Rapid Quantitation of Phenolic Compounds in Islay Single Malt Scotch Whiskies by Direct Injection Mass Spectrometry
Scotch whiskies from the Isle of Islay are renowned for their peaty and smoky aroma characteristics, which are imparted by phenolic compounds that are transferred from peat smoke to the malt during the kilning process. Conventional analysis of aroma compounds in beverages such as whisky is carried out by gas chromatography coupled to mass spectrometry (GC-MS), which delivers a comprehensive qualitative and quantitative picture of constituent compounds. Yet, GC-MS is time-consuming when quantifying aqueous-phase concentrations from liquid matrices. An alternative analytical approach to detecting constituent aroma compounds in (alcoholic) drinks based on direct injection mass spectrometry (DIMS) is presented here in a proof-of-concept study. Specifically, proton transfer reaction time-of-flight mass spectrometry (PTR-TOFMS) was used in combination with a liquid calibration unit (LCU) to quantify phenolic compounds in a selection of Islay whiskies in comparison to non-peaty whiskies. The LCU was used in a unconventional manner: Sample aliquots of aqueous whisky solutions were vaporized by the LCU to allow for direct analysis of their constituent volatile aroma compounds in the gas-phase by PTR-TOFMS with subsequent conversion to aqueous-phase concentrations in the sample matrix. This chapter presents this concept as a novel approach for the targeted analysis of specific volatile constituents of alcoholic beverages using a feasibility study focusing on the quantitation of phenol, methylphenols (cresols), and 2-methoxyphenol (guaiacol) in selected whiskies. This technique holds promise for the rapid liquid-phase quantitation of volatile aroma compounds via their direct detection in the gas-phase with minimal sample workup and high sample throughput yields.