Andrew M. Crouch

Stir bar sorptive extraction combined with GC-MS analysis and chemometric methods for the classification of South African wines according to the volatile composition

A simple method for the analysis of major wine volatiles and semivolatiles by stir bar sorptive extraction in combination with thermal desorption and gas chromatography-mass spectrometry (SBSE-TD-GC-MS) was developed. Significant experimental parameters such as extraction time, temperature, salt addition, pH, and thermal desorption parameters were optimized to provide a sensitive and robust analytical method. The method provided good repeatability (%RSD < 10%) for 38 major wine volatile compounds, including alcohols, acids, esters, phenols, aldehydes, ketones, and lactones. Quantitative data for 62 South African red and white wines were used to study the suitability of major volatile data for the differentiation of wine samples according to grape variety or cultivar. Principal component analysis (PCA) and cluster analysis (CA) showed that most of the variation in volatile composition between wine samples could be ascribed to differences in wine age, wood contact, and fermentation practices. Despite the contribution of these factors, discriminant analysis (DA) was successfully applied to the classification of red and white wine samples according to cultivar.

Solid phase extraction in combination with comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry for the detailed investigation of volatiles in South African red wines

Comprehensive two-dimensional gas chromatography in combination with time-of-flight mass spectrometry (GC×GC-TOFMS) has been applied for the analysis of volatile compounds in three young South African red wines. In spite of the significant benefits offered by GC×GC-TOFMS for the separation and identification of volatiles in such a complex matrix, previous results utilizing headspace solid phase micro extraction (HS-SPME) demonstrated certain limitations. These were primarily associated with the choice of sample preparation technique, which failed to extract some influential semi-volatile wine constituents. Therefore, in the current report, we utilized solid phase extraction (SPE) in combination with GC×GC-TOFMS for the detailed investigation of particularly low-level semi-volatiles in South African wine. 214 compounds previously reported in grapes and related beverages were tentatively identified based on mass spectral data and retention indices, while 62 additional compounds were positively identified using authentic standards. The method proved particularly beneficial for the analysis of terpenes, lactones and volatile phenols, and allowed us to report the presence of numerous volatile compounds for the first time in Pinotage wines.

Analysis of Volatiles in Pinotage Wines by Stir Bar Sorptive Extraction and Chemometric Profiling

A fast, simple, cost-effective, and reliable method based on stir bar sorptive extraction (SBSE) in the headspace mode was used for the analysis of 39 volatile components in Pinotage wines. The method was sensitive, with LODs ranging from 50.0 pg/L to 281 ng/L and LOQs between 180 pg/L and 938 ng/L. Precision was between 6 and 20%. The intermediate precision was within the acceptable range. Moreover, good calibration curves with R(2) > 0.99 for all compounds were achieved. The method was successfully applied for the analysis of 87 young Pinotage wines of vintages 2005 and 2006 collected from various South African regions. To characterize the results based on vintage and origin, the obtained concentrations of the compounds were subjected to chemometric analysis. Exploratory factor analysis (FA), principal component analysis (PCA), and analysis of variance (one-way ANOVA) were consecutively done. The chemometrics approach revealed a reasonable correlation among the volatile components of these wines, as well as with respect to their year of production.

Bidentate aryldichalcogenide complexes of [(diphosphino)ferrocene]palladium(II) and [(diphosphino)ferrocene]platinum(II). Synthesis, molecular structures and electrochemistry

Abstract A series of homochalcogenide and mixed-chalcogenide ligand complexes of palladium and platinum have been prepared from the reactions of Pd(dppf)Cl2, (dppf=1,1′-bis(diphenylphosphino)ferrocene), Pd(dippf)Cl2 (1,1′-bis(diisopropylphosphino)ferrocene), and Pt(dppf)Cl2 with 1,2-benzenedithiol (HSC6H4SH) (a), 3,4-toluenedithiol (HSC6H3MeSH) (b), 3,6-dichloro-1,2-benzenedithiol (HSC6H2Cl2SH) (c), 2-mercaptophenol (HSC6H4OH) (d), thiosalicylic acid (HSC6H4CO2H) (e) and thionicotinic acid (HSC6H3NCO2H) (f). Single-crystal X-ray diffraction studies show that all complexes have distorted square-planar geometry. The complexes undergo two quasi-reversible or irreversible one-electron redox processes that involve the chalcogen ligands and diphosphinoferrocene ligands. The oxidation potentials of the chalcogen ligands increase when they bear electron-withdrawing substituents.

Anionic Fischer-type carbene complexes as bidentate (N,O) ligandsElectronic supplementary information (ESI) available: molecular structures of 4b, 5, 7 and 8. See http://www.rsc.org/suppdata/dt/b3/b316998g/

New polynuclear complexes, (L1)3M2 [M2 = Cr(III) (4a,4b), Fe(III) (5), Co(III) (8)], (L1)2M2(L2)2 [M2 = Co(II) (7), Ni(II) (9)], (L1)2M2(O)L2 [M2 = V(IV) (6)] and L1M2Cp2 [M2 = Ti(III) (10)] with L1 = (CO)5M1=C[C=NC(CH3)=CHS](O-)(M1 = Cr or W) and L2 = 4-methylthiazole or THF, are described. The molecular structures of these complexes determined by X-ray diffraction show that the Fischer-type carbene complexes act as bidentate ligands towards the second metal centre, coordinating through C(carbene)-attached O-atoms and imine N-atoms of the thiazolyl groups to form five-membered chelates with the oxygen atoms in the mer configuration. Isostructural complexes have similar characteristic band patterns in their far-IR spectra. Cyclic voltammetry of selected complexes reveals the oxidation of the carbene complex ligand between 1.01 and 1.29 V. Oxidation of the central metal (M2) takes place at 0.56 and 0.86 V for 7 and 9, respectively. Three stepwise reductions of Cr(III) to Cr(0) occur for 4a and 4b in the region -0.51 to -1.58 V. These new ligand types and other variants thereof should find application in ligand design with the first metal -- and other ligands attached thereto -- in the carbene complex ligand, playing an important role.

The use of GC×GC with time‐of‐flight mass spectrometry to investigate dienes and Diels–Alder polymerisation products in high‐temperature Fischer–Tropsch‐based fuels

Comprehensive GC in the reversed polarity mode i.e. a polar main column and an apolar secondary column have been used to differentiate cyclic olefins, dienes and cyclic dienes in high temperature Fischer-Tropsch (FT)-based fuels. The elucidation of the blobs in the contour plot was based on spectral data obtained by TOF-MS. The effectiveness of selective catalytic hydrogenation in transforming dienes into olefins could be studied. Dienes are prone to polymerisation by Diels-Alder reactions and dimers and trimers were detected in the 2-D plot by operating the MS in the ion extraction mode. Polymerisation is thought to be partially responsible for gum formation in high temperature FT-derived products.

Comparative study of cyclic voltammetry with potentiometric analysis for determining formation constants for polyaminocarboxylate–metal ion complexes

Abstract Cyclic voltammetry offers a convenient route towards the determination of ligand protonation/deprotonation constants and also for metal–ligand complex stability constants in aqueous media. The utility of voltammetry is exemplified by a comparison of the metal–ligand stability constants of complexes of ethylenediamine (ED), ethylenediamine-N,N,N′,N′-tetra-acetic acid (H4EDTA) and diethylenetriamine-N,N,N′,N″,N″-penta-acetic acid (H5DTPA) with transition metals, with these stability constants being in good agreement with those obtained using potentiometric analysis. Further support for the usefulness of voltammetry as an analytical tool is the good correlation of the protonation constants for (S,S)-ethylenediamine-N,N′-disuccinic acid with those determined potentiometrically.

Acrolein in wine: importance of 3-hydroxypropionaldehyde and derivatives in production and detection.

Certain lactic acid bacteria strains belonging to the genus Lactobacillus have been implicated in the accumulation of 3-hydroxypropionaldehyde (3-HPA) during anaerobic glycerol fermentation. In aqueous solution 3-HPA undergoes reversible dimerization and hydration, resulting in an equilibrium state between different derivatives. Wine quality may be compromised by the presence of 3-HPA due to the potential for spontaneous conversion into acrolein under winemaking conditions. Acrolein is highly toxic and has been implicated in the development of bitterness in wine. Interconversion between 3-HPA derivatives and acrolein is a complex and highly dynamic process driven by hydration and dehydration reactions. Acrolein is furthermore highly reactive and its steady-state concentration in complex systems very low. As a result, analytical detection and quantification in solution is problematic. This paper reviews the biochemical and environmental conditions leading to accumulation of its precursor, 3-HPA. Recent advances in analytical detection are summarized, and the roles played by natural chemical derivatives are highlighted.

A voltammetric study on the speciation and stability constants for iminodisuccinic acid with selected transition metal ions

Abstract The stability constants for complexes between biodegradable racemic iminodisuccinic acid (H4IDS) and various metal ions have been determined by cyclic voltammetry at a glassy carbon microelectrode. Measured data compared favourably to the same values determined by potentiometric methods of titration, reiterating the utility of cyclic voltammetry as a facile means for collecting complex stability thermodynamic data. Speciation data of the investigated metal-ligand complexes were subsequently determined by applying these measured constants in a thermodynamic database, the Joint Expert Speciation System (JESS). A semi-empirical molecular orbital study of complex stabilities was also carried out in vacuo on H4IDS and its homologous series. Relative complex stabilities were determined by making use of isodesmic equations and were found to increase with an increase in the size of the chelate rings formed.