Screen-printed electrode-based electrochemical detector coupled with in-situ ionic-liquid-assisted dispersive liquid–liquid microextraction for determination of 2,4,6-trinitrotoluene

A novel method is reported, whereby screen-printed electrodes (SPELs) are combined with dispersive liquid–liquid microextraction. In-situ ionic liquid (IL) formation was used as an extractant phase in the microextraction technique and proved to be a simple, fast and inexpensive analytical method. This approach uses miniaturized systems both in sample preparation and in the detection stage, helping to develop environmentally friendly analytical methods and portable devices to enable rapid and onsite measurement. The microextraction method is based on a simple metathesis reaction, in which a water-immiscible IL (1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [Hmim][NTf₂]) is formed from a water-miscible IL (1-hexyl-3-methylimidazolium chloride, [Hmim][Cl]) and an ion-exchange reagent (lithium bis[(trifluoromethyl)sulfonyl]imide, LiNTf₂) in sample solutions. The explosive 2,4,6-trinitrotoluene (TNT) was used as a model analyte to develop the method. The electrochemical behavior of TNT in [Hmim][NTf₂] has been studied in SPELs. The extraction method was first optimized by use of a two-step multivariate optimization strategy, using Plackett–Burman and central composite designs. The method was then evaluated under optimum conditions and a good level of linearity was obtained, with a correlation coefficient of 0.9990. Limits of detection and quantification were 7 μg L^?1 and 9 μg L^?1, respectively. The repeatability of the proposed method was evaluated at two different spiking levels (20 and 50 μg L^?1), and coefficients of variation of 7 % and 5 % (n?=?5) were obtained. Tap water and industrial wastewater were selected as real-world water samples to assess the applicability of the method.

The Nicolas and Robin inequalities with sums of two squares

In 1984, G. Robin proved that the Riemann hypothesis is true if and only if the Robin inequality σ(n) < e ^γ n log log n holds for every integer n > 5040, where σ(n) is the sum of divisors function, and γ is the Euler–Mascheroni constant. We exhibit a broad class of subsets S{\mathcal {S}} of the natural numbers such that the Robin inequality holds for all but finitely many n ? S{n \in \mathcal {S}} . As a special case, we determine the finitely many numbers of the form n = a ² + b ² that do not satisfy the Robin inequality. In fact, we prove our assertions with the Nicolas inequality n/φ(n) < e ^γ log log n; since σ(n)/n < n/φ(n) for n > 1 our results for the Robin inequality follow at once.     

A model for HCMV infection in immunosuppressed patients

We propose a model for HCMV infection in healthy and immunosuppressed patients. First, we present the biological model and formulate a system of ordinary differential equations to describe the pathogenesis of primary HCMV infection in immunocompetent and immunosuppressed individuals. We then investigate how clinical data can be applied to this model. Approximate parameter values for the model are derived from data available in the literature and from mathematical and physiological considerations. Simulations with the approximated parameter values demonstrates that the model is capable of describing primary, latent, and secondary (reactivated) HCMV infection. Reactivation simulations with this model provide a window into the dynamics of HCMV infection in (D-R+) transplant situations, where latently-infected recipients (R+) receive transplant tissue from HCMV-naive donors (D-).     

A mathematical model for the first-pass dynamics of antibiotics acting on the cardiovascular system

We present a preliminary first-pass dynamic model for delivery of drug compounds to the lungs and heart. We use a compartmental mass-balance approach to develop a system of nonlinear differential equations for mass accumulated in the heart as a result of intravenous injection. We discuss sensitivity analysis as well as methodology for minimizing mass in the heart while maximizing mass delivered to the lungs on a first circulatory pass.     

A note on compressive limiting for two‐material flows

In this short note we describe a simple extension to the multi‐material shock‐capturing algorithm presented in (J. Comput. Phys. 2007; 223 :262–297) that can be used to maintain sharp material interfaces. The method takes the form of an artificial compression which is designed so that the material indicator jumps across only a few cells but which does not excite physical instabilities in the flow. The advantages of the approach include its simplicity and flexibility in that it provides a parameter that effectively determines the captured interface thickness. Copyright © 2009 John Wiley & Sons, Ltd.     

Spectrophotometric determination of aluminum and iron in calcium

A simple, rapid, and accurate method is described whereby micro amounts of aluminum and iron can be determined in calcium metal. The iron is determined spectrophotometrically by measuring the absorbency of the reddish-orange color of the tris(1,10-phenanthroline)iron(II)ion, [(C₁₂H₈N₂)₃Fe]⁺⁺. This complex effectively removes the iron interference, and the aluminum is determined spectrophotometrically in the solution remaining from the iron determination by extracting it with a chloroform solution of 8-quinolinol and measuring the absorbency of the yellow color of the tris(8-quinolinolo)aluminum(III) in the chloroform solution.     

1,2-Cycloheptanedionedioxime as a micro-gravimetric reagent for nickel

Summary 1,2-Cycloheptanedionedioxime (heptoxime), previously reported as a water-soluble gravimetric reagent for nickel, has been applied succesfully to the micro-gravimetric determination of this ion. The procedure, modified from the macro-determination, consists of precipitating nickelheptoxime in a micro-filter beaker and weighing the precipitate after drying at 110–120° C. The commonly encountered cations and metallic ions do not interfere or may be masked by complexing agents. The method has been tested on standard nickel solutions and standard steel samples.

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Zusammenfassung Über die Eignung des 1,2-Cycloheptandiondioxims (Heptoxim) als wasserlösliches Reagens für die gravimetrische Bestimmung von Nickel wurde in einer früheren Arbeit berichtet. Nun wird gezeigt, daß es sich auch für die Mikrobestimmung des Nickels eignet. Das Nickelheptoxim wird in einem Filterbecher gefällt und nach Trocknen bei 110 bis 120° C gewogen. Die gewöhnlich das Nickel begleitenden Metalle hindern die Bestimmung entweder nicht oder können durch Überführung in geeignete Komplexe zufriedenstellend maskiert werden. Das Mikroverfahren wurde an Nickellösungen bekannten Gehaltes und an Stahlproben des National Bureau of Standards geprüft.

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Résumé On a rapporté dans un travail précédent, la possibilité d'emploi de la cyclo-1,2 heptanedionedioxime (heptoxime) comme réactif hydrosoluble pour le dosage gravimétrique du nickel. On a montré maintenant qu'elle est convenable pour le microdosage du nickel. L'heptoximate de nickel est précipité au bécher-filtre et pesé après séchage à 110–120°. Les métaux qui accompagnent habituellement le nickel, ou bien ne gênent pas ou bien peuvent être masqués d'une façon satisfaisante. Le procédé microchimique a été essayé sur des solutions de nickel de titre connu ou sur des échantillons d'acier du National Bureau of Standards.