A unified approach for the measurement of individual or total volatile organic sulfur compounds in hydrocarbon matrices by dual-plasma chemiluminescence detector and low thermal mass gas chromatography

A novel gas chromatographic technique has been developed, offering dual-analytical capability of either speciation or rapid measurement of total volatile sulfur compounds in hydrocarbon matrices using the same hardware. The technique employs a pressurized liquid injection system for the delivery of volatile liquid hydrocarbons, low thermal mass gas chromatography, and a dual-plasma sulfur chemiluminescence detector to enable this dual capability with a high degree of sensitivity and selectivity towards sulfur-containing compounds. Using the technique described, a detection limit in the range of 20 ppb sulfur and less than 30 s analysis is attained. Response is linear over five orders of magnitude, with a high degree of repeatability.     

Ground-and Excited-State Characterization of an Electrostatic Complex between Tetrakis-(4-Sulfonatophenyl)porphyrin and 16-Pyrimidinium Crown-4

Abstract— We report the formation of an electrostatic complex between (16-pyrimidinium crown-4)tetranitrate (16PC4) and tetrakis-(4-sulfonatophenyl)porphyrin (4SP) in aqueous solution. Ground-state complex formation results in a red shift of the 4SP visible absorption bands and a decrease in absorbance of the Soret band. The equilibrium constant for complex formation (determined from optical titrations) is found to be (2.0 ± 0.2) × 10⁵ M ⁻¹. In addition, the data fit to an expression describing a 1:1 stoichiometry. Excitation of the complex results in quenching of both the excited singlet and triplet states of the associated porphyrin. The singlet-state lifetime decreases from 10 ns for the free porphyrin to 1.5 ns in the presence of 16PC4 at low solution ionic strengths. In addition, evidence is presented for triplet-state quenching within the complex with k _q= (1.1 ± 0.1) × 10⁴ s⁻¹. The mechanism of quenching is tentatively assigned to electron transfer from either the excited singlet or excited triplet state of the porphyrin to the ground state of the 16PC4.     

High field asymmetric waveform ion mobility spectrometry-mass spectrometry: an investigation of leucine enkephalin ions produced by electrospray ionization

High field asymmetric waveform ion mobility spectrometry (FAIMS) provides atmospheric pressure, room temperature, low-resolution separation of gas-phase ions. The FAIMS analyzer acts as an ion filter that can continuously transmit one type of ion, independent of m/z. The combination of FAIMS with electrospray ionization and mass spectrometry (ESI-FAIMS-MS) is a powerful technique and is used in this study to investigate the cluster ions of leucine enkephalin (YGGFL). Separation by FAIMS of leucine enkephalin ions having the same m/z (m/z 556.5), [M + H]⁺ and [2M + 2H]²⁺, was observed. In addition, four complex ions of leucine enkephalin, [2M + H]⁺, [4M + 2H]²⁺, [6M + 3H]³⁺, and [8M + 4H]⁴⁺, all having m/z 1112, were shown to be separated in FAIMS. Fragmentation of ions as the result of harsh conditions within the mass spectrometer interface (FAIMS-MS) was shown to provide similar information to that obtained from MS/MS experiments in conventional ESI-MS.     

Unit and proper bitolerance digraphs

In this paper we prove that the following statements about a directed graph G→ are equivalent. (1) G→ is a unit bitolerance digraph, (2) G→ is a proper bitolerance digraph, and (3) the digraph obtained by reversing all arc directions of G→ is an interval catch digraph (also known as a point-core digraph). This result combined with known algorithms for recognizing interval catch digraphs, gives the first known polynomial-time algorithm for recognizing a class of (bi)tolerance digraphs. © 1997 John Wiley & Sons, Inc.     

Rapid and sensitive gas chromatography–ion-trap tandem mass spectrometry method for the determination of tobacco-specific N-nitrosamines in secondhand smoke

Tobacco-specific nitrosamines (TSNAs) are some of the most potent carcinogens in tobacco and cigarette smoke. Accurate quantification of these chemicals is needed to help assess public health risks. We developed and validated a specific and sensitive method to measure four TSNAs adsorbed to model surfaces and secondhand smoke (SHS) particles using gas chromatography–ion-trap tandem mass spectrometry. In an 18-m³ room-sized chamber, a smoking machine generated realistic concentrations of SHS that were actively sampled on Teflon-coated fiber glass (TCFG) filters, and passively sampled on cellulose substrates. A simple solid–liquid extraction protocol using methanol as solvent was successfully applied to both substrates with recoveries ranging from 85 to 115%. For each TSNA, tandem MS parameters were optimized and the major fragmentation pathways were elucidated. The method showed excellent performance, with a linear dynamic range from 2 to 1000 ng mL⁻¹, low detection limits (S/N > 3) of 30–300 pg mL⁻¹ and precision with experimental errors below 10% for all compounds. Moreover, no interfering peaks were observed, indicating a high selectivity of MS/MS without the need for a sample clean-up step. This method provides a suitable analytical tool to detect and quantify traces of TSNA in indoor environments polluted with SHS.     

Stability of the parametrically excited damped inverted pendulum: theory and experiment

The parametrically driven, damped, inverted pendulum can be dynamically stabilized in particular regions of the parameter space. The impact of damping on dynamic stabilization can be stabilizing or destabilizing depending on the location in parameter space (i.e., drive frequency and amplitude). Floquet analysis and numerical simulations were used to determine the stable regions. An experiment was conducted that verifies the model. Physical explanations and simple bounding approximations are provided to summarize findings. The utility of the highly damped pendulum results are illustrated by drawing the analogy to dynamic stabilization of the Rayleigh-Taylor instability: it permits ready demonstration that dynamic stabilization is impossible in that system absent surface tension.     

Cyto-mechanoresponsive polyelectrolyte multilayer films

Cell adhesion processes take place through mechanotransduction mechanisms where stretching of proteins results in biological responses. In this work, we present the first cyto-mechanoresponsive surface that mimics such behavior by becoming cell-adhesive through exhibition of arginine-glycine-aspartic acid (RGD) adhesion peptides under stretching. This mechanoresponsive surface is based on polyelectrolyte multilayer films built on a silicone sheet and where RGD-grafted polyelectrolytes are embedded under antifouling phosphorylcholine-grafted polyelectrolytes. The stretching of this film induces an increase in fibroblast cell viability and adhesion.     

How Can Proteins Enter the Interior of a MOF? Investigation of Cytochrome c Translocation into a MOF Consisting of Mesoporous Cages with Microporous Windows

It has been demonstrated for the first time that the heme protein cytochrome c (Cyt c) can enter the interior of a MOF despite the larger molecular dimension of the protein relative to the access pore sizes. Mechanistic studies suggest that the Cyt c molecules must undergo a significant conformational change during translocation into the MOF interior through the relatively small nanopores.     

Hilbert reconstruction of phase-shifted second-harmonic holographic images

New techniques are presented that make phase-shifting holography viable for second-harmonic generation (SHG) holography with weak object fields. We developed an intrinsic phase shift calibration of SHG holograms, an algorithm that extracts the reference and object intensity directly from a set of phase-shifted holographic data, and a more robust phase-shifting holography reconstruction algorithm based on π-shifted hologram pairs that permits self-calibration of the phase shift and recovery of the complex field through a Hilbert transform.