Study of the factors affecting the performance of microextraction by packed sorbent (MEPS) using liquid scintillation counter and liquid chromatography-tandem mass spectrometry

Microextraction by packed sorbent (MEPS) is a new technique for sample preparation that can be connected on-line with LC or GC. In MEPS, approximately 1-2 mg of the solid packing material is inserted into a syringe (100-250 μL) as a plug. Sample preparation takes place on the packed bed. The bed can be packed or coated to provide selective and suitable sampling conditions. The new method is very promising for extraction of drugs and metabolites from biological samples.In this paper, some factors affecting the performance of MEPS such as recovery, carry-over, leakage, washing volume and elution volume were studied using C18 and hydroxylated polystyrene-divinylbenzene copolymer (ENV+) as sorbents. Radioactively labelled bupivacaine in plasma samples was used as test analyte. For the extraction of this drug, using methanol/water 95:5 (v/v) (0.25% ammonium hydroxide) was used as elution solvent. The analyte response increased with increasing the elution volume and it was linear upp up to 100 μL utilizing liquid scintillation counter. Further, for concentrating the sample, we found that MEPS may be used such that the sample can be drawn through the needle, up and down, several times. The analyte leakage increases as the volume washing increases, though higher washing volumes may also result in cleaner extracts. To eliminate analyte carry-over, the sorbents were washed first with 3 × 250 μL elution solution and then with 3 × 250 μL washing solution. In addition, the reproducibility measurements show relatively good relative standard deviation (RSD) % values concerning analyte recovery and analyte leakage. The present study provides an understanding of basic aspects when optimizing methods for MEPS. In this study, MEPS was used off-line with liquid scintillation counter and on-line with LC-MS/MS.     

A new approach to the Assad–Kirk fixed point theorem

In the present paper, considering the Wardowski’s technique, we give a new approach to the Assad–Kirk fixed point theorem on metrically convex metric spaces.We also provide a nontrivial example showing that our result is a proper extension of the Assad–Kirk fixed point theorem.     

Thermochemistry of N‐heterocyclic carbenes with 5‐, 4‐, 3‐, and 2‐membered rings

N‐heterocyclic carbenes (NHCs) based on imidazole‐2‐ylidene ( 1 ) or the saturated imidazolidine‐2‐ylidene ( 2 ) scaffolds are long‐lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p‐pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS‐QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four‐membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino‐substituted cyclopropenylidenes have been reported but three‐membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino‐substituted cyclopropenylidenes are comparable in stability to the four‐membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of “two‐membered ring” azapropadienenylidene cations of form :C?N?W with W = an electron‐withdrawing agent. We have studied W = NO₂, CH₂(+), CF₂(+), and (CN)₂C(+). Although these systems display a degree of stabilization and carbene‐like electronic structure, the stability of the NHCs is unsurpassed. © 2014 Wiley Periodicals, Inc.     

Computational characterization of isomeric C4H2O systems: Thermochemistry,vibrational frequencies,and optical spectra for butatrienone,ethynyl ketene,butadiynol, and triafulvenone

Species of empirical formula C₄H₂O have been invoked either as elusive intermediates in flames or oxidations on heterogeneous catalysts, or as long‐lived species in the interstellar medium. Butatrienone has been characterized experimentally, but isomers ethynyl ketene, butadiynol, and trifulvenone have been described only by computational modeling. Triafulvenone is of special interest as the ketene analog of the carbonyl compound cyclopropenone; both species contain seriously strained three‐membered rings. In contrast to cyclopropenone, which is detected in the interstellar medium, triafulvenone continues to elude experimental capture. The contrast is attributed to a degree of aromatic stabilization in cyclopropenone and anti‐aromatic destabilization in triafulvenone. In this report, we characterize the structure, vibrational and electronic spectra, and thermochemistry for triafulvenone and three of its isomers, butatrienone, ethynyl ketene, and butadiynol to assist experimental detection of these elusive species. Our calculations have shown that triafulvenone is the least stable of these four isomers; even the well‐known butatrienone, is not the most stable. The so far undetected ethynyl ketene is thermodynamically the most stable of these isomers. To facilitate experimental detection of these species we provide vibrational frequencies calculated using both B3LYP/cc‐pVTZ and MP2/cc‐pVTZ level model chemistry corrected for anharmonicity including the possibility that the spectra may include overtones and combination bands for these species The regions of intense IR absorption and most important frequencies are also underlined for all the species involved. To guide the search for short‐lived C₄H₂O species, we also characterize the optical spectrum. © 2015 Wiley Periodicals, Inc.     

Non-gray radiative transfer

The normal-mode-expansion technique is used to establish the solution of the Milne problem basic to a generalized equation of radiative transfer. The non-gray model used includes the effects of absorption, scattering and losses due to photo-electric ionizations and collisions of the second kind. Accurate numerical results are presented for such physical quantities as the extrapolation distance, the integrated Planck function and the angular distribution of the exit intensity for selected values of the basic parameters.     

Influence of Solute–Solvent Interaction on Acid Strength of Some Substituted Phenols in Ethanol–Water Media

The solute–solvent interactions of some phenol derivatives were investigated potentiometrically in 0–60 % (v/v) ethanol–water mixtures. The acidity constants values were correlated with either macroscopic parameters such as molar fraction, permittivity and the solvating ability or microscopic parameters, such as the Kamlet–Taft solvatochromic parameters. Moreover, it is demonstrated that the pK _a values in any ethanol–water mixtures are linearly related to the pK _a values of the phenols in pure water. The slope and intercept parameters of the linear correlations are related with the mole fraction of ethanol. These equations permit accurate calculation of the pK _a values of the studied phenols at any ethanol–water composition.     

Photoactivable peptides for identifying enzyme-substrate and protein-protein interactions

Photoactivated cross-linking of peptides to proteins is a useful strategy for identifying enzyme-substrate and protein-protein interactions in cell lysates as demonstrated by studies on the human hypoxia inducible factor system.     

Linear parameter varying feedforward control synthesis using parameter-dependent Lyapunov function

This paper presents the dynamic feedforward control synthesis for linear parameter varying (LPV) systems. It is assumed that all system matrices are dependent on varying parameters, which are measurable with sensor or observable. The parameters have bounded variation rates. Parameter-dependent Lyapunov function is used for the feedforward control synthesis such that the robust stability is assured for all varying parameters at the time of the operation. The method is formulated in terms of linear matrix inequalities for LPV feedforward controller that guarantees the stability of the transfer matrix having \(L_{2}\) -gain. This compensator is designed by adding on the feedback controller in two degrees of freedom control configuration. This controller can be used for the disturbance attenuation or decreasing the tracking error. The numerical examples and simulations are given to provide the applicability of the proposed solution.