Development and validation of a multiresidue method for the analysis of 151 pesticide residues in strawberry by gas chromatography coupled to a triple quadrupole mass analyzer

A new method was developed and validated for the simultaneous determination of 151 pesticide residues in strawberry by gas chromatography coupled to a triple quadrupole mass analyzer (GC/QqQ-MS/MS), mainly using the selected reaction monitoring (SRM) mode. The list of target compounds included various classes of pesticides such as organochlorine (OCPs), organophosphorus (OPPs), carbamates, pyrethroids, triazoles and dicarboximides. A single extraction of 10 g of sample with acetonitrile followed by liquid-liquid partition formed by the addition of 4 g of MgSO4 and 1 g of NaCl was applied in sample preparation. Cleanup of the extracts was carried out by applying dispersive solid-phase extraction (D-SPE) with primary secondary amine (PSA). The analysis time was 21 min. The method was subjected to a thorough validation procedure. The recovery data were obtained by spiking blank samples at two concentration levels (11.5 and 50 microg/kg), yielding recoveries in the range 70-110%. Precision values expressed as relative standard deviation (RSD) were lower than 18% and 22% for the intraday and interday precision, respectively. Linearity was studied in the range 10-200 microg/kg and determination coefficients (R(2)) were higher than 0.98% for all compounds. Limits of detection (LODs) and limits of quantification (LOQs) were established as 4 and 10 microg/kg, respectively. The overall uncertainty of the method was estimated at two different concentrations (11.5 and 50 microg/kg), being lower than 25% in both cases. According to the validation data and performance characteristics as well as the high sample throughput and low cost, the proposed method is suitable for routine application.     

Comparison of several extraction techniques for multiclass analysis of veterinary drugs in eggs using ultra-high pressure liquid chromatography-tandem mass spectrometry

This study compared four extraction methods for the simultaneous determination of tetracyclines, macrolides, quinolones, sulphonamides and anthelmintics (including benzimidazoles and avermectins) in eggs by ultra-high pressure liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). Solvent extraction, solid-phase extraction (SPE), matrix solid-phase dispersion (MSPD) and modified QuEChERS procedure were compared in terms of recovery and number of veterinary drugs extracted. The solvent extraction procedure with a clean-up step provided better results than the other tested procedures. The QuEChERS procedure was simpler and faster, but extracted fewer compounds than solvent extraction. MSPD did not extract tetracyclines and quinolones, whereas macrolides and tetracyclines were not extracted when SPE was applied. The solvent extraction procedure was validated, obtaining recoveries ranging from 60% (sulfaquinoxaline) to 119% (levamisole) with repeatability values (expressed as relative standard deviations, RSDs) lower than 20% at two concentration levels (10 and 100 μg kg⁻¹), except for erythromycin, emamectin and ivermectin that showed RSD values close to 25% at 10 μg kg⁻¹. Limits of quantification (LOQs) were always equal or lower than 5 μg kg⁻¹. Finally the method was applied to egg samples, and erythromycin, enrofloxacin, difloxacin, thiabendazole, emamectin and fenbendazole were detected in four samples.     

Super‐Resolution Imaging of Plasma Membrane Glycans

Much of the physiology of cells is controlled by the spatial organization of the plasma membrane and the glycosylation patterns of its components, however, studying the distribution, size, and composition of these components remains challenging. A bioorthogonal chemical reporter strategy was used for the efficient and specific labeling of membrane‐associated glycoconjugates with modified monosaccharide precursors and organic fluorophores. Super‐resolution fluorescence imaging was used to visualize plasma membrane glycans with single‐molecule sensitivity. Our results demonstrate a homogeneous distribution of N‐acetylmannosamine (ManNAc)‐, N‐acetylgalactosamine (GalNAc)‐, and O‐linked N‐acetylglucosamine (O‐GlcNAc)‐modified plasma membrane proteins in different cell lines with densities of several million glycans on each cell surface.     

Smooth C(alkyl)-H bond activation in rhodium complexes comprising abnormal carbene ligands

Rhodation of trimethylene-bridged diimidazolium salts induces the intramolecular activation of an alkane-type C-H bond and yields mono- and dimetallic complexes containing a formally monoanionic C,C,C-tridentate dicarbene ligand bound to each rhodium centre. Mechanistic investigation of the C(alkyl)-H bond activation revealed a significant rate enhancement when the carbene ligands are bound to the rhodium centre via C4 (instantaneous activation) as compared to C2-bound carbene homologues (activation incomplete after 2 days). The slow C-H activation in normal C2-bound carbene complexes allowed intermediates to be isolated and suggests a critical role of acetate in mediating the bond activation process. Computational modelling supported by spectroscopic analyses indicate that halide dissociation as well as formation of the agostic intermediate is substantially favoured with C4-bound carbenes. It is these processes that discriminate the C4- and C2-bound systems rather than the subsequent C-H bond activation, where the computed barriers are very similar in each case. The tridentate dicarbene ligand undergoes selective H/D exchange at the C5 position of the C4-bound carbene exclusively. A mechanism has been proposed for this process, which is based on the electronic separation of the abnormal carbene ligand into a cationic N-C-N amidinium unit and a metalla-allyl type M-C-C fragment.     

Determination of St. John's wort components in dietary supplements and functional foods by liquid chromatography

St. John's wort (Hypericum perforatum L.) preparations, a top-selling botanical dietary supplement used primarily as an antidepressant, has recently been used as an ingredient in some food products sold as functional foods. A rapid extraction technique followed by a liquid chromatographic (LC) method was developed to determine 4 characteristic bioactive compounds (pseudohypericin, hypericin, hyperforin, and adhyperforin) from St. John's wort in dietary supplements and functional foods to which it was added. Solid samples, including dried leaf/flower mixture, dietary supplement capsules, tea bags, puff and snack bar, were extracted with methanol by sonication. Noncarbonated, fruit-flavored drinks were centrifuged and mixed with methanol. Compounds were then determined by isocratic, reversed-phase LC with UV detection at 2 wavelengths and further identified or confirmed by photodiode array spectra and LC/mass spectrometry. Within-laboratory method variations (% RSD) were satisfactory. Very low amounts, if any, of the 4 components were found in drink and puff samples, and none was found in the snack bar. The methods developed provide a useful means for the determination of St. John's wort components in dietary supplements and functional foods.     

Optimization of microwave‐assisted extraction of analgesic and anti‐inflammatory drugs from human plasma and urine using response surface experimental designs†

The performance of microwave‐assisted extraction and HPLC with photodiode array detection method for determination of six analgesic and anti‐inflammatory drugs from plasma and urine, is described, optimized, and validated. Several parameters affecting the extraction technique were optimized using experimental designs. A four‐factor (temperature, phosphate buffer pH 4.0 volume, extraction solvent volume, and time) hybrid experimental design was used for extraction optimization in plasma, and three‐factor (temperature, extraction solvent volume, and time) Doehlert design was chosen to extraction optimization in urine. The use of desirability functions revealed the optimal extraction conditions as follows: 67°C, 4 mL phosphate buffer pH 4.0, 12 mL of ethyl acetate and 9 min, for plasma and the same volume of buffer and ethyl acetate, 115°C and 4 min for urine. Limits of detection ranged from 4 to 45 ng/mL in plasma and from 8 to 85 ng/mL in urine. The reproducibility evaluated at two concentration levels was less than 6.5% for both specimens. The recoveries were from 89 to 99% for plasma and from 83 to 99% for urine. The proposed method was successfully applied in plasma and urine samples obtained from analgesic users.     

Protein-matrix coupling/uncoupling in "dry" systems of photosynthetic reaction center embedded in trehalose/sucrose: the origin of trehalose peculiarity

Trehalose is a nonreducing disaccharide of glucose found in organisms, which can survive adverse conditions such as extreme drought and high temperatures. Furthermore, isolated structures, as enzymes or liposomes, embedded in trehalose are preserved against stressing conditions [see, e.g., Crowe, L. M. Comp. Biochem. Physiol. A 2002, 131, 505-513]. Among other hypotheses, such protective effect has been suggested to stem, in the case of proteins, from the formation of a water-mediated, hydrogen bond network, which anchors the protein surface to the water-sugar matrix, thus coupling the internal degrees of freedom of the biomolecule to those of the surroundings [Giuffrida, S.; et al. J. Phys. Chem. B 2003, 107, 13211-13217]. Analogous protective effect is also accomplished by other saccharides, although with a lower efficiency. Here, we studied the recombination kinetics of the primary, light-induced charge separated state (P(+)Q(A)(-)) and the thermal stability of the photosynthetic reaction center (RC) of Rhodobacter sphaeroides in trehalose-water and in sucrose-water matrixes of decreasing water content. Our data show that, in sucrose, at variance with trehalose, the system undergoes a "nanophase separation" when the water/sugar mole fraction is lower than the threshold level approximately 0.8. We rationalize this result assuming that the hydrogen bond network, which anchors the RC surface to its surrounding, is formed in trehalose but not in sucrose. We suggest that both the couplings, in the case of trehalose, and the nanophase separation, in the case of sucrose, start at low water content when the components of the system enter in competition for the residual water.     

On the use of solid phase ion exchangers for isolation of amino acids from liquid samples and their enantioselective gas chromatographic analysis

This work focuses on the development of a suitable working procedure for preconcentration of amino acids enantiomers from water samples using a solid phase extraction. The three types of ion exchangers with various capabilities have been used. The effect of experimental conditions in SPE procedure employing strong anion exchange (SAX), weak (WCX) and strong cation exchange (SCX) cartridges (such as sample volume, pH, origin of elution solvent and its volume) on effective preconcentration of the model set of amino acids has been studied in detail. The enantiomers of isolated and preconcentrated amino acids have been analysed by GC on three capillary columns coated with chiral selectors. The different amino acids derivatives have been investigated in order to achieve optimal resolution of biogenic amino acids and their enantiomers. The best separation of amino acid enantiomers has been obtained on a Chirasil-L-Val column analysing their N-TFA methyl esters. It has been shown that SCX-SPE cartridge with sulfonic groups attached on silicagel support is most suitable for isolation and preconcentration of amino acids from water samples. For this sample treatment procedure, the overall recovery of extraction process has been calculated as an average value from three measurements. It has been found, that recoveries are practically identical for both enantiomers of a particular amino acid and varies in the range 75-99% depending on the type of amino acid. The effectivity of this sample preparation and GC method has been verified by preconcentration of amino acids from orange juice fortified by racemic mixture of some selected amino acids.