Two-dimensional gas chromatography analysis of components in fuel and fuel additives using a simplified heart-cutting GC system

Multidimensional gas chromatography (MGC) using heart cutting is an old idea that can benefit from the performance of modern instruments and capillary columns to provide fast, reliable separation of target analytes from complex sample matrices. A simplified heart-cutting switch is described that uses these improvements to provide very narrow precise heart cuts between columns of different selectivity. This system is used to analyze ppm levels of 4,6-dimethydibenzothiophene in diesel fuel using a standard flame ionization detector instead of a complex sulfur-selective detector. MGC systems also offer the possibility of faster analysis speed by using two short columns of different selectivity instead of very long columns to resolve compounds from complex matrices. The analysis of alcohols in denatured fuel ethanol using the MGC system is performed over six times faster than the standard American Society for Testing and Materials methodology.     

Equal-time second-order moments of a harmonic oscillator with stochastic frequency and driving force

Using a simple matrix method, we have obtained exact second-order equilibrium moments for a linearly damped harmonic oscillator with a fluctuating frequency (t) and driven by a fluctuating forcef(t). We have assumed each of the fluctuating quantities to be delta-correlated. We demonstrate that the final answers are identical whetherf(t) and (t) are statistically independent or delta-correlated. We have also established the region of parameter space in which the oscillator is energetically stable. The results are shown to be completely determined by the coefficients of the first and second cumulants of the fluctuations.Supported in part by the Office of Naval Research, NSF Grant # CHE 78-21460, and by a grant from Charles and Renée Taubman.     

A computer simulation of XPD degradation due to multipath propagation

A computer program has been constructed to study degradation in Xpd (cross-polarization discrimination) due to multipath propagation. The geometric optics limit is used. The program will accept any analytic index of refraction including those with discontinuities in n or ?n. Antenna effects are included and all reflected, refracted, direct and ground reflected rays can be considered. Using a realistic index of refraction and realistic discontinuity, we have found that, after the antenna pattern, the major influence on Xpd is interference between the ground-reflected wave and the direct wave. There is also significant interference due to the waves reflected from the refractive index discontinuity which may arrive without ground reflection or may be reflected once or twice from the ground.     

Dynamic Characteristics of the Renal Arterial Microvasculature of the Dog Obtained by Simulation

To develop a measurement technique for microvascular stiffness and resistance, an iterative algorithm (NOLESQ) combining steepest descent and Taylor series (Gauss-Newton) methods was used to estimate model parameters to provide a minimal least squares error at 5-ms intervals of the predicted blood flow to the measured flow. Renal arterial pressure and flow data of high dynamic and static accuracy were used. The data were in blocks of 2500 values each (12.5-s samples). A reasonable fit was obtained with only three parameters: arterial conductance, compliance, and a steady pressure at or just beyond the glomerulae. Other models, incorporating large artery resistance, blood mass, pressure dependent conductance, and pressure dependent compliance were studied. Criteria for acceptance were reasonable values from prior knowledge of biological materials, similarity of parameter values between consecutive samples and between animals, convergence, and error of fit reduced to the noise level of the input data.     

Basic Derivations for Subarrangements of Coxeter Arrangements

We prove that various subarrangements of Coxeter hyperplane arrangements are free. We do this by exhibiting a basis for the corresponding module of derivations. Our method uses a theorem of Saito [24] and Terao [30] which checks for a basis using certain divisibility and determinantal criteria. As a corollary, we find the roots of the characteristic polynomials for these arrangements, since they are just one more than the degrees in any basis of the module. We will also see some interesting applications of symmetric and supersymmetric functions along the way.     

An improved branch and bound algorithm for minimum concave cost network flow problems

This paper formulates the minimum concave cost network flow (MCCNF) problem as a mixed integer program and solves this program using a new branch and bound algorithm. The algorithm combines Driebeek's up and down penalties with a new technique referred to as the simple bound improvement (SBI) procedure. An efficient numerical method for the SBI procedure is described and computational results are presented which show that the SBI procedure reduces both the in-core storage and the CPU time required to solve the MCCNF problem. In fact, for problems with over 200 binary decision variables, the SBI procedure reduced the in-core storage by more than one-third and the CPU time by more than 40 percent.     

Consensus scoring criteria for improving enrichment in virtual screening

MOTIVATION: Virtual screening of molecular compound libraries is a potentially powerful and inexpensive method for the discovery of novel lead compounds for drug development. The major weakness of virtual screening-the inability to consistently identify true positives (leads)-is likely due to our incomplete understanding of the chemistry involved in ligand binding and the subsequently imprecise scoring algorithms. It has been demonstrated that combining multiple scoring functions (consensus scoring) improves the enrichment of true positives. Previous efforts at consensus scoring have largely focused on empirical results, but they have yet to provide a theoretical analysis that gives insight into real features of combinations and data fusion for virtual screening. RESULTS: We demonstrate that combining multiple scoring functions improves the enrichment of true positives only if (a) each of the individual scoring functions has relatively high performance and (b) the individual scoring functions are distinctive. Notably, these two prediction variables are previously established criteria for the performance of data fusion approaches using either rank or score combinations. This work, thus, establishes a potential theoretical basis for the probable success of data fusion approaches to improve yields in in silico screening experiments. Furthermore, it is similarly established that the second criterion (b) can, in at least some cases, be functionally defined as the area between the rank versus score plots generated by the two (or more) algorithms. Because rank-score plots are independent of the performance of the individual scoring function, this establishes a second theoretically defined approach to determining the likely success of combining data from different predictive algorithms. This approach is, thus, useful in practical settings in the virtual screening process when the performance of at least two individual scoring functions (such as in criterion a) can be estimated as having a high likelihood of having high performance, even if no training sets are available. We provide initial validation of this theoretical approach using data from five scoring systems with two evolutionary docking algorithms on four targets, thymidine kinase, human dihydrofolate reductase, and estrogen receptors of antagonists and agonists. Our procedure is computationally efficient, able to adapt to different situations, and scalable to a large number of compounds as well as to a greater number of combinations. Results of the experiment show a fairly significant improvement (vs single algorithms) in several measures of scoring quality, specifically "goodness-of-hit" scores, false positive rates, and "enrichment". This approach (available online at http://gemdock.life. nctu.edu.tw/dock/download.php) has practical utility for cases where the basic tools are known or believed to be generally applicable, but where specific training sets are absent.     

Synergistic catalysis of anthrone Diels-Alder reactions

[reaction: see text] The combination of an amine base and Lewis acid (Li+) leads to synergistic catalysis of the Diels-Alder reaction of anthrone with methyl crotonate and some other less reactive dienophiles. These cycloadditions either do not occur with the individual catalysts used separately or they are greatly accelerated by the combined catalysts. DMSO solvent allows the use of LiCl as the Lewis acid source and can provide greater control of subsequent conversion to Michael adduct.     

Syntheses and crystal structures of ferrocenyl derivatives of biphenyl

Arylation of ferrocene with substituted biphenyldiazonium tetrafluoroborates afforded 4-bromo-, 4-nitro-, and 4-cyano-4"-ferrocenylbiphenyls. 4-Nitro- and 4-cyano-4"-ferrocenylbiphenyls were studied by X-ray diffraction analysis. In the crystals of 4-nitro-4"-ferrocenylbiphenyl, the molecules are linked via stacking - interactions.     

Solvation of Calix[4]arene-bis-crown-6 Molecules

Single crystal X-ray diffraction analyses of calix[4]arene-bis-crown-6 (1) and calix[4]arene-bis-benzocrown-61,2-dichloroethane (2) are reported. Thesestructures offer an opportunity to study the conformation of calix[4]arene-bis-crown-6 molecules in the absence of solvent, and when very weakly solvated. These structures exhibit different conformations of the crown ring, and limited flexibility of the calix[4]arene,but do not show solvent or crown inclusion in the calix[4]arene cavity. Comparisons to similar structures are made, and the implications for cesium binding are discussed.