Dynamic Field Gradient Research Articles

Dynamic fringe field effect of the rapid ramping quadrupole magnets at a Rapid Cycling Synchrotron

For a DC quadrupole magnet, the normalized magnetic field distribution along the beam trajectory is mainly determined by the mechanical structure of the magnet and remains almost independent of the exciting current. However, the magnetic field measurement results of the Rapid Cycling Synchrotron (RCS) at the China Spallation Neutron Source (CSNS) show that the normalized magnetic field distribution along the beam trajectory varies during the exciting current ramping process for rapid ramping magnets. Consequently, for rapid ramping quadrupole magnets, there are significant differences between the dynamic and static magnetic field gradient distribution. And the changes in the distribution of the magnetic field gradient can cause time dependent tune-shifts during the exciting current ramping process. For RCS, the tune is crucial, and the shifts of tune may lead to the beam approaching the resonance line, thereby causing beam loss. The impact of the fringe field effect of the rapid ramping quadrupole magnets on beam dynamics at CSNS-RCS was detailly studied. A new method for correcting the time-dependent tune shifts caused by the dynamic field distribution, based on waveform compensation at the quadrupole magnets, has also been proposed for the RCS of CSNS. Related simulation and experiment have been conducted, and the simulation results and experimental results are consistent with theoretical study results.

Quantum dynamics of trapped ions in a dynamic field gradient using dressed states

Novel ion traps that provide either a static or a dynamic magnetic gradient field allow for the use of radio-frequency radiation for coupling internal and motional states of ions, which is essential for conditional quantum logic. We show that the Hamiltonian describing this coupling in the presence of a resonant dynamic gradient, is identical, in a dressed state basis, to the Hamiltonian in the case of a static gradient. The coupling strength is in both cases described by the same effective Lamb-Dicke parameter. This insight can be used to overcome demanding experimental requirements when using a dynamic gradient field for state-of-the-art experiments with trapped ions, for example, in quantum information science. At the same time, this insight opens new experimental perspectives by way of using a single resonant or detuned dynamic gradient field, inducing long-range coupling, for conditional multi-qubit dynamics.

Dynamic Field Gradient and Erasure After Write for High Data Rate Recording

Extensive micromagnetic simulations are performed to investigate the impact of the current overshoot amplitude (OSA) on the writer dynamics. The field risetime, dynamic gradient, bubble width, as well as erasure risk due to large stray fields in the neighbouring tracks during the switching are all shown to be very sensitive to the OSA. Numerical results show that increasing OSA can sharply improve the field risetime. However, it also leads to a larger write bubble expansion during switching which could result in a wider track width and reduce track per inch (TPI) capability. The risk of side track erasure for different overshoot conditions are evaluated and also tend to increase with higher OSA. Futhermore, results for remnant fields in the media produced by the writer are shown to be extremely sensitive to the choice of damping parameter used in the simulations.

Detection of Magnetic Particles in Live DBA/2J Mouse Eyes Using Magnetomotive Optical Coherence Tomography

To demonstrate in vivo molecular imaging of the eye using spectral-domain magnetomotive optical coherence tomography (MMOCT). A custom-built, high-speed, and high-resolution MMOCT was developed for imaging magnetic particle-coupled molecules in living mouse eyes by applying an external dynamic magnetic field gradient during optical coherence tomography (OCT) scanning. The magnetomotive signals were tested in vitro by scanning magnetic beads embedded within an agarose gel (1.5%) and in vivo in the anterior segment of a mouse eye. Cross-sectional OCT images of the gel and the anterior segment of the eye were acquired by regular OCT structural scanning. Magnetomotive optical coherence tomography signals were successfully captured in the agarose gel with embedded magnetic beads. The signals were captured in the anterior segment of the mouse eyes after injecting the beads. The signal was overlaid successfully onto the structural OCT image. We demonstrated the ability to detect particles injected into the anterior chamber of the mouse eye using MMOCT. This suggests that MMOCT is effective for future live detection of molecular (protein) targets in various ocular diseases in mouse models.

Influence of the semi‐permeable membrane on the performance of dynamic field gradient focusing

This paper is part of our continued effort to understand the underlying principles of dynamic field gradient focusing. In this investigation, we examined three problems associated with the use of a semi-permeable membrane. First, the influence of steric and ionic exclusion of current carrying ions through the membrane was examined. It was found that resistance to the transport of ions across the membrane resulted in a shallowing of the electric field profile and an increase in the size of the defocusing zone, which is where the slope of the electric field is reversed so that it disperses rather than concentrates solutes. These problems could be reduced by using a membrane with large pores relative to the size of the buffering ions and completely void of fixed charges. Next, a numerical simulation was used to investigate concentration polarization of protein onto the surface of the membrane. Due to the presence of a transverse electric field, species were pulled toward the membrane. If the membrane is restrictive to those species, a concentrated, polarized layer will form on the surface. The simulation showed that by decreasing the channel to a depth of 20 microm, the concentrated region next to the membrane could be reduced. Finally, it was found that changes in column volume due to loss of membrane structural integrity could be mitigated by including a porous ceramic support. The variation in peak elution times was decreased from greater than 20% to less than 3%.

Development of a membrane-less dynamic field gradient focusing device for the separation of low-molecular-weight molecules.

Dynamic field gradient focusing uses an electric field gradient generated by controlling the voltage profile of an electrode array to separate and concentrate charged analytes according to their individual electrophoretic mobilities. This study describes a new instrument in which the electrodes have been placed within the separation channel. The major challenge faced with this device is that when applied voltages to the electrodes are larger than the redox potential of water, electrolysis will occur, producing hydrogen ions (H+) plus oxygen gas on the anodes and hydroxide (OH(-)) plus hydrogen gas on the cathodes. The resulting gas bubbles and pH excursions can cause problems with system performance and reproducibility. An on-column, degassing system that can remove gas bubbles "on-the-fly" is described. In addition, the use of a high capacity, low-conductivity buffer to address the problem of the pH shift that occurs due to the production of H+ on the anodes is illustrated. Finally, the successful separation of three, low-molecular-weight dyes (amaranth, bromophenol blue and methyl red) is described.

Simultaneous Separation of Negatively and Positively Charged Species in Dynamic Field Gradient Focusing Using a Dual Polarity Electric Field

Dynamic field gradient focusing (DFGF) utilizes an electric field gradient established by a computer-controlled electrode array to separate and concentrate charged analytes at unique axial positions. Traditionally, DFGF has been restricted to the analysis of negatively charged species due to limitations in the software of our voltage controller. This paper introduces a new voltage controller capable of operating under normal polarity (positive potentials applied to the electrode array) and reversed polarity (negative potentials applied to the electrode array) for the separation of negatively and positively charged analytes, respectively. The experiments conducted under normal polarity and reversed polarity illustrate the utility of the new controller to perform reproducible DFGF separations (elution times showing less than 1% run-to-run variation) over a wide pH range (3.08 to 8.5) regardless of the protein charge. A dual polarity experiment is then shown in which the separation channel has been divided into normal polarity and reversed polarity regions. This simultaneous separation of negatively charged R-phycoerythrin (R-PE) and positively charged cytochrome c (CYTC) within the same DFGF apparatus is shown.

Nonlinear modeling of protein separation in a preparative‐scale dynamic field gradient focusing instrument

AbstractDynamic field gradient focusing (DFGF) uses an electric field gradient opposed by a counter‐flow of buffer to separate milligrams of proteins according to their electrophoretic mobilities. A nonlinear model of protein separation in a preparative‐scale DFGF device was developed to aid in refining the instrument's design and finding optimal run conditions prior to performing experiments. The model predicted the focal points of bovine serum albumin (BSA), and bovine hemoglobin (Hb) to within the 95% confidence intervals about the means of the experimental values. The resolution between the proteins in the model was 2.08, which was 3% less than the lower limit of the 95% confidence interval about the experimental value. The model predicted 67% more dispersion than was present in the experimental device, which made the simulated BSA peak 22% wider than the experimentally measured width. © 2008 American Institute of Chemical Engineers AIChE J, 2009

Tailoring the Network Properties of Ca2+ Crosslinked Aloe vera Polysaccharide Hydrogels for in Situ Release of Therapeutic Agents

Properties of Aloe vera galacturonate hydrogels formed via Ca(2+) crosslinking have been studied in regard to key parameters influencing gel formation including molecular weight, ionic strength, and molar ratio of Ca(2+) to COO(-) functionality. Dynamic oscillatory rheology and pulsed field gradient NMR (PFG-NMR) studies have been conducted on hydrogels formed at specified Ca(2+) concentrations in the presence and absence of Na(+) and K(+) ions in order to assess the feasibility of in situ gelation for controlled delivery of therapeutics. Aqueous Ca(2+) concentrations similar to those present in nasal and subcutaneous fluids induce the formation of elastic Aloe vera polysaccharide (AvP) hydrogel networks. By altering the ratio of Ca(2+) to COO (-) functionality, networks may be tailored to provide elastic modulus (G') values between 20 and 20000 Pa. The Aloe vera polysaccharide exhibits time-dependent phase separation in the presence of monovalent electrolytes. Thus the relative rates of calcium induced gelation and phase separation become major considerations when designing a system for in situ delivery applications where both monovalent (Na(+), K(+)) and divalent (Ca(2+)) ions are present. PFG-NMR and fluorescence microscopy confirm that distinctly different morphologies are present in gels formed in the presence and absence of 0.15 M NaCl. Curve fitting of theoretical models to experimental release profiles of fluorescein labeled dextrans indicate diffusion rates are related to hydrogel morphology. These studies suggest that for efficient in situ release of therapeutic agents, polymer concentrations should be maintained above the critical entanglement concentration ( Ce, 0.60 wt %) when [Ca(2+)]/[COO(-)] ratios are less than 1. Additionally, the monovalent electrolyte concentration in AvP solutions should not exceed 0.10 M prior to Ca(2+) crosslinking.

Cover Picture: Electrophoresis 13/2008

AbstractIssue 13 is a regular issue including an Emphasis Section offering a series of 10 papers on “Fundamentals and Methodologies”. These papers are related to peptide isoelectric point calculation, DNA separation by MEKC, modeling mobility of apothioneins, protein expression in osteoarthritis, capillary coating, preparative separation of proteins by dynamic field gradient focusing, determination of pKa, speciation analysis by CE‐inductively coupled plasma MS, etc.

Meetings Diary: Electrophoresis 13/2008

Dynamic field gradient focusing uses an electric field gradient to separate and concentrate proteins in native buffers. A prototype preparative-scale dynamic field gradient focusing apparatus reproducibly separated hemoglobin and bovine serum albumin with a mean resolution of 2.64+/-0.503. Run-to-run variations in the hemoglobin's focal point and peak width appeared to be related to fluctuations in the shape of the electric field, rather than the 5% accuracy of the pump that provided the counter-flow in the separation annulus. The variation in the electric field gradient was probably due to the formation and expansion of an ion-depleted region at the top of the separation annulus.

Protein separation using preparative‐scale dynamic field gradient focusing

AbstractDynamic field gradient focusing uses an electric field gradient to separate and concentrate proteins in native buffers. A prototype preparative‐scale dynamic field gradient focusing apparatus reproducibly separated hemoglobin and bovine serum albumin with a mean resolution of 2.64 ± 0.503. Run‐to‐run variations in the hemoglobin's focal point and peak width appeared to be related to fluctuations in the shape of the electric field, rather than the 5% accuracy of the pump that provided the counter‐flow in the separation annulus. The variation in the electric field gradient was probably due to the formation and expansion of an ion‐depleted region at the top of the separation annulus.

Design and Construction of a Preparative‐Scale Dynamic Field Gradient Focusing Apparatus

A linear model is used to show that dynamic field gradient focusing (DFGF) can be scaled to preparative capacity, approximately O (10 mgs). This paper explains how the preparative-scale DFGF apparatus was designed and fabricated. Scaled-down experiments and mathematical modeling guided material selection and design changes during construction to increase the probability that the prototype preparative-scale DFGF apparatus would perform as intended. The finished prototype successfully focused bovine hemoglobin from an initial concentration of 6.82 to 15 mg/mL and allowed for 86% recovery of injected protein.

Characterization of voltage degradation in dynamic field gradient focusing.

Dynamic field gradient focusing (DFGF) is an equilibrium gradient method that utilizes an electric field gradient to simultaneously separate and concentrate charged analytes based on their individual electrophoretic mobilities. This work describes the use of a 2-D nonlinear, numerical simulation to examine the impact of voltage loss from the electrodes to the separation channel, termed voltage degradation, and distortions in the electric field on the performance of DFGF. One of the design parameters that has a large impact on the degree of voltage degradation is the placement of the electrodes in relation to the separation channel. The simulation shows that a distance of about 3 mm from the electrodes to the separation channel gives the electric field profile with least amount of voltage degradation. The simulation was also used to describe the elution of focused protein peaks. The simulation shows that elution under constant electric field gradient gives better performance than elution through shallowing of the electric field. Qualitative agreement between the numerical simulation and experimental results is shown. The simulation also illustrates that the presence of a defocusing region at the cathodic end of the separation channel causes peak dispersion during elution. The numerical model is then used to design a system that does not suffer from a defocusing region. Peaks eluted under this design experienced no band broadening in our simulations. Preliminary experimental results using the redesigned chamber are shown.

Assessing the scalability of dynamic field gradient focusing by linear modeling

Dynamic field gradient focusing (DFGF) separates and concentrates proteins in native buffers, where proteins are most soluble, using a computer-controlled electric field gradient which lets the operator adjust the pace and resolution of the separation in real-time. The work in this paper assessed whether DFGF could be scaled up from microgram analytical-scale protein loads to milligram preparative-scale loads. Linear modeling of the electric potential, protein transport, and heat transfer simulated the performance of a preparative-scale DFGF instrument. The electric potential model showed where the electrodes should be placed to optimize the shape and strength of the electric field gradient. Results from the protein transport model suggested that in 10 min the device should separate 10 mg each of two proteins whose electrophoretic mobilities differ by 5 x. Proteins with electrophoretic mobilities differing by only 5% should separate in 3 h. The heat transfer model showed that the preparative DFGF design could dissipate 1 kW of Joule heat while keeping the separation chamber at 25 degrees C. Model results pointed to DFGF successfully scaling up by 1000 x using the proposed instrument design.

Towards a miniaturised system for dynamic field gradient focused separation of proteins

Separation and focusing of proteins is described in a miniaturised dynamic field gradient focusing device with a 2.5 cm × 0.1 cm channel filled with a porous polymer monolith. The separation channel is in contact with a parallel electric field channel with five individually addressable electrodes through a porous glass membrane so that a variable field can be generated that drives charged proteins electroosmotically against a constant hydrodynamic flow. Separated pre-stained proteins were detected by means of a digital camera and background subtraction.

Nuclear acoustic resonance measurements of the temperature dependent dynamic electric field gradient V44 in palladium

The authors report on the temperature dependence of the relative electron contribution r44 to the dynamic electric field gradient V44 in single-crystal palladium. For this, NAR2 lineshape data (NAR identical to nuclear acoustic resonance) are presented in the temperature range between 11 and 210 K. By extrapolation, the value of r44 at very low temperature is found to fit well the systematic dependence of r44 on N(EF) among the d transition metals.

Relaxation op quadrupolar energy in cubic crystals

In Ideal cubic crystals there is no quadrupolar splitting of Zeeman nuclear spin levels since electric field gradient on nucleus equals zero. But if the nuclear spin-system is exited by the resonant acoustic field there is a principle difference in the energy level spectrum in the rotating frame. The spectrum should be determined by interaction between nuclear quadrupolar moment and dynamic electric field gradient on the nucleus, created by acoustic strains. Spin-lattice relaxation time of this quadrupolar energy should be different from the Zeeman energy one. This difference was firstly detected experimentally on 23Na spin-system in NaCl single crystal.

Interaction of Acoustic Phonons with Nuclear Spins in KTaO3

The nuclear spin resonance of $^{181}\mathrm{Ta}$ in a single crystal of KTa${\mathrm{O}}_{3}$ has been studied using the technique of direct ultrasonic excitation. Nuclear resonances have been observed for transitions where $\ensuremath{\Delta}m=\ifmmode\pm\else\textpm\fi{}1 \mathrm{and} \ifmmode\pm\else\textpm\fi{}2$, where $m$ is the magnetic quantum number. Longitudinal and both polarizations of transverse waves, set up along the [100] axis of the cubic crystal, were used to excite the nuclear spin resonances. The interaction which leads to the absorption of energy by the nuclear spins from the externally generated phonons is via the torque exerted on the nuclear electric quadrupole moment by the dynamic electric field gradient created by the passage of the sound wave. Expressions for the ultrasonic absorption by the nuclear spins are presented, and the predictions of the angular dependence of the resonant intensity are compared with experiment. The agreement between theory and experiment is found to be fair, and possible reasons for the discrepancy are suggested.