estimated as 17% owing to this effect. In addition, the uncertainty introduced by the software used for evaluation of the -ray spectra is discussed. Details of the uncertainty components, including their assessed probability distributions and degrees of freedom, and the conversion into standard uncertainties are reported in this paper, following the ISO Guide to the expression of uncertainty in measurement. A standard form for documenting and reporting the uncertainty budged is proposed and illustrated with an example.     

Analytic calculation of second-order perturbatior energy for the one-dimensional Hubbard model in the Hückel limit

The study of the Hückel limit for the one-dimensional Hubbard model is very complicated. It is necessary to prepare the Lieb-Wu system of equations for expansion and then to initialize the variables which appear in the system, which is not straightforward. However, it is remarkable that, for the second-order contribution to the energy in this limit, it is possible to obtain, after complicated manipulations, quite transparent expressions. The analytical solution is obtained order by order for the variables to ^–2 which appear in the Lieb-Wu solution and the ^–1 coefficient in the energy is calculated using these expansions. The energy can be calculated numerically using these analytic results for the variables for increasing values ofN.     

Does the choice of the crystal structure influence the results of the periodic DFT calculations? A case of glycine alpha polymorph GIPAW NMR parameters computations

Glycine is a common amino acid with relatively complex chemistry in solid state. Although several polymorphs (α, β, δ, γ, ε) of crystalline glycine are known, for NMR spectroscopy the most important is a polymorph, which is used as a standard for calibration of spectrometer performance and therefore it is intensively studied by both experimental methods and theoretical computation. The great scientific interest in a glycine results in a large number of crystallographic information files (CIFs) deposited in Cambridge Structural Database (CSD). The aim of this study was to evaluate the influence of the chosen crystal structure of α glycine obtained in different crystallographic experimental conditions (temperature, pressure and source of radiation of α glycine) on the results of periodic DFT calculation. For this purpose the total of 136 GIPAW calculations of α glycine NMR parameters were performed, preceded by the four approaches (“SP”, “only H”, “full”, “full+cell”) of structure preparation. The analysis of the results of those computations performed on the representative group of 34 structures obtained at various experimental conditions revealed that though the structures were generally characterized by good accuracy (R < 0.05 for most of them) the results of the periodic DFT calculations performed using the unoptimized structures differed significantly. The values of the standard deviations of the studied NMR parameters were in most cases decreasing with the number of optimized parameters. The most accurate results (of the calculations) were in most cases obtained using the structures with solely hydrogen atoms positions optimized. © 2018 Wiley Periodicals, Inc.     

Analysis of fourth-order Møller–Plesset limit energies: the importance of three-electron correlation effects

Fourth-order M?ller–Plesset (MP4) correlation energies are computed for 28 atoms and simple molecules employing Dunning's correlation-consistent polarized-valence m-zeta basis sets for m=2, 3, 4, and 5. Extrapolation formulas are used to predict MP4 energies for infinitely large basis sets. It is shown that both total and partial MP4 correlation energies can be extrapolated to limit values and that the sum of extrapolated partial MP4 energies equals the extrapolated total MP4 correlation energy within calculational accuracy. Therefore, partial MP4 correlation energies can be presented in the form of an MP4 spectrum reflecting the relative importance of different correlation effects. Typical trends in calculated correlation effects for a given class of electron systems are independent of the basis set used. As first found by Cremer and He [(1996) J Phys Chem 100:6173], one can use MP4 spectra to distinguish between electron systems with well-separated electron pairs and systems for which electrons cluster in a confined region of atomic or molecular space. MP4 spectra for increasing size of the basis set reveal that smaller basis set calculations underestimate the importance of three-electron correlation effects for both classes by overestimating the importance of pair correlation effects. The minimum size of a basis set required for reliable MP4 calculations is given by a valence triple-zeta polarized basis, which even in the case of anions performs better than a valence double-zeta basis augmented by diffuse functions. Received: 14 June 2000 / Accepted: 16 June 2000 / Published online: 24 October 2000     

Raman spectroscopy in the near infrared – a most capable method of vibrational spectroscopy

Raman spectroscopy has enjoyed a dramatic improvement during the last years: The interference by the fluorescence of impurities is virtually eliminated, the sample preparation is considerably easier as for infrared spectroscopy and many applications in routine analytics, quality control and process control in various branches of industry are now possible. It is shown that the up-to-date near-infrared Raman spectrometers now meet most demands for a modern analytical instrument concerning applicability, analytical information and convenience. It can be anticipated that Raman spectroscopy will catch up infrared spectroscopy, the current workhorse of vibrational spectroscopy.     

FT-IR spectra of NaCl�CH2O in the region from near to beyond the critical state

Water and NaCl?CH₂O solutions and their molecular spectra at high temperatures and pressures were observed and examined using a new design of hydrothermal diamond anvil cell (HDAC) connected to both a light and an infrared microscope. We have modified the diamond window of the HDAC to have a wide angle to allow the infrared beam to pass the window. Fourier-transform infrared (FT-IR) spectra of NaCl?CH₂O?CD₂O were examined at high temperatures and pressures up to 850 °C and 3 GPa. The effect of increasing temperature on water spectra differed from that of increasing pressure. The O?CH stretching frequency of water molecules increases with increasing temperature (from 20 to 600 °C), and also with increasing salinity of the solution.     

Determination of the natural abundance δ15N of taurine by gas chromatography-isotope ratio measurement mass spectrometry

The measurement of the nitrogen isotope ratio of taurine (2-aminoethanesulphonic acid) in biological samples has a large number of potential applications. Taurine is a small water-soluble molecule which is notoriously difficult to analyze due to its polarity and functionality. A method is described which allows the determination of the natural abundance δ(15)N values of taurine and structural analogues, such as 3-amino-1-propanesulphonic acid (APSA), by isotope ratio mass spectrometry interfaced to gas chromatography (GC-irm-MS). The one-step protocol exploits the simultaneous derivatization of both functionalities of these aminosulphonic acids by reaction with triethylorthoacetate (TEOA). Conditions have been established which ensure quantitative reaction thus avoiding any nitrogen isotope fractionation during derivatization and workup. The differences in the δ(15)N values of derivatized and non-derivatized taurine and APSA all fall within the working range of 0.4‰ (-0.02 to 0.39‰). When applied to four sources of taurine with various δ(15)N values, the method achieved excellent reproducibility and accuracy. The optimized method enables the determination of the natural abundance δ(15)N values of taurine over the concentration range 1.5-7.84 μmol.mL(-1) in samples of biological origin.     

Examples of the ‘characteristic’ function applied to instrumental precision in chemical measurement

The ‘characteristic function’ with two empirically determined parameters α and β, is proposed as a general purpose function to describe the variation of precision (in terms of standard deviation σ), or uncertainty, with analyte concentration c (here denoting any compositional quantity), for specific analytical methods applied to a defined type of test material. In this study it is applied to examples of analytical data collected under ‘instrumental’ conditions for estimating precision. The function fitted the data well, with no systematic lack of fit. The study therefore extends the range of applications of this function.