Femtosecond laser time-of-flight mass spectrometry of labile molecular analytes: laser-desorbed nitro-aromatic molecules.

Femtosecond laser time-of-flight mass spectra of solid samples of trinitrobenzene (TNB), trinitrotoluene (TNT) and trinitrophenol (TNP) have been recorded. Desorption of the solid samples was enacted by the fourth harmonic output (266 nm) of a 5 ns Nd:YAG laser. Subsequent femtosecond post-ionisation of the plume of neutral molecules was achieved using 800 nm laser pulses of 80 fs duration. Mass spectra have been recorded for desorption laser intensities from 2-6 x 10(9) W cm(-2) with ionisation laser intensities between 2 x 10(14) and 6 x 10(15) W cm(-2). Femtosecond laser ionisation has been shown to be capable of generating precursor and characteristic high-mass fragment ions for labile nitro-aromatic molecules commonly used in high-explosive materials. This feature is critical in the future development of femtosecond laser-based analytical instruments that can be used for complex molecular identification and quantitative analysis of environmentally important labile molecules. Furthermore, a comparison of femtosecond post-ionisation mass spectra with standard 70 eV electron impact data has revealed similarities in the spectra and hence the fragmentation processes.     

Syntheses and stereochemistry of 4-hydroxy tetrahydroisoquinolines in the 1-benzyl and 1-phenethyl series. Efficient routes to isopavines and homoisopavines

The title compounds have been prepared in high yield $\text{via}$ cyclisation of urethane acetals.     

A 768-lane microfabricated system for high-throughput DNA sequencing

A 768-lane DNA sequencing system based on microfluidic plates has been designed as a near-term successor to 96-lane capillary arrays. Electrophoretic separations are implemented for the first time in large-format (25 cm x 50 cm) microdevices, with the objective of proving realistic read length, parallelism, and the scaled sample requirements for long-read de novo sequencing. Two 384-lane plates are alternatively cycled between electrophoresis and regeneration via a robotic pipettor. A total of greater than 172000 bases, 99% accuracy (corresponding to quality score 20) is achieved for each iteration of a 384 lane plate. At current operating conditions, this implies a system throughput exceeding 4 megabases of raw sequence (Phred 20) per day on the new platform. Standard operation is at "1/32x" Sanger chemistry, equal to typical genome center operation on mature capillary array machines, and a 16-fold improvement in scaling relative to previous microfabricated devices. Experiments provide evidence that sample concentration can be further reduced to 1/256x Sanger chemistry in the microdevice. Life-testing indicates a usable life of >150 hours (more than 50 runs) for the 384 lane plates. The combined advances, particularly those in read length and sample requirement, directly address the cost model requirements for adaptation of the new technology as the next step beyond capillary array instruments.     

Chain length dependence of the thermodynamic properties of linear and cyclic alkanes and polymers

The specific heat capacity was measured with step-scan differential scanning calorimetry for linear alkanes from pentane (C(5)H(12)) to nonadecane (C(19)H(40)), for several cyclic alkanes, for linear and cyclic polyethylenes, and for a linear and a cyclic polystyrene. For the linear alkanes, the specific heat capacity in the equilibrium liquid state decreases as chain length increases; above a carbon number N of 10 (decane) the specific heat asymptotes to a constant value. For the cyclic alkanes, the heat capacity in the equilibrium liquid state is lower than that of the corresponding linear chains and increases with increasing chain length. At high enough molecular weights, the heat capacities of cyclic and linear molecules are expected to be equal, and this is found to be the case for the polyethylenes and polystyrenes studied. In addition, the thermal properties of the solid-liquid and the solid-solid transitions are examined for the linear and cyclic alkanes; solid-solid transitions are observed only in the odd-numbered alkanes. The thermal expansion coefficients and the specific volumes of the linear and cyclic alkanes are also calculated from literature data and compared with the trends in the specific heats.     

E-(hydroxyimino)(hydroxymethoxyphosphinyl)acetic acid: Synthesis and pH-dependent fragmentation

In contrast to both its parent “troika” acid (E-1, a phosphorylating agent at pH 7 and 25 °C) and its C-methyl isomer (E-2, which is stable at both acidic and neutral pH), (E)-(hydroxyimino)(hydroxymethoxyphosphinyl)acetic acid E-3 was unreactive at pH 7 and 25 °C but at pH 1.5 fragmented to methyl phosphate 10 (15%) and methyl phosphorocyanidate 11 (85%). The minor product is consistent with solvent phosphorylalion, the reaction exclusively observed with E-1. The non-phosphorylating fragmentation pathway is proposed to involve a preliminary E→ Z isomerizalion of 3 prior to C-C_β cleavage. Dual fragmentation pathways were also detected (³¹P NMR) when the DCHA⁺ salt of E-3 (E-9) was heated in acetonitrile or EtOH; in addition to phosphorylation products (16–19%), 11 was formed (81–84%). Reaction of E-9 in refluxing EtOH:t-BuOH (1:1) showed low stereoselectivity in product formation (~3:1 ethyl methyl phosphate:t-butyl methyl phosphate), supporting a dissociative phosphorylation process.     

Simultaneous determination of fluoxetine and norfluoxetine enantiomers in biological samples by gas chromatography with electron-capture detection.

An electron-capture gas chromatographic procedure was developed for the simultaneous analysis of the enantiomers of fluoxetine and norfluoxetine. The assay involves basic extraction of these enantiomers from the biological samples, followed by their conversion to diastereoisomers using the chiral derivatizing reagent (S)-(-)-N-trifluoroacetylprolyl chloride. The method was utilized to detect and measure the quantity of these enantiomers in plasma and urine of patients and in liver and brain tissue of rats treated with (R,S)-fluoxetine.     

A high-performance multilane microdevice system designed for the DNA forensics laboratory

We report preliminary testing of "GeneTrack", an instrument designed for the specific application of multiplexed short tandem repeat (STR) DNA analysis. The system supports a glass microdevice with 16 lanes of 20 cm effective length and double-T cross injectors. A high-speed galvanometer-scanned four-color detector was specially designed to accommodate the high elution rates on the microdevice. All aspects of the system were carefully matched to practical crime lab requirements for rapid reproducible analysis of crime-scene DNA evidence in conjunction with the United States DNA database (CODIS). Statistically significant studies demonstrate that an absolute, three-sigma, peak accuracy of 0.4-0.9 base pair (bp) can be achieved for the CODIS 13-locus multiplex, utilizing a single channel per sample. Only 0.5 microL of PCR product is needed per lane, a significant reduction in the consumption of costly chemicals in comparison to commercial capillary machines. The instrument is also designed to address problems in temperature-dependent decalibration and environmental sensitivity, which are weaknesses of the commercial capillary machines for the forensics application.     

Interaction mechanism of some alkyl iodides with femtosecond laser pulses

The interaction of 1-iodopropane, 2-iodopropane, 1-iodobutane, 2-iodobutane, and 1-iodopentane with (5 x 10(13-)5 x 10(15) W/cm2) femtosecond laser fields is studied by means of a time-of-flight mass spectrometer. It is found that multiphoton ionization (MPI) and field ionization (FI) processes are involved in the molecular ionization. The contribution of these processes can be distinguished using the peak profile of the ions in the mass spectra. Thus, from the mass spectra of 2-iodoropane and 2-iodobutane, it is concluded that MPI processes are taking place even for Keldysh parameter values gamma approximately 0.3. The field ionization process depends on the characteristics of the molecular binding potential well and leads to an asymmetric charge distribution of the transient multiply charged parent ions. In the case of 1-iodobutane, the MPI processes lead to a stable doubly charged parent ion production with a laser intensity threshold higher than that found for I2+ ions. In addition, the isomers studied exhibit distinct differences in their mass spectra and their origin is discussed in detail.     

Shear stress relaxation and physical aging study on simple glass-forming materials

Relaxation and aging behaviors in three supercooled liquids: m-toluidine, glycerol, and sucrose benzoate have been studied by shear stress relaxation experiments in the time domain above and below their nominal glass transition temperatures. For the equilibrium state, the current study provides new data on the behavior of organic complex fluids. The shape of the relaxation function as characterized by the stretching exponent beta is discussed considering that a time-temperature master curve can be constructed even though the beta's for the individual response curves at each temperature vary systematically. In the nonequilibrium state, isothermal physical aging experiments at different glassy structures reveal that the effect of the aging process on the mechanical shear relaxation in these simple glass formers is similar to that observed in polymeric and other systems. Departure from the Vogel-Fulcher-Tamman behavior after the samples have aged back to equilibrium in the glassy state is observed for m-toluidine and, less strongly, for glycerol but not for sucrose benzoate. An inherent structure-based energy landscape concept is briefly discussed to account for the slow dynamics during the physical aging process.     

Statistical analysis of C–H activation by oxo complexes supports diverse thermodynamic control over reactivity

Transition metal oxo species are key intermediates for the activation of strong C–H bonds. As such, there has been interest in understanding which structural or electronic parameters of metal oxo complexes determine their reactivity. Factors such as ground state thermodynamics, spin state, steric environment, oxygen radical character, and asynchronicity have all been cited as key contributors, yet there is no consensus on when each of these parameters is significant or the relative magnitude of their effects. Herein, we present a thorough statistical analysis of parameters that have been proposed to influence transition metal oxo mediated C–H activation. We used density functional theory (DFT) to compute parameters for transition metal oxo complexes and analyzed their ability to explain and predict an extensive data set of experimentally determined reaction barriers. We found that, in general, only thermodynamic parameters play a statistically significant role. Notably, however, there are independent and significant contributions from the oxidation potential and basicity of the oxo complexes which suggest a more complicated thermodynamic picture than what has been shown previously.

Statistical analysis of transition metal oxo mediated C–H activation indicates that thermodynamic factors dictate reactivity and that the energetics of proton and electron transfer have effects independent of the free energy of the reaction.