Cross-validation and evaluation of the performance of methods for the elemental analysis of forensic glass by μ-XRF, ICP-MS, and LA-ICP-MS

Elemental analysis of glass was conducted by 16 forensic science laboratories, providing a direct comparison between three analytical methods [micro-x-ray fluorescence spectroscopy (μ-XRF), solution analysis using inductively coupled plasma mass spectrometry (ICP-MS), and laser ablation inductively coupled plasma mass spectrometry]. Interlaboratory studies using glass standard reference materials and other glass samples were designed to (a) evaluate the analytical performance between different laboratories using the same method, (b) evaluate the analytical performance of the different methods, (c) evaluate the capabilities of the methods to correctly associate glass that originated from the same source and to correctly discriminate glass samples that do not share the same source, and (d) standardize the methods of analysis and interpretation of results. Reference materials NIST 612, NIST 1831, FGS 1, and FGS 2 were employed to cross-validate these sensitive techniques and to optimize and standardize the analytical protocols. The resulting figures of merit for the ICP-MS methods include repeatability better than 5 % RSD, reproducibility between laboratories better than 10 % RSD, bias better than 10 %, and limits of detection between 0.03 and 9 μg g^?1 for the majority of the elements monitored. The figures of merit for the μ-XRF methods include repeatability better than 11 % RSD, reproducibility between laboratories after normalization of the data better than 16 % RSD, and limits of detection between 5.8 and 7,400 μg g^?1. The results from this study also compare the analytical performance of different forensic science laboratories conducting elemental analysis of glass evidence fragments using the three analytical methods.

Quantitative analysis of amylopectin unit chains by means of high-performance anion-exchange chromatography with pulsed amperometric detection

High-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC–PAD) is a widely used technique to study the chain length distribution of amylopectin. The chromatograms, however, do not directly reflect this distribution, since the PAD response changes with the degree of polymerization. In this study, waxy maize starch was debranched and fractionated on a Bio-Gel P-6 column and the response factors for maltosaccharides with DP 3–65 were determined. The detector response per μg glucan chains was shown to decrease considerably for DP 3–7 while the curve leveled out for DP larger than 15.     

Electronic structure and spectrum of UO2 2+ and UO2Cl4 2-

A theoretical study is presented of the electronic spectra of the UO(2) (2+) and UO(2)Cl(4) (2-) ions, based on multiconfigurational perturbation theory (CASSCF/CASPT2), combined with a recently developed method to treat spin-orbit coupling [P.-A. Malmqvist et al., Chem. Phys. Lett. 357, 230 (2002); B. O. Roos and P.-A. Malmqvist, Phys. Chem. Chem. Phys. 6, 2919 (2004)]. The results are compared to the experimental spectroscopic data obtained for uranyl ions in Cs(2)UO(2)Cl(4) crystals from Denning [Struct. Bonding (Berlin) 79, 215 (1992)] and to previous theoretical calculations performed using a combined configuration-interaction spin-orbit treatment [Z. Zhang and R. M. Pitzer, J. Phys. Chem. A 103, 6880 (1999); S. Matsika and R. M. Pitzer, J. Phys. Chem. A. 105, 637 (2001)]. As opposed to the latter results, the calculations performed in this work point to a significant effect of the weakly bound equatorial chlorine ligands on the excitation energies.     

Solution structure of a DNA duplex containing a guanine-difluorotoluene pair: a wobble pair without hydrogen bonding?

The incorporation of synthetic nucleoside analogues into DNA duplexes provides a unique opportunity to probe both structure and function of nucleic acids. We used 1H and 19F NMR and molecular dynamics calculations to determine the solution structures of two similar DNA decamer duplexes, one containing a central G-T mismatched or "wobble" base pair, and one in which the thymine in this base pair is replaced by difluorotoluene (a thymine isostere) creating a G-F pair. Here, we show that the non-hydrogen-bonding G-F pair stacks relatively well into the helix and that the distortions caused by each non-Watson-Crick G-T or G-F base pair are quite localized to a three base pair site around the mismatch. A detailed structural analysis reveals that the absence of hydrogen bonding introduces more dynamic motion into the G-F pair relative to G-T and permits the G-F pair to exhibit stacking and conformational features characteristic of both a Watson-Crick base pair (on the guanine containing strand) and a wobble base pair (on the strand containing the difluorotoluene). We used these results to posit a rationale for recognition and repair of mismatch sites in DNA.     

Dianions of 7,7,8,8-tetracyano-p-quinodimethane and perfluorinated tetracyanoquinodimethane: information on excited states from lifetime measurements in an electrostatic storage ring and optical absorption spectroscopy

We have developed an experimental technique that allows us to study the physics of short lived molecular dianions in the gas phase. It is based on the formation of monoanions via electrospray ionization, acceleration of these ions to keV energies, and subsequent electron capture in a sodium vapor cell. The dianions are stored in an electrostatic ion storage ring in which they circulate with revolution times on the order of 100 micros. This enables lifetime studies in a time regime covering five orders of magnitude, 10(-5)-1 s. We have produced dianions of 7,7,8,8-tetracyano-p-quinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (TCNQ-F(4)) and measured their lifetimes with respect to electron autodetachment. Our data indicate that most of the dianions were initially formed in electronically excited states in the electron transfer process. Two levels of excitation were identified by spectroscopy on the dianion of TCNQ-F(4), and the absorption spectrum was compared with spectra obtained from spectroelectrochemistry of TCNQ-F(4) in acetonitrile solution.     

Electronic spectrum of UO2(2+) and [UO2Cl4]2- calculated with time-dependent density functional theory

The electronic spectra of UO(2) (2+) and [UO(2)Cl(4)](2-) are calculated with a recently proposed relativistic time-dependent density functional theory method based on the two-component zeroth-order regular approximation for the inclusion of spin-orbit coupling and a noncollinear exchange-correlation functional. All excitations out of the bonding sigma(u) (+) orbital into the nonbonding delta(u) or phi(u) orbitals for UO(2) (2+) and the corresponding excitations for [UO(2)Cl(4)](2-) are considered. Scalar relativistic vertical excitation energies are compared to values from previous calculations with the CASPT2 method. Two-component adiabatic excitation energies, U-O equilibrium distances, and symmetric stretching frequencies are compared to CASPT2 and combined configuration-interaction and spin-orbit coupling results, as well as to experimental data. The composition of the excited states in terms of the spin-orbit free states is analyzed. The results point to a significant effect of the chlorine ligands on the electronic spectrum, thereby confirming the CASPT2 results: The excitation energies are shifted and a different luminescent state is found.     

Derivation of a 3D pharmacophore model for the angiotensin-II site one receptor

Summary A systematic search has been used to derive a hypothesis for the receptor-bound conformation of A-II antagonists at the AT₁ receptor. The validity of the pharmacophore hypothesis has been tested using CoMFA, which included 50 diverse A-II antagonists, spanning four orders of magnitude in activity. The resulting cross-validated R² of 0.64 (conventional R² of 0.76) is indicative of a good predictive model of activity, and has been used to estimate potency for a variety of non-peptidyl antagonists. The structural model for the non-peptide has been compared with respect to the natural substrate, A-II, by generating peptide to non-peptide overlays.     

Solute- and temperature-responsive "smart" grafts and supported membranes formed by covalent layer-by-layer assembly

Polymers like poly(N-isopropylacrylamide) (PNIPAM) exhibit lower critical solution temperature (LCST) behavior. A variety of reports have shown that brush grafts of PNIPAM on surfaces exhibit similar temperature responsiveness. We recently described an alternative synthetic approach to such surfaces that affords surfaces with similar LCST-like behavior. We also noted how such surfaces' wettability can change in response to the identity and concentration of solutes. Here we show that this synthetic procedure can be extended to glass surfaces and to more complex surfaces present in porous glass frits. Functionalized glass surfaces exhibit solute-dependent wetting behavior analogous to that previously reported. We further show that the resulting responsive nanocomposite grafts on such frits exhibit "smart" responsive permeability with a greater than 1000-fold difference in permeability to water versus aqueous solutions of sodium sulfate. This "smart" permeability is ascribed to the solute-dependent wettability behavior of the responsive PNIPAM component of the nanocomposite graft, which is sensitive both to the identity and concentration of the solute anion and to temperature.     

Insights into the metathesis reaction involving M-M,C-C,and M-C triple bonds from computations employing density functional theory on model compounds M2(OH)6 and M2(SH)6, where M = Mo and W

Calculations employing density functional theory (Gaussian 98, B3LYP, LANL2DZ, 6-31G) have been undertaken to interrogate the factors influencing the metathesis reaction involving M-M, C-C, and M-C triple bonds for the model compounds M(2)(EH)(6), M(2)(EH)(6)(mu-C(2)H(2)), and [(HE)(3)M(tbd1;CH)](2), where M = Mo, W and E = O, S. Whereas in all cases the ethyne adducts are predicted to be enthalpically favored in the reactions between M(2)(EH)(6) compounds and ethyne, only when M = W and E = O is the alkylidyne product [(HO)(3)W(tbd1;CH)](2) predicted to be more stable than the alkyne adduct. For the reaction M(2)(EH)(6)(mu-C(2)H(2)) --> [(HE)(3)M(tbd1;CH)](2), the deltaG degrees values (kcal mol(-)(1)) are -6 (M = W, E = O), +5 (M = Mo, E = O), +18 (M = W, E = S), and +21 (M = Mo, E = S) and the free energies of activation are calculated to be deltaG() = +19 kcal mol(-)(1) (M = W, E = O) and +34 kcal mol(-)(1) (M = Mo, E = O), where the transition state involves an asymmetric bridged structure M(2)(OH)(4)(mu-OH)(2)(CH)(mu-CH) in which the C-C bond has broken; C.C = 1.89 and 1.98 A for W and Mo, respectively. These results are discussed in terms of the experimental observations of the reactions involving ethyne and the symmetrically substituted alkynes (RCCR, where R = Me, Et) with M(2)(O(t)()Bu)(6) and M(2)(O(t)()Bu)(2)(S(t)()Bu)(4) compounds, where M = Mo, W.     

Copper corroles: the question of noninnocence

In this paper, the results are presented from a comparative study of the electronic and geometric structure of copper corroles by means of either density functional theory (DFT, using both pure and hybrid functionals) and multiconfigurational ab initio methods, starting from either a complete active space (CASSCF) or restricted active space (RASSCF) reference wave function and including dynamic correlation by means of second-order perturbation theory (CASPT2/RASPT2). DFT geometry optimizations were performed for the lowest singlet and triplet states of copper corrole, both unsubstituted and meso-substituted with three phenyl groups. The effect of saddling on the electronic structure was investigated by comparing the results obtained for planar (C(2v)) and saddled (C(2)) structures. With DFT, the origin of the saddling distortion is found to be dependent on the applied functional: covalent Cu 3d-corrole π interactions with pure functionals (BP86, OLYP), antiferromagnetic exchange coupling between an electron in the corrolate (C(2)) b type π orbital, and an unpaired Cu(II) 3d electron with hybrid functionals (B3LYP, PBE0). The CASPT2 results essentially confirm the suggestion from the hybrid functionals that copper corroles are noninnocent, although the contribution of diradical character to the copper-corrole bond is found to be limited to 50% or less. The lowest triplet state is calculated at 0-10 kcal/mol and conform with the experimental observation (variable temperature NMR) that this state should be thermally accessible.