Radiation damping in microcoil NMR probes

Radiation damping arises from the field induced in the receiver coil by large bulk magnetization and tends to selectively drive this magnetization back to equilibrium much faster than relaxation processes. The demand for increased sensitivity in mass-limited samples has led to the development of microcoil NMR probes that are capable of obtaining high quality NMR spectra with small sample volumes (nL-microL). Microcoil probes are optimized to increase sensitivity by increasing either the sample-to-coil ratio (filling factor) of the probe or quality factor of the detection coil. Though radiation damping effects have been studied in standard NMR probes, these effects have not been measured in the microcoil probes. Here a systematic evaluation of radiation damping effects in a microcoil NMR probe is presented and the results are compared with similar measurements in conventional large volume samples. These results show that radiation-damping effects in microcoil probe is much more pronounced than in 5 mm probes, and that it is critically important to optimize NMR experiments to minimize these effects. As microcoil probes provide better control of the bulk magnetization, with good RF and B0 inhomogeneity, in addition to negligible dipolar field effects due to nearly spherical sample volumes, these probes can be used exclusively to study the complex behavior of radiation damping.     

Auxiliary probe design adaptable to existing probes for remote detection NMR, MRI, and time-of-flight tracing

A versatile, detection-only probe design is presented that can be adapted to any existing NMR or MRI probe with the purpose of making the remote detection concept generally applicable. Remote detection suggests freeing the NMR experiment from the confinement of using the same radio frequency (RF) coil and magnetic field for both information encoding and signal detection. Information is stored during the encoding step onto a fluid sensor medium whose magnetization is later measured in a different location. The choice of an RF probe and magnetic field for encoding can be made based solely on the size and characteristics of the sample and the desired information quality without considering detection sensitivity, as this aspect is dealt with by a separate detector. While early experiments required building probes that included two resonant circuits, one for encoding and one for detection, a modular approach with a detection-only probe as presented here can be used along with any existing NMR probe of choice for encoding. The design of two different detection-only probes is presented, one with a saddle coil for milliliter-sized detection volumes, and the other one with a microsolenoid coil for sub-microliter fluid quantities. As example applications, we present time-of-flight (TOF) tracing of hyperpolarized (129)Xe spins in a gas mixture through coiled tubing using the microsolenoid coil detector and TOF flow imaging through a nested glass container where the gas flow changes its direction twice between inlet and outlet using the saddle coil detector.     

Using low-E resonators to reduce RF heating in biological samples for static solid-state NMR up to 900 MHz

RF heating of solid-state biological samples is known to be a destabilizing factor in high-field NMR experiments that shortens the sample lifetime by continuous dehydration during the high-power cross-polarization and decoupling pulses. In this work, we describe specially designed, large volume, low-E 15N-1H solid-state NMR probes developed for 600 and 900 MHz PISEMA studies of dilute membrane proteins oriented in hydrated and dielectrically lossy lipid bilayers. The probes use an orthogonal coil design in which separate resonators pursue their own aims at the respective frequencies, resulting in a simplified and more efficient matching network. Sample heating at the 1H frequency is minimized by a loop-gap resonator which produces a homogeneous magnetic field B1 with low electric field E. Within the loop-gap resonator, a multi-turn solenoid closely matching the shape of the sample serves as an efficient observe coil. We compare power dissipation in a typical lossy bilayer sample in the new low-E probe and in a previously reported 15N-1H probe which uses a double-tuned 4-turn solenoid. RF loss in the sample is measured in each probe by observing changes in the 1H 360 degrees pulse lengths. For the same values of 1H B1 field, sample heating in the new probe was found to be smaller by an order of magnitude. Applications of the low-E design to the PISEMA study of membrane proteins in their native hydrated bilayer environment are demonstrated at 600 and 900 MHz.     

Cross polarization, radio frequency field homogeneity, and circuit balancing in high field solid state NMR probes

Homogeneous radio frequency (RF) fields are important for sensitivity and efficiency of magnetization transfer in solid state NMR experiments. If the fields are inhomogeneous the cross polarization (CP) experiment transfers magnetization in only a thin slice of sample rather than throughout the entire volume. Asymmetric patterns have been observed in plots of the CP signal versus RF field mismatch for an 800 MHz solid-state NMR probe where each channel is resonated in a single-ended mode. A simple model of CP shows these patterns can be reproduced if the RF fields for the two nuclei are centered at different places in the coil. Experimental measurements using B1 field imaging, nutation arrays on extremely short NMR samples, and de-tuning experiments involving disks of copper incrementally moved through the coil support this model of spatially offset RF fields. We have found that resonating the high frequency channel in a double-ended or "balanced" mode can alleviate this field offset problem, and have implemented this in a three-channel solid state NMR probe of our own design.     

Performance of cryogenic probes as a function of ionic strength and sample tube geometry

The pursuit for more sensitive NMR probes culminated with development of the cryogenic cooled NMR probe. A key factor for the sensitivity is the overall resistance of RF circuitry and sample. Lowering the coil temperature to approximately 25 K and the use of superconducting coil material has greatly reduced the resistance contribution of the hardware. However, the resistance of a salty sample remains the same and evolves as the major factor determining the signal-to-noise ratio. Several approaches have been proposed to reduce the resistance contribution of the sample. These range from encapsulating proteins in a water cavity formed by reverse micelles in low viscosity fluids to the optimal selection of low mobility, low conductivity buffer ions. Here we demonstrate that changing the sample diameter has a pronounced effect on the sample resistance and this results in dramatic improvements of the signal-to-noise ratio and shorter pi/2 pulses. We determined these parameters for common 5 mm NMR tubes under different experimental conditions and compared them to the 2, 3 and 4 mm tubes, in addition, 5mm Shigemi tubes were included since these are widely used. We demonstrate benefits and applicability of studying NMR samples with up to 4M salt concentrations in cryogenic probes. Under high salt conditions, best results in terms of short pi/2 pulses and high signal-to-noise ratios are obtained using 2 or 3mm NMR tubes, especially when limited sample is available. The 4 mm tube is preferred when sample amounts are abundant at intermediate salt conditions.     

A large volume flat coil probe for oriented membrane proteins

15N detection of mechanically aligned membrane proteins benefits from large sample volumes that compensate for the low sensitivity of the observe nuclei, dilute sample preparation, and for the poor filling factor arising from the presence of alignment plates. Use of larger multi-tuned solenoids, however, is limited by wavelength effects that lead to inhomogeneous RF fields across the sample, complicating cross-polarization experiments. We describe a 600 MHz 15N-1H solid-state NMR probe with large (580 mm3) RF solenoid for high-power, multi-pulse sequence experiments, such as polarization inversion spin exchange at the magic angle (PISEMA). In order to provide efficient detection for 15N, a 4-turn solenoidal sample coil is used that exceeds 0.27 lambda at the 600 MHz 1H resonance. A balanced tuning-matching circuit is employed to preserve RF homogeneity across the sample for adequate magnetization transfer from 1H to 15N. We describe a procedure for optimization of the shorted 1/4 lambda coaxial trap that allows for the sufficiently strong RF fields in both 1H and 15N channels to be achieved within the power limits of 300 W 1H and 1 kW 15N amplifiers. The 8 x 6 x 12 mm solenoid sustains simultaneous B1 irradiation of 100 kHz at 1H frequency and 51 kHz at 15N frequency for at least 5 ms with 265 and 700 W of input power in the respective channels. The probe functionality is demonstrated by 2D 15N-1H PISEMA spectroscopy for two applications at 600 MHz.     

Portable, low-cost NMR with laser-lathe lithography produced microcoils

Nuclear Magnetic Resonance (NMR) is unsurpassed in its ability to non-destructively probe chemical identity. Portable, low-cost NMR sensors would enable on-site identification of potentially hazardous substances, as well as the study of samples in a variety of industrial applications. Recent developments in RF microcoil construction (i.e. coils much smaller than the standard 5mm NMR RF coils), have dramatically increased NMR sensitivity and decreased the limits-of-detection (LOD). We are using advances in laser pantographic microfabrication techniques, unique to LLNL, to produce RF microcoils for field deployable, high sensitivity NMR-based detectors. This same fabrication technique can be used to produce imaging coils for MRI as well as for standard hardware shimming or "ex-situ" shimming of field inhomogeneities typically associated with inexpensive magnets. This paper describes a portable NMR system based on the use of a 2 kg hand-held permanent magnet, laser-fabricated microcoils, and a compact spectrometer. The main limitations for such a system are the low resolution and sensitivity associated with the low field values and quality of small permanent magnets, as well as the lack of large amounts of sample of interest in most cases. The focus of the paper is on the setting up of this system, initial results, sensitivity measurements, discussion of the limitations and future plans. The results, even though preliminary, are promising and provide the foundation for developing a portable, inexpensive NMR system for chemical analysis. Such a system will be ideal for chemical identification of trace substances on site.     

Reducing the NMR sample volume using a single organic liquid: Increased sensitivity for mass-limited samples with standard NMR probes

A simple inexpensive protocol for confining an aqueous sample to the active region of a standard NMR probe is examined for high-resolution NMR. The aqueous sample is sandwiched between an inert perfluorinated organic liquid that has been exploited in the design of micro-coil NMR probes. The procedure is demonstrated with 3 mm NMR tubes at ambient and elevated temperatures but should be equally applicable to smaller diameter tubes. It is shown that confinement has minimal effects on line shape and provides at least a two fold increase in sensitivity over a conventional sample, for the same mass of solute.     

On-line coupling of high temperature GPC and 1H NMR for the analysis of polymers

The on-line coupling of gel permeation chromatography (GPC) and 1H NMR operating at temperatures up to 130 degrees C is presented. A NMR flow probe with a cell volume of 120 microL and a stop-flow valve are developed for on-flow and stop-flow NMR measurements at high temperatures. To maintain high and constant temperatures through the whole probe, the flow probe contains two separate heating circuits. A modified stop-flow valve is developed as a control device for enabling on-flow and stop-flow experiments at high temperature conditions. Heated transfer lines connect the flow probe with the high temperature GPC system. Due to their semicrystalline nature, polyolefins can be studied by liquid chromatography only at temperatures above 100 degrees C. The novel high temperature GPC-NMR system is used for the separation of complex polyolefins regarding their molar mass and for the analysis of different chemical structures. Blends of polyethylene, poly(methyl methacrylate), and ethylene-methyl methacrylate copolymers are separated according to the molar masses of the components. The compositions of the components are directly studied by on-line NMR. Moreover, the chemical composition distribution of an ethylene-methyl methacrylate copolymer sample is analysed. Differences between results of on-flow and stop-flow measurements are discussed.     

Determination of moisture fraction in wood by mobile NMR device

A mobile NMR probe has been used as a non-destructive and non-invasive tool for water content analysis on wood samples. The porosity index, express as the fraction of the sensitivity volume of the probe occupied by water, is here proposed as an alternative to the moisture content index, namely the amount of water mass with respect to the mass of dried sample. In principle the method can be applied to any kind of porous media that has not detectable proton signal from the rigid matrix as, for instance, in building materials. In wood, where proton signal can be detected also from cellulose and others macromolecular components, some considerations and artifices are here proposed for eliminating this contribution. The method has allowed performing moisture volume fraction analysis on wood samples characterized by different wood species, cutting and moisture contents. The NMR data of moisture detection as volume fraction have successfully been compared with those obtained by the gravimetric method.     

Construction and performance of an NMR tube with a sample cavity formed within magnetic susceptibility-matched glass

We describe the construction and performance of an NMR tube with a magnetic susceptibility matched sample cavity that confines the solution within the detection zone in the axial direction and in a quasi-rectangular region in the radial direction. The slot-like sample cavity provides both good sample volume efficiency and tolerance to sensitivity loss in the sample space. The signal-to-noise ratio per unit volume of the constructed tube was 2.2 times higher than that of a cylindrical tube of 5mm outer diameter with a sample containing 300 mM NaCl at a static magnetic field of 14.1T. Even the overall signal-to-noise ratio of the slot tube was 35% higher than that of the conventional 5mm tube for a sample containing 300 mM NaCl. Similar improvements over existing sample tube geometries were obtained at 950 MHz. Moreover the temperature rise resulting from RF heating was found to be significantly lower for the slot tube even when compared to 3 and 4mm outer diameter cylindrical tubes as measured in a 5mm cryoprobe. A further advantage of this type of tube is that a sample cavity of any desired size and shape can be formed within a cylindrical tube for use in a single cryogenic probe.     

Reduction of RF-induced sample heating with a scroll coil resonator structure for solid-state NMR probes

Heating due to high power 1H decoupling limits the experimental lifetime of protein samples for solid-state NMR (SSNMR). Sample deterioration can be minimized by lowering the experimental salt concentration, temperature or decoupling fields; however, these approaches may compromise biological relevance and/or spectroscopic resolution and sensitivity. The desire to apply sophisticated multiple pulse experiments to proteins therefore motivates the development of probes that utilize the RF power more efficiently to generate a high ratio of magnetic to electric field in the sample. Here a novel scroll coil resonator structure is presented and compared to a traditional solenoid. The scroll coil is demonstrated to be more tolerant of high sample salt concentrations and cause less RF-induced sample heating. With it, the viable experimental lifetime of a microcrystalline ubiquitin sample has been extended by more than an order of magnitude. The higher B1 homogeneity and permissible decoupling fields enhance polarization transfer efficiency in 15N-13C correlation experiments employed for protein chemical shift assignments and structure determination.     

Adiabatic Mixing in the Liquid State

Adiabatic spin inversion has been used in the liquid state very efficiently for decoupling purposes. Here we show that it can also be adapted for spin mixing experiments, such as the TOCSY and clean TOCSY experiment, and is superior to previously employed mixing sequences. The main advantage of adiabatic mixing sequences over the conventional mixing schemes used in liquid state experiments is an extremely low sensitivity to RF field inhomogeneity and miscalibration of theB₁field strength. The method is evaluated experimentally by comparing results obtained with different mixing schemes in the basic 2D TOCSY experiment. In addition to higher reliability, adiabatic mixing provides a sensitivity improvement of ca. 20% as compared to conventional mixing schemes. This is explained by higher signal losses due to RF inhomogeneity in the experiments employing traditional mixing schemes. More significant sensitivity improvements can be expected in situations where RF homogeneity is traditionally poor, for example, in large volume probes and magnetic resonance imaging experiments.     

Low temperature probe for dynamic nuclear polarization and multiple-pulse solid-state NMR

Here, we describe the design and performance characteristics of a low temperature probe for dynamic nuclear polarization (DNP) experiments, which is compatible with demanding multiple-pulse experiments. The competing goals of a high-Q microwave cavity to achieve large DNP enhancements and a high efficiency NMR circuit for multiple-pulse control lead to inevitable engineering tradeoffs. We have designed two probes-one with a single-resonance RF circuit and a horn-mirror cavity configuration for the microwaves and a second with a double-resonance RF circuit and a double-horn cavity configuration. The advantage of the design is that the sample is in vacuum, the RF circuits are locally tuned, and the microwave resonator has a large internal volume that is compatible with the use of RF and gradient coils.     

High-precision nuclear magnetic resonance probe suitable for in situ studies of high-temperature metallic melts

High-temperature nuclear magnetic resonance (NMR) has proven to be very useful for detecting the temperature-induced structural evolution and dynamics in melts. However, the sensitivity and precision of high-temperature NMR probes are limited. Here we report a sensitive and stable high-temperature NMR probe based on laser-heating, suitable for in situ studies of metallic melts, which can work stably at the temperature of up to 2000 K. In our design, a well-designed optical path and the use of a water-cooled copper radio-frequency (RF) coil significantly optimize the signal-to-noise ratio (S/NR) at high temperatures. Additionally, a precise temperature controlling system with an error of less than ±1 K has been designed. After temperature calibration, the temperature measurement error is controlled within ±2 K. As a performance testing, ²⁷Al NMR spectra are measured in Zr-based metallic glass-forming liquid in situ. Results show that the S/NR reaches 45 within 90 s even when the sample's temperature is up to 1500 K and that the isothermal signal drift is better than 0.001 ppm per hour. This high-temperature NMR probe can be used to clarify some highly debated issues about metallic liquids, such as glass transition and liquid-liquid transition.     

Operating nanoliter scale NMR microcoils in a 1 tesla field

Microcoil probes enclosing sample volumes of 1.2, 3.3, 7.0, and 81 nanoliters are constructed as nuclear magnetic resonance (NMR) detectors for operation in a 1 tesla permanent magnet. The probes for the three smallest volumes utilize a novel auxiliary tuning inductor for which the design criteria are given. The signal-to-noise ratio (SNR) and line width of water samples are measured. Based on the measured DC resistance of the microcoils, together with the calculated radio frequency (RF) resistance of the tuning inductor, the SNR is calculated and shown to agree with the measured values. The details of the calculations indicate that the auxiliary inductor does not degrade the NMR probe performance. The diameter of the wire used to construct the microcoils is shown to affect the signal line widths.     

Optimal electric fields for different sample shapes in high resolution NMR spectroscopy

For many applications, reducing sample resistance, rather than increasing probe Q or filling factor, is the only way to further improve the signal-to-noise ratio of cryogenically cooled NMR probes. In this paper, bounds are calculated for the minimum sample resistance that can be achieved for various sample geometries. The sample resistance of 100 mM NaCl in H(2)O in 5 mm sample tubes was measured on a 600 MHz cold probe to be within 14% of the optimum value. The minimum sample resistance can however be lowered by altering the tube cross section. Rectangular tubes oriented with the long axis along the RF magnetic field are particularly favourable.     

NMR strong off-resonance irradiation without sample overheating

A prototype NMR probe for long RF pulse has been constructed. Its main elements are two coils mounted in the concentric position. The first bigger coil is wound around a glass dewar tube and the second smaller coil is placed inside the dewar. These two coils are thermally isolated by the dewar. A long and strong RF pulse is applied to the bigger coil. The smaller detection coil inside the dewar contains a sample and to this coil a short RF pulse is applied. The two coils are independently tuned and electrically isolated. During the operation of the strong RF pulse the smaller coil has a high resistance to ground (very low Q factor) and does not absorb energy from the bigger coil. During the operation of the short on-resonance RF pulse the bigger coil is detuned to a higher frequency, but the resonance circuit with the small coil is in the electrical resonance. The NMR probe may be used in off-resonance experiments in which long and strong RF pulses are applied to the bigger coil and thereby the problem of the sample overheating is avoided.     

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.     

An efficient (1)H/(31)P double-resonance solid-state NMR probe that utilizes a scroll coil

The construction and performance of a scroll coil double-resonance probe for solid-state NMR on stationary samples is described. The advantages of the scroll coil at the high resonance frequencies of (1)H and (31)P include: high efficiency, minimal perturbations of tuning by a wide range of samples, minimal RF sample heating of high dielectric samples of biopolymers in aqueous solution, and excellent RF homogeneity. The incorporation of a cable tie cinch for mechanical stability of the scroll coil is described. Experimental results obtained on a Hunter Killer Peptide 1 (HKP1) interacting with phospholipid bilayers of varying lipid composition demonstrate the capabilities of this probe on lossy aqueous samples.