Chemical-shift referencing and resolution stability in gradient LC-NMR (acetonitrile:water)

Enhancing Sensitivity in the Hyphenation of High-Performance Liquid Chromatography to Benchtop Nuclear Magnetic Resonance Spectroscopy at Isocratic and Onflow Conditions

The onflow hyphenation of high-performance liquid chromatography (HPLC) in adsorption mode with a benchtop 1H nuclear magnetic resonance (NMR) spectrometer is described for the first time. Protonated solvents and isocratic conditions are used. The sensitivity was increased by choosing suitable NMR acquisition parameter as well as optimizing injection parameters and postacquisition data processing methods. With optimized conditions, the limit of detection (LOD) and the limit of quantification (LOQ) were LOD = 0.010 g L-1 and LOQ = 0.031 g L-1 for the methoxy 1H of methyl paraben at 4.07 ppm, LOD = 0.038 g L-1 and LOQ = 0.134 g L-1 for the aromatic 1H of pentyl paraben between 7.00 and 8.50 ppm. These are expressed in injection concentration and are comparable to existing HPLC hyphenation with high-field NMR spectrometers. The analysis of a 2 g L-1 paraben mixture, far below the legal limits for usage in cosmetics, illustrates the applicability of the method. Taking advantage of the spectral resolution, chromatographically overlapping peaks are resolved using analyte-specific NMR elution traces. A methodology is detailed to facilitate the transfer of the optimized method to other (analyte) systems.

The integration of LC-MS and NMR for the analysis of low molecular weight trace analytes in complex matrices

This review discusses the integration of liquid chromatography (LC), mass spectrometry (MS), and nuclear magnetic resonance (NMR) in the comprehensive analysis of small molecules from complex matrices. We first discuss the steps taken toward making the three technologies compatible, so as to create an efficient analytical platform. The development of online LC-MS-NMR, highlighted by successful applications in the profiling of highly concentrated analytes (LODs 10 μg) is discussed next. This is followed by a detailed overview of the alternative approaches that have been developed to overcome the challenges associated with online LC-MS-NMR that primarily stem from the inherently low sensitivity of NMR. These alternative approaches include the use of stop-flow LC-MS-NMR, loop collection of LC peaks, LC-MS-SPE-NMR, and offline NMR. The potential and limitations of all these approaches is discussed in the context of applications in various fields, including metabolomics and natural product discovery.

Investigation into the structural composition of hydroalcoholic solutions as basis for the development of multiple suppression pulse sequences for NMR measurement of alcoholic beverages

An eight-fold suppression pulse sequence was recently developed to improve sensitivity in (1) H NMR measurements of alcoholic beverages [Magn. Res. Chem. 2011 (49): 734-739]. To ensure that only one combined hydroxyl peak from water and ethanol appears in the spectrum, adjustment to a certain range of ethanol concentrations was required. To explain this observation, the structure of water-ethanol solutions was studied. Hydroalcoholic solutions showed extreme behavior at 25% vol, 46% vol, and 83% vol ethanol according to (1) H NMR experiments. Near-infrared spectroscopy confirmed the occurrence of four significant compounds ('individual' ethanol and water structures as well as two water-ethanol complexes of defined composition - 1 : 1 and 1 : 3). The successful multiple suppression can be achieved for every kind of alcoholic beverage with different alcoholic strengths, when the final ethanol concentration is adjusted to a range between 25% vol and 46% vol (e.g. using dilution or pure ethanol addition). In this optimum region, an individual ethanol peak was not detected, because the 'individual' water structure and the 1 : 1 ethanol-water complex predominate. The nature of molecular association in ethanol-water solutions is essential to elucidate NMR method development for measurement of alcoholic beverages. The presented approach can be used to optimize other NMR suppression protocols for binary water-organic solvent mixtures, where hydrogen bonding plays a dominant role.

HPLC-NMR revisited: using time-slice high-performance liquid chromatography-solid-phase extraction-nuclear magnetic resonance with database-assisted dereplication

Time-based trapping of chromatographically separated compounds onto solid-phase extraction (SPE) cartridges and subsequent elution to NMR tubes was done to emulate the function of HPLC-NMR for dereplication purposes. Sufficient mass sensitivity was obtained by use of a state-of-the-art HPLC-SPE-NMR system with a cryogenically cooled probe head, designed for 1.7 mm NMR tubes. The resulting (1)H NMR spectra (600 MHz) were evaluated against a database of previously acquired and prepared spectra. The in-house-developed matching algorithm, based on partitioning of the spectra and allowing for changes in the chemical shifts, is described. Two mixtures of natural products were used to test the approach: an extract of Carthamus oxyacantha (wild safflower), containing an array of spiro compounds, and an extract of the endophytic fungus Penicillum namyslowski, containing griseofulvin and analogues. The database matching of the resulting spectra positively identified expected compounds, while the number of false positives was few and easily recognized.

On-line (HPLC-NMR) and Off-line Phytochemical Profiling of the Australian Plant, Lasiopetalum macrophyllum

On-line (HPLC-NMR) and off-line (HPLC, NMR and MS) methodologies were used to profile the constituents present in the crude extract of Lasiopetalum macrophyllum. On-flow and stop-flow HPLC-NMR supported the presence of trans-tiliroside and permitted partial identification of cis-tiliroside and 4′-methoxy- trans-tiliroside. Off-line isolation led to the unequivocal identification of four flavanoid glycosides including a new structural derivative, 4′-methoxy- cis-tiliroside. This is the first report of flavonoid glycosides occurring in this plant genus. In addition, a number of structure revisions have been proposed for previously reported flavonoid glycosides that were incorrectly assigned.

On-line (HPLC-NMR) and Off-line Phytochemical Profiling of the Australian Plant, Lasiopetalum macrophyllum

On-line (HPLC-NMR) and off-line (HPLC, NMR and MS) methodologies were used to profile the constituents present in the crude extract of Lasiopetalum macrophyllum. On-flow and stop-flow HPLC-NMR supported the presence of trans-tiliroside and permitted partial identification of cis-tiliroside and 4′-methoxy- trans-tiliroside. Off-line isolation led to the unequivocal identification of four flavanoid glycosides including a new structural derivative, 4′-methoxy- cis-tiliroside. This is the first report of flavonoid glycosides occurring in this plant genus. In addition, a number of structure revisions have been proposed for previously reported flavonoid glycosides that were incorrectly assigned.

Chemical-shift referencing and resolution stability in methanol:water gradient LC-NMR

Solvent-gradient LC-NMR can generate wide ranges of solvent conditions during an experiment. This complicates the chemical-shift referencing of the resulting NMR data. This problem and other experimental issues are evaluated here for LC-NMR in methanol:water, using solvent mixtures running from 0% to 100% methanol. It is shown that the use of the methanol methyl signal is superior to the use of the water signal in any form (either the 1H or the 2H signal), either as a secondary reference, as a signal for shimming, or as a lock signal. Also shown are the limitations of the referencing methods and other experimental parameters, and the limitations of the solvent-gradient ramp parameters, primarily as they affect lineshapes.