Improvement of an overloaded, multi-component, solvent gradient bioseparation through multiobjective optimization

Solvent gradient chromatography is quite often used in analytical studies for decreasing the analysis time of samples having components with widely different retention behaviour. Several studies, both theoretical and experimental, have been reported on the optimization of gradient profiles in improving analytical separation performance, suggesting various linear and non-linear gradients. In preparative chromatography, on the other hand, though solvent gradient is being increasingly used (especially in bioseparation) to improve the product yield and productivity, there is a dearth of literature and clearer understanding of the effect(s) of modifier gradients on the separation performance. For this, the gradients used in applications are of relatively simple profiles like step or linear gradients, obtained through hand optimization based on experience and intuition. Significant improvements, however, can be expected using the state-of-the art modelling of chromatographic processes and optimization routines running on widely available hi-speed desktop computers. In this work we are reporting such an optimization procedure to improve the purification of an industrial multi-component mixture, containing 65.8% of Calcitonin as the main product, in an overloaded reversed-phase column. The work comprises both theoretical simulations and their experimental validation using multilinear gradients as optimization variable. The study produced interesting insights for modifier gradient design, like using peak deformation of the target peptide to increase yield and productivity, and improved our understanding of the effect of modifier gradients in non-linear separations.     

A Dynamic Nuclear Polarization spectrometer at 95 GHz/144 MHz with EPR and NMR excitation and detection capabilities

A spectrometer specifically designed for systematic studies of the spin dynamics underlying Dynamic Nuclear Polarization (DNP) in solids at low temperatures is described. The spectrometer functions as a fully operational NMR spectrometer (144 MHz) and pulse EPR spectrometer (95 GHz) with a microwave (MW) power of up to 300 mW at the sample position, generating a MW B(1) field as high as 800 KHz. The combined NMR/EPR probe comprises of an open-structure horn-reflector configuration that functions as a low Q EPR cavity and an RF coil that can accommodate a 30-50 μl sample tube. The performance of the spectrometer is demonstrated through some basic pulsed EPR experiments, such as echo-detected EPR, saturation recovery and nutation measurements, that enable quantification of the actual intensity of MW irradiation at the position of the sample. In addition, DNP enhanced NMR signals of samples containing TEMPO and trityl are followed as a function of the MW frequency. Buildup curves of the nuclear polarization are recorded as a function of the microwave irradiation time period at different temperatures and for different MW powers.     

Experimental observation of optical bound states in the continuum

We present the experimental observation of bound states in the continuum. Our experiments are carried out in an optical waveguide array structure, where the bound state (guided mode) is decoupled from the continuum by virtue of symmetry only. We demonstrate that breaking the symmetry of the system couples this special bound state to continuum states, leading to radiative losses. These experiments demonstrate ideas initially proposed by von Neumann and Wigner in 1929 and offer new possibilities for integrated optical elements and analogous realizations with cold atoms and optical trapping of particles.     

A quantitative approach to soliton instability

We present an approach for instabilities of solitons that is based on the spectrum of a fourth-order linearized operator. Unlike the standard approach which is based on the slope (Vakhitov-Kolokolov) condition, this approach provides the quantitative value of the instability rate and the qualitative nature of the instability dynamics.     

Low energy charged particles interacting with amorphous solid water layers

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 μA) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 ± 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.