The most often used methods for profiling biofluids are NMR spectroscopy (Nuclear Magnetic Resonance) and LC-MS (Liquid Chromatograpyh Mass Spectroscopy). In the field of metabolomics of plant extracts, GC-MS (Gas Chromatography Mass Spectroscopy) is used in addition. All methods allow an identification of known metabolites, a quantifiation of known metabolites and partially a structure elucidation of unknown metabolites.
Analytical techniques typically used for profiling biofluids: NMR (left) and LC-MS (right)
LC-MS and NMR are neither redundant nor concurring methods, but complement each other. NMR is highly reproducible and allows exact quantifications of metabolites (linear relation between signals and concentrations of analytes). NMR spectroscopy without a chromatographic separation is faced by challenges due to overlapping peaks in the spectrum originating from the high number of metabolites in biofluids. In addition, NMR is less sensitive than LC-MS. Whereas NMR sees analytes with concentrations higher than mg/ml, LC-MS sees analyte with concentrations of pg/ml. NMR sees more polar molecules, wheras LC-MS sees the less polar molecules. For LC-MS the reproducibilty in terms of quantization of metabolites is lower due to matrix effects during the ionization process. On the one hand, signals of metabolites interfere less, but on the other hand the chromatography limts the throughput of samples. A chromatographic run needs half an hour at least, whereas a measurement by NMR of a typical urine sample needs about 5 minutes. In conclusion it can be said, that each method by itself only sees the top of the iceberg and that both methods are needed to see a bigger part of the complete metabolome.
NMR and LC-MS measurements complement each other. For example, using NMR as single technique is like looking at the tip of the iceberg only.
Permission to use image of iceberg kindly granted from Uwe Kils
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