Comparison of the Separation Characteristics of the Organic–Inorganic Hybrid Stationary Phases XBridge C8 and Phenyl and XTerra Phenyl in RP-LC

Differences in the system constants of the solvation parameter model and retention factor correlation plots for varied solutes are used to study the retention mechanism on XBridge C₈, XBridge Phenyl and XTerra Phenyl stationary phases with acetonitrile–water and methanol–water mobile phases containing from 10 to 70% (v/v) organic solvent. These stationary phases are compared with XBridge C₁₈ and XBridge Shield RP₁₈ characterized in an earlier report using the same protocol. The XBridge stationary phases are all quite similar in their retention properties with larger difference in absolute retention explained by differences in cohesion and the phase ratio, mainly, and smaller changes in relative retention (selectivity) by the differences in individual system constants and their variation with mobile phase type and composition. None of the XBridge stationary phases are selectivity equivalent but XBridge C₁₈ and XBridge Shield RP₁₈ have similar separation properties, likewise so do XBridge C₈ and XBridge Phenyl, while the differences between the two groups of two stationary phases is greater than the difference within either group. The limited range of changes in selectivity is demonstrated by the high coefficient of determination (>0.98) for plots of the retention factors for varied compounds on the different XBridge phases with the same mobile phase composition.     

Evaluation of the separation characteristics of application-specific (pesticides and dioxins) open-tubular columns for gas chromatography

The solvation parameter model is used to characterize the retention properties of four application-specific open-tubular columns (Rtx-CLPesticides, Rtx-OPPesticides, Rtx-Dioxin and Rtx-Dioxin2) at five equally spaced temperatures over the range 60-140 degrees C. Cluster analysis is used to compare the system constants to a database of forty open-tubular columns characterized according to the same method. System constants differences and retention factor correlation plots are then used to determine selectivity differences between the application-specific columns and their nearest neighbors identified by cluster analysis. The Rtx-CLPesticides and Rtx-OPPesticides columns are shown to belong to the selectivity group containing poly(dimethylmethyltrifluoroprpylsiloxane) stationary phases with Rtx-OPPesticides having a similar selectivity to a poly(dimethylmethyltrifluoropropylsiloxane) stationary phase containing 20% methyltrifluoropropylsiloxane monomer (DB-200) and Rtx-CLPesticides separation properties for a stationary phase containing less than 20% methyltrifluoropropylsiloxane monomer. The Rtx-Dioxin and Rtx-Dioxin2 columns are located in the selectivity group dominated by the poly(dimethyldiphenylsiloxane) stationary phases containing less than 20% diphenylsiloxane monomer. The Rtx-Dioxin and Rtx-Dioxin2 columns are shown to be selectivity equivalent to a (5% phenyl) carborane-siloxane copolymer stationary phase (Stx-500) and a second generation silarylene-siloxane copolymer stationary phase containing dimethylsiloxane and diphenylsiloxane monomers (DB-XLB), respectively.     

Soft due window assignment and scheduling of unit-time jobs on parallel machines

We study problems of scheduling n unit-time jobs on m identical parallel machines, in which a common due window has to be assigned to all jobs. If a job is completed within the due window, then no scheduling cost incurs. Otherwise, a job-dependent earliness or tardiness cost incurs. The job completion times, the due window location and the size are integer valued decision variables. The objective is to find a job schedule as well as the location and the size of the due window such that a weighted sum or maximum of costs associated with job earliness, job tardiness and due window location and size is minimized. We establish properties of optimal solutions of these min-sum and min-max problems and reduce them to min-sum (traditional) or min-max (bottleneck) assignment problems solvable in O(n ⁵/m ²) and O(n ^4.5log^0.5 n/m ²) time, respectively. More efficient solution procedures are given for the case in which the due window size cost does not exceed the due window start time cost, the single machine case, the case of proportional earliness and tardiness costs and the case of equal earliness and tardiness costs.     

Application of the Free Tangent Law in Quantification of Household Satisfaction from Durable Consumer Goods

This paper presents a postulate for a new approach in the measurement of households’ satisfaction from durable consumer goods, based on a modified inflation expectation measurement method used in survey research. The authors examine the application of a three-step qualitative evaluation, followed by the quantification of responses using a modified Carlson and Parkin method adopted in the context of the free tangent law.     

Chemical structure of polycaproamide obtained by anionic polymerization of caprolactam in solvent

Chemical structure of polycaproamide (nylon-6), obtained by low temperature anionic polymerization of caprolactam in the presence of the sodium salt of caprolactam and carbon dioxide in solvent has been investigated. The polymer formed in a heterogeneous system, depending upon its final treatment, is terminated with amino and cyclic lactam groups or, after hydrolysis, with amino and carboxyl group. i.r. Spectra and acid-base titrations have shown the presence of amino and carboxyl groups in macromolecules. The contents of basic and acidic groups in the final product and its fractions were determined by potentiometric titrations. Number-average molecular weights of samples based on the results of titrations were compared with the results of osmotic measurements. Equal numbers of basic and acid groups in the polymer suggests that macromolecules are terminated at one end by an amino group and at the other by a carboxyl group. The results indicate a linear and regular structure for the polycaproamide.     

Analysis and handling of bio-nanoparticles and environmental nanoparticles using electrostatic aerosol mobility

The successful application of differential mobility analysis for the characterization and manipulation of nanoparticles at atmospheric pressure has given rise to further development of this technique.The parallel differential mobility analyzer provides the possibility to simultaneously measure a size spectrum of nanoparticles and select a particular set of nanoparticles with a defined size for collection(as well as enrichment) and further orthogonal analysis(as for example electron microscopy,atomic force microscopy or mass spectrometry).Performing a high resolution measurement of electrical mobility diameters allows molecular weight determination of species with ultrahigh molecular masses in the mega Dalton range(e.g.protein complexes).The precise size measurement of the human rhinovirus has confirmed the potential of this technique to analyze even intact infectious human-pathogenic viruses. Moreover,the real-time measurement of nanoparticle occurrence in an urban environment confirms the versatility of the method presented here and its applicability also in other areas of importance.     

Criterion of multiple collisions in a simple mechanical system with viscous damping and dry friction

A criterion has been formulated for single and multiple central collisions of two rigid bodies of a simple mechanical system that can be either conservative or non-conservative. Analytical discussion has been confined to investigations of the possibility of appearance of four successive collisions between the bodies with damping neglected and two collisions when viscous damping and Coulomb dry friction are considered.     

On the equilibrium nature of thermodesorption processes. TPD-NH3 studies of surface acidity of Ni/MgO-Al2O3 catalysts

This article deals with a quantitative analysis of thermodesorption spectra of ammonia: a technique commonly applied to study the surface acidity of solids. The method used for determination of adsorption energy distributions of ammonia is the same as that published recently for the case of hydrogen thermodesorption (Panczyk, T.; et al. Langmuir 2005, 21, 7311). The developed theoretical expression describing the thermodesorption process is based on the statistical rate theory (SRT) and its analysis leads to the conclusion that majority of thermodesorption processes, carried out under flow conditions, are in fact quasi-equilibrium ones. Similar conclusion has already been drawn by some authors applying the classical absolute rate theory (ART) for analysis of thermodesorption data. This conclusion has important practical consequences. Namely, it greatly simplifies the quantitative analysis of thermodesorption processes since there is no need to use any kinetic approaches to that purpose. The quantitative analysis of thermodesorption spectra can thus be based on commonly accepted relations following from equilibrium thermodynamics. It is worth noting that in quasiequilibrium conditions either the SRT or the ART lead to this same expression with only a slightly different meaning of some constants. Thus, in quasiequilibrium conditions there is no need to decide which theoretical approach should be applied. As an illustration, the ammonia thermodesorption spectra from the modified nickel catalysts are analyzed. The catalysts were prepared by the coprecipitation method and differ by the amount of MgO and NiO, whereas the amount of Al(2)O(3) is constant and equals 30%. It was stated that the presence of MgO reduces the number of acid centers corresponding to high values of ammonia adsorption energy.     

Biosynthesis of lovastatin analogs with a broadly specific acyltransferase

The natural product lovastatin and its semisynthetic, more effective derivative, simvastatin, are important drugs for the treatment of hypercholesterolemia. Here, we report the biochemical characterization of a dedicated acyltransferase, LovD, encoded in the lovastatin biosynthetic pathway. We demonstrate that LovD has broad substrate specificity towards the acyl carrier, the acyl substrate, and the decalin acyl acceptor. LovD can efficiently catalyze the acyl transfer from coenzyme A thioesters or N-acetylcysteamine (SNAC) thioesters to monacolin J. When alpha-dimethylbutyryl-SNAC was used as the acyl donor, LovD was able to convert monacolin J and 6-hydroxyl-6-desmethylmonacolin J into simvastatin and huvastatin, respectively. Using the Escherichia coli LovD overexpression strain as a whole-cell biocatalyst, preparative amounts of simvastatin were synthesized in a single fermentation step. Our results demonstrate LovD is an attractive enzyme for engineered biosynthesis of pharmaceutically important cholesterol-lowering drugs.