LC‐NMR: An Overview of A Hyphenated Technique

Abstract

LC‐NMR , a hyphenated system of liquid chromatography ( LC ) and nuclear magnetic resonance ( NMR ), is an efficient technique used in various fields such as metabolites analysis, drug impurity profiling, structure elucidation, identification of volatile oils, analysis of pesticides, and other natural products. The technique provides quantitative information on chemical compositions, including the structure and type of functional groups. The major drawback of LC‐NMR is its low sensitivity that can be overcome by using cryogenic probes and higher magnetic field strength. In this article, the LC‐NMR instrumentation, modes of operation, and sample preparation are introduced. Applications, advancement in the field, and performance of LC‐NMR compared with other analytical techniques are also discussed.

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“…In order to avoid this ambiguity, LC-MS and LC-NMR are combined so that NMR and MS data can be acquired simultaneously from a single chromatographic run 48 . The hyphenation LC-NMR with MS resulted in LC-NMR-MS in the year 1996 49 . A splitter is incorporated after the HPLC to direct different sample loads to the MS and NMR units, respectively, since MS is more considerably sensitive than NMR 48 .…”

“…LC-NMR-MS is highly adaptable and helps in the identification and determination of active metabolites 49 . For a given LC peak, the MS data and acquired NMR spectra can be directly correlated 50 .…”

“…In case of on-flow LC-NMR, the LC eluent is directly directed to the NMR flow probe, where the NMR spectra are acquired continuously when the LC eluent is moving through a constant speed 58 . Due to its very short resident time in the NMR flow cell, its approach is limited to 1D NMR spectra acquisition only which can be used to analyse major components of the mixture by 1 H and 19 F NMR 48,49 . Instrumentation is simple as no interruption of chromatographic eluent occurs and allows detection of all NMR active compounds under identical conditions.…”

“…The major disadvantages of on-flow mode are that it is only limited to acquire high abundance 1 H or 19 F NMR spectra of major components in a sample due to limited NMR data acquisition time 58 , sample amounts in the range of 10-100 µg are commonly needed, which may lead to column overloading and have lower sensitivity and overlapping of chromatographic peaks 49 .…”

“…Stop-flow mode allows cautious optimization of acquisition parameters and field homogeneity in order to enhance the exposure time of the peak in the flow cell. This method excludes the dependency of the detector to position the sample in the NMR flow cell, which is the significant role of this approach 49 .…”