Monitoring Heterogeneously Catalyzed Hydrogenation Reactions at Elevated Pressures Using In-Line Flow NMR.

We present a novel setup that can be used for the in-line monitoring of solid-catalyzed gas-liquid reactions. The method combines the high sensitivity and resolution of a stripline NMR detector with a microfluidic network that can withstand elevated pressures. In our setup we dissolve hydrogen gas in the solvent, then flow it with the added substrate through a catalyst cartridge, and finally flow the reaction mixture directly through the stripline NMR detector. The method is quantitative and can be used to determine the solubility of hydrogen gas in liquids; it allows in-line monitoring of hydrogenation reactions and can be used to determine the reaction kinetics of these reactions. In this work, as proof of concept we demonstrate the optimization of the Pd-catalyzed hydrogenation reactions of styrene, phenylacetylene, cyclohexene, and hex-5-en-2-one in a microfluidic context.

Inline Reaction Monitoring of Amine-Catalyzed Acetylation of Benzyl Alcohol Using a Microfluidic Stripline Nuclear Magnetic Resonance Setup.

We present an in-depth study of the acetylation of benzyl alcohol in the presence of N, N-diisopropylethylamine (DIPEA) by nuclear magnetic resonance (NMR) monitoring of the reaction from 1.5 s to several minutes. We have adapted the NMR setup to be compatible to microreactor technology, scaling down the typical sample volume of commercial NMR probes (500 µL) to a microfluidic stripline setup with 150 nL detection volume. Inline spectra are obtained to monitor the kinetics and unravel the reaction mechanism of this industrially relevant reaction. The experiments are combined with conventional 2D NMR measurements to identify the reaction products. In addition, we replace DIPEA with triethylamine and pyridine to validate the reaction mechanism for different amine catalysts. In all three acetylation reactions, we find that the acetyl ammonium ion is a key intermediate. The formation of ketene is observed during the first minutes of the reaction when tertiary amines were present. The pyridine-catalyzed reaction proceeds via a different mechanism.

Hyphenation of Supercritical Fluid Chromatography and NMR with In-Line Sample Concentration.

By coupling supercritical fluid chromatography (SFC) and nuclear magnetic resonance (NMR) in-line, a powerful analytical method arises that enables chemically specific analysis of a broad range of complex mixtures. However, during chromatography, the compounds are diluted in the mobile phase, in this case supercritical CO2 (scCO2), often resulting in concentrations that are too low to be detected by NMR spectroscopy or at least requiring excessive signal averaging. We present a hyphenated SFC-NMR setup with an integrated approach for concentrating samples in-line, which are diluted in scCO2 during chromatography. This in-line concentration is achieved by controlled in-line expansion of the scCO2. As a proof of concept four isomers of vitamin E (tocopherol) were isolated by SFC, concentrated in-line by expanding CO2 from 120 to 50 bar, and finally shuttled to the NMR spectrometer fitted with a dedicated probehead for spectroscopic characterization of microfluidic samples. The abundant isomers were readily detected, supporting the viability of SFC-NMR as a powerful analytical tool.

Shim-on-Chip Design for Microfluidic NMR Detectors.

In this contribution we present a novel system for shimming capillary samples such as used in microfluidic NMR probe heads. Due to the small sample size, shimming microliter samples using regular shim coils is complicated. Here we demonstrate the use of a series of parallel wires placed perpendicular to B0 as a Shim-on-Chip shim system. This is achieved by placing a ribbon flat cable horizontally over the NMR detector, in our case a stripline. The current through each wire of the ribbon cable can be controlled independently employing a 16 channel DAC. This makes for a simple, cheap, and easy to construct alternative to regular shim systems. The Shim-on-Chip is, nevertheless, quite flexible in creating a magnetic field which matches the inhomogeneity of the magnet in one dimension. The capillary sample geometry is well suited for this type of shimming since its length  is much larger than its width. With this Shim-on-Chip system we have reached line widths of 2.2 Hz (at 50%) and 27 Hz (at 0.55%) on a 144 MHz NMR spectrometer without any other room temperature shims. Unlike regular shims, the Shim-on-Chip is located inside the NMR probe. It is always centered on the NMR sample, because of this the shims have an intuitive effect on the line shape. Therefore, the manual shimming is simpler when compared to a regular shim system, as it is difficult to position a microliter sample in the exact center of the shim coils. We furthermore demonstrate the use of a Shim-on-Chip method in a 400 MHz Rapid-Melt DNP system. Decent line widths were achieved even for a sample which is located off-center inside the NMR magnet.

B1 gradient coherence selection using a tapered stripline.

Pulsed-field gradients are common in modern liquid state NMR pulse sequences. They are often used instead of phase cycles for the selection of coherence pathways, thereby decreasing the time required for the NMR experiment. Soft off-resonance pulses with a B1 gradient result in a spatial encoding similar to that created by pulsed-field (B0) gradients. In this manuscript we show that pulse sequences with pulsed-field gradients can easily be converted to one which uses off-resonance B1 field gradient (OFFBEAT) pulses. The advantage of B1 gradient pulses for coherence selection is that the chemical shift evolution during the pulses is (partially) suppressed. Therefore no refocusing echos are required to correct for evolution during the gradient pulses. A tapered stripline is shown to be a convenient tool for creating a well-defined gradient in the B1 field strength. B1 gradient coherence selection using a tapered stripline is a simple and cheap alternative to B0 pulsed-field gradients.

Continuous Flow 1H and 13C NMR Spectroscopy in Microfluidic Stripline NMR Chips.

Microfluidic stripline NMR technology not only allows for NMR experiments to be performed on small sample volumes in the submicroliter range, but also experiments can easily be performed in continuous flow because of the stripline's favorable geometry. In this study we demonstrate the possibility of dual-channel operation of a microfluidic stripline NMR setup showing one- and two-dimensional 1H, 13C and heteronuclear NMR experiments under continuous flow. We performed experiments on ethyl crotonate and menthol, using three different types of NMR chips aiming for straightforward microfluidic connectivity. The detection volumes are approximately 150 and 250 nL, while flow rates ranging from 0.5 µL/min to 15 µL/min have been employed. We show that in continuous flow the pulse delay is determined by the replenishment time of the detector volume, if the sample trajectory in the magnet toward NMR detector is long enough to polarize the spin systems. This can considerably speed up quantitative measurement of samples needing signal averaging. So it can be beneficial to perform continuous flow measurements in this setup for analysis of, e.g., reactive, unstable, or mass-limited compounds.

A multi-nuclear magnetic resonance and density functional theory investigation of epitaxially grown InGaP2.

In this paper the short and long range order in In0.483Ga0.517P thin films is investigated by solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. To this end two samples were grown on a GaAs substrate by metal-organic vapor phase epitaxy at two different growth-pressures. From band gap energy measurements, CuPt long range order parameters of SCuPt = 0.22 and 0.39 were deduced, respectively. In the (31)P spectrum five resonances are observed corresponding to the five possible P(GanIn4-n), n = 0-4, coordinations whose relative intensities correspond to the order in the material, but the intensity variations for order parameters between 0 and 0.5 are minimal. (69)Ga, (71)Ga and (115)In (MQ)MAS spectra were acquired to analyze the quadrupolar and chemical shift distributions related to the (dis)order in these materials in more detail. All these spectra clearly reflect the disorder in the sample and do not show the presence of highly ordered domains. The difference in the order parameter in the sample is not clearly reflected in the spectra. (31)P chemical shifts were calculated using Density Functional Theory. The experimentally observed shifts are well reproduced with a simple random model of the disorder, thus confirming the assignment of the resonances. The (31)P chemical shifts are very sensitive to changes in the lattice parameter and chemical surroundings. These effects nearly compensate and explain why the (31)P chemical shifts in pure InP and GaP are nearly identical whereas a large difference would be expected based on the observed shift difference for the P[In4] and P[Ga4] coordinations in In0.483Ga0.517P.

Towards Overhauser DNP in supercritical CO(2).

Overhauser Dynamic Nuclear Polarization (ODNP) is a well known technique to improve NMR sensitivity in the liquid state, where the large polarization of an electron spin is transferred to a nucleus of interest by cross-relaxation. The efficiency of the Overhauser mechanism for dipolar interactions depends critically on fast local translational dynamics at the timescale of the inverse electron Larmor frequency. The maximum polarization enhancement that can be achieved for (1)H at high magnetic fields benefits from a low viscosity solvent. In this paper we investigate the option to use supercritical CO2 as a solvent for Overhauser DNP. We have investigated the diffusion constants and longitudinal nuclear relaxation rates of toluene in high pressure CO2. The change in (1)H T1 by addition of TEMPO radical was analyzed to determine the Overhauser cross-relaxation in such a mixture, and is compared with calculations based on the Force Free Hard Sphere (FFHS) model. By analyzing the relaxation data within this model we find translational correlation times in the range of 2-4ps, depending on temperature, pressure and toluene concentration. Such short correlation times may be instrumental for future Overhauser DNP applications at high magnetic fields, as are commonly used in NMR. Preliminary DNP experiments have been performed at 3.4T on high pressure superheated water and model systems such as toluene in high pressure CO2.

Solid Effect DNP in a Rapid-melt setup.

Dynamic Nuclear Polarization (DNP) has become a key element in nuclear magnetic resonance (NMR). Recently, we developed a novel approach to DNP enhanced liquid-state NMR based on rapid melting of a solid hyperpolarized sample followed by 'in situ' liquid-state NMR detection. This method allows (1)H detection with fast cycling options for signal averaging. In nonpolar solvents, doped with BDPA radicals, proton enhancement factors were achieved of up to 400. A short recycling delay of about 5s allows for a fast determination of the hyper-polarization dynamics as function of the microwave frequency and power. Here, we use the rapid melt dnp method to study the mechanisms for DNP in the solid phase in more detail. Solid Effect, Cross Effect, Solid Overhauser and Liquid-state (supercritical) Overhauser DNP enhancement can be observed in the same setup. In this paper, we concentrate on Solid Effect DNP observed with both homogeneous narrow line radicals such as BDPA and with wide line anisotropic nitroxide radicals such as TEMPOL. We find indications that BDPA protons play an important role in Solid Effect DNP with this radical. A simplified spin diffusion model for BDPA can give a semi-quantitative description of the enhancements as function of the microwave power and as function of the proton concentration in the solid solution. For aqueous frozen samples we observe a similar Solid Effect DNP enhancement, which is analyzed within the simplified spin diffusion model.

Spatially resolved spectroscopy using tapered stripline NMR.

Magnetic field B0 gradients are essential in modern Nuclear Magnetic Resonance spectroscopy and imaging. Although RF/B1 gradients can be used to fulfill a similar role, this is not used in common practice because of practical limitations in the design of B1 gradient coils. Here we present a new method to create B1 gradients using stripline RF coils. The conductor-width of a stripline NMR chip and the strength of its radiofrequency field are correlated, so a stripline chip can be tapered to produce any arbitrary shaped B1 field gradient. Here we show the characterization of this tapered stripline configuration and demonstrate three applications: magnetic resonance imaging on samples with nL-µL volumes, reaction monitoring of fast chemical reactions (10(-2)-10(1)s) and the compensation of B0 field gradients to obtain high-resolution spectra in inhomogeneous magnetic fields.

Analysis of mass-limited mixtures using supercritical-fluid chromatography and microcoil NMR.

A protocol is presented for offline microfluidic NMR analysis hyphenated with supercritical chromatographic separation. The method demonstrates quantitative detection with good sensitivity. Typical sample amounts of 10 nanomoles can be detected in a fast and cost-effective manner.

Direct dynamic measurement of intracellular and extracellular lactate in small-volume cell suspensions with (13)C hyperpolarised NMR.

Hyperpolarised (HP) (13)C NMR allows enzymatic activity to be probed in real time in live biological systems. The use of in vitro models gives excellent control of the cellular environment, crucial in the understanding of enzyme kinetics. The increased conversion of pyruvate to lactate in cancer cells has been well studied with HP (13)C NMR. Unfortunately, the equally important metabolic step of lactate transport out of the cell remains undetected, because intracellular and extracellular lactate are measured as a single resonance. Furthermore, typical experiments must be performed using tens of millions of cells, a large amount which can lead to a costly and sometimes highly challenging growing procedure. We present a relatively simple set-up that requires as little as two million cells with the spectral resolution to separate the intracellular and extracellular lactate resonances. The set-up is tested with suspensions of prostate cancer carcinoma cells (PC3) in combination with HP [1-(13)C]pyruvate. We obtained reproducible pyruvate to lactate label fluxes of 1.2 and 1.7 nmol/s per million cells at 2.5 and 5.0 mM pyruvate concentrations. The existence of a 3-Hz chemical shift difference between intracellular and extracellular lactate enabled us to determine the lactate transport rates in PC3. We deduced a lactate export rate of 0.3 s(-1) and observed a decrease in lactate transport on addition of the lactate transport inhibitor α-cyano-4-hydroxycinnamic acid.

Rapid-melt Dynamic Nuclear Polarization.

In recent years, Dynamic Nuclear Polarization (DNP) has re-emerged as a means to ameliorate the inherent problem of low sensitivity in nuclear magnetic resonance (NMR). Here, we present a novel approach to DNP enhanced liquid-state NMR based on rapid melting of a solid hyperpolarized sample followed by 'in situ' NMR detection. This method is applicable to small (10nl to 1µl) sized samples in a microfluidic setup. The method combines generic DNP enhancement in the solid state with the high sensitivity of stripline (1)H NMR detection in the liquid state. Fast cycling facilitates options for signal averaging or 2D structural analysis. Preliminary tests show solid-state (1)H enhancement factors of up to 500 for H2O/D2O/d6-glycerol samples doped with TEMPOL radicals. Fast paramagnetic relaxation with nitroxide radicals, In nonpolar solvents such as toluene, we find proton enhancement factors up to 400 with negligible relaxation losses in the liquid state, using commercially available BDPA radicals. A total recycling delay (including sample freezing, DNP polarization and melting) of about 5s can be used. The present setup allows for a fast determination of the hyper-polarization as function of the microwave frequency and power. Even at the relatively low field of 3.4T, the method of rapid melting DNP can facilitate the detection of small quantities of molecules in the picomole regime.

Liquid state dynamic nuclear polarization of ethanol at 3.4 T (95 GHz).

Dynamic Nuclear Polarization (DNP) in the liquid state has become the focus of attention to improve the NMR sensitivity of mass limited samples. The Overhauser model predicts a fast reduction in DNP enhancement at high magnetic fields where the electron Larmor frequency exceeds the typical inverse correlation time of the magnetic interaction between an unpaired electron spin of a radical and proton spins of the solvent molecules. The Overhauser hard sphere model is able to predict quantitatively the DNP enhancement for water TEMPOL solutions. The increase in temperature due to dielectric heating of the sample acts to reduce the correlation times and allows a substantial Overhauser DNP. In this paper we extend the work done on water towards other small molecules, such as ethanol. Experimentally we observe a similar enhancement for all three proton groups in the ethanol molecule. The classical interpretation of the low field Overhauser experiments on ethanol invokes a very fast anisotropic rotation of the hydrogen bonded TEMPOL-ethanol complex to explain the fast relaxation of the OH proton. Here we will discuss W-band relaxation and DNP enhancement within this classical model. Although the description can be made quantitative, the invoked parameters do not seem to be realistic. We will propose an alternative model based on the dynamic interaction both in free collision and due to modulation of the hydrogen bond length of the complex.

Screening for chronic hepatitis B and C in migrants from Afghanistan, Iran, Iraq, the former Soviet Republics, and Vietnam in the Arnhem region, The Netherlands.

Migrants born in hepatitis B virus (HBV) and hepatitis C virus (HCV) endemic countries are at increased risk of being infected with these viruses. The first symptoms may arise when liver damage has already occurred. The challenge is to identify these infections early, since effective treatment has become available. In 2011 we conducted a screening project in first-generation migrants (FGMs) born in Afghanistan, Iran, Iraq, the former Soviet Republics, and Vietnam and living in Arnhem and Rheden. All participants were offered free blood screening for HBV and HCV. In total 959 participants were tested, with the country of origin known for 927, equating to 28·7% of all registered FGMs from the chosen countries. Nineteen percent (n = 176) had serological signs of past or chronic HBV infection and 2·2% (n = 21) had chronic HBV infection. The highest prevalence of chronic HBV infection was found in the Vietnamese population (9·5%, n = 12). Chronic HCV was found in two persons from the former Soviet Republics and one from Vietnam. Twenty-four percent (n = 5) of the newly identified patients with chronic HBV and one of the three patients with chronic HCV received treatment. Three of the patients, two with HCV and one with HBV, already had liver cirrhosis. The highest (9·5%) HBV prevalence was found in FGMs from Vietnam, indicating a high need for focusing on that particular immigrant population in order to identify more people with silent HBV infection. The fact that three patients already had liver cirrhosis underlines the necessity of early identification of HBV and HCV infection in risk groups.

Repetitive sideband-selective double frequency sweeps for sensitivity enhancement of MAS NMR of half-integer quadrupolar nuclei.

A sensitivity enhancement scheme aiming at selective adiabatic inversion of a single set of satellite transition sidebands under magic angle spinning has been employed on samples of albite containing a single moderately distorted (27)Al site and zoisite containing two highly distorted octahedral (27)Al sites. Overall enhancements of ∼2.5 for albite and ∼3 for the two AlO(6) sites of zoisite are reported by applying this scheme at different spinning speeds reflecting the versatility of this enhancement scheme which achieves significant signal-to-noise enhancements for the systems with moderately high quadrupolar coupling and high quadrupolar coupling. Repeating the sensitivity enhancement scheme and signal readout several times without allowing for spin-lattice relaxation leads to sensitivity enhancements of factors of ∼4 for albite and ∼5 for zoisite which substantially increases the detectability of the quadrupolar sites. The effectiveness of this scheme at high magnetic field under very fast magic angle spinning has been demonstrated. Finally, the possibility of performing spectral editing by selective enhancement of one of the quadrupolar sites in zoisite whilst keeping the other site unaffected has been explored.

Fibroblast growth factor 23 in hypophosphataemic HIV-positive adults on tenofovir.

OBJECTIVES: Hypophosphataemia is common in HIV-positive patients, in particular in those using tenofovir disoproxil fumarate (TDF). Its pathogenesis is not well understood. The importance of fibroblast growth factor 23 (FGF-23), the most potent phosphaturic hormone known today, has not been studied in these patients. The aim of the study was to investigate whether FGF-23 might be involved in the aetiology of hypophosphataemia in HIV-positive patients on tenofovir. METHODS: Calcium and phosphate metabolism was studied in 36 HIV-positive patients on TDF. Hypophosphataemia was defined as a serum phosphate level<0.75 mmol/L. RESULTS: Fifteen patients (42%) had hypophosphataemia (group 1), and 21 had a normal serum phosphate level (group 2). The renal phosphate reabsorption threshold [tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/gfr)] was significantly lower in group 1 than in group 2 (0.58 ± 0.04 vs. 0.91 ± 0.03 mmol/L, respectively; P<0.0001). The serum phosphate concentration was strongly correlated with TmP/gfr (R=0.71; P<0.0001). Both groups had normal serum FGF-23 levels, and serum phosphate and TmP/gfr were not related to serum parathyroid hormone (PTH) or FGF-23 levels. CONCLUSION: FGF-23 is not involved in the pathogenesis of hypophosphataemia in HIV-positive patients on TDF. The data suggest that a PTH-like factor may be involved.

Quantitative analysis of high field liquid state Dynamic Nuclear Polarization.

Dynamic Nuclear Polarization (DNP) in the liquid state has become the focus of attention to improve the NMR sensitivity of mass limited samples. The Overhauser model predicts a fast reduction in DNP enhancement at high magnetic fields where the Electron Larmor frequency exceeds the typical inverse correlation time of the magnetic interaction between a radical spin and proton spins of the water molecules. Recent experiments have shown that an appreciable DNP enhancement in the liquid state is possible also at magnetic fields of 3 to 9 Tesla. At present it is not clear whether the Overhauser model needs to be adapted to explain these results. In the present paper we aim to resolve this question by a combination of in situ temperature dependent NMR relaxation measurements, EPR and DNP experiments. Enhancement factors of up to -165 are obtained with microwave powers below 500 mW. We conclude that at 3.4 Tesla (95 GHz) the various measurements are consistent with each other and in quantitative agreement with Overhauser theory. Microwave heating of the sample does play an important role to reduce the correlation times and allow a substantial Overhauser DNP. The typical enhancement factors may allow new applications in microfluidic NMR.

The role of Ni in increasing the reversibility of the hydrogen release from nanoconfined LiBH4.

Nanoconfinement and the use of catalysts are promising strategies to enhance the reversibility of hydrogen storage in light metal hydrides. We combined nanoconfinement of LiBH4 in nanoporous carbon with the addition of Ni. Samples were prepared by deposition of 5-6 nm Ni nanoparticles inside the porous carbon, followed by melt infiltration with LiBH4. The Ni addition has only a slight influence on the LiBH4 hydrogen desorption, but significantly enhances the subsequent uptake of hydrogen under mild conditions. Reversible, but limited, intercalation of Li is observed during hydrogen cycling. X-ray diffraction shows that the initial crystalline 5-6 nm Ni nanoparticles are not present anymore after melt infiltration with LiBH4. However, transmission electron microscopy showed Ni-containing nanoparticles in the samples. Extended X-ray absorption fine structure spectroscopy proved the presence of Ni(x)B phases with the Ni-B coordination numbers changing reversibly with dehydrogenation and rehydrogenation of the sample. Ni(x)B can act as a hydrogenation catalyst, but solid-state 11B NMR proved that the addition of Ni also enhanced the reversibility of the system by influencing the microstructure of the nanoconfined LiBH4 upon cycling.

A solid-state NMR and DFT study of compositional modulations in Al(x)Ga(1-x)As.

We have conducted (75)As and (69)Ga Nuclear Magnetic Resonance (NMR) experiments to investigate order/disorder in Al(x)Ga(1-x)As lift-off films with x∼ 0.297 and 0.489. We were able to identify all possible As(Al(n)Ga(4-n)) sites with n = 0-4 coordinations in (75)As NMR spectra using spin-echo experiments at 18.8 Tesla. This was achieved by employing high rf field strengths using a small solenoid coil and an NMR probe specifically designed for this purpose. Spectral deconvolution, using an evolutionary algorithm, complies with the absence of long-range order if a CuAu based order parameter is imposed. An unconstrained fit shows a deviation of the statistics imposed by this type of ordering. The occupational disorder in the Ga and Al positions is reflected in a distribution of the Electric Field Gradients (EFGs) experienced at the different arsenic sites. We established that this can be modelled by summing the effects of the first coordination sphere and a Czjzek type distribution resulting from the compositional variation in the Al/Ga sub-lattice in the higher coordination spheres. (69)Ga 3QMAS and nutation data exclude the presence of highly symmetric sites and also show a distribution in EFG. The experimentally obtained quadrupolar interactions are in good agreement with calculations based on Density Functional Theory (DFT). Using additivity of EFG tensors arising from distant charge perturbations, we could use DFT to model the EFG distributions of the n = 0-4 sites, reproducing the Czjzek and extended Czjzek distributions that were found experimentally. On the basis of these calculations we conclude that the (75)As quadrupolar interaction is sensitive to compositional modulations up to the 7th coordination shell in these systems.