Hop oil analysis—The power of stable isotope dilution assays for quantification at trace levels

Technical Session 04: Hops II Session
Leif A Garbe, TU Berlin / VLB Berlin
Co-author(s): Nils Rettberg, TU Berlin/VLB Berlin, Germany

ABSTRACT: Hop essential oils and their oxidation products are of special interest for brewing science and quality control as well as for practical brewers. They are volatile, chemical reactive, and usually are present in very low concentrations in the final product. In hops, they are embedded in complex matrices that hinder rapid analysis and make trace analysis a challenging discipline. Various sample cleanup and target isolation strategies have been established. The more intense the sample cleanup procedures are, the more sources of analyte losses and errors have to be considered in quantification and calibration. In trace analysis, external calibration is not reasonable, and the quality of internal standard assays strongly depends on the availability and properties of the internal standard. Stable isotope dilution assay (SIDA) is a special type of an internal standard assay. In SIDA standard and analyte are isotopologues, thus they are as nearly identical as possible. Their chemical and physical properties match, and their chromatographic and mass spectrometric characteristics are very alike. This paper highlights the advantages and challenges of SIDA in trace analysis on hop oils. The important hop oil terpene hydrocarbons and terpene alcohols myrcene, linalool, terpineol, nerol, geraniol, and farnesol, as well as caryophyllene and humulene, are not commercially available as stable isotope labeled standards. Therefore, their chemical synthesis as isotopologues carrying Deuterium (hydrogen-2), oxygen-18, or carbon-13 is of crucial necessity for SIDA. We have performed lab synthesis of these terpene compounds. Using SIDA, precise and valid quantification even at low concentrations from the complex beer matrix, raw hops, and any intermediate is strongly simplified. In the presented paper, a short introduction to SIDA and SIM-MS methods are given. The major focus of the paper deals with data from hop oil analysis via conventional and SIDA methods, respectively. One disadvantage of SIDA is the necessary instrumentation—chromatography coupled to mass spectrometry. However, SIDA is also proposed as a reference method in evaluation of routine assays preformed on cost-effective non-MS equipment like GC-FID analysis.

Leif-Alexander Garbe is professor for biochemical and technical analysis at the Berlin Institute of Technology (TUB). Additionally, he chairs the Department for Special Analyses at the Research and Teaching Institute for Brewing in Berlin (VLB). Leif graduated in 1996 from TUB with a diploma in chemistry. Then he worked as a researcher and teacher at VLB and TUB. He supervised biotechnology and brewing students and performed several research projects in brewing and life sciences. He finished his Ph.D. thesis in April 2002 on the “Metabolism of Hydroxy-Fatty Acids in Yeasts,” and his habilitation thesis in 2009 on “The Biochemistry of Oxidized Lipids: Analytical Characterization of Bioactive Metabolites” at TUB. Today Leif’s research interests focus on mass spectrometry, NMR, trace analysis, biotransformation, isotope dilution technique, and Maillard reaction of peptides/proteins.