Mass spectrometry (MS) is almost a hundred years old, but the coupling of MS with liquid chromatography (LC) is still in its youth and began in the mid-seventies. However, the real advances in this hyphenated technique began in the early eighties with the introduction of fast-atom bombardment (FAB) and thermospray ionization (TS), which led to the development of atmospheric-pressure chemical ionization (APCI).

The most famous ionization sources — electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) — followed five to seven years later. The next fundamental development for the incredible growth of the LC–MS instrumentation was catalysed by the popularization of "lightweight" mass analysers, such as quadrupoles and ion traps, the revival of time-of-flight (TOF) analysers, and the introduction of the new Orbitrap detector (Thermo Fisher Scientific, San Jose, California, USA).

Figure 1

These analysers are now available in a wide variety of combinations that allow users to work with tandem MS in a simple, problem-free manner. LC–MS has entered the field of organic analytics in a big way and changed the perspectives and direction of many research fields, including the composition of food and beverages. This article looks at the analysis of wine and the new insights that LC–MS has revealed regarding its composition, health aspects and winemaking practices (Figure 1).

The Colour of Wine

Keynotes:

Even to lay eyes, the different colour nuances of young and aged red wines are clearly visible. It has been known for at least 30 yearsv that this colour change results from the reactions of wine pigments called anthocyanidins with other compounds present in wine.¹ There are five anthocyanidins present in grapes as conjugates, mainly glycosides (anthocyanins). Their analysis by LC–MS can be accomplished using APCI, ESI or MALDI ionization, and different tandem MS scanning modes allow their determination in complex mixtures.² The study of anthocyanic composition can be used to characterize the grape/wine variety and addition of hybrid grape varieties.³ The true value of LC–MS, however, lies in the fact that it allows the identification of pigments formed during wine maturation and resulting from the anthocyanin condensation reactions with pyruvic acid, acetaldehyde,⁴ tannins,⁵ furfural derivatives,⁶ vinylepicatechin,⁷ to name just a few, and also from anthocyanin polymerization.^1,8 ESI, particularly in negative ionization mode, has become indispensable in these studies.^5,6,8 The new (non-polymeric) pigments have been named pyranoanthocyanins and are chemically more stable than grape anthocyanins, but their structural diversity is enormous and they continue to form endlessly during wine ageing.^1,7,8 To further complicate matters, a recent LC–ESI–MS analysis indicated that anthocyanin oligomers are already present in grape skin extract.⁹

The colour change of white wines involves a different mechanism: the browning has been shown to be induced by glycoaldehyde, resulting from malolactic fermentation of wine.³