Implementing solenoid microcoils for wide-line solid-state NMR

Exploration of Materials for Three-Dimensional NMR Microcoil Production via CNC Micromilling and Laser Etching

The excellent versatility of 5-axis computer numerical control (CNC) micromilling has led to its application for prototyping NMR microcoils tailored to mass-limited samples (reducing development time and cost). However, vibrations during 5-axis milling can hinder the creation of complex 3D volume microcoils (i.e., solenoids and saddle coils). To address these limitations, a high-resolution NSCNC ELARA 4-axis milling machine was developed with the extra precision required for making complex 3D volume microcoils. Upon investigating the performance of resonators made with various copper-coated dielectrics, resonators with poly(methyl methacrylate) (PMMA) provided the best SNR/line shape. Thus, complex 1.7 mm microcoil designs were machined from Cu-coated PMMA. A milled 6.4 mm solenoid also provided 6.6× the total carbon signal for a 13C-labeled broccoli seed compared to a commercial inverse 5 mm NMR probe (demonstrating potential for larger coil designs). However, the manufacture of coils <1.7 mm with copper-coated PMMA rods was challenging as ∼0.5 mm of remaining PMMA was needed to retain their structural integrity. To manufacture smaller microcoils, both a solenoid and saddle coil (both with 1 mm O.D., 0.1 mm thick walls) were etched from Cu-coated glass capillaries using a UV picosecond laser that was mounted onto an NSCNC 5-axis MiRA7L. Both resonators showed excellent signal and identified a wide range of metabolites in a 13C-labeled algae extract, while the solenoid was further tested on two copepod egg sacs (∼4 μg of total sample). In summary, the flexibility to prototype complex microcoils in-house allows laboratories to tailor microcoils to specific mass-limited samples while avoiding the costs of cleanrooms.

An automated multi-order phase correction routine for processing ultra-wideline NMR spectra

Efficient acquisition of wideline solid-state nuclear magnetic resonance (NMR) spectra with patterns affected by large inhomogeneous broadening is accomplished with the use of broadband pulse sequences. These specialized pulse sequences often use frequency-swept pulses, which feature time-dependent phase and amplitude modulations that in turn deliver broad and uniform excitation across large spectral bandwidths. However, the resulting NMR spectra are often affected by complex frequency-dependent phase dispersions, owing to the interplay between the frequency-swept excitations and anisotropic resonance frequencies. Such phase distortions necessitate the use of multi-order non-linear corrections in order to obtain absorptive, distortion-free patterns with uniform phasing. Performing such corrections is often challenging due to the complex interdependence of the linear and non-linear phase contributions, and how these may affect the NMR signal. Hence, processing of these data usually involves calculating the spectra in magnitude mode wherein the phase information is discarded. Herein, we present a fully automated phasing routine that is capable of processing and phase correcting such wideline NMR spectra. Its performance is corroborated via processing of NMR data acquired using both the WURST-CPMG (Wideband, Uniform-Rate, Smooth Truncation with Carr-Purcell Meiboom-Gill acquisition) and BRAIN-CP (BRoadband Adiabatic Inversion Cross Polarization) pulse sequences for a variety of nuclei (i.e., 119Sn, 195Pt, 35Cl, 87Rb, and 14N). Based on both simulated and experimental NMR datasets, it is demonstrated that automatic phase corrections up to and including second order can be readily achieved without a priori information regarding the nature of the phase-distorted NMR datasets, and independently of the exact manner in which time-domain NMR data are collected and subsequently processed. In addition, it is shown that NMR spectra acquired at both single and multiple transmitter frequencies that are processed with this automated phasing routine have improved signal-to-noise properties than those processed with conventional magnitude calculations, along with powder patterns that better match those of ideal NMR spectra, even for datasets possessing low signal-to-noise ratios and/or affected by spectral artifacts.

The normalized limit of detection in NMR spectroscopy

We derive the normalized limit of detection for frequency space (nLOD_f) as a parameter to measure the sensitivity of an NMR spectroscopy setup. nLOD_f is independent of measurement settings such as bandwidth or number of measurement points, and allows to compare performances of different setups. We demonstrate the usefulness of the new nLOD_f by comparing the sensitivity of NMR setups from various publications, which all use microcoils. Finally, we want to propose a standard measurement and report format for the sensitivity of new NMR setups.

Recent progress in solid-state nuclear magnetic resonance of half-integer spin low-γ quadrupolar nuclei applied to inorganic materials

An overview is presented of recent progress in the solid-state nuclear magnetic resonance (NMR) observation of low-γ nuclei, with a focus on applications to inorganic materials. The technological and methodological advances in the last 20 years, which have underpinned the increased accessibility of low-γ nuclei for study by solid-state NMR techniques, are summarised, including improvements in hardware, pulse sequences and associated computational methods (e.g., first principles calculations and spectral simulation). Some of the key initial observations from inorganic materials of these nuclei are highlighted along with some recent (most within the last 10 years) illustrations of their application to such materials. A summary of other recent reviews of the study of low-γ nuclei by solid-state NMR is provided so that a comprehensive understanding of what has been achieved to date is available.

Proton-detected solid-state NMR spectroscopy of spin-1/2 nuclei with large chemical shift anisotropy

Constant-time (CT) dipolar heteronuclear multiple quantum coherence (D-HMQC) has previously been demonstrated as a method for proton detection of high-resolution wideline NMR spectra of spin-1/2 nuclei with large chemical shift anisotropy (CSA). However, ¹H transverse relaxation and t₁-noise often reduce the sensitivity of D-HMQC experiments, preventing the theoretical gains in sensitivity provided by ¹H detection from being realized. Here we demonstrate a series of improved pulse sequences for ¹H detection of spin-1/2 nuclei under fast MAS, with ¹⁹⁵Pt SSNMR experiments on cisplatin as an example. First, a t₁-incrementation protocol for D-HMQC dubbed Arbitrary Indirect Dwell (AID) is demonstrated. AID allows the use of arbitrary, rotor asynchronous t₁-increments, but removes the constant time period from CT D-HMQC, resulting in improved sensitivity by reducing transverse relaxation losses. Next, we show that short high-power adiabatic pulses (SHAPs), which efficiently invert broad MAS sideband manifolds, can be effectively incorporated into ¹H detected symmetry-based resonance echo double resonance (S-REDOR) and t₁-noise eliminated (TONE) D-HMQC experiments. The S-REDOR experiments with SHAPs provide approximately double the dipolar dephasing, as compared to experiments with rectangular inversion pulses. We lastly show that sensitivity and resolution can be further enhanced with the use of swept excitation pulses as well as adiabatic magic angle turning (aMAT).

Broadband radio-frequency transmitter for fast nuclear spin control

The active manipulation of nuclear spins with radio-frequency (RF) coils is at the heart of nuclear magnetic resonance (NMR) spectroscopy and spin-based quantum devices. Here, we present a miniature RF transmitter designed to generate strong RF pulses over a broad bandwidth, allowing for fast spin rotations on arbitrary nuclear species. Our design incorporates (i) a planar multilayer geometry that generates a large field of 4.35 mT per unit current, (ii) a 50 Ω transmission circuit with a broad excitation bandwidth of ∼20 MHz, and (iii) an optimized thermal management leading to minimal heating at the sample location. Using individual ¹³C nuclear spins in the vicinity of a diamond nitrogen-vacancy center as a test system, we demonstrate Rabi frequencies exceeding 70 kHz and nuclear π/2 rotations within 3.4 μs. The extrapolated values for ¹H spins are about 240 kHz and 1 μs, respectively. Beyond enabling fast nuclear spin manipulations, our transmitter system is ideally suited for the incorporation of advanced pulse sequences into micro- and nanoscale NMR detectors operating at a low (<1 T) magnetic field.

3D printed microchannels for sub-nL NMR spectroscopy

Nuclear magnetic resonance (NMR) experiments on subnanoliter (sub-nL) volumes are hindered by the limited sensitivity of the detector and the difficulties in positioning and holding such small samples in proximity of the detector. In this work, we report on NMR experiments on liquid and biological entities immersed in liquids having volumes down to 100 pL. These measurements are enabled by the fabrication of high spatial resolution 3D printed microfluidic structures, specifically conceived to guide and confine sub-nL samples in the sub-nL most sensitive volume of a single-chip integrated NMR probe. The microfluidic structures are fabricated using a two-photon polymerization 3D printing technique having a resolution better than 1 μm3. The high spatial resolution 3D printing approach adopted here allows to rapidly fabricate complex microfluidic structures tailored to position, hold, and feed biological samples, with a design that maximizes the NMR signals amplitude and minimizes the static magnetic field inhomogeneities. The layer separating the sample from the microcoil, crucial to exploit the volume of maximum sensitivity of the detector, has a thickness of 10 μm. To demonstrate the potential of this approach, we report NMR experiments on sub-nL intact biological entities in liquid media, specifically ova of the tardigrade Richtersius coronifer and sections of Caenorhabditis elegans nematodes. We show a sensitivity of 2.5x1013 spins/Hz1/2 on 1H nuclei at 7 T, sufficient to detect 6 pmol of 1H nuclei of endogenous compounds in active volumes down to 100 pL and in a measurement time of 3 hours. Spectral resolutions of 0.01 ppm in liquid samples and of 0.1 ppm in the investigated biological entities are also demonstrated. The obtained results may indicate a route for NMR studies at the single unit level of important biological entities having sub-nL volumes, such as living microscopic organisms and eggs of several mammalians, humans included.

NMR interaction tensors of 51V and 207Pb in vanadinite, Pb5(VO4)3Cl, determined from DFT calculations and single-crystal NMR measurements, using only one general rotation axis

Orientation-dependent NMR spectra of a single crystal of the mineral vanadinite, Pb₅(VO₄)₃Cl, were acquired using only one rotation axis with a general orientation in the hexagonal crystal lattice (space group P6₃/m). The chemical shift (CS) tensors for the ²⁰⁷Pb on Wyckoff positions 6h and 4f, and both CS and quadrupole coupling tensor Q for ⁵¹V at the positions 6h were determined by including the NMR response of symmetry-related atoms in the unit cell (and in case of ²⁰⁷Pb at 4f, also the isotropic shift from MAS NMR spectra). This previously suggested 'single rotation method' greatly reduces the necessary amount of data acquisition and analysis. The precise orientation of the rotation axis could not be found by X-ray diffraction experiments because of the high linear absorption coefficient of vanadinite, which is chiefly due to its high lead content. The axis orientation was therefore included into the multi-parameter data fit routine. This NMR-based approach is widely applicable, and offers an alternative way of orienting single crystals. The NMR parameters derived from the tensor eigenvalues are δ_iso=(-1729±9) ppm, Δδ=(-1071±5) ppm, η_CS=0.362±0.008 for ²⁰⁷Pb at positions 6h, and δ_iso=(-1619±2) ppm, Δδ=(-780±58) ppm, η_CS=0.06±0.08 for positions 4f. For ⁵¹V, δ_iso=(-509±3) ppm, Δδ=(-37±2) ppm, η_CS=0.78±0.09, with the quadrupolar coupling described by χ=(2.52±0.01) MHz and η_Q=0.047±0.003. In contrast to the precisely determined tensor eigenvalues, the orientation of the eigenvectors in the crystal ab -plane of the vanadinite system could only be resolved by resorting to data obtained from density functional theory (DFT) calculations.

Magnetic flux tailoring through Lenz lenses for ultrasmall samples: A new pathway to high-pressure nuclear magnetic resonance

A new pathway to nuclear magnetic resonance (NMR) spectroscopy for picoliter-sized samples (including those kept in harsh and extreme environments, particularly in diamond anvil cells) is introduced, using inductively coupled broadband passive electromagnetic lenses, to locally amplify the magnetic field at the isolated sample, leading to an increase in sensitivity. The lenses are adopted for the geometrical restrictions imposed by a toroidal diamond indenter cell and yield signal-to-noise ratios at pressures as high as 72 GPa at initial sample volumes of only 230 pl. The corresponding levels of detection are found to be up to four orders of magnitude lower compared to formerly used solenoidal microcoils. Two-dimensional nutation experiments on long-chained alkanes, CnH2n+2 (n = 16 to 24), as well as homonuclear correlation spectroscopy on thymine, C5H6N2O2, were used to demonstrate the feasibility of this approach for higher-dimensional NMR experiments, with a spectral resolution of at least 2 parts per million. This approach opens up the field of ultrahigh-pressure sciences to one of the most versatile spectroscopic methods available in a pressure range unprecedented up to now.

Increasing the detection distance of remote NMR using wireless inductive coupling coil

Unilateral nuclear magnetic resonance (UNMR) spectrometers have been applied in a variety of fields such as petrochemistry, materials science, and process control¹. In UNMR measurements the sample is placed outside of the UNMR sensor and the signal intensity is reduced almost exponentially as the sample-to-sensor distances increases. To expand the detection limits of remote UNMR sensors, wireless inductive coupling was proposed and tested. This strategy was proved to reduce signal attenuation due to sample detachment from sensor, resulting in an increase in detection distance by one order of magnitude (i.e., from few millimeters to few centimeters). This novel approach broadens the potential applications of UNMR sensors and opens new opportunities in several areas, from chemical to biomedical applications.

Submillimeter coils for stimulated-echo spectroscopy of a solid sodium ion conductor by nonselective excitation of MHz broad 23Na quadrupolar satellite spectra

In solids the detection of ionic motion covering the time range of milliseconds and longer is often accomplished using stimulated-echo spectroscopy. For spectral line widths much below or much above 1MHz nonselective or fully selective radio-frequency pulse excitation, respectively, is typically applied in such experiments. To enable the study of samples with quadrupolarly broadened satellite spectra featuring intermediate widths (in the lower MHz range) the present work exploits microcoils. Using such coils, stimulated-echo spectroscopy can be performed under conditions of nonselective excitation for instance with ²³Na as a nuclear probe. Nutation experiments carried out used to assess the coil performance. The impact of second-order quadrupolar interactions is studied using explicit density-matrix calculations. The applicability of the present approach is successfully tested for a sodium orthophosphate based solid ion conductor.

Indirect detection of infinite-speed MAS solid-state NMR spectra

Heavy spin-1/2 nuclides are known to possess very large chemical shift anisotropies that can challenge even the most advanced magic-angle-spinning (MAS) techniques. Wide manifolds of overlapping spinning sidebands and insufficient excitation bandwidths often obfuscate meaningful spectral information and force the use of static, low-resolution solid-state (SS)NMR methods for the characterization of materials. To address these issues, we have merged fast-magic-angle-turning (MAT) and dipolar heteronuclear multiple-quantum coherence (D-HMQC) experiments to obtain D-HMQC-MAT pulse sequences which enable the rapid acquisition of 2D SSNMR spectra that correlate isotropic ¹H chemical shifts to the indirectly detected isotropic "infinite-MAS" spectra of heavy spin-1/2 nuclides. For these nuclides, the combination of fast MAS and ¹H detection provides a high sensitivity, which rivals the DNP-enhanced ultra-wideline SSNMR. The new pulse sequences were used to determine the Pt coordination environments in a complex mixture of decomposition products of transplatin and in a metal-organic framework with Pt ions coordinated to the linker ligands.

Enhanced efficiency of solid-state NMR investigations of energy materials using an external automatic tuning/matching (eATM) robot

We have developed and explored an external automatic tuning/matching (eATM) robot that can be attached to commercial and/or home-built magic angle spinning (MAS) or static nuclear magnetic resonance (NMR) probeheads. Complete synchronization and automation with Bruker and Tecmag spectrometers is ensured via transistor-transistor-logic (TTL) signals. The eATM robot enables an automated "on-the-fly" re-calibration of the radio frequency (rf) carrier frequency, which is beneficial whenever tuning/matching of the resonance circuit is required, e.g. variable temperature (VT) NMR, spin-echo mapping (variable offset cumulative spectroscopy, VOCS) and/or in situ NMR experiments of batteries. This allows a significant increase in efficiency for NMR experiments outside regular working hours (e.g. overnight) and, furthermore, enables measurements of quadrupolar nuclei which would not be possible in reasonable timeframes due to excessively large spectral widths. Additionally, different tuning/matching capacitor (and/or coil) settings for desired frequencies (e.g.⁷Li and ³¹P at 117 and 122MHz, respectively, at 7.05 T) can be saved and made directly accessible before automatic tuning/matching, thus enabling automated measurements of multiple nuclei for one sample with no manual adjustment required by the user. We have applied this new eATM approach in static and MAS spin-echo mapping NMR experiments in different magnetic fields on four energy storage materials, namely: (1) paramagnetic ⁷Li and ³¹P MAS NMR (without manual recalibration) of the Li-ion battery cathode material LiFePO₄; (2) paramagnetic ¹⁷O VT-NMR of the solid oxide fuel cell cathode material La₂NiO_4+δ; (3) broadband ⁹³Nb static NMR of the Li-ion battery material BNb₂O₅; and (4) broadband static ¹²⁷I NMR of a potential Li-air battery product LiIO₃. In each case, insight into local atomic structure and dynamics arises primarily from the highly broadened (1-25MHz) NMR lineshapes that the eATM robot is uniquely suited to collect. These new developments in automation of NMR experiments are likely to advance the application of in and ex situ NMR investigations to an ever-increasing range of energy storage materials and systems.

Ultrasensitive mechanical detection of magnetic moment using a commercial disk drive write head

Sensitive detection of weak magnetic moments is an essential capability in many areas of nanoscale science and technology, including nanomagnetism, quantum readout of spins and nanoscale magnetic resonance imaging. Here we show that the write head of a commercial hard drive may enable significant advances in nanoscale spin detection. By approaching a sharp diamond tip to within 5 nm from a write pole and measuring the induced diamagnetic moment with a nanomechanical force transducer, we demonstrate a spin sensitivity of 0.032 μB Hz(-1/2), equivalent to 21 proton magnetic moments. The high sensitivity is enabled in part by the pole's strong magnetic gradient of up to 28 × 10(6) T m(-1) and in part by the absence of non-contact friction due to the extremely flat writer surface. In addition, we demonstrate quantitative imaging of the pole field with ∼10 nm spatial resolution. We foresee diverse applications for write heads in experimental condensed matter physics, especially in spintronics, ultrafast spin manipulation and mesoscopic physics.

Susceptibility cancellation of a microcoil wound with a paramagnetic-liquid-filled copper capillary

Even though microcoils improve the sensitivity of NMR measurement of tiny samples, magnetic-field inhomogeneity due to the bulk susceptibility effect of the coil material can cause serious resonance-line broadening. Here, we propose to fabricate the microcoil using a thin, hollow copper capillary instead of a wire and fill paramagnetic liquid inside the capillary, so as to cancel the diamagnetic contribution of the copper. Susceptibility cancellation is demonstrated using aqueous solution of NiSO4. In addition, the paramagnetic liquid serves as coolant when it is circulated through the copper capillary, effectively transferring the heat generated by radiofrequency pulses.

Radiofrequency microcoils for magnetic resonance imaging and spectroscopy

Small radiofrequency coils, often termed "microcoils", have found extensive use in many areas of magnetic resonance. Their advantageous properties include a very high intrinsic sensitivity, a high (several MHz) excitation and reception bandwidth, the fact that large arrays can fit within the homogeneous volume of the static magnetic field, and the very high resonance frequencies (several GHz) that can be achieved. This review concentrates on recent developments in the construction of single and multiple RF microcoil systems, and new types of experiments that can be performed using such assemblies.

Microcoils and microsamples in solid-state NMR

Recent reports on microcoils are reviewed. The first part of the review includes a discussion of how the geometries of the sample and coil affect the NMR signal intensity. In addition to derivation of the well-known result that the signal intensity increases as the coil size decreases, the prediction that dilution of a small sample with magnetically inert matter leads to better sensitivity if a tiny coil is not available is given. The second part of the review focuses on the issues specific to solid-state NMR. They include realization of magic-angle spinning (MAS) using a microcoil and harnessing of such strong pulses that are feasible only with a microcoil. Two strategies for microcoil MAS, the piggyback method and magic-angle coil spinning (MACS), are reviewed. In addition, MAS of flat, disk-shaped samples is discussed in the context of solid-state NMR of small-volume samples. Strong RF irradiation, which has been exploited in wide-line spectral excitation, multiple-quantum MAS (MQMAS), and dipolar decoupling experiments, has been accompanied by new challenges regarding the Bloch-Siegert effect, the minimum time resolution of the spectrometer, and the time scale of pulse transient effects. For a possible solution to the latter problem, recent reports on active compensation of pulse transients are described.

Microfabricated inserts for magic angle coil spinning (MACS) wireless NMR spectroscopy

This article describes the development and testing of the first automatically microfabricated probes to be used in conjunction with the magic angle coil spinning (MACS) NMR technique. NMR spectroscopy is a versatile technique for a large range of applications, but its intrinsically low sensitivity poses significant difficulties in analyzing mass- and volume-limited samples. The combination of microfabrication technology and MACS addresses several well-known NMR issues in a concerted manner for the first time: (i) reproducible wafer-scale fabrication of the first-in-kind on-chip LC microresonator for inductive coupling of the NMR signal and reliable exploitation of MACS capabilities; (ii) improving the sensitivity and the spectral resolution by simultaneous spinning the detection microcoil together with the sample at the “magic angle” of 54.74° with respect to the direction of the magnetic field (magic angle spinning – MAS), accompanied by the wireless signal transmission between the microcoil and the primary circuit of the NMR spectrometer; (iii) given the high spinning rates (tens of kHz) involved in the MAS methodology, the microfabricated inserts exhibit a clear kinematic advantage over their previously demonstrated counterparts due to the inherent capability to produce small radius cylindrical geometries, thus tremendously reducing the mechanical stress and tearing forces on the sample. In order to demonstrate the versatility of the microfabrication technology, we have designed MACS probes for various Larmor frequencies (194, 500 and 700 MHz) testing several samples such as water, Drosophila pupae, adamantane solid and LiCl at different magic angle spinning speeds.

Inductively coupled NMR probe for versatile dynamic nuclear polarization operation at 7 T: observation of 61 ± 2% ¹H polarization at 4 K

We have performed dynamic nuclear polarization (DNP) experiments at liquid helium temperatures using a low-power (<70 mW) solid-state diode microwave source at 200 GHz-the electron paramagnetic resonance frequency of stable radicals at 7 T. We employed a home-built Alderman-Grant probe for the detection of ¹H NMR signal at 300 MHz, as such coils are well suited for higher frequency NMR detection. The Alderman-Grant coil is inductively coupled to the rest of the radiofrequency (rf) circuit, whose design allows probe components to be placed away from the sample area, and also enables easy switching of coils with different diameters and resonance frequencies. We have tested our DNP instrument on a frozen nitroxide model system consisting of 4-Amino TEMPO dissolved in a glycerol:water mixture. The largest nuclear spin polarization observed was 61 ± 2% with a sample containing 20 mM 4-Amino TEMPO dissolved in deuterated glycerol (d-glycerol):D₂O:H₂O (50:40:10), amounting to record polarization measured to date at an easily amenable temperature of 4 K.

New methods for the acquisition of ultra-wideline solid-state NMR spectra of spin-1/2 nuclides

The Wideband Uniform Rate Smooth Truncation - Carr-Purcell Meiboom-Gill (WURST-CPMG) pulse sequence was recently introduced as a new method of acquiring ultra-wideline solid-state NMR (SSNMR) patterns of quadrupolar nuclei (Chem. Phys. Lett. 464 (2008) 97). Herein, we describe the application of the WURST-CPMG pulse sequence to stationary samples (i.e., non-spinning or "static" samples) of various spin-1/2 nuclides ((119)Sn, (207)Pb, (199)Hg and (195)Pt) in order to examine its effectiveness for acquiring ultra-wideline SSNMR patterns. WURST-CPMG is compared to the CPMG and Cross Polarization (CP)-CPMG pulse sequences in select cases ((119)Sn and (207)Pb, respectively), and its usefulness in obtaining ultra-wideline SSNMR spectra in a piecewise fashion is explored. In addition, a preliminary investigation of pulses generated using optimal control theory (OCT) for the purpose of wideline excitation is presented; spectra acquired using these pulses are compared with standard, rectangular pulses of similar pulse powers. Both methods show much promise for acquiring high quality wideline patterns dominated by chemical shift anisotropy, with minimal distortions and significantly reduced experimental times.

Practical aspects of liquid-state NMR with inductively coupled solenoid coils

Sensitivity enhancement by the use of inductively coupled milli- and microcoils has been demonstrated in solid-state as well as liquid-state NMR. In this work, we discuss the practical aspects of using inductively coupled solenoid coils of different sizes in a liquid-state NMR spectrometer. The sensitivity and resolution enhancements from these resonant coils, with sizes ranging between 3.0 and 0.75 mm i.d., are measured for (23)Na single-pulse and multidimensional imaging experiments and compared to the results obtained with the conventional liquids NMR 5.0-mm saddle coil. Enhancements in voxel-based sensitivity (SNR per √scans) were measured in multidimensional MR images and were found to be as large as 20.4 with the 0.75-mm coil.

Microscopic structural analysis of fractured silk fibers from Bombyx mori and Samia cynthia ricini using 13C CP/MAS NMR with a 1mm microcoil MAS NMR probehead

Conformational changes have been studied in silk fibers from the domestic silkworm Bombyx mori and a wild silkworm Samia cynthia ricini as a result of fractured by stretching. About 300 samples consisting of only the fractured regions of [1-13C]Ala or [1-13C]Gly labeled silk fibers were collected and observed by 13C CP/MAS NMR spectra. The total amount of these fractured fibers is only about 1mg and therefore we used a home-built 1mm microcoil MAS NMR probehead. A very small increase in the fraction of random coil was noted for the alanine regions of both silk fibroins and for the glycine region of B. mori silk fibroin. However, there is no difference in the spectra before and after fractured for the glycine region of S. c. ricini silk fibroin. Thus, the influence of fracture occurs exclusively at the Ala region for S. c. ricini. The relationship between sequence, fracture and structure is discussed.

Recent technique developments and applications of solid state NMR in characterising inorganic materials

A broad overview is given of some key recent developments in solid state NMR techniques that have driven enhanced applications to inorganic materials science. Reference is made to advances in hardware, pulse sequences and associated computational methods (e.g. first principles calculations, spectral simulation), along with their combination to provide more information about solid phases. The resulting methodology has allowed more nuclei to be observed and more structural information to be extracted. Cross referencing between experimental parameters and their calculation from the structure has given an added dimension to NMR as a characterisation probe of materials. Emphasis is placed on the progress made in the last decade especially from those nuclei that were little studied previously. The general points about technique development and the increased range of nuclei observed are illustrated through some specific exemplars from inorganic materials science.

On the practical aspects of recording wideline QCPMG NMR spectra

The practical aspects of applying CPMG for acquisition of wideline powder patterns are examined. It is shown that most distortions/modulations of spikelet spectra can be traced to the incoherent signal averaging from multiple coherence transfer pathways. A strategy for minimizing these distortions/modulations is described. Also, a few interesting observations regarding the implementation of the wideline WURST-QCPMG experiment are presented, namely the accumulation of second-order signal phase and the effects of varying the sweep rate and rf field of chirp pulses.

Double-resonance magic angle coil spinning

We present an extension of magic angle coil spinning (MACS) solid-state NMR spectroscopy to double-resonance experiments, enabling implementation of powerful double-resonance solid-state NMR methodologies including cross polarization, proton decoupling, and two-dimensional correlation spectroscopy etc., while still enjoying the merits that are intrinsic to MACS, such as high concentration sensitivity, eliminated magnetic susceptibility-induced field distortion, and an easy-to-use approach with the conventional and widespread hardware.

High-field QCPMG NMR of large quadrupolar patterns using resistive magnets

Spectroscopy in a high magnetic field reduces second-order quadrupolar shift while increasing chemical shift. It changes the scale between quadrupolar and chemical shift of half-integer quadrupolar spins. The application of QCPMG multiple echo for acquiring large quadrupolar pattern under the high magnetic field of a 25 T resistive magnet is presented for acquiring large quadrupolar patterns. It shows that temporal field fluctuations and spatial homogeneity of the Keck magnet at the NHMFL contribute about +/- 20 ppm in line broadening. NMR patterns which have breadths of hundreds to thousands of kilohertz can be efficiently recorded using a combination of QCPMG and magnetic field stepping with negligible hindrance from the inhomogeneity and field fluctuations of powered magnets.

Experimental and numerical examination of eddy (Foucault) currents in rotating micro-coils: Generation of heat and its impact on sample temperature

The application of nuclear magnetic resonance (NMR) to systems of limited quantity has stimulated the use of micro-coils (diameter <1mm). One method recently proposed for the union of micro-coils with Magic Angle sample Spinning (MAS), involves the integration of a tuned micro-coil circuit within standard MAS rotors inductively coupled to the MAS probe coil, termed "magic-angle coil spinning" (MACS). The spinning of conductive materials results in the creation of circulating Foucault (eddy) currents, which generate heat. We report the first data acquired with a 4mm MACS system and spinning up to 10kHz. The need to spin faster necessitates improved methods to control heating. We propose an approximate solution to calculate the power losses (heat) from the eddy currents for a solenoidal coil, in order to provide insight into the functional dependencies of Foucault currents. Experimental tests of the dependencies reveal conditions which result in reduced sample heating and negligible temperature distributions over the sample volume.

Active compensation of rf-pulse transients

A new approach to compensate rf-pulse transients is proposed. Based on the idea of the response theory of a linear system, a formula is derived to obtain the required excitation voltage profile back from the intended target rf-pulse shape. The validity of the formula is experimentally confirmed by monitoring the rf-field created inside the sample coil with a pickup coil. Since this approach realizes accurate rf-pulse shapes without reducing the Q-factor of the tank circuit of the probe, it can be used not only to suppress the transient tail of the rf-pulse, but also as a general concept for accurate rf-pulsing.

The generation of intense radiofrequency fields in microcoils

Large amplitude radiofrequency (rf) fields hold great promise in wide line NMR where it becomes possible to excite the full breadth of the line in a quantitative fashion. Applications in quadrupole NMR and in NMR of paramagnetic systems benefit greatly from intense fields. Spin manipulations in multiple quantum NMR experiments, specifically, the generation of multiple quantum coherence in MQ NMR, are more efficiently produced using intense rf fields. In this work we describe a microcoil probe that produces an rf field of 25 MHz, more than five times larger than the greatest rf field reported in the literature. We accomplish this in a robust, 127 microm diameter solenoid coil using 1 kW of rf power.

Controlling coherence using the internal structure of hard pi pulses

The tiny difference between hard pi pulses and their delta-function approximation can be exploited to control coherence. Variants on the magic echo that work despite a large spread in resonance offsets are demonstrated using the zeroth- and first-order average Hamiltonian terms, for 13C NMR in 60C. The 29Si NMR linewidth of silicon has been reduced by a factor of about 70,00 using this approach, which also has potential applications in magnetic resonance microscopy and imaging of solids.

Stripline probes for nuclear magnetic resonance

A novel route towards chip integrated NMR analysis is evaluated. The basic element in the design is a stripline RF 'coil' which can be defined in a single layer lithographic process and which is fully scalable to smaller dimensions. The sensitivity of such a planar structure can be superior to that of a conventional 3D helix. The basic properties, such as RF field strength, homogeneity and susceptibility broadening are discussed in detail. Secondary effects related to the thermal characteristics are discussed in simplified models. Preliminary NMR tests of basic solid and liquid samples measured at 600 MHz confirm the central findings of the design study. It is concluded that the stripline structure can be a valuable addition to the NMR toolbox; it combines high sensitivity with low susceptibility broadening and high power handling capabilities in a simple scalable design.

High-resolution, high-sensitivity NMR of nanolitre anisotropic samples by coil spinning

Nuclear magnetic resonance (NMR) can probe the local structure and dynamic properties of liquids and solids, making it one of the most powerful and versatile analytical methods available today. However, its intrinsically low sensitivity precludes NMR analysis of very small samples-as frequently used when studying isotopically labelled biological molecules or advanced materials, or as preferred when conducting high-throughput screening of biological samples or 'lab-on-a-chip' studies. The sensitivity of NMR has been improved by using static micro-coils, alternative detection schemes and pre-polarization approaches. But these strategies cannot be easily used in NMR experiments involving the fast sample spinning essential for obtaining well-resolved spectra from non-liquid samples. Here we demonstrate that inductive coupling allows wireless transmission of radio-frequency pulses and the reception of NMR signals under fast spinning of both detector coil and sample. This enables NMR measurements characterized by an optimal filling factor, very high radio-frequency field amplitudes and enhanced sensitivity that increases with decreasing sample volume. Signals obtained for nanolitre-sized samples of organic powders and biological tissue increase by almost one order of magnitude (or, equivalently, are acquired two orders of magnitude faster), compared to standard NMR measurements. Our approach also offers optimal sensitivity when studying samples that need to be confined inside multiple safety barriers, such as radioactive materials. In principle, the co-rotation of a micrometre-sized detector coil with the sample and the use of inductive coupling (techniques that are at the heart of our method) should enable highly sensitive NMR measurements on any mass-limited sample that requires fast mechanical rotation to obtain well-resolved spectra. The method is easy to implement on a commercial NMR set-up and exhibits improved performance with miniaturization, and we accordingly expect that it will facilitate the development of novel solid-state NMR methodologies and find wide use in high-throughput chemical and biomedical analysis.

Microcoil High-Resolution Magic Angle Spinning NMR Spectroscopy

We report the construction of a dual-channel microcoil nuclear magnetic resonance probehead allowing magic-angle spinning for mass-limited samples. With coils down to 235 mum inner diameter, this allows high-resolution solid-state NMR spectra to be obtained for amounts of materials of a few nanoliters. This is demonstrated by the carbon-13 spectrum of a tripeptide and a single silk rod, prepared from the silk gland of the Bombyx mori silkworm. Furthermore, the microcoil allows for radio frequency field strengths well beyond current probe technology, aiding in getting the highest possible resolution by efficiently decoupling the observed nuclei from the abundantly present proton nuclei.

Use of Microcoil Probehead for Determination of the Structure of Oriented Silk Fibers by Solid-State NMR

A microcoil probehead for solid-state NMR was developed with a two-channel radio-frequency circuit, and 13C observation with a proton-decoupling probehead was performed to obtain information on the distribution of the orientation of silk fibroin molecules in the fiber. The coil (1 mm (diameter) x 5 mm (length)) of the probehead was placed at the angles 90 degrees and 30 degrees , relative to the static magnetic field. Only 70 mug of [1-13C]Gly silk fibroin fiber was used in a magnet of 9.4 T (400 MHz for proton channel).

Crossed-coil detection of two-photon excited nuclear quadrupole resonance

Applying a recently developed theoretical framework for determining two-photon excitation Hamiltonians using average Hamiltonian theory, we calculate the excitation produced by half-resonant irradiation of the pure quadrupole resonance of a spin-3/2 system. This formalism provides expressions for the single-quantum and double-quantum mutation frequencies as well as the Bloch-Siegert shift. The dependence of the excitation strength on RF field orientation and the appearance of the free-induction signal along an axis perpendicular to the excitation field provide an unmistakable signature of two-photon excitation. We demonstrate single- and double-quantum excitation in an axially symmetric system using 35Cl in a single crystal of potassium chlorate (omega(Q) = 28 MHz) with crossed-coil detection. A rotation plot verifies the orientation dependence of the two-photon excitation, and double-quantum coherences are observed directly with the application of a static external magnetic field.

Microcoil nuclear magnetic resonance spectroscopy

In comparison with most analytical chemistry techniques, nuclear magnetic resonance has an intrinsically low sensitivity, and many potential applications are therefore precluded by the limited available quantity of certain types of sample. In recent years, there has been a trend, both commercial and academic, towards miniaturization of the receiver coil in order to increase the mass sensitivity of NMR measurements. These small coils have also proved very useful in coupling NMR detection with commonly used microseparation techniques. A further development enabled by small detectors is parallel data acquisition from many samples simultaneously, made possible by incorporating multiple receiver coils into a single NMR probehead. This review article summarizes recent developments and applications of "microcoil" NMR spectroscopy.

Two-photon two-color nuclear magnetic resonance

Two-photon excitation has recently been demonstrated to be a practical means of exciting nuclear magnetic resonance (NMR) signals by radio-frequency (rf) irradiation at half the normal resonance frequency. In this work, two-photon excitation is treated with average Hamiltonian theory and shown to be a consequence of higher order terms in the Magnus expansion. It is shown that the excitation condition may be satisfied not only with rf at half resonance, but also with two independent rf fields, where the two frequencies sum to or differ by the resonance frequency. The technique is demonstrated by observation of proton NMR signals at 400 MHz while simultaneously exciting at 30 and 370 MHz. Advantages of this so-called two-color excitation, such as a dramatic increase in nutation rate over half-frequency excitation, along with a variety potential applications are discussed.

Multiple quantum spin counting techniques with quadrupolar nuclei

Phase incremented and continuous irradiation multiple spin correlation methods are applied to spin [Formula: see text] nuclei with small quadrupole couplings such as (7)Li in LiCl and are shown to successfully produce a coherently coupled dipolar spin network. Application to the analogous Na salt shows successful spin correlation evolving at a slower rate due to the weaker homonuclear dipolar coupling strength between Na nuclei. The results are analysed using a statistical approach. Spin counting is non-trivial as not only multiple quantum coherences between spins are generated but also within the quadrupolar spin levels. Na(2)C(2)O(4) is investigated as a material with non-negligible quadrupole coupling and it is in this limit that the spin correlation techniques are found to break down.

Towards nuclear magnetic resonance μ-spectroscopy and μ-imaging

The first successful experiments demonstrating Nuclear Magnetic Resonance (NMR) were a spin-off from the development of electromagnetic technology and its introduction into civilian life in the late forties. It was soon discovered that NMR spectra held chemically relevant information making it useful as an analytical tool. By introducing a new way of detection, moving away from continuous wave spectroscopy, Fourier Transform NMR helped to overcome sensitivity problems and subsequently opened the way for multi-dimensional spectroscopy. As a result NMR has developed into one of the most powerful analysis techniques with widespread applications. Still sensitivity is a limiting factor in the applicability of NMR. Therefore we witness a renaissance of technique development in magnetic resonance striving to improve its receptiveness. This tutorial review introduces the efforts currently made in miniaturizing inductive detection by designing optimal radio-frequency microcoils. A second approach is to introduce a new way of detecting magnetic resonance signals by means of very sensitive micromechanical force detectors. This shows that the detection limits in terms of absolute sensitivity or imaging resolution are still open to significant improvements.