4,831 research outputs found
Development of a novel LC-MS/MS method for the detection of adulteration of South African sauvignon blanc wines with 3-alkyl-2-methoxypyrazines
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2008.A method for the detection of adulteration of South African Sauvignon blanc
wines, by enrichment with foreign sources of 3-alkyl-2-methoxypyrazenes, is
described. The levels of three 3-alkyl-2-methoxypyrazenes (3-isobutyl-, 3-
isopropyl- and 3-sec-butyl-2-methoxypyrazine) in South African Sauvignon blanc
wines were measured with liquid chromatography-mass spectrometry. Sample
preparation involved clean-up and pre-concentration by distillation followed by
solvent extraction of the distillate with dichloromethane. Extracts were acidified
and concentrated by evaporation and finally reconstituted to a fixed volume to
affect quantitative pre-concentration of the samples. Sample extracts were
separated with reversed phase liquid chromatography utilizing a phenyl-hexyl
separation column. Residues were measured with liquid chromatography-mass
spectrometry utilizing a tandem quadrupole mass spectrometric detector
operated in multiple reaction monitoring mode for optimal trace level quantitation.
Atmospheric pressure chemical ionization was utilized as electrospray ionization
was found to suffer from quenching effects attributed to the sample matrix.
Qualitative information was obtained from the relevant molecular ions as well as
two secondary ion transitions (and one ion ratio) in each case. Recoveries
obtained by the extraction procedure were better than 90% with coefficient of
variance of better than 10% at concentrations from 1 to 100 ng/L. The limit of
detection of the method was 0.03 ng/L and the limit of quantification 0.10 ng/L for
the three analytes measured. The described LC-MS method is more sensitive for
the determination of 3-alkyl-2-methoxypyrazines in wine than GC methods
reported for the same purpose.
From the experimental data, a set of parameters were established to discriminate
adulterated South African Sauvignon blanc wines. It was demonstrated that the
3-isobutyl-2-methoxypyrazine concentration, despite showing considerable
variance, was confined to a relatively narrow range spanning approximately two
orders of magnitude (0.20 to 22 ng/L). A clear indication of possible maximum values for this compound in South African Sauvignon blanc wines was obtained
from the analysis of a large number of samples (577), spanning most relevant
wine producing regions and representing vintages 2003 to 2006. It was also
demonstrated that South African Sauvignon blanc wines contain the major 3-
alkyl-2-methoxypyrazenes in reasonably distinct relative amounts and that the
said ratios of abundance may be used to elucidate authenticity. The expected
effect of adulteration with green pepper extracts or some synthetic preparations
on the 3-isobutyl-2-methoxypyrazine concentration as well as the relative
abundances were also determined by characterizing the corresponding profiles in
green peppers and some synthetic flavor preparations. Two adulterated samples
in the dataset were identified by both outlined criteria. A limited number of wines
of other cultivars were also analyzed. The results represent the most complete
and accurate data on the 3-alkyl-2-methoxypyrazine content of South African
Sauvignon blanc wines to date.
A publication covering the work presented in this thesis is currently in
preparation
Melting of Branched RNA Molecules
Stability of the branching structure of an RNA molecule is an important
condition for its function. In this letter we show that the melting
thermodynamics of RNA molecules is very sensitive to their branching geometry
for the case of a molecule whose groundstate has the branching geometry of a
Cayley Tree and whose pairing interactions are described by the Go model.
Whereas RNA molecules with a linear geometry melt via a conventional continuous
phase transition with classical exponents, molecules with a Cayley Tree
geometry are found to have a free energy that seems smooth, at least within our
precision. Yet, we show analytically that this free energy in fact has a
mathematical singularity at the stability limit of the ordered structure. The
correlation length appears to diverge on the high-temperature side of this
singularity.Comment: 4 pages, 3 figure
Anomalously Slow Domain Growth in Fluid Membranes with Asymmetric Transbilayer Lipid Distribution
The effect of asymmetry in the transbilayer lipid distribution on the
dynamics of phase separation in fluid vesicles is investigated numerically for
the first time. This asymmetry is shown to set a spontaneous curvature for the
domains that alter the morphology and dynamics considerably. For moderate
tension, the domains are capped and the spontaneous curvature leads to
anomalously slow dynamics, as compared to the case of symmetric bilayers. In
contrast, in the limiting cases of high and low tensions, the dynamics proceeds
towards full phase separation.Comment: 4 pages, 5 figure
Molecular Electroporation and the Transduction of Oligoarginines
Certain short polycations, such as TAT and polyarginine, rapidly pass through
the plasma membranes of mammalian cells by an unknown mechanism called
transduction as well as by endocytosis and macropinocytosis. These
cell-penetrating peptides (CPPs) promise to be medically useful when fused to
biologically active peptides. I offer a simple model in which one or more CPPs
and the phosphatidylserines of the inner leaflet form a kind of capacitor with
a voltage in excess of 180 mV, high enough to create a molecular electropore.
The model is consistent with an empirical upper limit on the cargo peptide of
40--60 amino acids and with experimental data on how the transduction of a
polyarginine-fluorophore into mouse C2C12 myoblasts depends on the number of
arginines in the CPP and on the CPP concentration. The model makes three
testable predictions.Comment: 15 pages, 5 figure
Development of an automatic system to monitor the performance of a dense medium (mineral) separation process
Includes bibliographical references.Dense Medium Separation (DMS) is a process used extensively in the minerals processing industry to separate dense from less dense material in a dense fluid. It may be considered to be a simple “sink-float” separation process. DMS is used on a large scale in South African coal, iron ore and diamond operations. There are, however, no commercially available systems that can determine the separation efficiency of a DMS process on-line. This presents severe problems to those operating DMS processes. The present study attempts to provide a measurement technique for on-line application
Quiescence: a mechanism for escaping the effects of drug on cell populations
We point out that a simple and generic strategy to lower the risk for
extinction consists in the developing a dormant stage in which the organism is
unable to multiply but may die. The dormant organism is protected against the
poisonous environment. The result is to increase the survival probability of
the entire population by introducing a type of zero reproductive fitness. This
is possible, because the reservoir of dormant individuals act as a buffer that
can cushion fatal fluctuations in the number of births and deaths which without
the dormant population would have driven the entire population to extinction.Comment: 18 pages and 9 figure
Domain Growth, Budding, and Fission in Phase Separating Self-Assembled Fluid Bilayers
A systematic investigation of the phase separation dynamics in self-assembled
multi-component bilayer fluid vesicles and open membranes is presented. We use
large-scale dissipative particle dynamics to explicitly account for solvent,
thereby allowing for numerical investigation of the effects of hydrodynamics
and area-to-volume constraints. In the case of asymmetric lipid composition, we
observed regimes corresponding to coalescence of flat patches, budding,
vesiculation and coalescence of caps. The area-to-volume constraint and
hydrodynamics have a strong influence on these regimes and the crossovers
between them. In the case of symmetric mixtures, irrespective of the
area-to-volume ratio, we observed a growth regime with an exponent of 1/2. The
same exponent is also found in the case of open membranes with symmetric
composition
Competition and norms: a self-defeating combination?
This paper investigates the effects of information feedback mechanisms on electricity and heating usage at a student hall of residence in London. In a randomised control trial, we formulate different treatments such as feedback information and norms, as well as prize competition among subjects. We show that information and norms lead to a sharp – more than 20% - reduction in overall energy consumption. Because participants do not pay for their energy consumption this response cannot be driven by cost saving incentives. Interestingly, when combining feedback and norms with a prize competition for achieving low energy consumption, the reduction effect – while present initially – disappears in the long run. This could suggest that external rewards reduce and even destroy intrinsic motivation to change behaviour
A Classical Density-Functional Theory for Describing Water Interfaces
We develop a classical density functional for water which combines the White
Bear fundamental-measure theory (FMT) functional for the hard sphere fluid with
attractive interactions based on the Statistical Associating Fluid Theory
(SAFT-VR). This functional reproduces the properties of water at both long and
short length scales over a wide range of temperatures, and is computationally
efficient, comparable to the cost of FMT itself. We demonstrate our functional
by applying it to systems composed of two hard rods, four hard rods arranged in
a square and hard spheres in water
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