Chup Yew Mok

Solid-phase microextraction using pencil lead as sorbent for analysis of organic pollutants in water

Most solid-phase microextraction methods are based on poly (dimethylsiloxane)-coated silica fibers. Is the present study, pencil lead was used as an alternative sorbent for solid-phase imcroextraction. Its application to three organic pollutants with different polarity in water was investigated. The detection limit for determination by gas chromatography with electron capture detector was 0.005 ng ml−1 for lindane, 0.05 ng ml−1 for methyl parathion and 1 ng ml−1 for 2-chlorophenol. The presence of dissolved humic substances (10 mg l−1) in water did not affect the extraction of the three analytes.

Orthogonal array designs for the optimization of liquid chromatographic analysis of pesticides

Abstract Orthogonal array desings were applied to the optimization of liquid chromatographic analysis of pesticides used in golf courses. Three variables related to the composition of the mobile phase were studied: the percentage of acetonitrile, percentage of methanol, and the buffer pH. In the first experiment, each variable was tested at three levels using an OA9 (34) matrix, in which the interactions between the parameters were temporarily neglected. Based on the results of the first experiment, the second experiment was carried out following a two-level orthogonal array design with an OA8 (27) matrix, in which more exact levels for each variable were chosen, and the interaction effects were considered separately. Finally, the optimum conditions for liquid chromatographic analysis of the pesticides were proposed. The advantages and the disadvantages of three-level and two-level orthogonal array designs were discussed.

Small-scale multi-residue method for the determination of organochlorine and pyrethroid pesticides in vegetables

A simple and inexpensive multi-residue method is described for the determination of organochlorine and pyrethroid pesticides in vegetables. Pesticides in vegetables were extracted with ethanol and partitioned into toluene. A mini-column packed with 0.5 g of Florisil was used for further clean-up prior to gas chromatographic determination. The detection limits were 0.02-0.05 microgram/g without concentrating the extract, which are below the maximum residue limits set by the Singapore government. The recoveries of the pesticides from fortified samples were 65-97% at the 0.1 microgram/g level and 87-114% at the 0.5 microgram/g level. The amounts of the reagents required for analysing one sample are only 100 ml of ethanol, 6 ml of toluene and 0.5 g of Florisil. Among fifteen vegetable samples collected from the Singapore local market and were analysed by this method, five were found to contain detectable amount of organochlorine pesticides. One sample contained 22 micrograms/g of endosulfan but the residue levels in other four samples were below 1 microgram/g.

The alternative thermal decomposition mode of 2-oxetanone and 2-azetidinone: a DFT and PES study

Abstract We have found that, while 2-oxetanone undergoes thermal decarboxylation at 350°C to give CO 2 and ethene, 2-azetidinone proceeds in two different modes at 550°C, giving isocyanic acid, ethene, ketene, hydrogen cyanide and carbon monoxide among the pyrolysis products. B3LYP/6-31G ** calculations were performed to investigate the two alternative cycloreversion pathways for the two compounds. Theoretical thermodynamic data predicted that decarboxylation of 2-oxetanone has among the lowest activation barrier (159 kJ mol −1 ) and free energy (Δ G =−183 kJ mol −1 ). While the cycloreversion to ethene can be classified as a concerted asynchronous process, natural bond orbital (NBO) analysis suggested that the cycloreversion to ketene is more asynchronous for 2-azetidinone than for 2-oxetanone.

Gas phase pyrolysis of γ-butyrolactone and γ-thiobutyrolactone

In the gas phase the title lactone pyrolyses via decarboxylation, while the thiolactone pyrolyses via decarbonylation, as the major modes of decomposition.

A photoelectron spectroscopic study of the electron donor-acceptor complexes of trimethylamine with boron trihalides BX3 (X=F, Cl, Br)

Abstract HeI photoelectron spectra in the vapour phase and X-ray photoelectron spectra in the solid phase of the addition compounds of trimethylamine (TMA) with boron trihalides (BX 3 ; X=F, Cl, Br) are reported, and interpreted with the aid of ab initio SCF MO calculations. The shift in the binding energy of the amine lone-pair orbital n N correlates well with the calculated net charge transferred and structural changes upon complexation. The results confirm that the donor-acceptor interaction is stronger in the complexes TMA·BCl 3 and TMA·BBr 3 than in the fluoro analogue.

He(I) and He(II) photoelectron spectra of simple nitroalkanes

Abstract He(II) photoelectron spectra of nitromethane, nitroethane, 1- and 2-nitropropane have been recorded and compared with the corresponding He(I) spectra. The relative He(I)/He(II) spectral band intensities provide clear evidence for ascribing the first two maxima in the range 10–12 eV to ionization processes involving the removal of electrons from the orbitals n 0 + , n 0 − and π 2 . Similarly the band at ≈ 17 eV is attributed to ejection of electrons from the π 1 and σ NO − orbitals. Detailed assignment of spectral bands is made on the basis of bandshape and intensity, as well as ab initio SCF MO calculations using the 6-31G basis set. In the He(II) spectra, two bands in the region 18–22 eV are assigned to σ NO + and an orbital with 2s N and 2s O character.

Ethenethiol and 1-propene-1-thiol: a photoelectron spectroscopic study

Abstract Ethenethiol and 1-propene-1-thiol were produced through the thermolysis of thiirane and 2-methylthiirane, respectively, under reduced pressure in a flow system. The photoelectron spectra of the olefin thiols are characterised by the presence of a sharp first band which is well separated from the other bands by more than 2 eV. The spectra are discussed with the aid of AM1 and ab initio SCF MO calculations, and a diagram which correlates the binding energies of the first three orbitals, viz. π CCS * , π CCS and σ CS , with those of the corresponding orbitals of related molecules.

Penning ionization electron spectra of nitro compounds

Abstract He * (2 3 S) Penning ionization electron spectra (PIES) of some nitroalkanes, nitropropenes and nitrobenzene have been recorded and compared with their respective He(I) photoelectron spectra (UPS). The spatial electron density distributions of the out-of-plane π orbitals of the nitro group are more extended than the in-plane n 0 orbitals, resulting in relatively strong π bands in the PIES. Similarly, the π CC bands of nitropropenes and the π ring bands of nitrobenzene are enhanced in the PIES owing to the more extended nature of these orbitals. This has enabled the unequivocal assignment of the first three ionic states of nitromethane. The assignment is also supported by multi-deference determinant configuration interaction (MRD-CI) calculations. Variations of the intensity enhancement among these compounds are discussed in terms of local steric shielding effects.

A UV photoelectron spectroscopic study of addition compounds of boron trifluoride with methyl-substituted pyridines

Abstract Vertical ionization energies of the 1 : 1 molecular complexes of boron trifluoride with the three picolines and 3,5-lutidine are reported along with assignments based on their HeI photoelectron spectra and semi-empirical molecular orbital calculations. The shift in the lone pair (n N ) orbital of the donor molecules towards a higher ionization energy (IE) upon complexation is found to be essentially constant in all the complexes (nearly 2.8 eV) except in 3,5-lutidine·BF 3 where the shift is 3.3 eV. Such shifts in the IEs of the π orbital are about 1.1 eV for the 2- and 3-picoline complexes and 1.3 eV for the complexes of 4-picoline and 3,5-lutidine. These shifts reflect the relative stabilities of the complexes. The complex 2,6-lutidine·BF 3 undergoes dissociation into the free donor and acceptor molecules under the conditions of our experiment. A comparison of the shifts is made with the experimental heats of reaction of the pyridine bases with boron trifluoride. Relative stabilities of the complexes as deduced from ab initio and AM1 calculations are compared with predictions of gas-phase basicities of the bases.