Comparison of linear and non-linear equations for univariate calibration

Univariate data accumulated for the purpose of calibration of chromatographic and spectroscopic methods often exhibit slight but definite curvature. In this paper the performance of a non-linear calibration equation with the capacity to account empirically for the curvature, y = a + bx(m), (m not equal to 1) is compared with the commonly used linear equation, y = a + bx, as well as the quadratic equation, y = a + bx + cx2. All equations were applied to high quality HPLC calibration data using unweighted least squares. Parameter estimates and their standard errors were calculated for each equation. Standard errors and 95% prediction intervals in analyte concentrations were estimated with the aid of the fitted equations and their respective covariance matrices. Results indicate that the non-linear and quadratic equations each provide a better fit than the linear equation to the data considered here, as judged by the Akaikes information criterion (AIC), the adjusted coefficient of multiple determination, the magnitude and scatter of residuals, standard errors in estimated analyte concentrations and lack of fit analysis of variance (ANOVA). While the difference between the equations y = a + bx + cx2 and y = a + bx(m) as judged by the same criteria is more marginal, this work suggests that the non-linear calibration equation should be considered when a curve is required to be fitted to low noise calibration data which exhibit slight curvature.     

Expert systems for chemical analysis (ESPRIT Project 1570)

Summary This paper gives an overview of the structure and processes going on in the project Expert Systems for Chemical Analysis (ESCA), an international project within the ESPRIT research programme of the European Community. The application area, HPLC method development, is explained and broken down into four domains. The methods used for representing knowledge in the project are discussed.     

The absolute configurations of (+)-usnic and (+)-isousnic acid. X-ray analyses of the (−)-α-phenylethylamine derivative of (+)-usnic acid and of (−)-pseudoplacodiolic acid,a new dibenzofuran,from the lichen Rhizoplaca chrysoleuca.

The absolute configuration of (+)-usnic acid has been shown to be (4aR) as in (1) by X-ray analysis of the (?)-α-phenylethylamine derivative (2). The structure and absolute stereochemistry of (?)-pseudoplacodiolic acid (4) have been established.     

Instrumentation and Measurement Issues for Nanometer Particles: Workshop Summary

Journal of Nanoparticle Research -     

QM/MM modelling of oleamide hydrolysis in fatty acid amide hydrolase (FAAH) reveals a new mechanism of nucleophile activation

Fatty acid amide hydrolase (FAAH), a promising target for the treatment of several central and peripheral nervous system disorders, such as anxiety, pain and hypertension, has an unusual catalytic site, and its mechanism has been uncertain; hybrid quantum mechanics/molecular mechanics (QM/MM) calculations reveal a new mechanism of nucleophile activation (involving a Lys-Ser-Ser catalytic triad), with potentially crucial insights for the design of potent and selective inhibitors.     

Effect of non-uniform distribution of absorption on reverberation time

An attempt is made to develop a geometric theory of reverberation without bringing in the simplifying assumptions that are required by statistical considerations. The reverberation phenomenon is simulated on a digital computer by the ray tracing method. Experimental investigations to check the validity of the computer model are described. Large disagreements are found between the computed and experimental results. Consequently, the computer model is re-examined and modified to incorporate the scattering of sound that occurs at the edges and corners of an enclosure. Good agreement is reported between the modified computer results and the experimental results in situations where the absorbing material covers the enclosure surface(s) completely.     

A survey of service system noise faults at the University of Liverpool

This paper reports on the findings of a survey that investigated buildings for acoustic intrusions from the services. During the course of the survey it was found that fifty per cent of the buildings looked at had failures and thirty per cent serious faults.The failures are investigated and some general conclusions are made concerning noise from service systems.     

Expedient asymmetric synthesis of all four isomers of N,N'-protected 2,3-diaminobutanoic acid

2,3-Diaminobutanoic acid (DAB) is found in several peptide antibiotics, toxins, and biologically active molecules. This paper describes the practical and highly enantioselective synthesis of all four N,N'-protected DAB stereoisomers using an asymmetric Rh(I)-phosphine-catalyzed hydrogenation of isomeric enamides as the key step. Thermal and photochemical isomerization of the enamide hydrogenation substrates coupled with catalyst-geometric isomer pairing allows targeted synthesis of single DAB isomers in maximum yield.