Comparing the aroma profiles of hops by TD–GC–TOF MS with Select-eV variable-energy electron ionisation

Summary

This Application Note demonstrates the high performance of a TD–GC–TOF MS system for the analysis of complex aroma profiles from hops. The use of complementary soft electron ionisation provides enhanced confidence in the identification of terpenoids, while the comparison of hop varieties is simplified by near-real-time analysis in the accompanying software package.

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Introduction

Beer contains hundreds of organic ingredients, with concentrations spanning many orders of magnitude. Monoterpenes (C₁₀) and sesquiterpenes (C₁₅) are aroma-active hydrocarbons found in the essential oils of various plants. Most notably for the brewing industry, they are found in hops, which provide much of the characteristic ‘bitterness’ of the finished beer.¹ Of the greatest importance for beer is the monoterpene β-myrcene, and the sesquiterpenes caryophyllene, β-farnesene and α-humulene. However, there are hundreds of other terpenes that may also be present and have an impact on the final aroma and flavour. Many of these compounds have very low odour thresholds, so a highly sensitive analytical approach is needed to assess the quality of the hops before brewing commences. A number of factors can affect VOC profiles of hops, including seasonal variations, packaging, storage and ageing, so it is essential that robust quality control is applied. This study investigates the use of Markes’ Micro-Chamber/ Thermal Extractor™ (μ-CTE™) for dynamic headspace extraction of hop ‘cones’, with collection of vapours onto a sorbent tube and analysis by thermal desorption–GC–TOF MS. The use of thermal desorption (TD) offers pre-concentration of the aroma compounds, while coupling to highly sensitive time-of-flight MS detection with Select-eV^® variable-energy ionisation technology allows a comprehensive aroma profile to be investigated in a single sequence.

Background to BenchTOF systems

BenchTOF™ mass spectrometers are time-of-flight instruments designed specifically for gas chromatography. They are particularly appropriate for confident identification of trace-level compounds in hops for the following reasons:

• Sensitivity: Highly efficient direct-extraction technology allows BenchTOF systems to acquire full-range spectra with SIM-like sensitivity, allowing them to reliably detect trace-level targets and unknowns in a single run, which would be difficult or impossible on a quadrupole system.
• Spectral quality: The ‘reference-quality’ spectra produced by BenchTOF systems are a close match for those in commercial libraries such as NIST or Wiley. This enables quick and confident matching of analytes.
• Speed: The ability to record full-range mass spectral information to extremely high densities (10,000 transient spectral accumulations per second) enables advanced spectral deconvolution and ‘data-mining’ algorithms to extract maximum information from weak, matrix-masked signals.

>> Download the full Application Note as PDF

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