Adulterators caught over a barrel by IRMS

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  • Published: Mar 1, 2018
  • Author: Ryan De Vooght-Johnson
  • Channels: Gas Chromatography
thumbnail image: Adulterators caught over a barrel by IRMS

Distinguishing between synthetic and natural flavour compounds is a challenge

A wide variety of distilled alcoholic spirits, including whisky, brandy and grappa, are typically aged in wooden barrels of various types. The wood imparts flavour compounds to the spirit, improving its taste. Vanillin is one of the main flavour compounds arising from wood. Sometimes synthetic vanillin is added to poor quality spirits to fake the ageing process, a practice forbidden by EU law.

The researchers from Trento and Wagga Wagga used gas chromatography to separate vanillin from other compounds present in spirits. They then used combustion followed by IRMS to find the amount of 13C in the vanillin, which differs between vanillin from wood, synthetic vanillin (which has the lowest amount of 13C) and vanillin from vanilla flavouring, which comes from the vanilla plant. The amount of 13C was given in terms of the δ13C value: the more negative this value, the less 13C is present. The researchers examined 32 samples of spirit: 20 Scotch whiskies, four Bourbon whiskeys, three rum samples, one grappa, three cognacs and one other brandy. Synthetic vanillin samples, vanillin from wood tannin samples and lignin, and products containing vanilla were also examined.

Origin of vanillin in spirits determined by GC and IRMS

The spirit samples were acidified to a pH of 1 with 1 M hydrochloric acid and the ethanol present was largely removed by rotary evaporation. The remaining liquid was filtered and extracted with dichloromethane (DCM). The combined DCM extracts were dried with sodium sulfate and evaporated to dryness. The residue was taken up in DCM prior to GC. In some spirit samples samples 5-(hydroxymethyl)furfural (5-HMF) was present, which tended to co-elute with vanillin under the GC conditions used. Where 5-HMF was present, a little potassium metabisulfite was added to the sample prior to extraction, with the aim of converting any 5-HMF to its bisulfite adduct. The removal, however, was not complete.

A Thermo Scientific Trace GC Ultra instrument fitted with a Phenomenex ZB-FFAP column was used for GC. For some samples, the oven temperature was kept at 65 °C for 1 min, then raised to 250 °C at a rate of 20 °C/min and finally kept at 250 °C for 16 min (the so-called ‘short method’). For other, more complex, samples, the temperature was kept at 50 °C for 10 minutes, then raised to 90 °C at 3 °/min, held at 90 °C for 2 min, raised to 250 °C at 6 °C/min and finally kept at 250 °C for 20 min (the so-called ‘long method’).

The substances from the GC column were split 90:10 between a Thermo Scientific DELTA V IRMS and a Thermo Scientific ISQ single quadrupole mass spectrometer. Before the molecules reached the former instrument they underwent combustion, converting them to carbon dioxide and water, in an alumina tube reactor at 1030 °C. A Nafion membrane and a water trap served to remove water vapour, leaving the carbon dioxide to enter the IRMS.

The δ13C values of vanillin from commercial wood tannins and lignin ranged from -29.5% to -26.7%, while synthetic vanillin samples gave values -32.6% to -29.3%, and vanillin from vanilla gave -21.0% to -19.3%. The spirit samples mostly gave δ13C values within the range -28.9% to -25.7%, as expected from wood-derived vanillin. One sample of rum gave a value of -32.5%, showing that the vanillin was clearly synthetic in origin. The ‘other brandy’ sample contained ethyl vanillin rather than vanillin; the former is a synthetic, not a natural, compound. δ13C values could not be obtained for two rum samples as 5-HMP interfered with the vanillin peak.

Adulteration of spirits can be detected by GC and IRMS

The new method allows the provenance of vanillin in spirits to be identified, showing cases where the shortcut of adding synthetic vanillin had been taken. Further improvement of the method is needed to completely separate the interfering 5-HMP.

Related Links

Rapid Communications in Mass Spectrometry, 2018, 32, 311-318. Van Leeuwen et al. Differentiation of wood-derived vanillin from synthetic vanillin in distillates using gas chromatography/ combustion/ isotope ratio mass spectrometry for δ13C analysis.

Wikipedia, δ13C

Wikipedia, Isotope-Ratio Mass Spectrometry

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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