A probe design for the acquisition of homonuclear, heteronuclear, and inverse detected NMR spectra from multiple samples

Sample-centred shimming enables independent parallel NMR detection

Two major technical challenges facing parallel nuclear magnetic resonance (NMR) spectroscopy, at the onset, include the need to achieve exceptional [Formula: see text] homogeneity, and good inter-detector radiofrequency signal decoupling, and have remained as technical obstacles that limit high throughput compound screening via NMR. In this contribution, we consider a compact detector system, consisting of two NMR 'unit cell' resonators that implement parallel [Formula: see text] shimming with parallel radiofrequency detection, as a prototype NMR environment, pointing the way towards achieving accelerated NMR analysis. The utility of our approach is established by achieving local field correction within the bore of a 1.05T permanent magnet MRI. Our forerunner platform suppresses signal cross-coupling in the range of [Formula: see text] dB to [Formula: see text] dB, under a geometrically decoupled scheme, leading to a halving of the necessary inter-coil separation. In this permanent magnet environment, two decoupled parallel NMR detector sites simultaneously achieve narrow spectral linewidth, overcoming the spatial inhomogeneity of the magnet from 400 to 28 Hz.

Multiplexing experiments in NMR and multi-nuclear MRI

Multiplexing NMR experiments by direct detection of multiple free induction decays (FIDs) in a single experiment offers a dramatic increase in the spectral information content and often yields significant improvement in sensitivity per unit time. Experiments with multi-FID detection have been designed with both homonuclear and multinuclear acquisition, and the advent of multiple receivers on commercial spectrometers opens up new possibilities for recording spectra from different nuclear species in parallel. Here we provide an extensive overview of such techniques, designed for applications in liquid- and solid-state NMR as well as in hyperpolarized samples. A brief overview of multinuclear MRI is also provided, to stimulate cross fertilization of ideas between the two areas of research (NMR and MRI). It is shown how such techniques enable the design of experiments that allow structure elucidation of small molecules from a single measurement. Likewise, in biomolecular NMR experiments multi-FID detection allows complete resonance assignment in proteins. Probes with multiple RF microcoils routed to multiple NMR receivers provide an alternative way of increasing the throughput of modern NMR systems, effectively reducing the cost of NMR analysis and increasing the information content at the same time. Solid-state NMR experiments have also benefited immensely from both parallel and sequential multi-FID detection in a variety of multi-dimensional pulse schemes. We are confident that multi-FID detection will become an essential component of future NMR methodologies, effectively increasing the sensitivity and information content of NMR measurements.

Broadband and multi-resonant sensors for NMR

It has always been of considerable interest to study the nuclear magnetic resonance response of multiple nuclei simultaneously, whether these signals arise from internuclear couplings within the same molecule, or from uncoupled nuclei within sample mixtures. The literature contains numerous uncorrelated reports on techniques employed to achieve multi-nuclear NMR detection. This paper consolidates the subset of techniques in which single coil detectors are utilized, and highlights the strengths and weaknesses of each approach, at the same time pointing the way towards future developments in the field of multi-nuclear NMR. We compare the different multi-nuclear NMR techniques in terms of performance, and present a guide to NMR probe designers towards application-based optimum design. We also review the applicability of micro-coils in the context of multi-nuclear methods. Micro-coils benefit from compact geometries and exhibit lower impedance, which provide new opportunities and challenges for the NMR probe designer.

Heteronuclear Micro-Helmholtz Coil Facilitates µm-Range Spatial and Sub-Hz Spectral Resolution NMR of nL-Volume Samples on Customisable Microfluidic Chips

We present a completely revised generation of a modular micro-NMR detector, featuring an active sample volume of ∼ 100 nL, and an improvement of 87% in probe efficiency. The detector is capable of rapidly screening different samples using exchangeable, application-specific, MEMS-fabricated, microfluidic sample containers. In contrast to our previous design, the sample holder chips can be simply sealed with adhesive tape, with excellent adhesion due to the smooth surfaces surrounding the fluidic ports, and so withstand pressures of ∼2.5 bar, while simultaneously enabling high spectral resolution up to 0.62 Hz for H2O, due to its optimised geometry. We have additionally reworked the coil design and fabrication processes, replacing liquid photoresists by dry film stock, whose final thickness does not depend on accurate volume dispensing or precise levelling during curing. We further introduced mechanical alignment structures to avoid time-intensive optical alignment of the chip stacks during assembly, while we exchanged the laser-cut, PMMA spacers by diced glass spacers, which are not susceptible to melting during cutting. Doing so led to an overall simplification of the entire fabrication chain, while simultaneously increasing the yield, due to an improved uniformity of thickness of the individual layers, and in addition, due to more accurate vertical positioning of the wirebonded coils, now delimited by a post base plateau. We demonstrate the capability of the design by acquiring a 1H spectrum of ∼ 11 nmol sucrose dissolved in D2O, where we achieved a linewidth of 1.25 Hz for the TSP reference peak. Chemical shift imaging experiments were further recorded from voxel volumes of only ∼ 1.5 nL, which corresponded to amounts of just 1.5 nmol per voxel for a 1 M concentration. To extend the micro-detector to other nuclei of interest, we have implemented a trap circuit, enabling heteronuclear spectroscopy, demonstrated by two 1H/13C 2D HSQC experiments.

Miniaturized multi-coil arrays for functional planar imaging with a single-sided NMR sensor

Nowadays most low-field NMR sensors, such as the single-sided Profile NMR-MOUSE®, still suffer from poor sensitivity, either resulting from low magnetic field strengths and correspondingly low NMR frequencies, or lack of sensitivity. Generally, micro-coils can improve sensitivity, but due to their small size, and thus small inductance, they are mainly used for high-field NMR. Their main application field is parallel imaging, where those coils are typically assembled to receive-only coil-arrays and increase the field-of-view. Prominent signal combination techniques such as GRAPPA and SENSE are used to combine the spatially independent NMR signals to images in order to increase acquisition speed. A decisive disadvantage of today's single-sided NMR probes is the limited accessibility for NMR imaging. Although it is possible to use flat gradient coils on top of the NMR-MOUSE® to apply imaging techniques, such images can only be recorded with very long acquisition times, excluding the NMR-MOUSE® for lateral imaging of time-dependent processes. In this study sensitivity improved micro-structured RF coils, optimized for low frequencies, and correspondingly arrays of these coils, were employed to improve sensitivity and gave access to lateral spatial resolution within the sensitive plane at several observation points at the same time. Recently developed three- and four-coil arrays were combined with a Profile NMR-MOUSE® and characterized in terms of coil coupling, noise correlation and signal combination. The three-coil array was used for lateral imaging of moisture transport in travertine rock samples and to study the one-dimensional drying of paint.

Multiplexed NMR: an automated CapNMR dual-sample probe

A new generation of microscale, nuclear magnetic resonance (CapNMR) probe technology employs two independent detection elements to accommodate two samples simultaneously. Each detection element in the dual-sample CapNMR probe (DSP) delivers the same spectral resolution and S/N as in a CapNMR probe configured to accommodate one sample at a time. A high degree of electrical isolation allows the DSP to be used in a variety of data acquisition modes. Both samples are shimmed simultaneously to achieve high spectral resolution for simultaneous data acquisition, or alternatively, a flowcell-specific shim set is readily called via spectrometer subroutines to enable acquisition from one sample while the other is being loaded. An automation system accommodates loading of two samples via dual injection ports on an autosampler and two completely independent flowpaths leading to dedicated flowcells in the DSP probe.

Design and construction of a versatile dual volume heteronuclear double resonance microcoil NMR probe

Improved NMR detection of mass limited samples can be obtained by taking advantage of the mass sensitivity of microcoil NMR, while throughput issues can be addressed using multiple, parallel sample detection coils. We present the design and construction of a double resonance 300-MHz dual volume microcoil NMR probe with thermally etched 440-nL detection volumes and fused silica transfer lines for high-throughput stopped-flow or flow-through sample analysis. Two orthogonal solenoidal detection coils and the novel use of shielded inductors allowed the construction of a probe with negligible radio-frequency cross talk. The probe was resonated at (1)H-(2)D (upper coil) and (1)H-(13)C (lower coil) frequencies such that it could perform 1D and 2D experiments with active locking frequency. The coils exhibited line widths of 0.8-1.1 Hz with good mass sensitivity for both (1)H and (13)C NMR detection. (13)C-directly detected (2)D HETCOR spectra of 5% v/v (13)C labeled acetic acid were obtained in less than 5 min. Demonstration of the probe characteristics as well as applications of the versatile two-coil double resonance probe are discussed.

Development of a dual cell, flow-injection sample holder, and NMR probe for comparative ligand-binding studies

NMR based ligand screening is becoming increasingly important for the very early stages of drug discovery. We have proposed a method that makes highly efficient use of a single sample of a scarce target, or one with poor or limited solubility, to screen an entire compound library. This comparative method is based on immobilizing the target for the screening procedure. In order to support the method, a dual cell, flow injection probe with a single receiver coil has been constructed. The flow injection probe has been mated to a single high performance pump and sample handling system to enable the automated analysis of large numbers of compound mixes for binding to the target. The probe, having an 8 mm 1H/2H dual tuned coil and triple axis gradients, is easily shimmed and yields NMR spectra of comparable quality to a standard 5 mm high-resolution probe. The lineshape in the presence of a solid support is identical to that in glass NMR tubes in a 5 mm probe. Control spectra of each cell are identical and well separated, while ligand binding in a complex mixture can be readily detected in 20-30 min, thus paving the way for use of the probe for actual drug discovery efforts.

Spectral unraveling by space-selective Hadamard spectroscopy

Spectral unraveling by space-selective Hadamard spectroscopy (SUSHY) enables recording of NMR spectra of multiple samples loaded in multiple sample tubes in a modified spinner turbine and a regular 5mm liquids NMR probe equipped with a tri-axis pulsed field gradient coil. The individual spectrum from each sample is extracted by adding and subtracting data that are simultaneously obtained from all the tubes based on the principles of spatially resolved Hadamard spectroscopy. The well-known Hadamard spectroscopy has been applied for spatial selection and the method utilizes standard configuration of NMR instrument hardware. The SUSHY method can be easily incorporated in multi-dimensional multi-tube NMR experiments. This method combines the excitation multiplexing, natural advantage of FTNMR, and sample multiplexing and offers high-throughput by reducing the total experimental time by up to a factor of four in a 4-tube mode.

Multiple-sample probe for solid-state NMR studies of pharmaceuticals

Solid-state NMR spectroscopy (SSNMR) is an extremely powerful technique for the analysis of pharmaceutical dosage forms. A major limitation of SSNMR is the number of samples that can be analyzed in a given period of time. A solid-state magic-angle spinning (MAS) probe that can simultaneously acquire up to seven SSNMR spectra is being developed to increase throughput/signal-to-noise ratios. A prototype probe incorporating two MAS modules has been developed and spectra of ibuprofen and aspirin have been acquired simultaneously. This version is limited to being a two-module probe due to large amounts of space required for the tuning elements located next to the MAS modules. A new probe design incorporating coaxial transmission lines and smaller MAS modules has been constructed. This probe allows for close proximity of the MAS modules (within 3 cm), adequate proton decoupling power (>50 kHz), and the capability of remote tuning and sample changing. Spectra of hexamethylbenzene (HMB) have been acquired and show signal-to-noise ratios comparable to existing SSNMR probes. Adamantane line widths are also comparable to conventional probe technology. Decoupling powers of 70 kHz have been achieved using a MAS module suitable for 3 cm spacing between modules. Remote tuning has also been achieved with this new coaxial transmission line design. This probe design can be easily scaled to incorporate multiple MAS modules, which is a limitation of the previous design. The number of modules that can be incorporated is only limited by the number of transmission lines that will fit in a cross-sectional diameter of the bore and the axial field length of the magnet.

Solvent-localized NMR spectroscopy using the distant dipolar field: a method for NMR separations with a single gradient

Solvent-localized NMR (SOLO) is a new method which allows the separation of NMR spectra of substances dissolved in different solvents. It uses the selective HOMOGENIZED pulse sequence to produce a two-dimensional NMR spectrum resulting from intermolecular zero-quantum coherences in one distinct solvent. The detected signal is locally refocused by the action of the distant dipolar field, which is created by a frequency selective pulse only in regions containing the selected solvent. The prerequisites are that the different solvents have sufficiently different chemical shifts to be excited separately and that compartments with different solvents are spatially separated by more than the typical diffusion distance. Here, the method is demonstrated for the solvents water and DMSO on a length scale of 0.5 mm. Because signal in the spectra is refocused locally, SOLO is insensitive to variations in the magnetic field which may result from inhomogeneities or structures in the sample. This makes applications in strongly structured samples possible. SOLO is the first method that achieves localization of NMR signal with a single gradient pulse. Therefore, it can be used in conventional NMR spectrometers with one-axis gradient systems and lends itself to a wide range of applications including in vivo NMR.

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.

An eight-coil high-frequency probehead design for high-throughput nuclear magnetic resonance spectroscopy

In order to increase the throughput of high-resolution nuclear magnetic resonance spectroscopy a multiple-coil probe, which enables the simultaneous analysis of eight different samples, was designed. The probe, consisting of eight identical solenoidal coils, was constructed for operation at 600 MHz. By using four receivers and radiofrequency switches, spectra from eight different chemical solutions were acquired in the time normally required for one. Two-dimensional COSY, gradient COSY, and TOCSY data have been acquired. Intercoil electrical isolation was between 25 and 45 dB, with signal cross-talk between approximately 1 and 5% measured by NMR. The spectral linewidths for the eight coils were between 3 and 6Hz for a single optimized shim setting.

Dual Microcoil NMR Probe Coupled to Cyclic CE for Continuous Separation and Analyte Isolation

Capillary electrophoresis (CE)-nuclear magnetic resonance (NMR) spectroscopy combines the separation efficiency of CE and the information-rich detection capabilities of NMR. However, the temporally narrow CE peaks reduce NMR sensitivity and prevent on-line multidimensional NMR acquisitions. In this work, cyclic CE with multicoil NMR instrumentation is developed to perform CE in multiple closed loops. As a proof of concept, a two-loop five-junction capillary configuration creates two connected yet independently operable fluidic loops. With appropriate voltage switching, analytes can be directed as desired around or between the loops, and a particular analyte band can be parked in one NMR detector coil while CE continues in the second loop and monitored with a second NMR detector coil. The separation of a mixture of amino acids (Ala, Val, Thr) is achieved in two cycles. After one CE cycle, Ala is separated and COSY data are recorded in one loop while Val and Thr are separated in the second loop. At the end of the second cycle, both Val and Thr are separated and multidimensional NMR spectra acquired. With this instrumentation and appropriate protocols, two-dimensional NMR data acquisition and CE separation are achieved simultaneously.

Microflow NMR: Concepts and Capabilities

The principles and parameters to consider when choosing an NMR probe for analysis of a volume- or mass-limited sample are identified and discussed. In particular, a capillary-based microflow probe is described which has a mass sensitivity comparable to cryoprobes (observe volume approximately 40 microL), but with several distinct advantages. The microflow probe has a flowcell volume of 5 microL and an observe volume of 1.5 microL and is equipped with proton and carbon observe channels, deuterium lock, and z-gradient capability. The entire flow path is fused silica; inlet and outlet capillary inner diameters are 50 microm to minimize sample dispersion, making it well-suited to volume-limited samples. An injected sample of 1 nmol of sucrose (0.34 microg in 3 microL, 0.33 mM; MW = 342 g/mol) yields a 1D proton spectrum in 10 min on a spectrometer of 500 MHz or higher. In another example, 15 microg of sucrose (in 3 microL; 15 mM, 45 nmol) is injected and parked in the probe to yield a heteronuclear multiple-quantum coherence (HMQC) spectrum in less than 15 h. The natural product muristerone A (75 microg in 3 microL, 50 mM, 150 nmol; MW = 497 g/mol) was delivered to the flow cell, and a gradient correlation spectroscopy spectrum was acquired in 7 min, a gradient HMQC in 4 h, and a gradient heteronuclear multiple-bond correlation in 11 h. Four basic modes of sample injection into the probe vary in degree of user intervention, speed, solvent consumption, and sample delivery efficiency. Manual, manual-assisted (employing a micropump), automated (using an autosampler), and capillary HPLC modes of operation are described.

Design of small volume HX and triple-resonance probes for improved limits of detection in protein NMR experiments

Three- and four-frequency nuclear magnetic-resonance probes have been designed for the study of small amounts of protein. Both "HX" (1H, X, and 2H channels) and "triple-resonance" (1H, 15N, 13C, and 2H) probes were implemented using a single transmit/receive coil and multiple-frequency impedance matching circuits. The coil used was a six-turn solenoid with an observe volume of 15 microl. A variable pitch design was used to improve the B1 homogeneity of the coil. Two-dimensional HSQC spectra of approximately 1mM single labeled 15N- and double labeled 15N/13C-proteins were acquired in experimental times of approximately 2h. Triple-resonance capability of the small-volume triple-resonance probe was demonstrated by acquiring three-dimensional HNCO spectra from the same protein samples. In addition to enabling very small quantities of protein to be used, the extremely short pulse widths (1H = 4, 15N = 4, and 13C = 2 micros) of this particular design result in low power decoupling and wide-bandwidth coverage, an important factor for the ever-higher operating frequencies used for protein NMR studies.

High-Throughput Nuclear Magnetic Resonance Analysis Using a Multiple Coil Flow Probe

An automated method for high-throughput nuclear magnetic resonance (NMR) spectroscopy has been developed using a four-coil Multiplex NMR probe. The probe is constructed with solenoidal microcoils optimized for detection of small volume, mass-limited samples and a flow-through design. Four samples can be simultaneously injected into the Multiplex probe with a robotics liquid handler and then analyzed in rapid succession using a selective excitation experiment. Due to the simultaneous injection of four samples and the reduced analysis time with rapid selective excitation, the analysis rate achieved thus far is as low as 1 sample/34 s for 1D 1H NMR.

Flow NMR applications in combinatorial chemistry

Flow NMR techniques are now well accepted and widely used in many areas of drug discovery. Although natural-product-, rational-drug-design-, and NMR-screening-programs have begun to use flow NMR more routinely, flow NMR has not yet gained widespread acceptance in combinatorial chemistry, even though it has been shown to be a potentially useful tool. Recent developments in DI-NMR, FIA-NMR, and LC-NMR will help flow NMR eventually gain a wider acceptance within combinatorial chemistry. These developments include LC-NMR-MS instrumentation, flow probe improvements, new pulse sequences, improved automation of NMR data analysis, and the application of flow NMR to related fields in drug discovery.

Micromixer-based time-resolved NMR: applications to ubiquitin protein conformation

Time-resolved NMR spectroscopy is used to studychanges in protein conformation based on the elapsed time after a change in the solvent composition of a protein solution. The use of a micromixer and a continuous-flow method is described where the contents of two capillary flows are mixed rapidly, and then the NMR spectra of the combined flow are recorded at precise time points. The distance after mixing the two fluids and flow rates define the solvent-protein interaction time; this method allows the measurement of NMR spectra at precise mixing time points independent of spectral acquisition time. Integration of a micromixer and a microcoil NMR probe enables low-microliter volumes to be used without losing significant sensitivity in the NMR measurement. Ubiquitin, the model compound, changes its conformation from native to A-state at low pH and in 40% or higher methanol/water solvents. Proton NMR resonances of the His-68 and the Tyr-59 of ubiquitin are used to probe the conformational changes. Mixing ubiquitin and methanol solutions under low pH at microliter per minute flow rates yields both native and A-states. As the flow rate decreases, yielding longer reaction times, the population of the A-state increases. The micromixer-NMR system can probe reaction kinetics on a time scale of seconds.

NMR detection with multiple solenoidal microcoils for continuous-flow capillary electrophoresis

Nuclear magnetic resonance (NMR) spectroscopy represents a promising on-line detector for capillary electrophoresis (CE). The inherent poor sensitivity of NMR mandates the use of NMR probes with the highest mass sensitivity, such as those containing solenoidal microcoils, for CE/NMR hyphenation. However, electrophoretic current degrades the resolution of NMR spectra obtained from solenoidal coils. A new method to avoid microcoil NMR spectral degradation during continuous-flow CE is demonstrated using a unique multiple solenoidal coil NMR probe. The electrophoretic flow from a single separation capillary is split into multiple outlets, each possessing its own NMR detection coil. While the CE electrophoretic flow is directed through one outlet, stopped-flow, high-resolution NMR spectra are obtained from the coil at the other outlet. The electrophoretic flow and NMR measurements are cycled between the outlets to allow a continuous CE separation with "stopped-flow" detection. As a new approach for improving multiple coil probe performance, the magnetic field homogeneity is automatically adjusted (via the shim coils of the magnet) for the active coil. The multiple microcoil CE/NMR coupling has been used to analyze a <3 nmole mixture of amines while obtaining between 1 and 2 Hz line width, demonstrating the ability to avoid electrophoretic current-induced line broadening.

Microscale NMR

NMR spectroscopy is increasingly being used to characterize microliter and smaller-volume samples. Substances at picomole levels have been identified using NMR spectrometers equipped with microcoil-based probes. NMR probes that incorporate multiple sample chambers enable higher-throughput NMR experiments. Hyphenation of capillary-scale separations and microcoil NMR has also decreased analysis time of mixtures. For example, capillary isotachophoresis/NMR allows the highest mass sensitivity nanoliter-volume flow cells to be used with low microliter volume samples because isotachophoresis concentrates the microliter volume sample into the nanoliter volume NMR detection probe. In addition, the diagnostic capabilities of NMR spectroscopy allow the physico-chemical aspects of a capillary separation process to be characterized on-line. Because of such advances, the application of NMR to smaller samples continues to grow.