Quantitative Nuclear Magnetic Resonance (qNMR) Spectroscopy

Quantitative nuclear magnetic resonance (qNMR) spectroscopy is a field of NMR spectroscopy dedicated to the measurement of analytes by signal intensity and its linear relationship with analyte concentration. One of the most important advantages of qNMR spectroscopy is its precise external calibration, the absence of any requirement for identical reference materials, its high precision and accuracy when properly validated, and its ability to quantify multiple analytes simultaneously. Today, qNMR spectroscopy has become a mature technology in many fields, such as drugs, excipients, vaccines, natural products, peptides, agrochemicals, food and beverages, metabolic profiling/fingerprinting of plant extracts and tinctures as well as body fluids, e.g. metabolomics for disease diagnosis and drug treatment control, combinatorial chemistry, and on-flow monitoring of reaction processes ^[1].

Basic Principle

The basic principle of qNMR is to determine the number of protons at each position in the molecule from the integral area and to quantify it according to the amount of substance in the analyte that is proportional to the integral area of the corresponding resonance peak ^[2]. There are two fundamental modes of qNMR spectroscopy: relative quantitation and absolute quantitation. That is the measurement of the ratio of a target component present within the sample being measured and the measurement of the effective amount of the target component present within the sample being measured in each case respectfully. Of relative and absolute quantification, the latter is the more commonly used method. Absolute quantification involves several techniques such as the internal standard method, external standard method, and standard substance addition method, among which the internal standard method is the most prevalent.

Figure 1. Internal standard and external standard method ^[2].

Advantages

qNMR spectroscopy is a fast, reliable, versatile, flexible and powerful analytical technique. The technique has several advantages, including but not limited to the following.

Accurate Quantification: qNMR spectroscopy provides accurate and precise quantification of compounds, without the need for calibration curves or external standards. This makes it a reliable method for determining the purity of compounds, and for measuring the concentration of active ingredients in pharmaceuticals, natural products, and food products.

Non-Destructive: qNMR spectroscopy is a non-destructive method, which means that samples can be reused or analyzed further after the measurement. This is particularly useful when working with limited or expensive samples, or when a single sample needs to be analyzed multiple times.

Rapid Analysis: qNMR spectroscopy is a relatively fast method, with measurements typically taking only a few minutes to complete. This makes it a convenient technique for routine analysis, and for high-throughput screening of large numbers of samples.

Versatility: qNMR spectroscopy can be used to analyze a wide range of compounds, including small molecules, natural products, peptides, and proteins. It can also be used to determine the purity of mixtures, and to identify unknown compounds.

Minimal Sample Preparation: qNMR spectroscopy requires minimal sample preparation, which simplifies the analysis process and reduces the risk of introducing errors. In many cases, samples can be analyzed directly without the need for extraction or derivatization.

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References

1. Holzgrabe, U. Quantitative NMR spectroscopy in pharmaceutical applications. Progress in Nuclear Magnetic Resonance Spectroscopy. 2010, 57(2): 229-240.
2. Wang, Z. F.; et al. Research progress of NMR in natural product quantification. Molecules. 2021, 26(20): 6308.

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