Determination of hydrogen cyanide in air using mass amplification by heavy ligand replacement on a coated quartz piezoelectric crystal

Piezoelectric crystals coated with bis(pentan-2,4-dionato)nickel are used to detect the presence of hydrogen cyanide in the range 13–93 ppm (10^?4 mol mol^?1). The displacement of the heavy ligand pentan-2,4-dione by a light molecule (HCN) yields an increase in sensitivity, compared to simple absorption of the analyte (i.e., mass amplification). A continuous flow system is used at relative humidities of 40–92%. The lifetime of such a device is limited by hydrolysis. Reproducibility from coated-crystal to coated-crystal is poor, probably owing to inconsistencies in coating techniques.     

Applicability of gradient liquid chromatography with tandem mass spectrometry to the simultaneous screening for about 100 pesticides in crops

A method was developed for screening crops for a range of pesticide residues by liquid chromatography/tandem mass spectrometry (LC/MS/MS). A complete set of LC, electrospray ionization (ESI), and tandem MS acquisition parameters was established for the determination of 108 analytes; these parameters were used for the simultaneous acquisition of 98 analytes in the positive ESI mode and 10 analytes in an additional MS/MS method in the negative ESI mode. The entire procedure involves extraction of residues with methanol-water and partition into dichloromethane. The utility of the method is demonstrated by the analysis of crops of 5 matrix types (water-containing, acidic, dry, sugar-containing, and fatty). Of 108 pesticides/metabolites tested, 104 showed sufficient stability in most matrixes for determination by LC/MS/MS. These analytes belong to 20 chemical classes, which demonstrate the general applicability of the method for multiclass analysis. By using matrix-matched standards, 67 compounds could be determined in most matrixes with recoveries of 70-120% and a relative standard deviation of < or = 25% at the 0.01 mg/kg level.     

Bis(diethylamino)carbene and the mechanism of dimerisation for simple diaminocarbenes

Bis(diethylamino)carbene is kinetically stable to dimerization in THF at ambient temperature; dimer formed during carbene generation arises from reaction of the carbene with the precursor formamidinium ion; this is probably the commonest route to tetraaminoethene dimers, and in a related case the intermediate protonated tetraaminoethene can be observed by NMR.     

Effect of substituents on the T1 energy of trans-stilbenes

Oxygen-induced singlet—triplet absorption and electrochemiluminescence quenching experiments with substituted stilbenes indicate a small influence of monosubstitution or donor—acceptor disubstitution on the triplet energy. This implies a decrease of the S₁T₁ energy difference particularly in the case of donor—acceptor substituted stilbenes.     

Hydrogen bonding in supercritical tert-butanol assessed by vibrational spectroscopies and molecular-dynamics simulations

We have investigated the state of aggregation in supercritical tert-butanol (T = 523 K,0.05 < rho < 0.4 g cm(-3)) by means of vibrational spectroscopies (infrared and Raman) and molecular-dynamics (MD) simulations. A quantitative band shape analysis of the spectra associated with the OH stretching mode of tert-butanol has been done using activities computed by ab initio calculations on small clusters. This allows us to determine the degree of hydrogen bonding and populations of oligomers. These latter quantities have been derived from MD simulations and very consistent results are found with experiments. These results show that hydrogen bond still exist in supercritical tert-butanol and that the fluid mainly consists of oligomers smaller than tetramers.     

Substituentenabhängigkeit bei der direktentrans →cis-Photoisomerisierung von donator-substituierten 4′-Diphenylphosphinyl-trans-stilbenen

From the fluorescence quantum yield _f , the fluorescence decay time _f ^w and the quantum yield _tc of thetrans cis photoisomerization results the fraction of perpendicular singlet (perp-S₁) decaying to ground statetrans-stilbenes1. The values of the donor substituted 4-diphenylphosphinyl-trans-stilbenes1 in the solvents toluene andn-propanol are dependent on the donor substituent in1 and the solvent. It is concluded for1 that increasing polarity of the first excited singlet state (tr-S₁) promotes thetrans cis photoisomerization.

     

11H-Pyrido[3',2':4,5]pyrrolo[2,3-g]isoquinoléines (aza-7 ellipticines) substituées sur leur position 6

6-Dialkylaminoalkylamino substituted 11H-pyrido[3',2':4,5]pyrrolo[2,3-g]isoquinolines (7-aza ellipti-cines) were obtained by a six step synthesis starting from 2-chloro-3-nitro pyridine and 6-amino-5-methyl (and 5,8-dimethyl) isoquinoline-1-2H-ones already described. A brief survey of biological results shows that derivatives of this new heterocyclic ring system are less interesting than their 5H-pyrido(3',4':4,5]pyrrolo[2,3-g]isoquinolines (9-aza ellipticines) and pyrido[4,3-b]carbazoles (ellipticines) analogues.     

Fast multiresidue screening of 300 pesticides in water for human consumption by LC-MS/MS

The study tested the determination of 300 pesticides in mineral water at levels of 0.1 and 1.0 μg/L. Measurements were conducted by direct sample injection into a liquid chromatograph coupled to a tandem mass spectrometer without any sample enrichment and/or cleanup. Two separate injections enabled the recording of two transitions per analyte (600 selected reaction monitoring transitions in total). For 285 analytes the sensitivity of direct sample injection (100 μL) was sufficient to quantify residues at 0.1 μg/L. All remaining pesticides were detected at 1.0 μg/L. Calibration functions were linear for more than 80% of analytes. Signal suppression or enhancement compared with signals in high-performance liquid chromatography water was equal to or smaller than 20% for 240 analytes. Even the largest matrix-induced suppression did not result in the disappearance of peaks. Combining the results of seven mineral waters, the relative standard deviation of “recovery” was 20% or less for 87% of the substances. A second transition for confirmatory purposes was often available. Consequently, the proposed direct injection of samples without any sample enrichment and/or cleanup is suitable for screening of many pesticides in mineral and drinking water.     

Boundary-mediated electron-electron interactions in quantum point contacts

An unusual increase of the conductance with temperature is observed in clean quantum point contacts for conductances larger than 2(e2/h). At the same time, a positive magnetoresistance arises at high temperatures. A model accounting for electron-electron interactions mediated by boundaries (scattering on Friedel oscillations) qualitatively describes the observation. It is supported by a numerical simulation at zero magnetic field.     

Dzyaloshinsky-Moriya anisotropy in the spin-1/2 kagome compound ZnCu3(OH)6Cl2

We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the kagome spin-1/2 compound ZnCu3(OH)6Cl2. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component |Dz|=15(1) K to be perpendicular to the kagome planes. Through the temperature dependent ESR linewidth, we observe a building up of nearest-neighbor spin-spin correlations below approximately 150 K.