Capillary HPLC−NMR Coupling: High-Resolution 1H NMR Spectroscopy in the Nanoliter Scale

High information spectroscopic detection techniques for gas chromatography

Gas chromatography has always been a simple and widely used technique for the separation of volatile compounds and their quantitation. However, the common detectors used with this technique are mostly universal and do not provide any specific qualitative information. There have been some attempts to combine the separation power of GC with the qualitative capabilities of "high-information" spectroscopic techniques including infrared spectroscopy, nuclear magnetic resonance spectroscopy, molecular rotational resonance spectroscopy, and vacuum ultraviolet spectroscopy. Some of these hyphenations have proven to be quite successful while others were less so. The history of such attempts, up to the most recent studies in this area, are discussed. Most recently, the hyphenation of GC with molecular rotational resonance spectroscopy which provides promising results and is a newly developed technique is reviewed and compared to previous high-information spectroscopic detection approaches. The history, description and features of each method along with their applications and challenges are discussed.

Taking compact NMR to monitoring real reactions in large-scale chemical industries-General considerations and learnings from a lab-scale test case

The considerations for use of compact nuclear magnetic resonance in a large-scale industrial environment clearly differ from those in academic and educational settings and even from those in smaller companies. In the first part of this article, these differences will be discussed along with the additional requirements that need to be fulfilled for successful applicability in different use cases. In the second part of the article, outcomes from different research activities aiming to fulfill these requirements will be presented with a focus on an online reaction-monitoring study on a lab-scale nucleophilic chlorination reaction.

Implementation of infrared and Raman modalities for glycosaminoglycan characterization in complex systems

Glycosaminoglycans (GAGs) are natural, linear and negatively charged heteropolysaccharides which are incident in every mammalian tissue. They consist of repeating disaccharide units, which are composed of either sulfated or non-sulfated monosaccharides. Depending on tissue types, GAGs exhibit structural heterogeneity such as the position and degree of sulfation or within their disaccharide units composition being heparin, heparan sulfate, chondroitine sulfate, dermatan sulfate, keratan sulfate, and hyaluronic acid. They are covalently linked to a core protein (proteoglycans) or as free chains (hyaluronan). GAGs affect cell properties and functions either by direct interaction with cell receptors or by sequestration of growth factors. These evidences of divert biological roles of GAGs make their characterization at cell and tissue levels of importance. Thus, non-invasive techniques are interesting to investigate, to qualitatively and quantitatively characterize GAGs in vitro in order to use them as diagnostic biomarkers and/or as therapeutic targets in several human diseases including cancer. Infrared and Raman microspectroscopies and imaging are sensitive enough to differentiate and classify GAG types and subtypes in spite of their close molecular structures. Spectroscopic markers characteristic of reference GAG molecules were identified. Beyond these investigations of the standard GAG spectral signature, infrared and Raman spectral signatures of GAG were searched in complex biological systems like cells. The aim of the present review is to describe the implementation of these complementary vibrational spectroscopy techniques, and to discuss their potentials, advantages and disadvantages for GAG analysis. In addition, this review presents new data as we show for the first time GAG infrared and Raman spectral signatures from conditioned media and live cells, respectively.

Analysis of glycosaminoglycan-derived disaccharides by capillary electrophoresis using laser-induced fluorescence detection

A quantitative and highly sensitive method for the analysis of glycosaminoglycan (GAG)-derived disaccharides that relies on capillary electrophoresis (CE) with laser-induced fluorescence detection is presented. This method enables complete separation of 17 GAG-derived disaccharides in a single run. Unsaturated disaccharides were derivatized with 2-aminoacridone to improve sensitivity. The limit of detection was at the attomole level and approximately 100-fold more sensitive than traditional CE-ultraviolet detection. A CE separation timetable was developed to achieve complete resolution and shorten analysis time. The relative standard deviations of migration time and peak areas at both low and high concentrations of unsaturated disaccharides are all less than 2.7 and 3.2%, respectively, demonstrating that this is a reproducible method. This analysis was successfully applied to cultured Chinese hamster ovary cell samples for determination of GAG disaccharides. The current method simplifies GAG extraction steps and reduces inaccuracy in calculating ratios of heparin/heparan sulfate to chondroitin sulfate/dermatan sulfate resulting from the separate analyses of a single sample.

From single to multiple microcoil flow probe NMR and related capillary techniques: a review

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important and powerful instrumental analytical techniques for structural elucidation of unknown small and large (complex) isolated and synthesized compounds in organic and inorganic chemistry. X-ray crystallography, neutron scattering (neutron diffraction), and NMR spectroscopy are the only suitable methods for three-dimensional structure determination at atomic resolution. Moreover, these methods are complementary. However, by means of NMR spectroscopy, reaction dynamics and interaction processes can also be investigated. Unfortunately, this technique is very insensitive in comparison with other spectrometric (e.g., mass spectrometry) and spectroscopic (e.g., infrared spectroscopy) methods. Mainly through the development of stronger magnets and more sensitive solenoidal microcoil flow probes, this drawback has been successfully counteracted. Capillary NMR spectroscopy increases the mass-based sensitivity of the NMR spectroscopic analysis up to 100-fold compared with conventional 5-mm NMR probes, and thus can be coupled online and off-line with other microseparation and detection techniques. It offers not only higher sensitivity, but in many cases provides better quality spectra than traditional methods. Owing to the immense number of compounds (e.g., of natural product extracts and compound libraries) to be examined, single microcoil flow probe NMR spectroscopy will soon be far from being sufficiently effective as a screening method. For this reason, an inevitable trend towards coupled microseparation-multiple microcoil flow probe NMR techniques, which allow simultaneous online and off-line detection of several compounds, will occur. In this review we describe the current status and possible future developments of single and multiple microcoil capillary flow probe NMR spectroscopy and its application as a high-throughput tool for the analysis of a large number of mass-limited samples. The advantages and drawbacks of different coupled microseparation-capillary NMR spectroscopy techniques, such as capillary high-performance liquid chromatography-NMR spectroscopy, capillary electrophoresis-NMR spectroscopy, and capillary gas chromatography-NMR spectroscopy, are discussed and demonstrated by specific applications. Another subject of discussion is the progress in parallel NMR detection techniques. Furthermore, the applicability and mixing capability of tiny reactor systems, termed "microreactors" or "micromixers," implemented in NMR probes is demonstrated by carbamate- and imine-forming reactions.

Small-volume nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is one of the most information-rich analytical techniques available. However, it is also inherently insensitive, and this drawback precludes the application of NMR spectroscopy to mass- and volume-limited samples. We review a particular approach to increase the sensitivity of NMR experiments, namely the use of miniaturized coils. When the size of the coil is reduced, the sample volume can be brought down to the nanoliter range. We compare the main coil geometries (solenoidal, planar, and microslot/stripline) and discuss their applications to the analysis of mass-limited samples. We also provide an overview of the hyphenation of microcoil NMR spectroscopy to separation techniques and of the integration with lab-on-a-chip devices and microreactors.

Hyphenated techniques for the analysis of heparin and heparan sulfate

The elucidation of the structure of glycosaminoglycan has proven to be challenging for analytical chemists. Molecules of glycosaminoglycan have a high negative charge and are polydisperse and microheterogeneous, thus requiring the application of multiple analytical techniques and methods. Heparin and heparan sulfate are the most structurally complex of the glycosaminoglycans and are widely distributed in nature. They play critical roles in physiological and pathophysiological processes through their interaction with heparin-binding proteins. Moreover, heparin and low-molecular weight heparin are currently used as pharmaceutical drugs to control blood coagulation. In 2008, the health crisis resulting from the contamination of pharmaceutical heparin led to considerable attention regarding their analysis and structural characterization. Modern analytical techniques, including high-performance liquid chromatography, capillary electrophoresis, mass spectrometry, and nuclear magnetic resonance spectroscopy, played critical roles in this effort. A successful combination of separation and spectral techniques will clearly provide a critical advantage in the future analysis of heparin and heparan sulfate. This review focuses on recent efforts to develop hyphenated techniques for the analysis of heparin and heparan sulfate.

Application of polymer based stationary phases in high performance liquid chromatography and capillary high performance liquid chromatography hyphenated to microcoil 1H nuclear magnetic resonance spectroscopy

The increased demand for chromatographic materials that are able to achieve a fast separation of large quantities of structure analogues is a great challenge. It is known that polymer based chromatographic materials have a higher loadability, compared to silica based sorbents. Unfortunately these polymer materials cannot be used under high pressure which is necessary in order to obtain high flow rates, and hence long times are needed to perform a separation. However, by immobilizing a polymer on a mechanically stable porous silica core, this problem can be circumvented and higher flows become feasible on these materials. Especially for capillary liquid chromatography hyphenated with nuclear magnetic resonance a high loadability is of great importance in order to obtain sharp, resolved, and concentrated peaks thus resulting in a good signal to noise ratio in the NMR experiment. Therefore, a highly shape selective chromatographic sorbent was developed by covalently immobilizing a poly(ethylene-co-acrylic) acid copolymer (-CH(2)CH(2)-)(x)[CH(2)CH(CO(2)H)-](y) (x=119, y=2.4) with a mass fraction of acrylic acid of 5% as stationary phase on silica via a spacer molecule (3-glycidoxypropyltrimethoxysilane). First, the loadability of this sorbent compared to C(30) is demonstrated by the HPLC separation of two xanthophyll isomers. Subsequently, it has been successfully employed in the hyphenation of capillary HPLC with microcoil (1)H NMR spectroscopy by separating and identifying a highly concentrated solution of the tocopherol homologues.

Identification of natural products using HPLC-SPE combined with CapNMR

Two major development areas in HPLC-NMR hyphenation are postcolumn solid-phase extraction (HPLC-SPE-NMR) and capillary separations with NMR detection by means of solenoidal microcoils (CapNMR). These two techniques were combined off-line into HPLC-SPE-CapNMR, which combines the advantage of high loadability of normal-bore HPLC columns with high mass sensitivity of capillary NMR probes with an active volume of 1.5 microL. The technique was used for rapid identification of complex sesquiterpene lactones and esterified phenylpropanoids present in an essentially crude plant extract (toluene fraction of an ethanolic extract of Thapsia garganica fruits). Elution profiles of 10 x 1 mm i.d. SPE cartridges filled with poly(divinylbenzene) resin were found to be only marginally broader than those observed upon direct injection of 6-microL samples into the probe. Thus, the technique focuses analytes emerging in the HPLC elution bands of 0.5-1 mL into volumes of approximately 10 microL, compatible with the CapNMR probe. Using this technique, nine natural products (1-9) present in the plant extract in amounts varying from 0.1 to 20% were identified by means of 1D and 2D NMR spectra, supported by parallel HPLC-ESIMS measurements. Therefore, HPLC-SPE-CapNMR should be regarded as an attractive alternative to other applications of CapNMR for mixture analysis.

Hyphenation of Gas Chromatography to Microcoil 1H Nuclear Magnetic Resonance Spectroscopy

Whereas the hyphenation of gas chromatography (GC) with mass spectrometry is of great importance, little is known about the coupling to nuclear magnetic resonance spectroscopy (NMR). The investigation of this technique is an attractive proposition because of the valuable information given by NMR on molecular structure. The experiments shown here are to our knowledge the first hyphenating capillary GC to microcoil NMR. In contrast to liquids, gases have rarely been investigated by NMR, mainly due to the experimental difficulties in handling gases and the low signal-to-noise-ratio (SNR) of the NMR signal obtained at atmospheric pressure. With advances in NMR sensitivity (higher magnetic fields and solenoidal microprobes), this limitation can be largely overcome. In this paper, we describe the use of a custom-built solenoidal NMR microprobe with an active volume of 2 microL for the NMR detection of several compounds at 400 MHz, first in a mixture, and then with full coupling to capillary GC to identify them separately. The injected amounts of each analyte in the hyphenated experiments are in the range of 15-50 micromol, resulting in reasonable SNR for sample masses of 1-2 microg.

Monitoring of fluid motion in a micromixer by dynamic NMR microscopy

The velocity distribution of liquid flowing in a commercial micromixer has been determined directly by using pulsed-field gradient NMR. Velocity maps with a spatial resolution of 29 microm x 43 microm were obtained by combining standard imaging gradient units with a homebuilt rectangular surface coil matching the mixer geometry. The technique provides access to mixers and reactors of arbitrary shape regardless of optical transparency. Local heterogeneities in the signal intensity and the velocity pattern were found and serve to investigate the quality and functionality of a micromixer, revealing clogging and inhomogeneous flow distributions.

Nuclear magnetic resonance coupled microseparations

The increased separation efficiency afforded by reducing the size of the separation column has resulted in 'microseparations' becoming an important component in many chemical and biochemical applications. The coupling of microseparations with NMR detection is an area of increasing interest owing to the high structural information of NMR. In order to couple efficiently with the separation, the NMR detector must be reduced in size to correspond to that of the separation peak. This paper summarizes some of the approaches used in coupling NMR detection with pressure-driven and electrophoretic microseparations, the design of small NMR detectors and applications of this technology.

LC-NMR coupling technology: recent advancements and applications in natural products analysis

An overview of recent advances in nuclear magnetic resonance (NMR) coupled with separation technologies and their application in natural product analysis is given and discussed. The different modes of LC-NMR operation are described, as well as how technical improvements assist in establishing LC-NMR as an important tool in the analysis of plant-derived compounds. On-flow, stopped-flow and loop-storage procedures are mentioned, together with the new LC-SPE-NMR configuration. The implementation of mass spectrometry in LC-NMR is also useful on account of the molecular weight and fragmentation information that it provides, especially when new plant species are studied. Cryogenic technology and capillary LC-NMR are the other important recent developments. Since the plant kingdom is endless in producing potential drug candidates, development and optimization of LC-NMR techniques convert the study of natural products to a less-time-consuming task, speeding up identification.

Determination of regulatory phosphorylation sites in nanogram amounts of a synthetic fragment of ZAP-70 using microprobe NMR and on-line coupled capillary HPLC-NMR

The protein kinase ZAP-70 is involved in T-cell activation and interacts with tyrosine-phosphorylated peptide sequences known as immunoreceptor tyrosine activation motifs (ITAMs). We have studied the regulatory phosphorylation sites in the tryptic fragment containing amino acids 485-496 (ALGADDSYYTAR). The four possible peptides with phosphorylation at none, one, or both of the Y-492 and Y-493 tyrosines were specifically synthesized and analyzed by (1)H/(13)C-NMR at 600 MHz using a capillary HPLC-NMR microprobe. Unambiguous discrimination of the peptides was possible via effect of chemical shifts of phosphorylation on the aromatic tyrosine protons. With the microprobe and the detection volume of 1.5 microl, it was possible to perform structure elucidation with the very small amounts available for the various peptides. For the syringe injection, 15 microg of the analyte were used (corresponding to ca 2 mg in classical 5-mm tubes). Capillary HPLC-NMR spectra were recorded in the stopped-flow mode from less than 400 ng of each peptide, using 1D and 2D techniques ((1)H,(1)H-COSY-90, (1)H/(13)C-HSQC, and (1)H/(13)C-HMBC).

A comparison of capillary-scale LC-NMR with alternative techniques: spectroscopic and practical considerations

Experimental and practical details for the use of capillary LC (CapLC)-NMR are reported. The capillary NMR probe has high sensitivity and excellent flow characteristics and we found CapLC-NMR to be best suited to samples that are truly mass limited. CapLC-NMR relies on good capillary-scale chromatography where highly concentrated peaks with a volume closely matched to the NMR flow cell are achievable. Provided that the loading capacity of the capillary column is not limiting, the combination of high sensitivity and high solvent suppression quality makes CapLC-NMR an excellent choice. For many real samples, however, the loading is limiting and we found the combination of LC-SPE-MS-NMR with a cryoprobe enables more material to be purified for NMR analysis, while retaining sensitivity.

Hyphenation of capillary high-performance liquid chromatography to microcoil magnetic resonance spectroscopy—determination of various carotenoids in a small-sized spinach sample

The development of miniaturized hyphenated systems such as capillary high-performance liquid chromatography--and nuclear magnetic resonance spectroscopy (HPLC-NMR) remains challenging in the field of structure elucidation. In combination with a highly specific sample preparation technique, matrix solid-phase dispersion (MSPD), and a highly selective C30 reverse phase HPLC-NMR enables the identification of small amounts of natural compounds. Here, the investigation of five carotenoids in a standard solution and two carotenoids from a spinach sample demonstrate the potential of this new development. The separation of the carotenoids is performed with self-packed fused-silica capillaries with a binary solvent gradient consisting of acetone and water. The miniaturized system allows the use of fully deuterated solvents for on-line HPLC-NMR coupling. The 1H NMR spectra of the various carotenoids obtained in stopped-flow mode gave a high signal-to-noise ratio with a sample amount in the low nanogram range. All necessary parameters for structure elucidation such as multiplet structure, coupling constants and integration values can be detected unambiguously.

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.

Capillary liquid chromatography–microcoil 1H nuclear magnetic resonance spectroscopy and liquid chromatography–ion trap mass spectrometry for on-line structure elucidation of isoflavones in Radix astragali

Miniaturization and hyphenation of chromatographic separation techniques to nuclear magnetic resonance spectroscopy is being increasingly demanded in the field of biomedical, drug metabolite and natural product analysis. Herein, capillary liquid chromatography was coupled on-line to microcoil 1H nuclear magnetic resonance spectroscopy (capLC-NMR) equipped with a 1.5 microL solenoidal probe for structure elucidation of isoflavones in Radix astragali. The extract was screened by HPLC-UV-MS as the preliminary step and four major peaks were identified tentatively by ion trap mass spectrometry molecular weights and characteristic fragments. Then, stopped-flow capLC-UV-NMR was performed using 33 microg extract injected on-column. The four peaks were parked manually in the micro probe one by one and corresponding 1H NMR spectra were recorded with good resolutions under the applied capLC-NMR conditions (120 and 220 ng injected on-column for peaks 2 and 4, respectively). All aromatic regions of 1H NMR spectra correlated well to the characteristic signals of isoflavone aglycone protons. And the signal corresponding to the anomeric proton of the glucopyranoside of isoflavone glycoside was also obtained for peak 1. Therefore, these four peaks are determined as calycosin-7-O-beta-D-glucopyranoside (1), ononin (2), calycosin (3) and formononetin (4) unambiguously. The capLC-NMR results indicate that this hyphenated technique could be used for the determination of a great variety of natural products from small sample amounts, e.g., only 5 g R. astragali in this study.

Comparative analysis of a neurotoxin from Calliostoma canaliculatum by on-line capillary isotachophoresis/1H NMR and diffusion 1H NMR

NMR spectroscopy has been coupled on-line to capillary isotachophoresis (cITP) to enhance structural analyses of dilute charged species through separation and sample concentration. Microcoils, the most mass-sensitive NMR probes available, provide optimal detection for cITP/NMR. To evaluate the utility of cITP/NMR for natural product analysis, a homogenate of the hypobranchial gland from the marine snail Calliostoma canaliculatum containing a cationic neurotoxin (1, a disulfide-bonded dimer of 6-bromo-2-mercaptotryptamine) was studied. For comparison, hypobranchial gland homogenate was also examined by diffusion-NMR, an alternative approach for NMR mixture analysis. cITP/NMR concentrated the neurotoxin by almost 20-fold and isolated it from some of the other components present in the matrix. However, a minor component, likely a precursor or degradant, co-migrated with compound 1. Diffusion-NMR also did not resolve the two, indicating that the compounds possessed similar diffusion coefficients and electrophoretic mobilities. The strengths and limitations of the two approaches for NMR mixture analysis are discussed.

Hyphenation of Capillary HPLC to Microcoil 1H NMR Spectroscopy for the Determination of Tocopherol Homologues

Highly selective reversed phases (C(30) phases) are self-packed in 250 microm inner diameter fused-silica capillaries and employed for capillary HPLC separation of shape-constrained natural compounds (tocopherol homologues, vitamin E). Miniaturized hyphenated systems such as capillary HPLC-ESI-MS (positive ionization mode) and, with special emphasis, continuous-flow capillary HPLC- NMR are used for structural determination of the separated compounds. Despite the small amount of sample available (1.33 microg of each tocopherol), the authors have been able to monitor the capillary HPLC separation under continuous-flow (1)H NMR conditions, thus allowing an immediate peak identification. Further structural assignment was carried out in the stopped-flow NMR mode as shown, for example, by a 2D (1)H,(1)H COSY NMR spectrum of alpha-tocopherol. We demonstrate in this paper the considerable potential of hyphenated capillary separations coupled to MS and NMR for the investigation of restricted amounts of sample.

Direct on-line hyphenation of capillary liquid chromatography to nuclear magnetic resonance spectroscopy: Practical aspects and application to drug metabolite identification

In combining the high peak concentrations of capillary liquid chromatography (CapLC) with the high mass sensitivity of micro scale nuclear magnetic resonance (NMR) the hyphenation of CapLC to micro NMR offers a substantial gain in overall sensitivity. This paper deals with our experiences gained using a commercial CapLC-NMR system which has very recently become available. The limits of detection (SNR > 3) for a test compound of a molecular weight of M 318 were found to be approximately 100 ng (0.35 nmol) within an hour acquisition time and approximately 25 ng over night (85 pmol). Practical aspects such as the feasibility of stopped-flow experiments and sample handling issues are discussed in detail and first possible drug metabolite applications to hepatocyte incubations and direct analysis of plasma samples are presented.

Hyphenation of capillary separations with nuclear magnetic resonance spectroscopy

The hyphenation of small-volume separations to information-rich detection offers the promise of unmatched analytical information on the components of complex mixtures. Nuclear magnetic resonance (NMR) spectroscopy provides information about molecular structure, although sensitivity remains an issue for on-line NMR detection. This is especially true when hyphenating NMR to capillary separations as the observation time and analyte mass are decreased to the point where reduced information is obtained from the eluting analytes. Because of these limitations, advances in instrumental performance have a large impact on the overall performance of a separation-NMR system. Instrumental aspects and the capabilities of cLC-NMR, CEC-NMR and CE-NMR are reviewed, and applications that have used this technology highlighted. Recent trends towards small volume capillary scale separations are emphasized, as is the recent success of capillary-isotachophoresis (cITP)-NMR.

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.

High-resolution capillary tube NMR. A miniaturized 5-microL high-sensitivity TXI probe for mass-limited samples, off-line LC NMR, and HT NMR

A new triple-resonance (TXI) (1H, 13C, 15N) high-resolution nuclear magnetic resonance (NMR) capillary probe with 2.5-microL NMR-active sample volume (V(obs)) was built and tested for applications with mass- and volume-limited samples and for coupling of microbore liquid chromatography to NMR. This is the first microliter probe with optimized coil geometry for use with individual capillary tubes with an outer diameter of 1 mm. The 90 degree pulse lengths of the 1-mm microliter probe were below 2 micros for proton, below 8 micros for carbon, and below 20 micros for nitrogen, and a spectral line width at signal half-height below 1 Hz was obtained. Compared to a conventional 5-mm probe, the new 600-MHz 1-mm TXI microliter probe with z-gradient shows an increase in mass sensitivity by a factor of 5, corresponding to a 25-fold reduction in measuring time. The consumption of costly deuterated solvent is reduced by at least 2 orders of magnitude. The 1-mm TXI microliter probe with z-gradient allows the measurement of one-dimensional 1H NMR and two-dimensional heteronuclear NMR spectra with a few nanomoles (micrograms) of compound with high sensitivity, speed, and quality. This is a breakthrough for discrete sample NMR spectroscopy with paramount importance for structure elucidation in natural compound chemistry and metabolic research. It offers also advantages for linking chromatographic methods to NMR in a nindustrial environment. Capillary tube NMR may find new applications in areas where high sample throughput is essential, e.g., in the quality control of large sample arrays from parallel chemistry, screening, and compound depositories. It has the potential to increase the sample throughput by 1 order of magnitude or more if new hardware for fast sample handling and exchange becomes available.

Influence of pressure upon coupling pressurized capillary electrochromatography with nuclear magnetic resonance spectroscopy

In this work, the influence of supplementary pressure on the separation efficiency of pressurized capillary electrochromatography (pCEC) was examined. At low pressures of up to 30 bar, which is more than sufficient to prevent bubble formation, no significant loss in separation efficiency is observed. Even at 100 bar, the efficiency of pCEC is still significantly better than without application of an electric field. In addition, analysis times are drastically reduced compared to both capillary electrochromatography (CEC) and capillary HPLC. On the basis of these results, an improved interface for capillary NMR coupling is described and used for the separation and identification of a mixture of unsaturated fatty acid methyl esters. Under these conditions, the analysis time could be shortened by up to a factor of 10 when pCEC is coupled to NMR spectroscopy.

Union of capillary high-performance liquid chromatography and microcoil nuclear magnetic resonance spectroscopy applied to the separation and identification of terpenoids

This paper describes the first coupling of a commercial capillary HPLC system with a diode array spectrophotometric detector and a custom-built nuclear magnetic resonance (NMR) flow microprobe. The eluent from a 3-microm diameter C18 HPLC column is linked to a 500 MHz 1H-NMR microcoil probe with an observe volume of 1.1 microl. The separation and structurally-rich detection of a mixture of terpenoids under both isocratic and gradient solvent elution conditions is presented. The lowest limits of detection yet reported for capillary HPLC on-line measurement (i.e., 37 ng for alpha-pinene) are achieved with this system. The complementary nature of diode array and NMR detection allows stopped-flow data collection from analytes which would otherwise go unnoticed in continuous-flow NMR. Moreover, stopped-flow NMR data is presented for the detection of a trace (sub-nmol) impurity in the sample mixture. Since NMR signals degrade and shift during solvent gradients, flow injection analysis studies are conducted with injected solvent plugs differing in mobile phase composition. The NMR signal degradation accompanying these injections is largely due to the variance in chemical shift with the solvent composition rather than to changes in magnetic susceptibility of the solvent. Characterization of such effects enables the development of improved NMR probes for the coupling of capillary HPLC and NMR.

A photometric screening method for dimeric naphthylisoquinoline alkaloids and complete on-line structural elucidation of a dimer in crude plant extracts, by the LC-MS/LC-NMR/LC-CD triad

An efficient evaluation procedure for the chemical screening and on-line structural elucidation of dimeric naphthylisoquinoline alkaloids has been developed. The method is based on the lead tetraacetate oxidation of the central binaphthalene core of the alkaloids. UV spectra of the extracts after addition of the oxidant show, in the presence of naphthylisoquinoline dimers, the appearance of a characteristic long-wavelength absorption indicative of dinaphthoquinones. The efficiency and relevance of the method has been demonstrated in the discovery of a constitutionally and configurationally new dimeric naphthylisoquinoline alkaloid, named ancistrogriffithine A (4), from the previously uninvestigated Asian liana Ancistrocladus griffithii. After verification of this screening result by LC-ESI-MS/MS, the constitution and the relative configuration of the compound were elucidated on line, by LC-NMR and LC-CD on the extract. Using an LC-NMR-WET-ROESY experiment, itwas possible for the first time to determine the relative axial configuration of a natural biaryl compound on line, by observing long-range ROE interactions. Finally, an oxidative degradation right on the extract delivered the absolute configuration of 4, without isolation of the alkaloid. Ancistrogriffithine A is the as yet only dimeric naphthylisoquinoline from an Asian Ancistrocladaceae plant.

The potential of LC-NMR in phytochemical analysis

The coupling of high performance liquid chromatography with nuclear magnetic resonance spectroscopy (LC-NMR) is one of the most powerful methods for the separation and structural elucidation of unknown compounds in mixtures. The recent progress in pulse field gradients and solvent suppression, the improvement in probe technology, and the construction of high field magnets have given a new stimulus to this technique, which has emerged since the mid 1990s as a very efficient method for the on-line identification of organic molecules. LC-NMR thus represents a potentially interesting complementary technique to LC-UV-MS in phytochemical analysis for the detailed on-line structural analysis of natural products. Recent applications have fully demonstrated the usefulness of this technique. A brief review of the applications of LC-NMR in natural product chemistry is presented in this paper, and a summary of the basic principles and modes of operation of LC-NMR is provided. Selected examples of LC-NMR analyses of plant metabolites in crude extracts or in enriched fractions are outlined and used to illustrate the different strategies for employing the technique. The practical possibilities and limitations of LC-NMR in its application to the analysis of crude plant extracts are discussed by means of several examples. Analytical strategies involving LC multi-coupled (hyphenated) techniques for the chemical screening and dereplication of crude plant extracts are presented. An analysis of the future development of the technique with respect to its application in phytochemical analysis is also given.

Directly coupled HPLC-NMR and HPLC-NMR-MS in pharmaceutical research and development

The methodology for the direct coupling of HPLC with NMR spectroscopy and the simultaneous double coupling of HPLC with NMR and mass spectrometry (MS) is described. Indications of the necessary technical developments to achieve this are given, and the applications of these new techniques to studies of pharmaceutical relevance are reviewed. These include studies of combinatorial chemistry libraries, synthetic chemical impurities, characterisation of drug mixtures, identification of natural products of possible pharmaceutical interest and identification of xenobiotic metabolites in human, animal and in vitro systems. In addition, HPLC-NMR has been used to investigate xenobiotic metabolite reactivity. Finally, the potential future directions of the techniques are discussed.

Improved column preparation and performance in capillary electrochromatography

Problems encountered in capillary electrochromatography, i.e. non-reproducible column manufacture, bubble formation during usage, short column lifetimes and limited choice of packing particles are addressed by the development of fritless or single-frit, internally tapered, segmented and dead-volume free coupled capillary columns. The Van Deemter plots measured demonstrate the performance of these high-quality capillaries which are suitable for capillary electrochromatography as well as for capillary high-performance liquid chromatographic applications.

A microcoil NMR probe for coupling microscale HPLC with on-line NMR spectroscopy

An HPLC NMR system is presented that integrates a commercial microbore HPLC system using a 0.5-mm column with a 500-MHz proton NMR spectrometer using a custom NMR probe with an observe volume of 1.1 microL and a coil fill factor of 68%. Careful attention to capillary connections and NMR flow cell design allows on-line NMR detection with no significant loss in separation efficiency when compared with a UV chromatogram. HPLC NMR is performed on mixtures of amino acids and small peptides with analyte injection amounts as small as 750 ng; the separations are accomplished in less than 10 min and individual NMR spectra are acquired with 12 s time resolution. Stopped-flow NMR is achieved by diversion of the chromatographic flow after observation of the beginning of the analyte band within the NMR flow cell. Isolation of the compound of interest within the NMR detection cell allows multidimensional experiments to be performed. A stopped-flow COSY spectrum of the peptide Phe-Ala is acquired in 3.5 h with an injected amount of 5 micrograms.

Liquid chromatography-nuclear magnetic resonance spectroscopy

A general overview of the experimental set-up for performing analytical-scale and nanoliter-scale liquid chromatography-1H nuclear magnetic resonance spectroscopy (LC-1H-NMR) experiments is given. The high power of combining LC with 1H-NMR spectroscopy is demonstrated by two examples, where NMR acquisition was performed either in the continuous-flow mode on the analytical scale or in the stopped-flow mode on the nanoliter scale. Current developments employing the on-line coupling of capillary as well as supercritical fluid separation methods with 1H-NMR spectroscopy together with LC-13C-NMR spectroscopy are discussed.

NMR tools for biotechnology

Recent developments in NMR spectroscopy verify that NMR continues to be an exciting area of research. These advances can be placed into three general categories: new hardware; new techniques; and novel applications. The hardware developments include many advances in the area of flow NMR and some new probe designs. The new techniques include several ways to edit the NMR spectra of mixtures without using chromatographic separation. These new NMR tools are now allowing us to analyze complex mixtures, combinatorial-chemistry libraries, bound drugs, unstable compounds, very small samples, and heterogeneous samples.

Gradient elution capillary electrochromatography and hyphenation with nuclear magnetic resonance

Coupling of gradient capillary electrochromatography (gradient CEC) and capillary zone electrophoresis (CZE) with nuclear magnetic resonance spectroscopy (NMR) was performed using a recently developed capillary NMR interface. This technique was applied for the analysis of pharmaceuticals and food. An analgesic was investigated using isocratic and gradient continuous-flow CEC-NMR. Comparison of the results demonstrated the superiority of gradient CEC over isocratic CEC. Aspartame and caffeine, both ingredients of soft beverages, were separated and analyzed by continuous flow CZE-NMR. The order of elution could be reversed by altering the pH. This reversal led to an increased sample concentration in the NMR detection cell, thus allowing the acquisition of a totally correlated spectroscopy (TOCSY) two-dimensional (2-D) spectrum of the synthetic peptide aspartame.

Analyte identification in capillary electrophoretic separation techniques

A review on applications of on-line hyphenation in capillary electrophoresis and capillary electrochromatography for the identification of migrating analytes is presented. There is an urgent need for unambiguous analyte identification by combining spectral information and observed migration times, because the parameters influencing the migration times and separation efficiencies in these separation techniques are not easily controlled, especially when real samples containing unknown interferences have to be analyzed. The spectrometric techniques covered here are ultraviolet and visible radiation (UV/Vis) absorption, fluorescence including fluorescence line-narrowing spectroscopy, Raman spectroscopy, nuclear magnetic resonance and mass spectrometry. Attention is essentially confined to literature reports in which the extra information provided by the detector is really used for identification purposes, especially in real-life samples, while the interfacing as such and analyte detectabilities in standard solutions are only briefly discussed. This article covers an extensive fraction of the literature published on this topic until the beginning of 1998.

NMR spectroscopy in pharmacy

Since drugs in clinical use are mostly synthetic or natural products, NMR spectroscopy has been mainly used for the elucidation and confirmation of structures. For the last decade, NMR methods have been introduced to quantitative analysis in order to determine the impurity profile of a drug, to characteristic the composition of drug products, and to investigate metabolites of drugs in body fluids. For pharmaceutical technologists, solid state measurements can provide information about polymorphism of drug powders, conformation of drugs in tablets etc. Micro-imaging can be used to study the dissolution of tablets, and whole-body imaging is a powerful tool in clinical diagnostics. Taken together, this review covers applications of NMR spectroscopy in drugs analysis, in particular, methods of international pharmacopoeiae, pharmaceutics and pharmacokinetics. The authors have repeated many of the methods describe in their own laboratories.

NMR methods in combinatorial chemistry

The use of NMR spectroscopy in combinatorial chemistry has provided a versatile tool for monitoring combinatorial chemistry reactions and for assessing ligand-receptor interactions. The application of magic angle spinning NMR is widespread and has allowed structure determination to be performed on compounds attached to solid supports. A variety of two-dimensional NMR techniques have been applied to enhance the usability of the magic angle spinning NMR data. New developments for solution NMR analysis include high performance liquid chromatography, NMR, mass spectroscopy and flow NMR. NMR based methods currently being investigated may prove valuable as compound screening tools.