Measurement uncertainty distributions and uncertainty propagation by the simulation approach

A complete and accurate evaluation of measurement uncertainty requires the knowledge of the uncertainty distributions. The latter are rarely determined or verified experimentally, and hence up to now only crude estimates or assumptions based on intuition have been used. The simulation of experimental results is readily accessible and provides a more relia-ble solution to this problem. When using an appropriate model of measurement and after determina-tion of input value parameters by present state-of-the-art techniques, simulation data supply reliable information about the distribution of the output results of a complex measurement. The method permits simple variation of preposition and therefore ready analysis of various features influencing the measurement of uncertainty intervals. In the paper we described examples of such evaluations related to the preparation of certified reference materials, where there is excellent agreement between the traditional and simulation approaches. And evaluation of more complex measurements of diffusion coefficients by the open capillary method, where uncertainty of the simulated result is more realistic than the re-sult from the traditional error method due to non-linearity and probably Cauchy distribution in some steps. Received: 17 March 2000 Accepted: 25 July 2000     

Knowledge-based probabilistic representations of branching ratios in chemical networks: the case of dissociative recombinations

Experimental data about branching ratios for the products of dissociative recombination of polyatomic ions are presently the unique information source available to modelers of natural or laboratory chemical plasmas. Yet, because of limitations in the measurement techniques, data for many ions are incomplete. In particular, the repartition of hydrogen atoms among the fragments of hydrocarbons ions is often not available. A consequence is that proper implementation of dissociative recombination processes in chemical models is difficult, and many models ignore invaluable data. We propose a novel probabilistic approach based on Dirichlet-type distributions, enabling modelers to fully account for the available information. As an application, we consider the production rate of radicals through dissociative recombination in an ionospheric chemistry model of Titan, the largest moon of Saturn. We show how the complete scheme of dissociative recombination products derived with our method dramatically affects these rates in comparison with the simplistic H-loss mechanism implemented by default in all recent models.     

Introduction to measurement uncertainty in chemical analysis

 This paper reviews the experience of the use of the Eurachem Guide and gives a brief overview of the principles of evaluating uncertainty. This is followed by discussion of the comments received on the Guide and highlights some of the issues that need to be considered in the next version. Accepted: 21 October 1997     

Modeling toxicity by using supervised kohonen neural networks

Counterprogation neural network is shown to be a powerful and suitable tool for the investigation of toxicity. This study mined a data set of 568 chemicals. Two hundred eighty-two objects were used as the training set and 286 as the test set. The final model developed presents high performances on the data set R(2) = 0.83 (R(2) = 0.97 on the training set, R(2) = 0.59 on the test set). This technique distinguishes itself also for the ability to give to the expert two-dimensional maps suitable for the study of the distribution/clustering of the data and the identification of outliers.     

Thermal stability of hydrogen networks by chemical stress relaxation

The thermal stability of hydrocarbon networks by chemical stress relaxation was investigated in vacuo and in the temperature range 300–350°C. The polymers studied were low-density polyethylene, high-density polyethylene, and ethylene–propylene terpolymer. Carbon-carbon crosslinked networks were produced either by dicumyl peroxide or by radiation. The results show that the overall thermal stability is in the order: peroxide-cured EPT > peroxide-cured LDPE > radiation-cured HDPE. The results are in apparent contradiction to the belief that linear structures are more stable than branched structures. We believed that this can be explained in terms of weak linkages introduced during curing.     

Determination of chemical composition distributions in synthetic polymers

A characteristic feature of synthetic polymers is their dispersity in molar mass and, in many cases, chemical composition. Since dispersity is highly relevant in relation to polymer properties, ongoing efforts are being put in the development of appropriate analysis methods. In this respect, size-exclusion chromatography (SEC) is well known for the determination of molar mass distributions. Methods for chemical composition distributions are less mature than SEC and mainly include liquid chromatography and mass spectrometry and the combination of these techniques. The term chemical composition distribution is considered broad in this paper, i.e. for the chemical composition distribution of a (co)polymer backbone, for the functionality type distribution of a polymers' functional end groups, for the block length distribution of a block copolymer, for the branching distribution and for the tacticity distribution. In this paper, analysis methods for all types of chemical composition distributions are reviewed. Special attention is paid to practical requirements and common misconceptions that sometimes arise. Applications within the last 5 years are summarized.     

Modeling chemical reactivity in emulsions

Until relatively recently, modeling chemical reactivity in emulsions has proved refractory. The problem lies in developing good methods for determining the distributions of reactants because classical separation of the phases by physical methods do not work, which prevents measurement of the distributions of reactants and components between the oil, interfacial and aqueous regions in emulsions. Without this understanding, they cannot be used efficiently as reaction media. We are using a physical-organic chemistry approach grounded in thermodynamics and inspired by the prior success of pseudophase kinetic models developed for association colloids such as micelles microemulsions, and vesicles. In emulsions, as in association colloids, the observed rate constant depends on the concentrations of reactants in each region and on medium effects. The medium effects reflect the solvent properties of a reaction region and the distributions of reactants depend on their solubilities in each region. Here we introduce: (a) the current concepts and basic assumptions employed to interpret chemical reactivity in nonionic and ionic emulsions; (b) several approaches for estimating the partition constants for substrates between the oil-interfacial, P_O^I, and water-interfacial, P_W^I, regions of the emulsions; and (c) methods for determining the rate constant in the interfacial region, k_I. The results demonstrate that pseudophase kinetic analyses provide a unique, versatile, and robust solution to interpreting chemical reactivity in emulsions. The approach permits identifying the relative importance of various emulsion properties such as oil hydrophobicity, emulsifier structure and HLB, temperature, and acidity on reactant distributions. Representative results for antioxidants are included. The approach offers a new route for identifying most efficient antioxidant for a particular food application.     

Coherent phase control of the product branching ratio in the photodissociation of dimethylsulfide

Coherent phase control of the photodissociation reaction of the dimethylsulfide has been achieved by means of quantum-mechanical interference between one- and three-photon transitions. Dimethylsulfide was irradiated by fundamental and frequency-tripled outputs of a visible laser (600.5-602.5 nm), simultaneously to yield CH3S+ and CH3SCH2+ fragment ions. The branching ratio of the two product channels could be modulated with variation of the phase difference between the light fields. This accounted for the difference between the molecular phases of the two product channels. The phase lag was observed to have a maximum value of 8 degrees at 601.5 nm. This is the first result of a selective bond breaking in a polyatomic molecule by the coherent phase control.     

Identifiability of chemical reaction networks

We consider the dynamics of chemical reaction networks under the assumption of mass-action kinetics. We show that there exist reaction networks for which the reaction rate constants are not uniquely identifiable, even if we are given complete information on the dynamics of concentrations for all chemical species of . Also, we show that there exist reaction networks such that their dynamics are identical under appropriate choices of reaction rate constants, and present theorems that characterize the properties of , , that make this possible. We use these facts to show how we can determine dynamical properties of some chemical networks by analyzing other chemical networks.     

Charge distributions and chemical effects in simple alkanes

A simple parametric procedure for calculating the electronic structure of hydrocarbons has been tested using a set of parametric relations suggested by Fliszár. The calculated inductive charges are satisfactory and are compared with values derived from a least-squares analysis of theoretical charges. Further improvement is possible by applying the least-squares analysis first and then the parametric equations. The procedure is also used to calculate the energy of atomization of the hypothetical vibrationless state, giving results which agreee well with experimental values.     

Atoms of multistationarity in chemical reaction networks

Chemical reaction systems are dynamical systems that arise in chemical engineering and systems biology. In this work, we consider the question of whether the minimal (in a precise sense) multistationary chemical reaction networks, which we propose to call ‘atoms of multistationarity,’ characterize the entire set of multistationary networks. Our main result states that the answer to this question is ‘yes’ in the context of fully open continuous-flow stirred-tank reactors (CFSTRs), which are networks in which all chemical species take part in the inflow and outflow. In order to prove this result, we show that if a subnetwork admits multiple steady states, then these steady states can be lifted to a larger network, provided that the two networks share the same stoichiometric subspace. We also prove an analogous result when a smaller network is obtained from a larger network by ‘removing species.’ Our results provide the mathematical foundation for a technique used by Siegal- Gaskins et al. of establishing bistability by way of ‘network ancestry.’ Additionally, our work provides sufficient conditions for establishing multistationarity by way of atoms and moreover reduces the problem of classifying multistationary CFSTRs to that of cataloging atoms of multistationarity. As an application, we enumerate and classify all 386 bimolecular and reversible two-reaction networks. Of these, exactly 35 admit multiple positive steady states. Moreover, each admits a unique minimal multistationary subnetwork, and these subnetworks form a poset (with respect to the relation of ‘removing species’) which has 11 minimal elements (the atoms of multistationarity).     

The chemical synthesis of a cyclic oligosaccharide derivative with branching

A cyclic oligosaccharide derivative was synthesized by cationic ring-opening polymerization of an anhydrodisaccharide derivative under high vacuum in dichloromethane with 20 mol% of PF₅ as initiator. Analysis of the spectral results showed that the oligomer chain is composed of only 3 glucose units connected by -1,6 linkages with a glucopyranosyl branching unit at C-4 of each sugar residue in the main chain.     

Kinetics and branching ratio studies of the reaction of C2H5O2 + HO2 using chemical ionisation mass spectrometry

The overall rate coefficient for the reaction of C(2)H(5)O(2) with HO(2) was determined using a turbulent flow chemical ionization mass spectrometer (TF-CIMS) system over the pressure range of 75 to 200 Torr and temperatures between 195 and 298 K. The temperature dependence of the overall rate coefficient for the reaction between C(2)H(5)O(2) and HO(2) was fitted using the following Arrhenius expression: k(T) = (2.08) x 10(-13) exp [(864 +/- 79)/T] cm(-3) molecule(-1) s(-1). The upper limits for the branching ratios for reactive channels leading to O(3) and OH production were quantified for the first time. A tropospheric model has been used to assess the impact of the experimental error of the rate coefficients determined in this study on predicted concentrations of a number of key species, including O(3), OH, HO(2), NO and NO(2). In all cases it is found that the propagated error is very small and will not in itself be a major cause of uncertainty in modelled concentrations. However, at low temperatures, where there is a wide discrepancy between existing kinetic studies, modelling using the range of kinetic data in the literature shows a small but significant variation for [C(2)H(5)O(2)], [C(2)H(5)OOH], [NO(x)] and the HO(2) : OH ratio. Furthermore, a structure-activity relationship (SAR) was developed to rationalise the reactivity of the reaction between RO(2) and HO(2).     

Pressure dependence of chemical branching in the oxygen-atom reaction with allyl chloride

The pressure dependence of the rate constant of the O-atom reaction with allyl chloride and its branching fraction to produce HCO + CH₂ClCH₂ were measured over a seven-fold pressure range (0.5-3.5 Torr) at 299 K. Both are independent of pressure (<20% change). The results suggest that the rearrangements in the energy-rich adduct which are required to yield the products of the route studied are not collision induced.     

Modeling and analysis of uranium isotope enrichment by chemical exchange

Starting with an accurate mathematical model a theoretical study for the analysis of the separation of uranium isotopes by chemical exchange has been presented. The experimental data used in this study were obtained by reverse breakthrough technique and the numerical algorithm developed for simulation in previous studies was adapted and found to be suitable for this kind of processes. The model parameters were identified from experimental data and simulations were carried out for different experimental conditions. This revised version was published online in July 2006 with corrections to the Cover Date.