N. Chandrakumar

Indian Institute of Technology Madras, Chennai, Tamil Nādu, India

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Publications (20)27.29 Total impact

  • Christy George, N. Chandrakumar
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    ABSTRACT: We report a novel rare spin correlation experiment termed ADEQUATE with composite refocusing (CR), which is the 1H-detected version of 2D INADEQUATE CR. ADEQUATE CR begins with a polarization transfer from protons to the attached carbon, followed by 13C–13C double-quantum (DQ) preparation. Unlike the ADEQUATE class of experiments, 13C DQ coherence is converted after evolution to single-quantum single transitions (SQ-STs) by CR. 13C SQ-ST is then transferred back to the coupled protons by a coherence order selective reconversion. The present sequence produces partial transition selectivity in the 1H dimension as does 1H Indirect detected 13C Low-Abundance Single-transition correlation Spectroscopy (HICLASS), thereby mitigating the reduction in sensitivity enhancement because of the presence of homonuclear proton couplings. However, unlike HICLASS (which is an experiment that involves SQ-TS evolution), no homonuclear zero quantum mixing is required on the 13C channel in the present experiment. Experimental results are demonstrated on a variety of samples, establishing the efficiency of the proposed method. Copyright © 2014 John Wiley & Sons, Ltd.
    Magnetic Resonance in Chemistry 03/2014; · 1.53 Impact Factor
  • Abhishek Banerjee, N Chandrakumar
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    ABSTRACT: Two dimensional homonuclear (1)H correlation spectroscopy is of considerable interest for volume localized spectral studies, both in vivo and in vitro, of biological as well as material objects. The information principally sought from correlation spectra resides in the cross-peaks, which are often masked however by the presence of diagonal peaks in COSY, or 'pseudo-diagonal' peaks at F1=0 in SECSY. It has therefore been a concern to suppress these diagonal or 'pseudo-diagonal' peaks, in order to ensure that cross-peak information is fully discernible. We present here a report of our work on volume localized DIagonal Suppressed Spin Echo Correlation specTroscopy (LDISSECT) and demonstrate its performance in comparison to the standard volume localized SECSY experiment, employing brain metabolite phantoms in a gel. The sequence works in the inhomogeneous, multi-component environment by exploiting the short acquisition time to suppress undesired information by employing an additional rf pulse. A brief description of the pulse sequence, its theory, and simulations are also included, besides experimental benchmarking on two brain metabolite phantoms in gel phase.
    Journal of Magnetic Resonance 12/2013; 239C:69-74. · 2.30 Impact Factor
  • Christy George, N. Chandrakumar
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    ABSTRACT: The influence of homonuclear scalar couplings on multiple quantum coherences evolving under isotropic mixing is investigated. In suitable clusters of four or more spins-1/2 (or two or more spins >½), zero and multiple quantum coherences are shown to evolve at high frequencies that involve multiples of the coupling constant. This behavior under spin lock is consequent on the cluster being in a superposition of spin coupling eigenstates that differ in composite spin by more than one unit. The resulting accelerated spin dynamics also offers opportunity to measure unresolved couplings in MRS and in high resolution NMR.
    Chemical Physics Letters 03/2013; 563:107–111. · 2.15 Impact Factor
  • Christy George, N Chandrakumar
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    ABSTRACT: A novel proton-detected (13)C homonuclear correlation experiment is reported at natural abundance, viz., (1)H Indirect detected (13)C Low-Abundance Single-transition correlation Spectroscopy (HICLASS). HICLASS is based on the evolution of (13)C single-quantum single transitions, followed by their mixing, and (1)H detection subsequent to heteronuclear transfer. Reduced relaxation losses during the evolution time and partial selectivity in the (1)H multiplet structure result in enhanced sensitivity of HICLASS. The superior performance of HICLASS is demonstrated for (1)H-detected (13)C correlation work.
    Organic Letters 09/2011; 13(20):5448-51. · 6.14 Impact Factor
  • Abhishek Banerjee, N Chandrakumar
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    ABSTRACT: Some novel techniques for volume localized, chemical shift selective (13) C spectroscopy are described in this work. These techniques are based on rotating frame J cross polarization and are reported for both direct and indirect modes of (13) C detection. The performance of two selective mixing sequences, viz., pulsed rotating frame transfer sequences with windows (PRAWN) and PRAWN-π has been studied systematically with different liquid and gel phantoms. Two different front-end modules are used for volume localization, viz., point resolved spectroscopy (PRESS) and localized distortionless enhancement by polarization transfer (LODEPT). It is shown experimentally that both the selective J cross polarization sequences can operate efficiently with very low radiofrequency duty cycle; further, they have considerable tolerance to Hartmann-Hahn mismatch. A simple theoretical analysis is also presented to understand J cross-polarization dynamics at low RF field amplitudes. Finally, the performance of LODEPT-PRAWN-π is demonstrated for the selective detection of saturated fat in pigeon egg in indirect detection mode.
    Magnetic Resonance in Medicine 04/2011; 66(5):1209-17. · 3.27 Impact Factor
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    ABSTRACT: A new class of bio-composite polymer electrolyte membranes comprising chitosan (CS) and certain biomolecules in particular, plant hormones such as 3-indole acetic acid (IAA), 4-chlorophenoxy acetic acid (CAA) and 1-naphthalene acetic acid (NAA) are explored to realize proton-conducting bio-composite membranes for application in direct methanol fuel cells (DMFCs). The sorption capability, proton conductivity and ion-exchange capacity of the membranes are characterized in conjunction with their thermal and mechanical behaviour. A novel approach to measure the permeability of the membranes to both water and methanol is also reported, employing NMR imaging and volume localized NMR spectroscopy, using a two compartment permeability cell. A DMFC using CS-IAA composite membrane, operating with 2M aqueous methanol and air at 70°C delivers a peak power density of 25 mW/cm2 at a load current density of 150 mA/cm2. The study opens up the use of bio-compatible membranes in polymer-electrolyte-membrane fuel cells.
    Journal of The Electrochemical Society. 12/2010; 158(11):B1319-B1328.
  • Christy George, Narayanan Chandrakumar
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    ABSTRACT: We report the implementation of our novel rare-spin homonuclear correlation experiment, namely, Low-Abundance Single-transition correlation SpectroscopY (LASSY), for (119/117/115)Sn NMR at natural abundance. Our pulse sequence results in diagonal suppressed COSY-style display and outperforms the optimal homonuclear correlation experiment for rare spins, which involves double quantum evolution (INADEQUATE CR). The new experiment maximizes efficiency both in respect of pulse transformations as well as relaxation effects, and gives rise to a simplified two-dimensional (2D) spectrum with considerably reduced crowding, exhibiting only one transition in each cross peak, instead of four. Performance optimization of LASSY is carried out in light of the relatively 'large' line widths typical of Sn NMR in solution state. The superior performance of the sequence is demonstrated on dimeric tetraorganodistannoxane samples.
    Magnetic Resonance in Chemistry 09/2010; 48(12):912-7. · 1.53 Impact Factor
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    N Chandrakumar
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    ABSTRACT: Dynamic nuclear polarization was first predicted — and, shortly thereafter, established experimentally — in 1953, the first demonstration being on Lithium metal. The basic approach involves the saturation of the ESR of a paramagnetic species in the system, while the NMR is observed. Initial applications of DNP involved low and moderate field studies that focused especially on investigations of molecular hydrodynamics. Applications to MRI provided a subsequent fillip to the technique. In the meanwhile, the closely related nuclear Overhauser effect (NOE) — which involves saturation, as well as observation of different NMR signals — had become an essential technique for the structure elucidation of both small molecules, as well as biomolecules. Most recently, DNP is witnessing rejuvenation, with high field applications to sensitivity enhancement in NMR. We present in the following an overview of Dynamic nuclear polarization (DNP). The elementary general theory of the phenomenon is discussed. Four different DNP mechanisms that are currently recognized are briefly introduced and different modes of the experiment — involving either cw ESR irradiation, or pulsed ESR excitation — are pointed out. A brief run down of various possible implementations is presented, including our own early work at moderate fields in cw mode, as well as hardware configurations and requirements for high field DNP. Different current implementations of DNP experiments are summarized, including solid state, as well as in situ and ex situ dissolution DNP variants. Typical results of DNP enhanced high resolution NMR are then briefly discussed, including the results of our own early work on differential 19 F enhancements at moderate fields. Design of free radicals that satisfy the requirements to establish an efficient cross effect DNP is discussed. Recent experiments that have succeeded in detecting an intermediate in the photocycle of bacteriorhodopsin are alluded to. Finally, the implementation of ultrafast multi-dimensional NMR techniques under DNP conditions is briefly discussed, as an approach to further exploitation of the prospects that are on offer.
    01/2010;
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    ABSTRACT: Stabilized forms of heteropolyacids (HPAs), namely phosphomolybdic acid (PMA), phosphotungstic acid (PTA), and silicotungstic acid (STA), are incorporated into poly (vinyl alcohol) (PVA) cross-linked with sulfosuccinic acid (SSA) to form mixed-matrix membranes for application in direct methanol fuel cells (DMFCs). Bridging SSA between PVA molecules not only strengthens the network but also facilitates proton conduction in HPAs. The mixed-matrix membranes are characterized for their mechanical stability, sorption capability, ion-exchange capacity, and wetting in conjunction with their proton conductivity, methanol permeability, and DMFC performance. Methanol-release kinetics is studied ex situ by volume-localized NMR spectroscopy (employing point-resolved spectroscopy'') with the results clearly demonstrating that the incorporation of certain inorganic fillers in PVA-SSA viz., STA and PTA, retards the methanol-release kinetics under osmotic drag compared to Nafion, although PVA-SSA itself exhibits a still lower methanol permeability. The methanol crossover rate for PVA-SSA-HPA-bridged-mixed-matrix membranes decreases dramatically with increasing current density rendering higher DMFC performance in relation to a DMFC using a pristine PVA-SSA membrane. A peak power density of 150 mW/cm(2) at a load current density of 500 mA/cm(2) is achieved for the DMFC using a PVA-SSA-STA-bridged-mixed-matrix-membrane electrolyte. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3465653] All rights reserved.
    01/2010;
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    ABSTRACT: Spatially resolved NMR--especially volume-localized spectroscopy (VLS)is useful in various fields including clinical diagnosis, process monitoring, etc. VLS carries high significance because of its ability to identify molecular species and hence track molecular events. This paper reports the application of VLS at 200 MHz to study the postharvest ripening of sweet lime ( Citrus limettioides ) in situ, including a comparative study of normal and acetylene-mediated ripening. Localization to a cubic voxel of 64 microL was achieved with point-resolved spectroscopy (PRESS). Glucose, sucrose, fructose, and citric acid are found to be among the main constituents in the fruit. In the natural process, the sugar to acid ratio increases with ripening. Ethanol generation is seen to occur at a faster rate in acetylene-mediated ripening. Whereas NMR imaging experiments including parametric imaging (e.g., T(1) or T(2) maps) may be employed for "macro" monitoring of processes such as these, this work demonstrates that the molecular imprint of the process may be tracked noninvasively by VLS.
    Journal of Agricultural and Food Chemistry 02/2009; 57(4):1183-7. · 2.91 Impact Factor
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    ABSTRACT: Organic–inorganic composite membranes comprising Nafion with inorganic materials such as silica, mesoporous zirconium phosphate (MZP) and mesoporous titanium phosphate (MTP) are fabricated and evaluated as proton-exchange-membrane electrolytes for direct methanol fuel cells (DMFCs). For Nafion–silica composite membrane, silica is impregnated into Nafion matrix as a sol by a novel water hydrolysis process precluding the external use of an acid. Instead, the acidic nature of Nafion facilitates in situ polymerization reaction with Nafion leading to a uniform composite membrane. The rapid hydrolysis and polymerization reaction while preparing zirconia and titania sols leads to uncontrolled thickness and volume reduction in the composite membranes, and hence is not conducive for casting membranes. Nafion–MZP and Nafion–MTP composite membranes are prepared by mixing pre-formed porous MZP and MTP with Nafion matrix. MZP and MTP are synthesised by co-assembly of a tri-block co-polymer, namely pluronic-F127, as a structure-directing agent, and a mixture of zirconium butoxide/titanium isopropoxide and phosphorous trichloride as inorganic precursors. Methanol release kinetics is studied by volume-localized NMR spectroscopy (employing “point resolved spectroscopy”, PRESS), the results clearly demonstrating that the incorporation of inorganic fillers in Nafion retards the methanol release kinetics under osmotic drag. Appreciable proton conductivity with reduced methanol permeability across the composite membranes leads to improved performance of DMFCs in relation to commercially available Nafion-117 membrane.
    Journal of Membrane Science. 01/2009;
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    ABSTRACT: Composite membranes with mordenite (MOR) incorporated in poly vinyl alcohol (PVA)–polystyrene sulfonic acid (PSSA) blend tailored with varying degree of sulfonation are reported. Such a membrane comprises a dispersed phase of mordenite and a continuous phase of the polymer that help tuning the flow of methanol and water across it. The membranes on prolonged testing in a direct methanol fuel cell (DMFC) exhibit mitigated methanol cross-over from anode to the cathode. The membranes have been tested for their sorption behaviour, ion-exchange capacity, electrochemical selectivity and mechanical strength as also characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis. Water release kinetics has been measured by magnetic resonance imaging (NMR imaging) and is found to be in agreement with the sorption data. Similarly, methanol release kinetics studied by volume-localized NMR spectroscopy (point resolved spectroscopy, PRESS) clearly demonstrates that the dispersion of mordenite in PVA–PSSA retards the methanol release kinetics considerably. A peak power-density of 74mW/cm2 is achieved for the DMFC using a PVA–PSSA membrane electrolyte with 50% degree of sulfonation and 10wt.% dispersed mordenite phase. A methanol cross-over current as low as 7.5mA/cm2 with 2M methanol feed at the DMFC anode is observed while using the optimized composite membrane as electrolyte in the DMFC, which is about 60% and 46% lower than Nafion-117 and PVA–PSSA membranes, respectively, when tested under identical conditions.
    Journal of Membrane Science - J MEMBRANE SCI. 01/2009; 340(1):73-83.
  • V. Ramesh, C. George, N. Chandrakumar
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    ABSTRACT: An alternative approach for rare spin homonuclear correlation spectroscopy is proposed and examined, which does not involve double quantum evolution. It relies instead on double quantum filtration resulting in a set of single transitions. This is followed by evolution and zero quantum mixing, generating a diagonal suppressed, single transition correlation spectrum that attains an efficiency matching the bound on unitary coherence transfer, but in fact benefits from reduced relaxation losses. This generally results in modest signal intensity improvement in a given measurement time, compared to the most efficient INADEQUATE variant. Results on 13C are reported and confirm the expected behavior.
    Chemical Physics Letters 01/2009; 474(4):375-380. · 2.15 Impact Factor
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    Samanwita Pal, N. Chandrakumar
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    ABSTRACT: The paper reports studies of the dynamic process in pentachlorophenol (PCP) in the solid state by one-dimensional Nuclear Quadrupole Resonance (NQR) exchange spectroscopy employing shaped rf pulses. Pentachlorophenol exhibits intramolecular hydrogen bonding between the hydroxyl proton and one of the o-chlorines. Reorientation of the hydrogen bond with migration of the hydroxyl proton from one o-chlorine to the other is possible in a wide temperature range. In this process the two o-chlorines are exchanged; at the same time this implies the exchange of the two m-chlorines as well, which we have chosen to monitor, modeling it as a two-site exchange process. A detailed analysis of the appropriate Bloch-McConnell equation is performed to formulate the relevant kinetic matrix. The exchange pulse sequence implemented by us involves suitably modulated shaped RF pulses to achieve simultaneous on-resonance excitation of the two exchanging sites. This approach results in a clear definition of the exchange time window and requires only a short experiment time. Kinetic parameters, namely, the spin-lattice relaxation rate and the exchange rate are deduced from the experimental signal intensities. Variable temperature measurements in the range 213 K–318 K have been carried out to extract the activation parameters as well.
    Molecular Physics 01/2009; 107(14):1505-1511. · 1.67 Impact Factor
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    Samanwita Pal, N. Chandrakumar
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    ABSTRACT: Recent work on the nuclear quadrupole resonance (NQR) investigation of molecular dynamics in the solid state has relied on 2D methods. We report our studies of dynamic processes by 1D shaped pulse NQR spectroscopy. Significant advantages include considerably shorter experimental duration, clear definition of the exchange time window, and avoidance of off-resonance effects. The reorientation of the Cl3C˜ group in polycrystalline chloral hydrate [Cl3C–CH(OH)2] is considered as a test case. This may be modelled as a three-site exchange process. An analysis of the generalised Bloch–McConnell equation is performed to formulate the kinetic matrix. The present approach involves simultaneous excitation of the sites that undergo chemical exchange by employing a suitably modulated shaped RF pulse, followed by a mixing time, and finally a suitable read pulse for signal detection. The experimental signal intensities are plotted against the mixing time to extract the kinetic parameters, i.e. the exchange rate and the spin-lattice relaxation rate. Variable temperature measurements are carried out to determine the activation parameters. Short experiment times are possible in our 1D mode, enabling a large number of runs to be readily performed as a function of mixing time and temperature. The kinetic and activation parameters obtained in the case of chloral hydrate are in good agreement with recent literature values.
    Molecular Physics. 03/2008; 106(6):759-767.
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    ABSTRACT: A membrane with interpenetrating networks between poly(vinyl alcohol) (PVA) and poly(styrene sulfonic acid) (PSSA) coupled with a high proton conductivity is realized and evaluated as a proton exchange membrane electrolyte for a direct methanol fuel cell (DMFC). Its reduced methanol permeability and improved performance in DMFCs suggest the new blend as an alternative membrane to Nafion membranes. The membrane has been characterized by powder X-ray diffraction, scanning electron microscopy, time-modulated differential scanning calorimetry, and thermogravimetric analysis in conjunction with its mechanical strength. The maximum proton conductivity of 3.3×10−2 S/cm for the PVA–PSSA blend membrane is observed at 373 K. From nuclear magnetic resonance imaging and volume localized spectroscopy experiments, the PVA–PSSA membrane has been found to exhibit a promising methanol impermeability, in DMFCs. On evaluating its utility in a DMFC, it has been found that a peak power density of 90 mW/cm2 at a load current density of 320 mA/cm2 is achieved with the PVA–PSSA membrane compared to a peak power density of 75 mW/cm2 at a load current density of 250 mA/cm2 achievable for a DMFC employing Nafion membrane electrolyte while operating under identical conditions; this is attributed primarily to the methanol crossover mitigating property of the PVA–PSSA membrane.
    01/2008;
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    V. Vimalan, N. Chandrakumar
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    ABSTRACT: We have developed methodology for NMR quantum computing focusing on enhancing the efficiency of initialization, of logic gate implementation and of readout. Our general strategy involves the application of rotating frame pulse sequences to prepare pseudopure states and to perform logic operations. We demonstrate experimentally our methodology for both homonuclear and heteronuclear spin ensembles. On model two-spin systems, the initialization time of one of our sequences is three-fourths (in the heteronuclear case) or one-fourth (in the homonuclear case), of the typical pulsed free precession sequences, attaining the same initialization efficiency. We have implemented the logical SWAP operation in homonuclear AMX spin systems using selective isotropic mixing, reducing the duration taken to a third compared to the standard re-focused INEPT-type sequence. We introduce the 1D version for readout of the rotating frame SWAP operation, in an attempt to reduce readout time. We further demonstrate the Hadamard mode of 1D SWAP, which offers 2N-fold reduction in experiment time for a system with N-working bits, attaining the same sensitivity as the standard 1D version.
    Journal of Chemical Sciences 01/2008; 120(1):57-69. · 1.30 Impact Factor
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    ABSTRACT: Poly(styrene sulfonic acid) (PSSA) content in poly(vinyl alcohol) (PVA) and PSSA blend membrane is varied and its effect on proton conductivity is studied at varying relative humidity (RH) values. The maximum proton conductivity is observed for the PVA–PSSA membrane with about 35 wt. % PSSA at all humidity values. At 30% RH value, the conductivity of PVA–PSSA blend membrane is 1.20 × 10−3 S/cm, which is about two orders of magnitude higher than the conductivity value of 2.27 × 10−5 S/cm observed for pristine PVA membrane. Water self-diffusion coefficients and water release kinetics of these materials have been characterized by nuclear magnetic resonance (NMR) imaging technique, which validate the use of this membrane in polymer electrolyte fuel cells (PEFCs). A peak power density of 210 mW/cm2 at a load current-density of 500 mA/cm2 is achieved for the PEFC with the optimized PVA–PSSA membrane as electrolyte compared to a peak power density of only 38 mW/cm2 observed at a load current-density of 80 mA/cm2 for the PEFC with pristine PVA membrane as electrolyte while operating at 75 °C with H2 and O2 feeds to the fuel cell maintained at atmospheric pressure.
    Journal of Membrane Science. 01/2008;
  • V Ramesh, N Chandrakumar
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    ABSTRACT: We propose and demonstrate a 1D pulse sequence to convert double quantum coherence (DQC) of y phase with optimal efficiency, relying on single transition selection. Our sequence has a larger high-sensitivity bandwidth with respect to the coupling, compared to other reconversion strategies. A modified version of the new pulse sequence provides the missing chemical shift and coupling information, at minor cost in sensitivity. Application to 1D 13C INADEQUATE is demonstrated. Our new sequence is also applied to quadrupole coupled spin-1 systems, such as 2H in lyotropic phase. Performance of the sequence may be fine-tuned by pulse flip angle optimization, taking into account relaxation effects.
    Magnetic Resonance in Chemistry 11/2006; 44(10):936-42. · 1.53 Impact Factor
  • Abhishek Banerjee, N. Chandrakumar
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    ABSTRACT: An approach to measure in vitro drug dissolution rates employing spatially resolved nuclear magnetic resonance (NMR) is reported, as a complement to standard United States Pharmacopeia protocols. Measurements are performed under conditions that mimic the physiological (pH 1, temperature 37° C). In order to register realistic dissolution rates, the sample is stirred in the magnetic field, employing a setup that is described. While the stirrer in the sample cell does degrade spectral resolution even in volume-localized mode, it proves possible nevertheless to acquire ‘high resolution’ information. Measurements are performed employing ‘point resolved spectroscopy’ (PRESS) by tracking drug concentration in a selected voxel in the dissolution medium as a function of time. The dissolution of the tablets in vitro follows first-order kinetics under the non-sink conditions of these experiments, the measured rate constants reflecting characteristic differences in dissolution rates of different formulations. We supplement this study with diffusion-weighted imaging of water ingress into the tablets, confirming the trends.
    Applied Magnetic Resonance 40(2):251-259. · 0.83 Impact Factor