Random replicators with high-order interactions

We use tools of the equilibrium statistical mechanics of disordered systems to study analytically the statistical properties of an ecosystem composed of N species interacting via random mutual interactions, as well as via deterministic self-interactions of order p>/=2. We show that the main effect of increasing the order of the interactions among the species is to make the system less competitive, in the sense that the fraction of extinct species is greatly reduced. In addition, we find that for p>2 there is a threshold value which gives a lower bound to the concentration of the surviving species, preventing then the existence of rare species and, consequently, increasing the robustness of the ecosystem to external perturbations.     

Reversed-phase liquid chromatography using mandelic acid as an eluent for the determination of uranium in presence of large amounts of thorium

Studies were carried out for the separation of uranium (U) and thorium (Th) on reversed-phase (RP) C₁₈ columns using mandelic acid as an eluent. Retention of thorium–mandelate on the unmodified stationary phase was found to be greater than that of uranyl–mandelate under the pH conditions employed. Th retention capacity of the stationary phase was determined as a function of pH and MeOH content of the mobile phase. The optimised parameters allowing U elution prior to Th were utilized for the determination of small amounts of U in the presence of large amounts of Th. The method has been used for the determination of U in synthetic samples with Th/U amount ratios up to 100,000 (10 μg/g of U) without any pre-separation, employing a particulate C₁₈ column. Effect of concentration of ion interaction reagents (IIRs) on the retention was studied to understand the mechanism of adsorption of their mandelate complexes onto the stationary phase. The experiments conducted unequivocally prove that thorium–mandelate complex is neutral whereas uranyl–mandelate complex is anionic in nature.     

High performance liquid chromatographic separation of <Superscript>228,229</Superscript>Th and <Superscript>232,233</Superscript>U and their estimation by α- and γ-ray spectrometry

The activities of ^228,229Th and ^232,233U from an irradiated ThO₂ sample were radiochemicaly separated by using high performance liquid chromatography. Plancheted sources of the separated samples were made and the amount of ^232,233U and ^228,229Th were estimated by using alpha and gamma-ray spectrometric techniques. These estimations are important for the Th–U fuel reprocessing cycle of advanced heavy water reactor and accelerator driven sub-critical system.     

Maturation of auditory temporal integration and inhibition assessed with event-related potentials (ERPs)

Background

We examined development of auditory temporal integration and inhibition by assessing electrophysiological responses to tone pairs separated by interstimulus intervals (ISIs) of 25, 50, 100, 200, 400, and 800 ms in 28 children aged 7 to 9 years, and 15 adults.

Results

In adults a distinct neural response was elicited to tones presented at ISIs of 25 ms or longer, whereas in children this was only seen in response to tones presented at ISIs above 100 ms. In adults, late N1 amplitude was larger for the second tone of the tone pair when separated by ISIs as short as 100 ms, consistent with the perceptual integration of successive stimuli within the temporal window of integration. In contrast, children showed enhanced negativity only when tone pairs were separated by ISIs of 200 ms. In children, the amplitude of the P1 component was attenuated at ISIs below 200 ms, consistent with a refractory process.

Conclusions

These results indicate that adults integrate sequential auditory information into smaller temporal segments than children. These results suggest that there are marked maturational changes from childhood to adulthood in the perceptual processes underpinning the grouping of incoming auditory sensory information, and that electrophysiological measures provide a sensitive, non-invasive method allowing further examination of these changes.     

Separation of urea,hexamethylenetetramine, and their reaction products in sol–gel feed solution by zwitterionic HILIC

Studies were carried out to develop a chromatographic methodology based on hydrophilic interaction liquid chromatography (HILIC) for the separation of various reaction products of hexamethylenetetramine (HMTA) –urea used in the sol–gel process for the preparation of ceramic microspheres. Different chromatographic parameters such as organic modifiers, pH of mobile phase, buffer concentration, column temperature, etc. were studied to arrive at the optimum conditions for separation. Compounds such as urea, monomethylolurea (MMU), dimethylolurea (DMU), and HMTA were separated from a synthetic mixture using a mobile phase consisting of acetonitrile and acetate buffer of pH 6. The methodology developed based on HILIC stationary phase is simple and amicable for integration with electrospray ionization mass spectrometry (ESI-MS) to identify the unknown reaction products. The methodology was applied for the separation of reaction products in pre-boiled and untreated urea–HMTA mixtures used as feed in the sol–gel process.     

Determination of uranium in seawater samples by liquid chromatography using mandelic acid as a complexing agent

The determination of uranium at different stages of the recovery process as well as in seawater is important in its recovery study. A previous study developed a high-performance liquid chromatography (HPLC) method for uranium determination in seawater using α-hydroxy isobutyric acid as a chelating agent. However, this method causes turbidity in process samples containing high amounts of iron, resulting in the clogging of the HPLC column. In the present work, use of mandelic acid as a chelating agent for uranium has been explored. Elution conditions were optimized for the separation of iron [Fe(III)] and uranium [U(VI)] by studying the effect of an ion interaction reagent, the concentration of mandelic acid, and methanol content in the mobile phase. Different parameters were optimized to develop off- line pre-concentration of uranyl-mandelate on the reversed stationary phase. The method offers quantitative recovery of uranium and linearity in the U(VI) concentration range of 0.5 ppb to 500 ppb and can be used for the determination of U(VI) in process samples with Fe/U amount ratios up to 3,000. The method has been successfully used for the determination of U(VI) in seawater samples and process samples. The developed methodology was validated by comparing the results with those of isotope dilution-thermal ionization mass spectrometry.