SPME Arrow Improves Upon Conventional SPME in GC×GC–MS Analysis of Whiskey Flavor Compounds

In a study published in the Journal of Chromatography A, a collaborative of researchers across Austria, Greece, and Italy analyzed volatile organic compounds (VOCs) in Irish and Scotch whiskeys using a solid-phase microextraction (SPME) Arrow followed by comprehensive two-dimensional gas chromatography coupled to mass spectrometry (GC×GC–MS), seeking more information about the organoleptic characteristics that influence consumer tastes and preferences (1).

whiskey and natural ice | Image Credit: © Igor Normann - stock.adobe.com

The SPME Arrow is a fiber that extracts volatile compounds from the headspace of the whiskey sample, allowing for preconcentration and enrichment of the VOCs. The extracted compounds are then separated and identified using GC×GC–MS, a technique that provides high-resolution separation of complex mixtures. The technique allows for the detection and quantification of a wide range of VOCs in whiskey samples, including those that may contribute to flavor and aroma, as well as potential contaminants. The method is highly sensitive and specific, making it a valuable tool for quality control and authenticity testing in the whiskey industry.

The researchers hail from Aristotle University of Thessaloniki in Thessaloniki, Greece, the University Campus Bio-Medico of Rome in Rome, Italy, the University of Messina in Messina, Italy, and Vienna University of Technology in Vienna, Austria, and purchased samples of Irish whiskey, single malt Scotch whiskey, and blended Scotch whiskey from a local market in Vienna. Among the most abundant aromatic VOCs in these and other whiskeys are esters, alcohols, aldehydes, ketones, terpenes, and furanic and sulfur compounds.

The use of the SPME Arrow in the study, compared to conventional SPME, exhibited higher sensitivity by up to a factor of six and better repeatability by up to a factor of five, depending on the class of compound. In this approach, extraction is achieved using a coated fiber with an arrow-shaped tip attached to a stainless steel backbone. This design overcomes the shortcomings of conventional carboxen/polydimethylsiloxane (CAR/PDMS) SPME fibers including poor mechanical durability and low extraction phase volume. The SPME Arrow has been used for a wide variety of food analysis applications such as fish samples, grape skins, brown rice vinegar, milk, soy sauce, and Korean salt-fermented fish sauce, but even more recently has proven successful in analyzing distilled spirits including Chinese Baijiu and Korean Soju liquors.

Similarly, one-dimensional gas chromatography hyphenated to a mass spectrometer (GC–MS) is a well-established technique to determine aroma compounds in complex food samples, but that complexity can lead to insufficient separation and co-elution of target analytes. GC×GC overcomes this, especially systems equipped with cryogenic modulators, and when coupled to MS provides powerful fingerprinting of VOCs in both food and beverage. This study combined the SPME Arrow with GC×GC–MS for the first time in terms of analyzing whiskey compounds.

In total, the processes tentatively identified 167 VOCs, whereas 126 were found using conventional CAR/PDMS SPME. The compounds included several classes that one might expect to find in whiskey: esters, alcohols, ketones, terpenes, and carboxylic acids. The study therefore confirmed the coupling of the SPME Arrow with GC×GC–MS as a combination that delivered more comprehensive information about Irish and Scotch whiskeys than previously attempted methods.

Reference

(1) Ferracane, A.; Manousi, N.; Tranchida, P.Q.; Zachariadis, G.A.; Mondello, L.; Rosenberg, E. Exploring the volatile profile of whiskey samples using solid-phase microextraction Arrow and comprehensive two-dimensional gas chromatography–mass spectrometry. J. Chromatogr. A 2022, 1676, 463241. DOI: 10.1016/j.chroma.2022.463241