Analysis of pesticide residues using the Quick Easy Cheap Effective Rugged and Safe (QuEChERS) pesticide multiresidue method in combination with gas and liquid chromatography and tandem mass spectrometric detection

The Quick Easy Cheap Effective Rugged and Safe multiresidue method (QuEChERS) has been validated for the extraction of 80 pesticides belonging to various chemical classes from various types of representative commodities with low lipid contents. A mixture of 38 pesticides amenable to gas chromatography (GC) were quantitatively recovered from spiked lemon, raisins, wheat flour and cucumber, and determined using gas chromatography-tandem mass spectrometry (GC-MS/MS). An additional mixture of 42 pesticides were recovered from oranges, red wine, red grapes, raisins and wheat flour, using liquid chromatography-tandem mass spectrometry (LC-MS/MS) for determination. The pesticides chosen for this study included many of the most frequently detected ones and/or those that are most often found to violate the maximum residue limit (MRL) in food samples, some compounds that have only recently been introduced, as well as a few other miscellaneous compounds. The method employed involved initial extraction in a water/acetonitrile system, an extraction/partitioning step after the addition of salt, and a cleanup step utilizing dispersive solid-phase extraction (D-SPE); this combination ensured that it was a rapid, simple and cost-effective procedure. The spiking levels for the recovery experiments were 0.005, 0.01, 0.02 and 0.2 mg kg(-1) for GC-MS/MS analyses, and 0.01 and 0.1 mg kg(-1) for LC-MS/MS analyses. Adequate pesticide quantification and identity confirmation were attained, even at the lowest concentration levels, considering the high signal-to-noise ratios, the very good accuracies and precisions, as well as the good matches between the observed ion ratios. Mean recoveries mostly ranged between 70 and 110% (98% on average), and relative standard deviations (RSD) were generally below 10% (4.3% on average). The use of analyte protectants during GC analysis was demonstrated to provide a good alternative to the use of matrix-matched standards to minimize matrix-effect-related errors. Based on these results, the methodology has been proven to be highly efficient and robust and thus suitable for monitoring the MRL compliance of a wide range of commodity/pesticide combinations.     

Aldehydselektivität von alkylhafnium-komplexen in aldehyd-keton-konkurrenzversuchen; vergleich zu zirkonium- und titan-analogen komplexen sowie theoretische deutung

Cl₃Hf-Me, in contrast to MeLi is not able to attack an ester or a nitrile group and fails to discriminate in competition experiments between heptanal and diethylketone. To the contrary, Cl ₃Hf-Bu and (EtO)₃Hf-Me are highly aldehyde-selective. An explanation of these findings in comparison with alkylzirconium and alkyltitanium complexes is given.     

Molecular thermodynamics of fluid mixtures containing molecules differing in size and potential energy

Recent computer-simulation work by Shing and Gubbins (1983) for binary mixtures has shown that common semiempirical models (van der Waals n-fluid models) are in error when the molecules of the two components differ appreciably in size; the error is most severe in the dilute region. While perturbation theories are much better, they (like computer simulations) are not as yet useful for engineering work because of prohibitive computer requirements.This work proposes an algebraic expression for the Helmholtz energy of a mixture which gives results in very good agreement with those reported by Shing and Gubbins. This expression, using the local-composition concept, is based on a simplified but realistic picture of a fluid mixture: short-range order and long-range disorder. The proposed expression uses the Mansoori-Carnahan-Starling-Leland equation for the contribution of repulsive forces. For the contribution of attractive forces, it uses a new expression based on not one but several radii for the first-neighbor shell, one radius for each component.With reasonable simplifications, the resulting equation for the Helmholtz energy indicates that the van der Waals “constant” a is a strict quadratic function of mole fraction only at very low densities; at advanced densities, there are small deviations from the quadratic mixing rule. For practical calculations, the computer requirements are nearly the same as those for conventional engineering models.     

PIGMENT ORGANIZATION IN THE LIGHT-HARVESTING CHLOROPHYLL-a/b PROTEIN COMPLEX OF LETTUCE CHLOROPLASTS. EVIDENCE OBTAINED FROM PROTECTION OF THE CHLOROPHYLLS AGAINST PROTON ATTACK and FROM EXCITATION ENERGY TRANSFER

Abstract— The light-harvesting Chl-a/b protein complex (LHC) of Lactuca sativa L. was examined for pigment content, excitation energy transfer and behavior under acidic conditions:
(1) Lettuce LHC contains Chl-a, Chl-b and xanthophylls (lutein, neoxanthin, lactucaxanthin, viola-xanthin) at a molar ratio of 6:4:3; their contribution to the absorbance of the LHC between 390 and 530 nm is estimated to be about 31% (Chl-a), 26% (Chl-h) and 43% (xanthophylls).
(2) Energy transfer from xanthophylls and Chl-fe to Chl-a takes place at 100% transfer efficiency.
(3) LHC exhibits an unusual acid stability: in contrast to complexes of photosystem I or II, LHC-bound chlorophylls are not converted to phaeophytin and LHC apoprotein is not denatured at pH 1.5; also, energy transfer is maintained.
(4) Pronase or trypsin treatment do not affect acid stability and energy transfer.
(5) Treatments that break down acid stability (heat, urea or TritonX–100) also inhibit energy transfer.
The coincidental breakdown of energy transfer and acid stability points at one underlying process, namely, the breakdown of a structure that enables protection of chlorophylls from proton attack and close contiguity of xanthophylls and chlorophylls as required for energy transfer. Dense packing of xanthophylls and chlorophylls within lipophilic crevices of the LHC is suggested.