Modeling retention and selectivity as a function of pH and column temperature in liquid chromatography

In reversed-phase liquid chromatography (RPLC), the retention of weak acids and bases is a sigmoidal function of the mobile-phase pH. Therefore, pH is a key chromatographic variable to optimize retention and selectivity. Furthermore, at an eluent pH close to the pKa of the solute, the dependence of ionization of the buffer and solute on temperature can be used to improve chromatographic separations involving ionizable solutes by an adequate handling of column temperature. In this paper, we derive a general equation for the prediction of the retentive behavior of ionizable compounds upon simultaneous changes in mobile-phase pH and column temperature. Four experiments, two limiting pH values and two temperatures, provide the input data that allow predictions in the whole range of these two variables, based on the thermodynamic fundamentals of the involved equilibria. Also, the study demonstrates the significant role that the choice of the buffer compound would have on selectivity factors in RPLC at temperatures higher than 25 degrees C.     

Nanoscale capacitance imaging with attofarad resolution using ac current sensing atomic force microscopy

Nanoscale capacitance imaging with attofarad resolution (～1?aF) of a nano-structured oxide thin film, using ac current sensing atomic force microscopy, is reported. Capacitance images are shown to follow the topographic profile of the oxide closely, with nanometre vertical resolution. A comparison between experimental data and theoretical models shows that the capacitance variations observed in the measurements can be mainly associated with the capacitance probed by the tip apex and not with positional changes of stray capacitance contributions. Capacitance versus distance measurements further support this conclusion. The application of this technique to the characterization of samples with non-voltage-dependent capacitance, such as very thin dielectric films, self-assembled monolayers and biological membranes, can provide new insight into the dielectric properties at the nanoscale.     

Optical tomography of transparent objects with phase-shifting interferometry and stepwise-shifted Ronchi ruling

An experimental setup for tomographic inspection of phase objects is presented. The system uses a common-path interferometer consisting of two windows in the input plane and a translating grating as its pupil. In the output, interference of the fields associated with replicated windows is achieved by a proper choice of the windows' spacing with respect to the grating period. With a rotating object in one window and a plane wave in the second one, the phase distribution of each projection is encoded as a corresponding digital image row, which, in turn, constructs a composite interferogram over the plane of a traditional sinogram. Phase stepping of composite interferograms can be achieved by a proper translation of the grating in order to obtain the unwrapped phase distribution as the corresponding sinogram. This sinogram allows tomographic reconstruction of phase slices by standard procedures. Composite interferograms and reconstructions for some transparent samples are shown.     

High-intensity pulsed electric field variables affecting Staphylococcus aureus inoculated in milk

Staphylococcus aureus is an important milk-related pathogen that is inactivated by high-intensity pulsed electric fields (HIPEF). In this study, inactivation of Staph. aureus suspended in milk by HIPEF was studied using a response surface methodology, in which electric field intensity, pulse number, pulse width, pulse polarity, and the fat content of milk were the controlled variables. It was found that the fat content of milk did not significantly affect the microbial inactivation of Staph. aureus. A maximum value of 4.5 log reductions was obtained by applying 150 bipolar pulses of 8 μs each at 35 kV/cm. Bipolar pulses were more effective than those applied in the monopolar mode. An increase in electric field intensity, pulse number, or pulse width resulted in a drop in the survival fraction of Staph. aureus. Pulse widths close to 6.7 μs lead to greater microbial death with a minimum number of applied pulses. At a constant treatment time, a greater number of shorter pulses achieved better inactivation than those treatments performed at a lower number of longer pulses. The combined action of pulse number and electric field intensity followed a similar pattern, indicating that the same fraction of microbial death can be reached with different combinations of the variables. The behavior and relationship among the electrical variables suggest that the energy input of HIPEF processing might be optimized without decreasing the microbial death.     

Energy efficient design of building: A review

Energy saving is a high-priority in developed countries. For this reason, energy-efficient measures are being increasingly implemented in all sectors. The residential sector is responsible for an important part of the energy consumption in the world. Most of this energy is used in heating, cooling, and artificial ventilation systems.With a view to developing energy-efficient structures, this article provides an overview of building design criteria that can reduce the energy demand for the heating and cooling of residential buildings. These criteria are based on the adoption of suitable parameters for building orientation, shape, envelope system, passive heating and cooling mechanisms, shading, and glazing. An analysis was made of previous studies that evaluated the influence of these parameters on the total energy demand and suggested the best design options. This study is useful for professionals who are responsible for decision-making during the design phase of energy-efficient residential buildings.     

Modeling nonspecific toxicity of organic compounds to the fathead minnow fish by means of chromatographic systems

The performance of chromatographic systems to mimic aquatic toxicity to the fathead minnow fish is evaluated taking into account the factors that contribute to the variance of biological-chromatographic correlations. These factors are the precision to measure the fathead minnow toxicity, the precision of the surrogate chromatographic system, and the error from the dissimilarity between the fathead minnow and chromatographic systems. The precisions are estimated through the characterization of the systems by the solvation parameter model. Several chromatographic systems as well as the common reference octanol-water partition system have been selected to test their ability to model the nonspecific toxicity to the fathead minnow by means of the proposed approach. Predictions and experimental tests show that the micellar electrokinetic chromatography system of sodium taurocholate and chromatographic measurements using an immobilized artificial membrane column provide the most precise estimations of this biopartitioning property. The octanol-water partition system, the conventional C18 high-performance liquid chromatography systems, and the micellar electrokinetic chromatography system of sodium dodecylsulfate show worse performances.     

Back-focal-plane position detection with extended linear range for photonic force microscopy

In photonic force microscopes, the position detection with high temporal and spatial resolution is usually implemented by a quadrant position detector placed in the back focal plane of a condenser. An objective with high numerical aperture (NA) for the optical trap has also been used to focus a detection beam. In that case the displacement of the probe at a fixed position of the detector produces a unique and linear response only in a restricted region of the probe displacement, usually several hundred nanometers. There are specific experiments where the absolute position of the probe is a relevant measure together with the probe position relative the optical trap focus. In our scheme we introduce the detection beam into the condenser with low NA through a pinhole with tunable size. This combination permits us to create a wide detection spot and to achieve the linear range of several micrometers by the probe position detection without reducing the trapping force.     

Force mapping of an optical trap using an acousto-optical deflector in a time-sharing regime

We suggest and study experimentally a time-sharing protocol for acousto-optical deflectors (AODs) that permits one to map the radial optical trapping force of optical tweezers without using a controllable flux control or an additional beam. Variations of the trapping potential due to modifications of the optical system are easily detected in terms of the force map. The protocol can be used in optical tweezers that already include an AOD without adding new elements in the existing optical system.     

Temperature and velocity measurement fields of fluids using a schlieren system

This paper proposes a combined method for two-dimensional temperature and velocity measurements in liquid and gas flow using a schlieren system. Temperature measurements are made by relating the intensity level of each pixel in a schlieren image to the corresponding knife-edge position measured at the exit focal plane of the schlieren system. The same schlieren images were also used to measure the velocity of the fluid flow. The measurement is made by using particle image velocimetry (PIV). The PIV software used in this work analyzes motion between consecutive schlieren frames to obtain velocity fields. The proposed technique was applied to measure the temperature and velocity fields in the natural convection of water provoked by a heated rectangular plate.     

Tailoring fiber grating sensors for assessment of highly refractive fuels

Three approaches that allow the tailoring of long period gratings based refractometric sensors for concentration measurement in fuel blends are employed to assess the fuel quality in biodiesel and biodiesel-petrodiesel blend. To allow the analysis of fuel samples with refractive index higher than fiber cladding one, the samples refractive indices were changed by thermo-optic effect and by dilution in a standard substance with low refractive index. The obtained results show the sensor can detect oil concentration in biodiesel samples with resolution as better as 0.07% and biodiesel concentration in biodiesel-petrodiesel samples with average resolution of 0.09%.