Evaluation of sample processing and sub-sampling performance

A methodology for the estimation of sample processing and sub-sampling performance based on the comparison of the global method experimental dispersion of results with the uncertainty estimated from developed models for the subsequent analytical steps is presented. This approach is a valuable alternative to the evaluation of adequate experimental information using a classical ANOVA, since the significance of the sample processing and sub-sampling is evaluated with a higher number of degrees of freedom for the same number of experimental assays, due to the high number of degrees of freedom associated with the uncertainty estimated for the subsequent analytical steps from the combination of the involved sources of uncertainty.Considering the construction of a model to describe the performance of the analytical steps following sample processing and sub-sampling over a broad concentration range, the experimental assays involved at the evaluation of the sample processing and sub-sampling can be performed at any concentration meeting the previously validated range and several months after the development of that model once its adequacy has been proven over time.This approach, which also allows the construction of a detailed performance model for the global analytical method over a broad concentration range, was applied to the determination of pesticide residues in apples by gas-chromatography with electron capture detector.Considering that no information was available regarding the samples heterogeneity, sub-sampling performance was evaluated considering a sample representing the worst expectable homogeneity. This was accomplished by spiking just one out of the 10 halves of apples processed in each sample.The developed model for the performance of the analytical method was successfully and easily applied to routine analysis through an automated link between the information generated by the chromatograph software with a file containing the model.     

Sampling and local models for multivariate image analysis

Local models are a very important concept for microscopic and macroscopic imaging. Different methods of sub-sampling a multivariate image are described both in general and for three examples. The need for sub-sampling and its influence on multivariate image analysis and visualization are studied. Examples from MRI (256 × 256), satellite imaging (7 × 512 × 512) and biofuel studies (6 × 512 × 512) are used to illustrate some of the principles involved.     

Advances in surface-enhanced Raman spectroscopy for analysis of pharmaceuticals: A review

Recently, Raman spectroscopy become a popular and potential analytical technique for the analysis of pharmaceuticals as a result of its advancement. The innovation of laser technology, Fourier Transform-Raman spectrometers with charge coupled device (CCD) detectors, ease of sample preparation and handling, mitigation of sub-sampling problems using different geometric laser irradiance patterns and invention of different optical components of Raman spectrometers are contributors of the advancement of Raman spectroscopy. Transmission Raman Spectroscopy is a useful tool in pharmaceutical analysis to address the problems related with sub-sampling in conventional Raman back scattering. More importantly, the development of surface-enhanced Raman scattering (SERS) has been a prominent advancement for Raman spectroscopy to be applied for pharmaceuticals analysis as it avoids the inherent insensitivity and fluorescence problems. As the active pharmaceutical ingredients (APIs) contain aromatic or conjugated domains with strong Raman scattering activity, Raman spectroscopy is an attractive alternative conventional analytical method for pharmaceuticals. Coupling of Raman spectroscopy with separation techniques is also another advancement applied to reduce or avoid possible spectral interferences. Therefore, in this review, transmission Raman spectroscopy, SERS, and SERS coupled with various separation techniques for pharmaceutical analysis are presented.     

A case study in the practical estimation of measurement uncertainty

This case study is written for analytical laboratories, in order to give support to the implementation of the concept of measurement uncertainty for routine measurements. The aim is to provide a practical, understandable and common way of performing measurement uncertainty calculations, based mainly on pre-existing quality control and validation data. Practical examples taken directly from environmental laboratory monitoring are presented and explained. However, the approach is very general and should be applicable to most testing laboratories in the chemical field. Following the protocol of evaluation illustrated in the case study, it is possible to ensure that most relevant uncertainty components associated with the method are covered. Contributions associated with sampling, homogenisation, sub-sampling, and so on, are, however, excluded.     

Transition from patchlike to clusterlike inhomogeneity arising from hydrogen bonding in water

Assembling of water molecules via hydrogen bonding has been studied by molecular dynamics simulations using flexible potential model. The relationship between the number of H-bonds per molecule, n(HB), the size of H-bonded nets, k, and the size of patches of four-bonded molecules, k(4), has been examined for several thermodynamic states of water ranging from ambient to supercritical conditions. Two kinds of structural inhomogeneity have been found: the patchlike associated with the mean n(HB)> 2.0 and the clusterlike for n(HB)< 1.9. In compressed water up to ~473 K patches coexist with less ordered nets, both constituting the gel-like H-bonded network. The size of patches steeply decreases with the increasing temperature and the decreasing density of water. The inhomogeneity resulting from the presence of patches disappears above 473 K. This feature is associated with the rapid increase in the fraction of unbound molecules and with the breakage of the gel-like network into a variety of H-bonded clusters leading to the clusterlike structural inhomogeneity. In contrast to the patchlike inhomogeneity an increase in temperature and a decrease in density make this kind of inhomogeneity more pronounced. A degree of connectivity of H-bonds has been characterized by a parameter P(g) defined as the total fraction of molecules belonging to the H-bonded clusters of size k ≥ 5. The simulation-derived values of P(g) agree well with the predictions of the random bond theory giving the explicit expression for P(g) as a function of the mean n(HB). Going from ambient to supercritical conditions, we have found that the patchlike inhomogeneity is connected with the very slight reduction in P(g), whereas the clusterlike inhomogeneity generates a steep linear decrease of P(g) with the decreasing mean n(HB). The self-diffusion coefficient calculated for the thermodynamic states of water showing the clusterlike inhomogeneity has occurred to be inversely proportional to the density. We have also found that the clusterlike inhomogeneity is associated with the linear correlation between P(g) and the macroscopic properties of water: the static dielectric constant, the viscosity, and the density. The provided relationships allow one to estimate the degree of connectivity of hydrogen bonds from the measured macroscopic quantities.     

Ellipsometric characterization of inhomogeneous non‐stoichiometric silicon nitride films

Variable‐angle spectroscopic ellipsometry is employed for the optical characterization of non‐stoichiometric silicon nitride thin films exhibiting inhomogeneity formed by refractive index and extinction index changes through the film thickness. For all the film samples, the best fit of the experimental data is achieved if, in addition to the inhomogeneity, an overlayer or roughness of the upper boundary is included. However, distinguishing of these two defects is found not to be possible. The influence of working gas ratio, deposition temperature and on/off time on the film properties is studied. The refractive index and extinction coefficient is found to increase with increasing working gas ratio and less significantly with decreasing deposition temperature. It is also found that the inhomogeneity increases with decreasing deposition temperature, and the deposition rate of the films decreases with increasing working gas ratio. The influence of the on/off time on the film properties is practically unimportant. Copyright © 2013 John Wiley & Sons, Ltd.     

Large sample neutron activation analysis of a reference inhomogeneous sample

A benchmark experiment was performed for Neutron Activation Analysis (NAA) of a large inhomogeneous sample. The reference sample was developed in-house and consisted of SiO₂ matrix and an Al–Zn alloy “inhomogeneity” body. Monte Carlo simulations were employed to derive appropriate correction factors for neutron self-shielding during irradiation as well as self-attenuation of gamma rays and sample geometry during counting. The large sample neutron activation analysis (LSNAA) results were compared against reference values and the trueness of the technique was evaluated. An agreement within ±10% was observed between LSNAA and reference elemental mass values, for all matrix and inhomogeneity elements except Samarium, provided that the inhomogeneity body was fully simulated. However, in cases that the inhomogeneity was treated as not known, the results showed a reasonable agreement for most matrix elements, while large discrepancies were observed for the inhomogeneity elements. This study provided a quantification of the uncertainties associated with inhomogeneity in large sample analysis and contributed to the identification of the needs for future development of LSNAA facilities for analysis of inhomogeneous samples.     

Dynamically Harmonized FT-ICR Cell with Specially Shaped Electrodes for Compensation of Inhomogeneity of the Magnetic Field. Computer Simulations of the Electric Field and Ion Motion Dynamics

The recently introduced ion trap for FT-ICR mass spectrometers with dynamic harmonization showed the highest resolving power ever achieved both for ions with moderate masses 500?C1000?Da (peptides) as well as ions with very high masses of up to 200?kDa (proteins). Such results were obtained for superconducting magnets of very high homogeneity of the magnetic field. For magnets with lower homogeneity, the time of transient duration would be smaller. In superconducting magnets used in FT-ICR mass spectrometry the inhomogeneity of the magnetic field in its axial direction prevails over the inhomogeneity in other directions and should be considered as the main factor influencing the synchronic motion of the ion cloud. The inhomogeneity leads to a dependence of the cyclotron frequency from the amplitude of axial oscillation in the potential well of the ion trap. As a consequence, ions in an ion cloud become dephased, which leads to signal attenuation and decrease in the resolving power. Ion cyclotron frequency is also affected by the radial component of the electric field. Hence, by appropriately adjusting the electric field one can compensate the inhomogeneity of the magnetic field and align the cyclotron frequency in the whole range of amplitudes of z-oscillations. A method of magnetic field inhomogeneity compensation in a dynamically harmonized FT-ICR cell is presented, based on adding of extra electrodes into the cell shaped in such a way that the averaged electric field created by these electrodes produces a counter force to the forces caused by the inhomogeneous magnetic field.     

Characterisation of the chemical homogeneity of solid-state materials by chemometric methods: Further multivariate aspects

Some further possibilities of widespread multivariate methods are checked for usefulness in chemometrical characterising the inhomogeneity of solid materials. The methods are applied to data of ten elements obtained by spark source mass spectrometric milliprobing along radial and azimuthal co-ordinates on the surface of a copper disc sample. Special multivariate control charting reveals locations with significant concentration deviations. Two-factor analysis of variance can distinguish very sensitively between radial and azimuthal contributions. Principal components analysis and hierarchical cluster analysis need three or four main components to explain sufficiently some groups of chemical elements with similar behaviour. Factor analysis clearly confirms four element groups as background factors causing inhomogeneity in a different manner. An extended strategy is proposed how to use which chemometrical method in which sequence for characterising the inhomogeneity.Dedicated to Professor Dr. K. Doerffel on the occasion of his 70th birthday     

A model for the evaluation of uncertainty in routine multi-element analysis

A model for calculating uncertainty in routine multi-element analysis is described. The model is constructed according to the principles of GUM/EURACHEM. Control chart results are combined with other existing data and results from the actual measurement into a concentration-dependent estimate of combined standard uncertainty. Since possible sources of bias are included in the calculation, overall bias as estimated from the data is used only as a control to identify needs for modification of the model and/or the analytical procedure. For each individual sample, uncertainty can be calculated automatically based on two pre-calculated parameters together with measured concentration and instrumental standard deviation. As an example, the model is demonstrated for inductively coupled plasma-mass spectrometry (ICP-MS) analysis of sewage sludge including laboratory sub-sampling, sample preparation, and instrumental determination.     

Atomic shell structure based on inhomogeneity measures of the electron density

A measure of electron density inhomogeneity is proposed based on the distance of function values to the average value within a chosen region. The choice of the examined regions follows the approach of restricted space partitioning with fixed inhomogeneity as the restriction. The integration of electron density over the regions of the partitioning yields discrete distribution of charges, which is analyzed. The resulting functionals depend on the particular definition of distance. Two possibilities are selected for the distance definition, and the functionals are applied to examine the shell structure of the atoms Li to Xe.     

Defeating Radiation Damping and Magnetic Field Inhomogeneity with Spatially Encoded Noise

A simple NMR experiment capable of providing well resolved spectra under conditions where either radiation damping or static magnetic field inhomogeneity would broaden otherwise high‐resolution NMR spectra is introduced. The approach involves using a strong pulsed magnetic field gradient and a selective radio‐frequency pulse to encode a predetermined noise pattern into the spatial distribution of magnetization. Following readout in a much smaller field gradient, the noise sequence may be deconvolved from the acquired data and a high‐resolution spectrum is obtained, eliminating the effects of either radiation damping or the static field inhomogeneity. In the presence of field inhomogeneity a field map is also obtained from the same single transient. A quasi‐two‐dimensional version of the experiment eliminates the need for deconvolution and produces improved results with simplified processing, but without requiring a full two‐dimensional experiment. Example spectra are shown for both radiation damping and one‐dimensional field inhomogeneity with improvement in linewidths of more than a factor of 40.     

Microchemical inhomogeneity to characterize atomic configurations in the heating and quenching of a CuHf2 alloy

Based on the constructed Cu-Hf interatomic potential, Monte Carlo simulations were conducted to reveal the atomic configurations in heating and quenching of a CuHf(2) alloy through scrutinizing the evolution of microchemical inhomogeneity. Simulations show that the CuHf(2) crystalline structure becomes more homogeneous during heating but an obvious drop in microchemical inhomogeneity appears when reaching its melting point. During the quenching process, the CuHf(2) melt becomes increasingly inhomogeneous and shows a change in the slope in the microchemical inhomogeneity around glass transition temperature. Simulation results were evidenced by the atomic packing analysis through the Voronoi tessellation method. The implications of our study suggest that the glass transition could be visualized as a process involving increase of microchemical inhomogeneity from the liquid to glassy state.     

Exciton Derealization in the Light-Harvesting LH2 Complex of Photosynthetic Purple Bacteria

Abstract— Absorption and circular dichroism spectra for both long-wavelength bands of the antenna LH2 complex from photosynthetic purple bacteria have been calculated. Calculations were performed taking into account resonant interactions between all 27 bacteriochlorophyll (BChl) molecules for different values of site inhomogeneity. The exciton derealization length within BChl800 and BChl850 rings was estimated on the basis of calculated exciton wavefunctions for different temperatures and site inhomogeneity values. The relationship between anomalously high bleaching values in pump-probe spectra and the exciton derealization length is discussed.     

Minimization of spatial inhomogeneity in polystyrene gels formed by free-radical mechanism

The spatial inhomogeneity in polystyrene (PS) gels has been investigated with the static light scattering technique. PS gels were prepared starting from styrene monomer and ethylene glycol dimethacrylate crosslinker in a homogeneous solution. The gel synthesis parameters varied were the crosslinker concentration, the primary chains length and the quality of the polymerization solvent. The gels were characterized by elasticity tests as well as by light scattering measurements at a gel state just after their preparation. The degree of spatial gel inhomogeneity decreased with decreasing crosslinker content, with decreasing primary chain length or, with increasing quality of the polymerization solvent. It was shown that the gel synthesis parameters varied mainly affect the distance between the pendant vinyl groups locating on the same macromolecule during the gel formation process. Increasing the distance between the pendant vinyl groups reduces the rate of the multiple crosslinking reactions so that the resulting PS gels exhibit a lesser degree of inhomogeneity.     

Suppressing One‐Bond Correlations in HMBC Spectra: Improved Performance for the BIRD–HMBC Pulse Sequence

An improved version of the BIRD–HMBC experiment is proposed. In comparison to the original version, the filtering (suppression of ¹ J_CH signals) is accomplished using a double tuned G‐BIRD filter positioned in the middle of the long‐range correlations evolution period. Compensation of offset dependence by replacing the rectangular 180° pulses with the broadband inversion pulses (BIPs), with superior inversion performance and improved tolerance to B₁ field inhomogeneity, significantly improves the sensitivity of the original BIRD–HMBC experiment. For usual one‐bond coupling constants ranges (115–180 Hz), optimal results are easily obtained by adjusting the delays, δ, of the BIRD elements to an average J value. For larger ranges (e.g. 110–260 Hz), the use of a double tuned G‐BIRD filter allows excellent suppression degrees for all types of one‐bond constants present in a molecule, superior to the original scheme and other purging schemes. These attributes make the improved version of the BIRD–HMBC experiment a valuable and robust tool for rapid spectral analysis and rapid checks of molecular skeletons with a minimum spectrometer time. Copyright © 2009 John Wiley & Sons, Ltd.     

Scheme of detecting microscopic inhomogeneity in binary liquid mixtures utilizing resonantly coupled vibrational modes

Influence of microscopic inhomogeneity in binary liquid mixtures on their vibrational spectra is studied by doing calculations on a model liquid system. The concentration dependence of the noncoincidence effect (NCE), which is a feature of vibrational bands related to the intermolecular resonant coupling of vibrational modes, is analyzed. It is suggested that observation of convex behavior of the NCEs for the vibrational bands of both species, especially that of the less polar species, in a binary liquid mixture is an indication of the occurrence of microscopic inhomogeneity.     

A Phenomenological Model of the Electrical Double Layer in the Anthracite–Electrolyte System with Allowance for Energy Surface Inhomogeneity

A model for the structure of the electrical double layer taking into account the structural inhomogeneity of the solid surface was proposed. An interpretation was suggested for high capacity values of anthracites in electrolyte solutions. Calculated and experimental values of the capacity of the electrical double layer at the zero-charge potential were compared. Integrals allowing for the effect of the energy surface inhomogeneity of coals on their electrosurface properties were calculated.     

Elimination of Anti-spiral Waves by Local Inhomogeneity in Oscillatory Systems

Anti-spiral waves are controlled in an oscillatory system by using a local inhomogeneity. The inhomogeneity acts as a wave source, and gives rise to the propagating plane waves. It is found that there is a critical pacemaking domain size below which no wave will be created at all. Two types of ordered waves (target waves and traveling waves) are created depending on the geometry of the local inhomogeneity. The competition between the anti-spiral waves and the ordered waves is discussed. Two different competition mechanisms were observed, which are related to the ordered waves obtained from different local inhomogeneities. It is found that traveling waves with either lower frequency or higher frequency can both eliminate the anti-spiral waves, while only the target waves with lower absolute value of frequency can eliminate the anti-spiral waves.This method also applies to outwardly rotating spiral waves. The control mechanism is intuitively explained and the control method is easily operative.     

Design of experiment for evaluation of uncertainty from sampling in the framework of the fitness for purpose concept: a case study

The design of an experiment for the evaluation of sampling uncertainty in the framework of the fitness for purpose concept is described in terms of probabilities (risks of the user) of type 1 and type 2 errors in decisions concerning the significance of effects influencing the sampling uncertainty and the measurement results. As a case study, an experiment based on the duplicate method for quantification of the sampling uncertainty and inhomogeneity (heterogeneity) of a melt of tin-free bronze produced in a 10-ton reflective oven is analyzed. The melt is defined as the sampling target. It is shown that the number of such targets (melts), the number of samples under analysis and the number of replicate analyses can be minimized, i.e., the size and cost of the experiment can be reduced, if the user knows which risks are acceptable. When inhomogeneity of the sampling target has a systematic character, like the decrease of the mass fraction of aluminum from the beginning to the end of the melt pouring in the case study, the inhomogeneity effect can be separated from the sampling uncertainty and evaluated according to the user’s needs.