LC-NMR combines two techniques - liquid chromatography (LC) and nuclear magnetic resonance (NMR). It involves a HPLC separation followed by the detection of separated components by UV or other methods and ultimately NMR analysis. LC-NMR is a powerful analytical tool used to resolve complex mixtures. The coupling of LC separation with NMR characterization has been employed in a wide range of applications.
LC-NMR is also combined with other techniques such as two-dimensional NMR measurements (LC-2D NMR), mass spectroscopy or MS (LC-NMR/MS) and solid phase extraction or SPE (LC-SPE-NMR) to give rise to a whole new range of separation and characterization applications that have a high sensitivity. LC-NMR/MS provides insights into the mass and fragmentation of different components in a sample while the LC-SPE-NMR system helps in full structural elucidation of the sample components.
Applications of LC-NMR
The LC-NMR technique and its variants are widely used in the following applications:
- Analysis of crude extracts of natural products and plant-derived compounds - the technique is optimized for rapid identification of potential drug candidates in plant products.
- Detection of bulk drug impurities during drug-stability tests - the data from LC-NMR/MS allows full characterization of all impurities present in the drug.
- Analysis of unstable compounds or compounds formed in situ that cannot be detected or isolated using other techniques.
- Studying drug metabolism - analysis of biofluids such as plasma or urine. Tracers such as 19F are linked to drugs and they can be tracked clearly by NMR.
- Analysis of ring-fused and heterocyclic compounds that have a small number of protons - the LC-2D-NMR technique provides carbon shift and bonding information for these compounds, which is very useful for analysis of their structure.
- Composition profiling to analyze the content and structure of components in a chemical mixture, thus providing valuable insights for the development of a manufacturing process in the chemical industry.
- Studying polymers – the high resolution of LC-NMR is ideal for analysis of the microstructures in synthetic polymers and biopolymers including proteins.
- Studying uncharacterized, complex, non-living natural organic matter (NOM) present in the atmosphere, oceans, soil and sediments. LC-NMR and LC-SPE-NMR have been used to study dissolved NOM from freshwater and alkaline soil extracts for the separation and characterization of components in the complex mixture.
- Metabolomics to determine disease states using body fluids analysis.
The LC-NMR technique has matured over the years with respect to both sensitivity and practicality. This is due to advancements such as the production of high-sensitivity probes and powerful field magnets, development of new techniques that enhance the sensitivity, and automatic measurement software.
Advanced LC-NMR with such built in features are commercially available for a number of applications. LC-NMR systems from Bruker offer automated analysis of unstable and sensitive compounds. Its LC-SPE-NMR systems provide a significant increase in the signal-to-noise ratio of the NMR spectra obtained, making them ideal for structural analysis in experiments involving low concentrated impurities. The trapping (elution) process on the SPE cartridge provides a 100% increase in sensitivity as compared to LC-NMR.
Coupling LC-NMR with other analytical techniques such as MS has provided novel insights into the structure of different compounds. The use of spectral databases and software for structure analysis will help speed up structure elucidation. Developing the technical know-how and the continuous sensitivity improvement efforts will maximize the applications of LC-NMR in research and development.
Reviewed by Afsaneh Khetrapal BSc (Hons)