Alcoholic spirits classified: SPME-MS of volatiles exposes plant origins

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  • Published: Dec 15, 2010
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: Alcoholic spirits classified: SPME-MS of volatiles exposes plant origins

The spirit of Christmas: but do you know where your booze comes from?

When you open that new bottle of spirits in time for the festive season, do you ever speculate on the original source of the alcohol? Strictly speaking, alcohol can be produced from many plant sources by distillation then used in alcoholic beverages, so your seasonal tipple might have its origins in one of several crops.

This is certainly the case in eastern and central Europe, where vodka is produced by diluting rectified spirit that has been distilled from potatoes and cereal grains such as corn and rye. In Poland, each source has its own merits. Rye has the longest tradition for vodka production, potato spirit imparts a characteristic flavour to potato vodkas, and corn is becoming more popular as a relatively inexpensive source of ethanol.

The Polish vodka industry is populated by many small, local distilleries that produce raw spirits that are sold on to rectifying units to prepare the product. Since joining the EU, the legislation for spirits must be followed. The alcoholic strength of the ethanol must be at least 96% and the levels of certain regulated compounds, including methanol, aldehydes, esters and higher alcohols, must be within specific limits.

In addition, there should be a declaration of the raw material used for its production. This information is important for the production and rectifying plants, quality control labs and, of course, the consumer.

Spirit sources: average mass spectra acquired

Alcoholic beverages have been characterised in the past by studying the profile of volatile compounds. The most favoured approaches are GC and GC/MS, sometimes linked to SPME for extraction of the volatiles, and the use of electronic noses based on solid-state sensors or mass spectrometers.

Now, a group of Polish scientists have adopted a mass spectrometry method that does not involve chromatographic pre-separation in which the volatile compounds are used to identify the botanical source of the alcohol in spirits. Henryk Jelen, Angelika Ziolkowska and Anna Kaczmarek from the Poznan University of Life Sciences combined the method with a rapid SPME-based extraction that allows the analysis to be completed within 7 minutes.

The SPME process was optimised so that extraction on a carboxen-divinylbenzene-polydimethylsiloxane fibre for 2 minutes at a sample temperature of 50°C provided the best response. The fibre was inserted into the injection port and desorbed for 2 minutes at 260°C in splitless mode.

The desorbed compounds passed through uncoated fused silica tubing to the mass spectrometer source for electron ionisation and full-scan mass spectra were recorded over the range m/z 29-289. The mass spectra of all the compounds present were measured over 5 minutes for volatiles derived from raw spirits based on rye (84 samples), corn (27) and potato (27).

The spectra contained many of the expected ions, such as the base peak at m/z 88 which was formed from ethyl esters by the McLafferty rearrangement. Other McLafferty ions at m/z 74 and 44 resulted from methyl esters and aldehydes. Further ions were loosely correlated with various alcohols and acetates.

However, the point of the analysis is not to identify individual compounds but to measure an average mass spectrum of the volatiles from a given spirit.

Discriminating data: sourcing crops

The m/z values taken from average mass spectra were subjected to a principal components analysis in an attempt to distinguish the three spirit plant sources but resolution was incomplete. The first principal component separated rye spirits from the others but overlapping clusters were obtained for the potato and corn spirits.

A linear discriminant analysis (LDA) was more successful. A training set of 107 samples was used to build the model and the remaining 31 samples were used to test it. This resulted in a classification ability of 100% and a predictive ability of 96%, so that virtually all of the spirits were correctly identified as originating from corn, rye or potato.

The process was not ideal, because the predictive ability depended on the selection of samples for the training and test sets but Jelen considered that this might be due to the relatively small size of the test set.

Nevertheless, an almost unequivocal botanical classification of spirits was achieved by LDA following a very short extraction and analysis period. It should prove useful for confirming the authenticity of the drinks against the manufacturer information and is sufficiently quick to be useful in the quality control environment.

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

 An SPME-MS method with no chromatographic separation has been used to distinguish the botanical origin of raw spirits between potato, rye and corn, with 100% classification accuracy and 96% prediction ability

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