Liquid chromatographic analysis of vitamin B6 in reconstituted infant formula: collaborative study

A liquid chromatographic (LC) method was validated for the determination of total vitamin B6 in infant formula. Total vitamin B6 was quantified by converting the phosphorylated and free vitamers into pyridoxine. Pyridoxine was determined by ion pair reversed-phase LC with fluorescence detection. The method was subjected to an AOAC collaborative study involving a factory-manufactured, milk- and soy-based infant formula. Each was spiked at 3 concentrations in the range of 0-1 microg/g and sent as blind duplicate to participant laboratories. Nine laboratories returned valid data which were statistically analyzed for outliers and precision parameters. The repeatability relative standard deviation (RSD(r)) ranges were 2.0-4.0 and 3.5-5.9% for fortified milk- and soy-based formulas, respectively. The reproducibility relative standard deviation (RSD(R)) ranges were 8.2-8.4 and 6.7-11.2% for fortified milk- and soy-based formulas, respectively. HORRAT values ranged from 0.42 to 0.53, indicating that the precision of the method is acceptable. The mean RSD(r):RSD(R) values were 0.60 and 0.55 for milk- and soy-based formulas, respectively. As expected, RSDs for the unfortified samples were higher, but their HORRAT values (0.81 and 2.06) helped define a realistic limit of quantitation as 0.05 microg/g. Recovery data were quantitative and varied between 81.4 and 98.0% (mean = 89.8%) for each of 6 spiked materials.     

Antifreeze glycoproteins from the antarctic fish Dissostichus mawsoni studied by differential scanning calorimetry (DSC) in combination with nanolitre osmometry

This study investigates in detail the freezing events during cooling of solutions of various size classes of antifreeze glycoproteins. Differential scanning calorimetry and nanolitre osmometry were used to observe ice growth at temperatures within the hysteresis gap between the melting point and non-equilibrium freezing point (hysteresis freezing point) of solutions of the various sized antifreeze glycoproteins (AFGPs). The ice growth within the hysteresis gap is presumably due to both the expression of primary or near primary prism planes and also some growth at the basal plane. The binding of the AFGPs to the ice causes a particular ice crystal morphology. With the smaller AFGPs, substantial microscopic ice growth was observed in the form of a hexagonal bipyramids within the hysteresis gap.     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

Adhesive fracture of a lap shear joint

In many respects, adhesive and cohesive fractures are similar. It has been demonstrated in both cases that a Griffith-type energy balance can often be used to predict failure, e.g., crack growth. The only essential difference involves the interpretation of the energy required to create new (adhesive or cohesive) surface area. In the cohesive case, the specific fracture energy γ _c is that required to create a new surface in the same material, while in the adhesive case, the specific fracture energy γ _a is the energy per unit area required to separate different materials. The mechanical analysis, including a stress analysis to determine the strain energy and energy balance in principle remains unchanged. Generally speaking adhesive-bonded joints involve sharp corners or other “singularities” between adjacent materials which act as stress concentrators, particularly if a crack or other sharp imperfection is present or arises at such a location. The Griffith energy approach circumvents the problem of just how large this mathematically infinite stress must be to initiate failure. Recently, this method had been successfully applied to a number of different adhesive geometries; this paper discusses the case of a single-lap shear joint. This geometry is important because the lap-joint test is a common method for comparing adhesive strengths; in addition, the configuration itself is often used in engineering practice. Adhesive fracture is, therefore, compared on the basis of both energy and maximum stress criteria. Experimental data suggest the former to yield more accurate predictions.     

A 1H/19F minicoil NMR probe for solid-state NMR: application to 5-fluoroindoles

We show that it is feasible to use a minicoil for solid-state 19F 1H NMR experiments that has short pulse widths, good RF homogeneity, and excellent signal-to-noise for small samples while using low power amplifiers typical to liquid-state NMR. The closely spaced resonant frequencies of 1H and 19F and the ubiquitous use of fluorine in modern plastics and electronic components present two major challenges in the design of a high-sensitivity, high-field 1H/19F probe. Through the selection of specific components, circuit design, and pulse sequence, we were able to build a probe that has low 19F background and excellent separation of 1H and 19F signals. We determine the principle components of the chemical shift anisotropy tensor of 5-fluoroindole-3-acetic acid (5FIAA) and 5-fluorotryptophan. We also solve the crystal structure of 5FIAA, determine the orientation dependence of the chemical shift of a single crystal of 5FIAA, and predict the 19F chemical shift based on the orientation of the fluorine in the crystal. The results show that this 1H/19F probe is suitable for solid-state NMR experiments with low amounts of biological molecules that have been labeled with 19F.