Extraction rate in liquid-liquid segmented flow injection analysis

The extraction mechanism in liquid-liquid segmented flow-injection systems is investigated. A film is formed on the tubing wall by the phase with the highest affinity to the tubing material. This film surrounds the segments of the other phase. To a great extent the extraction takes place at the interface of this film. The extraction rate is influenced by the segment length, the inner diameter of the tubing and the flow velocity. Short segments, small inner diameter and high linear flow velocity lead to a high extraction rate. These findings indicate that miniaturization of the flow system will lead to faster extraction and to decreased sample-zone broadening.     

Theoretical study of the human DNA repair protein HOGG1 activity

We have examined the role of the catalytic lysine (Lys 249) in breaking the glycosidic bond of 8-oxoguanine in the enzyme human 8-oxoguanine DNA glycosylase. Until quite recently it has been assumed that this lysine acts as a nucleophile in an S(N)2 type of reaction after being activated through a donation of a proton to a strictly conserved aspartate, also located in the active site. However, evidence from crystallographic, as well as biochemical studies, questions this assumption mainly because the lysine is not ideally positioned for such an attack. In addition, the catalytic activity is preserved even after that aspartate is mutated to a residue not accepting protons, but still keeping the interactions in the active site. In this study, we have investigated several different reaction mechanisms to discover plausible ways where the lysine could assist in breaking the glycosidic bond. We use hybrid density functional theory to characterize both associative and dissociative pathways. We find that the smallest energetical barrier involves an S(N)1 type of mechanism where the lysine electrostatically stabilizes the dissociating base and then donates a proton with a very small barrier and then finally attacks the sugar ring to create the covalently bound protein-DNA intermediate complex. The S(N)2 mechanism also has a lower barrier than the "spontaneous" bond breaking but considerably above that of the S(N)1 reaction. However, in current conditions, the reactants placed in a conformation posed for an S(N)2 reaction is substantially more stable than if posed for the S(N)1 reaction, indicating that the active site has to bind stronger to the latter in order to achieve a full catalytic effect. An analysis of the polarization of the transition states shows that the polarization is largest for the S(N)1 reaction, indicating that this path will gain most by being placed in a prepolarized active site. These findings give further support to the hypothesis that a dissociative mechanism may be the preferred mode of action for this type of enzymes.     

Modified screen-printed electrodes for the investigation of the interaction of non-electroactive quinazoline derivatives with DNA

Five morpholino-quinazoline derivatives have been investigated voltammetrically using a competition with the tris(o-phenanthroline) cobalt(III) redox marker for the accumulation at dsDNA modified screen-printed electrodes. An association of quinazolines with DNA was observed at the modified electrodes polarized by the negative potential of –0.4 V vs. Ag/AgCl. This was confirmed by a potentiometric stripping analysis based on the DNA guanine signal. Calibration curves for quinazolines within a concentration range of μmol/L were obtained with DP voltammetry using 5 × 10^–7 mol/L Co(phen)₃ ³⁺ marker. The quinazolines exhibit no effect on the DNA complex with the fluorescent thiazole orange derivative TO-PRO-3. The role of the accumulation potential in the association interaction with DNA is discussed.     

The applicability of the superposition principle in differential pulse polarography

It is shown theoretically that the superposition principle is applicable to pulse voltammetry if the electrochemical system can be separated into a potential-dependent nonlinear part and a linear part. For systems not complicated by adsorption or electrode kinetics, the applicability of the principle depends on diffusion coefficients, electrode and cell geometry. For plane semi-infinite diffusion, applicability is expected; this is generally not the case for spherical electrodes or bounded regions (film electrode or thinlayer cell). The implications of the theory on differential pulse polarography are discussed and an experimental study on the applicability for iron(III), cadmium(II) and lead(II) is presented.     

Using a Bootstrap Method to Choose the Sample Fraction in Tail Index Estimation

Tail index estimation depends for its accuracy on a precise choice of the sample fraction, i.e., the number of extreme order statistics on which the estimation is based. A complete solution to the sample fraction selection is given by means of a two-step subsample bootstrap method. This method adaptively determines the sample fraction that minimizes the asymptotic mean-squared error. Unlike previous methods, prior knowledge of the second-order parameter is not required. In addition, we are able to dispense with the need for a prior estimate of the tail index which already converges roughly at the optimal rate. The only arbitrary choice of parameters is the number of Monte Carlo replications.     

Effects of temperature and flow regulated carbon dioxide cooling in longitudinally modulated cryogenic systems for comprehensive two-dimensional gas chromatography

Two different modes of temperature regulation in longitudinally modulated cryogenic systems (LMCSs) for comprehensive two-dimensional gas chromatography (GC x GC) were compared. Carbon dioxide was used as coolant. In the first mode of operation, the temperature of the trap was regulated to pre-set temperature using a digital temperature controller ("the constant temperature mode"). In the second, the temperature was regulated to a fixed negative offset to the oven temperature by using a constant flow of CO2 ("the constant flow mode"). A number of problems were occasionally observed using the constant temperature mode: (1) severe band broadening of high boiling analytes in the second dimension; (2) non-Gaussian reconstructed first-dimension peak profiles; (3) high background due to modulation of first-dimension column bleed. It was concluded that these problems were associated with inefficient solute remobilization at low LMCS trap temperatures (1 and 2) or large trap temperature fluctuations (3). These problems could be avoided or significantly reduced by using the constant flow mode. Best results were obtained as the trap temperature was kept about 70 degrees C below the oven temperature.     

Trace analysis of polychlorinated dibenzo-p-dioxins, dibenzofurans and WHO polychlorinated biphenyls in food using comprehensive two-dimensional gas chromatography with electron-capture detection

Trace analysis of 2,3,7,8-polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and the 12 WHO-PCBs (four non-ortho and eight mono-ortho congeners that have been assigned toxic equivalence factors, TEFs, by the World Health Organisation) was conducted by comprehensive two-dimensional gas chromatography with a micro electron-capture detector (GC x GC-microECD). Four food matrices (fish oil from herring, spiked cows' milk, vegetable oil and an eel extract) were analysed by two GC x GC laboratories, and four GC-HRMS laboratories generated reference values. The two GC x GC laboratories used different column combinations for separating the target analytes. For the first dimension, non-polar DB-XLB and VF-1 columns were used, and for the second dimension, an LC-50 liquid crystalline column with unique selectivity for planar compounds. The congener-specific and total toxic equivalence (TEQ) data obtained using DB-XLB x LC-50 were in good agreement with results obtained by the GC-HRMS laboratories. The WHO-PCB data obtained with the VF-1 x LC-50 combination was also good, but the PCDD/F concentrations were sometimes overestimated due to matrix interferences. GC x GC-microECD using DB-XLB x LC-50 seems to fulfil the European Community requirements of a screening method for PCDD/F and WHO-PCB TEQ in food.     

Upgrading of wood pre-hydrolysis liquor for renewable barrier design: a techno-economic consideration

A techno-economic assessment of an upgrading procedure and outtake of a pre-hydrolysate in a presumed dissolving pulp mill was performed. Pre-hydrolysis of spruce wood chips in pilot scale produced input data for energy and mass balances and was performed with and without subsequent membrane filtration to produce hydrolysate fractions rich in galactoglucomannan and with some lignin. The hydrolysate is a viable raw material for the production of renewable thin oxygen barrier films as demonstrated herein in the formulation of free standing films with very low oxygen permeability at both moderate and high relative humidities. Approximately 50,000 ton dry solid upgraded pre-hydrolysate suitable for production of oxygen barriers could be produced according to the presumed dissolving pulp mill producing about 500,000 air dry ton dissolving pulp per year and applying a liquor to wood ratio of 4:1. Utilization of the pre-hydrolysis liquor hence adds value to and realizes the dissolving plant as a biorefinery. A sensitivity analysis indicates that the market price of the upgraded pre-hydrolysate has the largest positive effect on the return on investment for separation and upgrading of a pre-hydrolysate. Increased investment cost and increased annuity factor show negative effects.     

A study of glycoprotein-lectin interactions using quartz crystal microbalance

A study of biospecific interactions between lectins and glycoproteins using a quartz crystal microbalance biosensor with dissipation monitoring (QCM-D) was reported. Four lectins were covalently immobilised on the thiol-modified gold electrode of the QCM chips in order to obtain sensing surfaces. The frequency shift served as analytical signal and the dissipation shift provided additional information about the viscoelastic properties of the glycoprotein-lectin complex formed on the surface of the QCM chip. The working conditions of the assay were optimised. The interaction between different lectins and glycoproteins was characterised by specific frequency shifts and each glycoprotein displayed its own unique lectin-binding pattern. This lectin pattern can serve as a finger print for the discrimination between various glycoproteins. The biosensor enabled quantitative determination of glycoproteins in the concentration range of 50 μg mL⁻¹ to 1 mg mL⁻¹ with good linearity and R.S.D. of less than 6.0%. An additional advantage of the proposed biosensor was the possibility to re-use the same lectin surfaces during a long period of time (2 month) without changes in analytical response. This was experimentally achieved by the application of a proper regeneration solution (10 mM glycine-HCl, pH 2.5). The lectin-based quartz crystal microbalance technique is suitable both for rapid screening and for quantitative assay of serum glycoproteins.