Characterization of the Chemical Composition of Beverages by NMR Spectroscopy

Abstract

Characterization of the chemical composition of beverages is important for establishing their quality and authenticity and in connection with their organoleptic properties. It enables screening for the possible addition of toxic materials and the determination of compounds important for human health. A fine example of the latter are polyphenols found in wine, fruit juices, and tea; they have a wide range of biological and pharmacological effects, including anticarcinogenic, anti-inflammatory, and antioxidant activity [1,2]. Beverages are complex mixtures of different classes of compounds present at a broad range of concentrations. In the past, various analytical techniques have been used in the analysis of beverages, most frequently HPLC, GC, capillary electrophoresis, and conductometry. Because of the compositional complexity of beverages and the wide concentration range of components classical chemical analysis requires time consuming sample preparation. In the case of compounds at low concentration, separation, derivatizaion, and preconcentration are common steps in the procedure. To date, NMR has assumed an outstanding position in the field of chemical analysis of food products. Though NMR is less sensitive than the mentioned methods, sample preparation for NMR is simpler and less time consuming. NMR is nondestructive, selective, and capable of simultaneous detection of a great number of low molecular mass components in complex mixtures. The great advantage of NMR also is the possibility of detecting different nuclei in different spatial and electronic environments. As a consequence, it is the preferable technique for molecular structure determination and the study of molecular interactions in solution. With development of high-field (500–900 MHz) NMR spectrometers with superconducting magnets and the possibility of recording twoand multi-dimensional spectra, NMR has become a powerful method for analyzing at the molecular level complex mixtures like beverages. The use of 2D homoand hetero-nuclear experiments, pulse sequences for the suppression of strong signals, and special probes, such as the so-called nano probe for μl quantities of samples, has enabled the characterization of minor compounds like anthocyanins [3]. The cold or cryogenic probe also is very promising. By cooling the key probe components to cryogenic temperature significant gains in sensitivity (up to 4 times in comparison to the room temperature probe) can be achieved and for a given amount of sample the experiment time is reduced by a factor of 16. The problems with signal overlapping and the limit detection may be overcome by the use of NMR hyphenation with techniques such as HPLC and MS spectrometry (LC-NMR, fully automated capillary scale LC-NMR, and LC-NMR/MS) [4–6]. Another way to overcome the problem of overlapping in NMR spectra of complex liquid mixtures is the application of diffusion-ordered spectroscopy (DOSY) [7]. NMR sensitivity can be further improved by combining hyphenated techniques with cryoprobe and by the incorporation of an online postcolumn solid-phase extraction (SPE) system for online preconcentration of analytes prior to transfer to NMR tube [4].