On-Line Coupling of Capillary Electrochromatography, Capillary Electrophoresis, and Capillary HPLC with Nuclear Magnetic Resonance Spectroscopy

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.

Pharmaceutical impurities and degradation products: uses and applications of NMR techniques

Current standards and regulations demand the pharmaceutical industry not only to produce highly pure drug substances, but to achieve a thorough understanding of the impurities accompanying their manufactured drug substances and products. These challenges have become important goals of process chemistry and have steadily stimulated the search of impurities after accelerated or forced degradation procedures. As a result, impurity profiling is one of the most attractive, active and relevant fields of modern pharmaceutical analysis. This activity includes the identification, structural elucidation and quantitative determination of impurities and degradation products in bulk drugs and their pharmaceutical formulations. Nuclear magnetic resonance (NMR) spectroscopy has evolved into an irreplaceable approach for pharmaceutical quality assessment, currently playing a critical role in unequivocal structure identification as well as structural confirmation (qualitative detection), enabling the understanding of the underlying mechanisms of the formation of process and/or degradation impurities. NMR is able to provide qualitative information without the need of standards of the unknown compounds and multiple components can be quantified in a complex sample without previous separation. When coupled to separative techniques, the resulting hyphenated methodologies enhance the analytical power of this spectroscopy to previously unknown levels. As a result, and by enabling the implementation of rational decisions regarding the identity and level of impurities, NMR contributes to the goal of making better and safer medicines. Herein are discussed the applications of NMR spectroscopy and its hyphenated derivate techniques to the study of a wide range pharmaceutical impurities. Details on the advantages and disadvantages of the methodology and well as specific challenges with regards to the different analytical problems are also presented.

The importance of hyphenated techniques in the discovery of new lead compounds from nature

Nature represents an extraordinary reservoir of novel molecules and there is currently a resurgence of interest in natural products as a possible source of new lead compounds for introduction into therapeutical screening programmes. To discover new bioactive natural products, the dereplication of crude extracts performed prior to isolation work is of crucial importance for avoiding the tedious isolation of known constituents. In this respect, chemical screening strategies based on hyphenated techniques such as liquid chromatography-ultraviolet photodiode array detection, liquid chromatography-mass spectrometry, liquid chromatography tandom mass spectrometry and liquid chromatography-nuclear magnetic resonance (LC-NMR) are more and more extensively used. In the laboratory of Hostettmann's group, these analytical methods have been fully integrated into the isolation process and are used for the chemical screening of crude plant extracts, in complement with online or at-line bioassays, for rapid localisation and identification of new bioactive compounds. In this paper, possibilities and limitations of hyphenated techniques for de novo online natural product identification are discussed. As LC-NMR is playing a key role in this respect, the main part of the paper is dedicated to this technique. In particular, various ways of integrating NMR in the dereplication process are illustrated and strategies involving either direct or indirect hyphenation are presented.

Identification of Alternaria alternata mycotoxins by LC-SPE-NMR and their cytotoxic effects to soybean (Glycine max) cell suspension culture

This present work describes the application of liquid chromatography-solid phase extraction-nuclear magnetic resonance spectroscopy to analyse Alternaria alternata crude extracts. Altenusin (1), alternariol (2), 3'-hydroxyalternariol monomethyl ether (3), and alternariol monomethyl ether (4), were separated and identified. High-resolution mass spectrometry confirmed the proposed structures. The cytotoxic effects of these compounds towards plants were determined using soybean (Glycine max) cell cultures as a model. EC(50) values which range from 0.11 (± 0.02) to 4.69 (± 0.47) μM showed the high cytotoxicity of these compounds.

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.

Susceptibility-matched plugs for microcoil NMR probes

For mass-limited samples, the residual sample volume outside the detection coil is an important concern, as is good base line resolution. Here, we present the construction and evaluation of magnetic susceptibility-matched plugs for microcoil NMR sample cells which address these issues. Mixed-epoxy glue and ultem tube plugs that have susceptibility values close to those of perfluorocarbon FC-43 (fluorinert) and copper were used in small volume (0.5-2 microL) and larger volume (15-20 microL) thin glass capillary sample cells. Using these plugs, the sample volume efficiency (i.e. ratio of active volume to total sample volume in the microcoil NMR cell) was improved by 6-12-fold without sensitivity and resolution trade-offs. Comparison with laser etched or heat etched microcoil sample cells is provided. The approaches described are potentially useful in metabolomics for biomarkers detection in mass limited biological samples.

Analysis of phenolic xenoestrogens by pressurized CEC with amperometric detection

A new method, pressurized CEC with end-column amperometric detection using carbon paste electrode, has been developed for the separation and determination of five phenolic xenoestrogens in chicken eggs and milk powder samples. Efficient separation of five analytes was performed by pressurized CEC using a mobile phase consisting of 60% v/v ACN and 40% v/v Tris buffer (5 mmol/L, pH 8.0), +6 kV of applied voltage and 7.0 MPa of supplementary pressure. Detection limits of 50, 5, 2, 10 and 20 ng/mL for pentachlorophenol, bisphenol-A, 2,4-dichlorophenol, 4-tert-octylphenol and 4-nonylphenol, respectively, were achieved using carbon paste electrode as working electrode and +0.8 V as detection potential. Matrix solid phase dispersion extraction method had been employed during sample preparation procedure, and mean recoveries ranged from 79.2 to 102.6% at different concentrations of phenolic xenoestrogens for spiked egg and milk powder samples were obtained.

[Recent advances in capillary electrochromatography and its coupling techniques]

As a novel micro-separation technique, capillary electrochromatography (CEC) has the merits of high efficiency, high selectivity, high resolution and rapid analysis. However, the small-volume injection manipulated in capillary dimensions poses a great challenge for detectors in achieving high sensitivity. Currently, one of the major researches into CEC involves the development of some sensitive detection modes. The general introduction, which includes the historical perspectives and the principles of CEC, is briefly described. The recent advances about CEC coupled with various detectors and its applications in the separation of complex samples are summarized. A total of 141 references are reviewed.

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.

End-column chemiluminescence detection for pressurized capillary electrochromatographic analysis of norepinephrine and epinephrine

A simple and convenient end-column chemiluminescence (CL) detection coupled to pressurized capillary electrochromatography (pCEC) was described. Luminol and N-(4-aminobutyl)-N-ethylisoluminol (ABEI) were adopted as mode compounds to evaluate the feasibility of end-column reactor. Detailed analysis of ABEI revealed that the high sensitivity could be obtained with the reactor. Furthermore, determination of norepinephrine (NE) and epinephrine (EP), which were labeled with ABEI, was accomplished by using the end-column pCEC-CL detection based on ABEI-potassium ferricyanide-alkaline medium CL reaction system. Under the optimum conditions, the detection limit (S/N=3) of NE and EP was 0.08 microM and 0.06 microM, respectively. The proposed method has also been successfully applied to the analysis of adrenaline hydrochloride injection sample.

Wide-bore monolithic column for electrochromatography

A new wide-bore electrophoresis (WE) system adopting an inner cooling device was set up to perform electrochromatography. In this system, a quartz tube of 1.2 mm inner diameter was used as the separation channel. The Joule heat generated during electrophoresis was removed timely through the outer surface of the quartz tube and a cooling capillary inserted into the quartz tube. A proper coolant passed through the cooling capillary to further improve the cooling efficiency. In the primary research, a polyacrylamide monolithic column was successfully prepared in this quartz tube. Then it was evaluated in the electrochromatographic mode. An electric field strength as high as 625 V/cm can be applied to this system without obvious deviation of the current from the linear curve of the Ohm plot. Sample volume as high as 1 microL was injected into the WE system and reasonable efficiency was obtained for separation of the test compounds.

On-Column Coaxial Flow Chemiluminescence Detection for Underivatized Amino Acids by Pressurized Capillary Electrochromatography Using a Monolithic Column

A new method for pressurized capillary electrochromatography (pCEC) coupling with chemiluminescence (CL) detection using a modified on-column coaxial flow detection interface was developed. To evaluate the feasibility and reliability of the experimental setup, the typical CL compounds luminol and isoluminol were separated and detected by using this pCEC-CL system. A detailed investigation of CL detection interface and postcolumn CL reagent flow rate parameters was described. The excellent resolution and detection sensitivity was achieved by using 3-microm ODS-C18 packed column with 30% ACN (v/v), 5 mmol/L phosphate buffer (pH 8.0). Moreover, with the presence of Co(II) (1.0 x 10(-4) mol/L) in the mobile phase, the linear range of the concentration for luminol was 2.0 x 10(-9)-2.0 x 10(-6) mol/L with a detection limit (S/N = 3) of 2.0 x 10(-10) mol/L, and 2.5 x 10(4) theoretical plates was achieved. In addition, separation and detection of the underivatized amino acids (l-threonine and l-tyrosine) were accomplished by using a polymerized monolithic column based on the principle of the luminol-H2O2-Cu(II)-amino acid CL system. Under the optimum conditions, the mixture of amino acids was efficiently separated with satisfactory results.

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.

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.

On-line coupling of pressurized capillary electrochromatography with end-column amperometric detection for analysis of estrogens

An improved technique, pressurized capillary electrochromatography (pCEC) coupling with end-column amperometric detection (AD), was developed and used for the separation and determination of estrogens. The effects of pH value, composition of mobile phase, concentration of the surfactant sodium dodecyl sulfate (SDS) and applied voltage on separation were investigated. The electrochemical oxidation of diethylstilbestrol (DES), dienestrol (DE), and hexestrol (HEX) could be reliably monitored with a carbon electrode at 0.9 V (vs. Ag/AgCl). The pCEC analyses were performed on a capillary separation column packed with 3 microm C18 particles with an acetonitrile/water (31%: 69%) mobile phase containing Tris buffer (5 mmol/L, pH 4.5) and 4 mmol/L SDS. High voltage up to 12 kV reduced the retention time dramatically and still provided a baseline resolution. In addition, supplementary pressure prevented bubble formation and provided reliability and reproducibility of the pCEC performance. The detection limits for the three estrogens ranged from 1.2 to 2.2x10(-7) mol/L, about 10 20-fold lower than those obtained with pCEC-UV detection. To evaluate the feasibility and reliability of this system, the proposed pCEC-AD method was further demonstrated with fish muscle samples spiked with estrogens.

Magnetic resonance microimaging and numerical simulations of velocity fields inside enlarged flow cells used for coupled NMR microseparations

The coupling of various chemical microseparation methods with small-scale NMR detection is a growing area in analytical chemistry. The formation of enlarged flow cells within the active volume of the NMR detector can significantly increase the coil filling factor and hence the signal-to-noise ratio of the NMR spectra. However, flow cells can also lead to deterioration of the separation efficiency due to the development of complex flow patterns, the form of which depend on the particular geometry of the flow cell and the flow rate used. In this study, we investigated the flow characteristics in different flow cell geometries relevant to the coupling of capillary liquid chromatography and NMR. Computational fluid dynamics was used to simulate fluid flow inside flow cells with a volume of approximately 1 microL. Magnetic resonance microimaging was used to measure experimentally the velocity fields inside these flow cells. The results showed good agreement between experiment and simulation and demonstrated that a relatively gradual expansion and contraction is necessary to avoid areas of weak recirculation and strong radial velocities, both of which can potentially compromise separation efficiency.

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.

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.

Reduced-bore monolithic silica column modified with C8-TEOS for reversed-phase electrochromatography

Monolithic silica columns of 2.7 mm ID were prepared and derivatized with C8-TEOS and TEOS by on-column sol-gel reaction. These C8 large diameter monolithic silica columns gave 21 000 theoretical plates for aromatic hydrocarbons in 60% acetonitrile and 40% Tris-HCI buffer. The surface areas as well as the separation reproducibility were improved on coating by the sol-gel approach. Joule heating was greatly reduced by using monolithic columns to which fine quartz sand had been added during column preparation. Since this is a preliminary investigation on a monolithic column with such a large inner diameter, the separation efficiency was not so high as that presently achieved in normal capillary electrochromatography (CEC). However, use of the columns improved sample loadability and concentration detectability of electrochromatography, and semi-preparative separations could be performed.

Multidimensional separations in the pharmaceutical arena

The introduction of novel, powerful and rapid multidimensional separation and characterization methods has produced revolutionary global changes at the genome, proteome and metabolome level, bringing about a radical transition in our views of living systems, at the molecular level. The age of proteomics and metabolomics demands high-resolution multidimensional separation techniques. Multidimensional gas and liquid chromatography techniques, in addition to capillary and microchip electrophoresis methods, offer increased resolution and sensitivity, while also affording adequate throughput and reproducibility to meet the demands of the modern pharmaceutical industry. Coupled with MS, these techniques provide not only separation but also reliable identification of the sample components. The resolving power of these methods has proved to be superior over individual one-dimensional approaches, enabling the comprehensive separation of complex biological mixtures, with excellent resolution and reproducibility. High capacity computer systems that are capable of rigorous qualitative and quantitative analysis of the separation profiles allow the establishment and mining of large databases. Examples of various modern multidimensional separation techniques, and their integration with MS, are reviewed, here, with respect to pharmaceutical analysis.

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.

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.

LC-NMR-MS in drug discovery

Nuclear magnetic resonance spectroscopy (NMR) is arguably the most versatile analytical platform for complex mixture analysis. Specifically, interfacing liquid chromatography with parallel NMR and mass spectrometry (LC-NMR-MS) gives comprehensive structural data on metabolites of novel drugs in development. Applications in natural product, combinatorial chemistry and drug metabolism studies are reviewed. Microcoil probes and capillary separation methods have enormous potential. Recent innovations to improve NMR detection limits include CryoFlowProbes and on-line solid-phase extraction (LC-SPE-NMR). These state-of-the-art analytical platforms are widely applicable to identifying novel candidate drugs from diverse complex mixtures within a drug discovery strategy.

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.

Spectral restoration from low signal-to-noise, distorted NMR signals: application to hyphenated capillary electrophoresis-NMR

In capillary electrophoresis separations coupled to NMR signal detection using small solenoidal coils, electrophoretic currents cause substantial distortion in the NMR spectral linewidths and peak heights, distortions which cannot be fully counteracted through shimming. The NMR spectra also have a low signal-to-noise ratio due to the small amounts of material, typically <1nmol, associated with such microseparations. This study proposes a two-step, signal processing method to restore spectral lines from the distorted NMR spectrum. First, a reference signal is acquired to estimate the broadening function, as a combination of several Lorentzian functions, using a gradient descent method. Then multi-resolution wavelet analysis is applied to the distorted spectrum to determine an initial estimate of the frequencies of the spectral lines. Convergence to the final spectrum, a second set of Lorentzians, involves deconvolution with the estimated broadening function using a gradient descent method. Experimental CE-NMR data show that considerable improvements in spectral quality are possible using this approach, although fine splittings can not be resolved if the broadening function is large.

Electrochromatography with a 2.7 mm inner diameter monolithic column

Monolithic columns of 2.7 mm I.D. have been prepared and used in electrochromatography (EC) separation. Although capillary electrochromatography (CEC) has higher separation efficiency, it displays some shortcomings, such as limited sample loadability and restricted concentration detectability etc. In this paper, we investigate the feasibility of EC separation with millimeter diameter monolithic columns. By using a designed preparation method of monolithic column packed with about 150 microm quartz sand, the effect of Joule heating can be reduced, and the processes of frit making and column packing can be avoided. The concentration detectability of the EC is improved comparing with that of CEC. Moreover, the separation efficiency of 52,000 plates/m was achieved with a 70 mm length and 2.7 mm I.D. monolithic column.

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.

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.

Capillary isotachophoresis/NMR: extension to trace impurity analysis and improved instrumental coupling

Building upon its promising initial performance, the online coupling of capillary isotachophoresis (cITP) to nuclear magnetic resonance (NMR) is extended to trace impurity analysis. By simultaneously concentrating and separating dilute charged species on the basis of their electrophoretic mobility, cITP greatly facilitates NMR structural elucidation. cITP/NMR appears particularly attractive for identifying trace charged synthetic and natural organic compounds obscured by large excesses of other components. A 9.4 microL injection of 200 microM (1.9 nmol) atenolol in a 1000-fold excess of sucrose (200 mM) is analyzed by cITP/NMR. A microcoil, the most mass sensitive NMR probe, serves as the detector as it provides optimal NMR observation of the capillary-scale separation. cITP successfully isolates the atenolol from the sucrose while concentrating it 200-fold to 40 mM before presentation to the 30 nL observe volume microcoil, thereby enabling rapid 1H NMR spectral acquisition of atenolol (experimental time of 10 s) without obstruction from sucrose. For this particular probe and sample, the stacking efficiency is near the theoretical limit as 67% of the sample occupies the 1 mm long microcoil during peak maximum. A multiple-coil probe with two serial 1 mm long microcoils arranged 1 cm apart has been developed to facilitate peak trapping and sample band positioning during cITP/NMR.

1H NMR characterization of the product from single solid-phase resin beads using capillary NMR flow probes

A capillary NMR flow probe was designed to generate high-resolution (1)H NMR spectra at 600 MHz from the cleaved product of individual 160-microm Tentagel combinatorial chemistry beads. By injecting a dissolved sample sandwiched between an immiscible, perfluorinated organic liquid directly into the probe, NMR spectra of the product cleaved from single beads were acquired in just 1 h of spectrometer time without diffusional dilution. Sample handling efficiency on the single bead scale was comparable to that obtained with a bulk sample. Using the relative intensity of the DMSO-d(5)H versus the analyte signals in a fully relaxed CPMG spectrum, the amount of product cleaved from a single bead was determined to be 540+/-170 pmol in one of the samples. Following the NMR data collection, the samples were examined with electrospray ionization mass spectrometry to provide additional structural information. By coupling with microliter-volume fluidic capabilities, the capillary flow probe described here will enable multidimensional characterization of single solid-phase resin products in an online manner.

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.