Application of in-situ formed polymer-based dispersive solid phase extraction in combination with solidification of floating organic droplet-based dispersive liquid–liquid microextraction for the extraction of neonicotinoid pesticides from milk samples

An in-situ formed polymer–based dispersive solid phase extraction in combination with solidification of floating organic droplet-based dispersive liquid–liquid microextraction was developed for the extraction of neonicotinoid pesticides from milk samples. The extracted analytes were determined using high-performance liquid chromatography–diode array detector. In this approach, after precipitating the proteins of milk using a zinc sulfate solution, the supernatant phase (containing sodium chloride) was transferred into another glass test tube, and a homogenous solution of polyvinylpyrrolidone and a suitable water-miscible organic solvent was rapidly injected into it. By this step, the polymer particles were re-produced and the analytes were extracted onto the sorbent surface. In the following step, the analytes were eluted with an appropriate organic solvent to use in the following solidification of floating organic droplet-based dispersive liquid–liquid microextraction step that was done to acquire the low limits of detection. Under the optimized conditions, satisfactory results consisting of low limits of detection (0.13–0.21 ng/ml) and quantification (0.43–0.70 ng/ml), high extraction recoveries (73%–85%), and enrichment factors (365–425), and good repeatability (relative standard deviations equal or less than 5.1% and 5.9% for intra- and inter-day precisions, respectively) were obtained.     

Identification of potential antioxidant compounds in the essential oil of thyme by gas chromatography with mass spectrometry and multivariate calibration techniques

Thyme species are used in traditional medicine throughout the world and are known for their antiseptic, antispasmodic, and antitussive properties. Also, antioxidant activity is one of the interesting properties of thyme essential oil. In this research, we aim to identify peaks potentially responsible for the antioxidant activity of thyme oil from chromatographic fingerprints. Therefore, the chemical compositions of hydrodistilled essential oil of thyme species from different regions were analyzed by gas chromatography with mass spectrometry and antioxidant activities of essential oils were measured by a 1,1‐diphenyl‐2‐picrylhydrazyl radical scavenging test. Several linear multivariate calibration techniques with different preprocessing methods were applied to the chromatograms of thyme essential oils to indicate the peaks responsible for the antioxidant activity. These techniques were applied on data both before and after alignment of chromatograms with correlation optimized warping. In this study, orthogonal projection to latent structures model was found to be a good technique to indicate the potential antioxidant active compounds in the thyme oil due to its simplicity and repeatability.     

Semi-analytic solutions for the free in-plane vibrations of confocal annular elliptic plates with elastically restrained edges

A two-dimensional analytical model is developed to describe the free extensional vibrations of thin elastic plates of elliptical planform with or without a confocal cutout under general elastically restrained edge conditions, based on the Navier displacement equation of motion for a state of plane stress. The model has been simplified by invoking the Helmholtz decomposition theorem, and the method of separation of variables in elliptic coordinates is used to solve the resulting uncoupled governing equations in terms of products of (even and odd) angular and radial Mathieu functions. Extensive numerical results are presented in an orderly fashion for the first three anti-symmetric/symmetric natural frequencies of elliptical plates of selected geometries under different combinations of classical (clamped and free) and flexible boundary conditions. Also, the occurrences of “frequency veering” between various modes of the same symmetry group and interchange of the associated mode shapes in the veering region are noted and discussed. Moreover, selected 2D deformed mode shapes are presented in vivid graphical form. The accuracy of solutions is checked through appropriate convergence studies, and the validity of results is established with the aid of a commercial finite element package as well as by comparison with the data in the existing literature. The set of data reported herein is believed to be the first rigorous attempt to obtain the in-plane vibration frequencies of solid and annular thin elastic elliptical plates for a wide range of plate eccentricities.     

An efficient algorithm based on the differential quadrature method for solving Navier–Stokes equations

In this paper, an approach to improve the application of the differential quadrature method for the solution of Navier–Stokes equations is presented. In using the conventional differential quadrature method for solving Navier–Stokes equations, difficulties such as boundary conditions' implementation, generation of an ill conditioned set of linear equations, large memory storage requirement to store data, and matrix coefficients, are usually encountered. Also, the solution of the generated set of equations takes a long running time and needs high computational efforts. An approach based on the point pressure–velocity iteration method, which is a variant of the Newton–Raphson relaxation technique, is presented to overcome these problems without losing accuracy. To verify its performance, four cases of two‐dimensional flows in single and staggered double lid‐driven cavity and flows past backward facing step and square cylinder, which have been often solved by researchers as benchmark solution, are simulated for different Reynolds numbers. The results are compared with existing solutions in the open literature. Very good agreement with low computational efforts of the approach is shown. It has been concluded that the method can be applied easily and is very time efficient. Copyright © 2012 John Wiley & Sons, Ltd.     

Exact solution for free vibration analysis of an eccentric elliptical plate

The method of separation of variables in elliptical coordinates in conjunction with the translational addition theorems for Mathieu functions is used to investigate the free flexural vibrations of a fully clamped thin elastic panel of elliptical planform containing an elliptical cutout of arbitrary size, location, and orientation. The first five natural frequencies are calculated for various plate/cutout aspect ratios and selected cutout location/orientation parameters. Also, a number of representative vibration mode shapes are depicted in graphical form. The accuracy of solutions is demonstrated through proper convergence studies, and the validity of results is established with the aid of a commercial finite element package as well as by comparison with those in the existing literature.     

Free vibration analysis of a finite-length isotropic solid elliptic cylinder using exact three dimensional elasticity theory

A three-dimensional analytical model based on Navier’s displacement equation of motion is developed to describe the free vibrations of a simply supported elastic isotropic solid elliptical cylinder of finite length. Helmholtz decomposition theorem is invoked and the classical technique of separation of variables in elliptical coordinates is employed to solve the subsequent uncoupled governing differential equations as products of ordinary and modified Mathieu functions. Inclusive numerical data are provided in a systematic manner for the first several symmetric/anti-symmetric eigen-frequencies as a function of cross sectional eccentricity for selected cylinder length ratios. The incidences of “mode crossing” and “frequency veering” among different modes of the same symmetry group as well as swapping of the corresponding mode shapes in the veering region are examined. Also, selected three dimensional deformation mode shapes are illustrated. The precision of calculations is checked by performing proper convergence studies, and the correctness of results is demonstrated using a commercial finite element software as well as by comparison with the information in the accessible literature. The collection of presented data is believed to be the first meticulous attempt to determine the free vibration frequencies of finite-length simply-supported elastic elliptical cylinders in an exact manner.     

Quantification of sofosbuvir in human serum by liquid chromatography with negative ionization mass spectrometry using the parent peak and its source‐induced fragment: Application to a bioequivalence study

In the mass spectrometry of sofosbuvir, a new orally administered antihepatitis C drug, a weak peak is detected at the m/z value of the parent ion (m/z 530) as a result of in‐source dissociation and current methods to its quantification, is based on monitoring of the parent peak using ultra high‐performance liquid chromatography with tandem mass spectrometry. With these methods serum concentration of the drug is quantifiable only up to 4–5 h postdose. However, the fragmentation of the molecule generates a more stable ion at m/z 287 (base peak) with a signal intensity of about tenfold compared to the parent ion. Our study was aimed to improve sensitivity of analysis by acquisition of the m/z value of the daughter ion from which it originated instead of the parent molecule. This novelty allows us to measure serum concentrations of the drug for a longer time postdose and provides more opportunity for pharmacokinetic studies of sofosbuvir. Our method was linear over the concentration range of 2–2560 ng/mL of sofosbuvir in human serum with a limit of quantification of 2 ng/mL compared to 10 ng/mL reported previously. The coefficient variation values of both inter and intraday analysis were less than 13.8%, and the percentage error was less than 6.3.     

Investigation of fragrance stability used in the formulation of cosmetic and hygienic products using headspace solid‐phase microextraction by nanostructured materials followed by gas chromatography with mass spectrometry

A new composite coating of polypyrrole and sodium lauryl ether sulfate was electrochemically prepared on a stainless‐steel wire using cyclic voltammetry. The application and performance of the fiber was evaluated for the headspace solid‐phase microextraction of a fragrance in aqueous bleach samples followed by gas chromatography combined with mass spectrometry to assess the fragrance stability in this kind of household cleaning product. To obtain a stable and efficient composite coating, parameters related to the coating process such as scan rate and numbers of cycles were optimized using a central composite design. In addition, the effects of various parameters on the extraction efficiency of the headspace solid‐phase microextraction process such as extraction temperature and time, ionic strength, sample volume, and stirring rate were investigated by experimental design methods using Plackett–Burman and Doehlert designs. The optimum values of 53°C and 28 min for sample temperature and time, respectively, were found through response surface methodology. Results show that the combination of polypyrrole and sodium lauryl ether sulfate in a composite form presents desirable opportunities to produce new materials to study fragrance stability by headspace solid‐phase microextraction.