Gas-phase volatilomic approaches for quality control of brewing hops based on simultaneous GC-MS-IMS and machine learning

Abstract

For the first time, a prototype HS-GC-MS-IMS dual-detection system is presented for the analysis of volatile organic compounds (VOCs) in fields of quality control of brewing hop. With a soft ionization and drift time-based ion separation in IMS and a hard ionization and m/z-based separation in MS, substance identification in the case of co-elution was improved, substantially. Machine learning tools were used for a non-targeted screening of the complex VOC profiles of 65 different hop samples for similarity search by principal component analysis (PCA) followed by hierarchical cluster analysis (HCA). Partial least square regression (PLSR) was applied to investigate the observed correlation between the volatile profile and the α-acid content of hops and resulted in a standard error of prediction of only 1.04% α-acid. This promising volatilomic approach shows clearly the potential of HS-GC-MS-IMS in combination with machine learning for the enhancement of future quality assurance of hops.

Nitrogen monoxide as dopant for enhanced selectivity of isomeric monoterpenes in drift tube ion mobility spectrometry with 3H ionization

“…Based on mass-to-mobility correlations and the comparison with values from literature, the dominating product ions obtained for isomeric monoterpenes in GC-IMS with 3 H radioactive ionization could be assigned to monomeric, dimeric, and trimeric product ions. Although the investigated monoterpenes clearly differ in their molecular structure, they form product ions of highly similar reduced mobilities.…”

“…Ion mobility spectrometry (IMS) becomes increasingly important for the analysis of VOCs in a number of application fields. Thereby, monoterpenes are of special interest as flavour or fragrance compounds in several food and cosmetic products [1][2][3]. Primarily, drift tube IMS operated at atmospheric pressure with radioactive ionization is known as a highly sensitive, selective, and easy-to-handle technique that allows its implementation in portable devices for quality control of food [4].…”

“…However, the sensitivity is limited and highly dependent on the proton affinity of the analyte. Commonly, 63 Ni or the less hazardous 3 H ionization sources are used as β-particle emitters and are often preferred to non-radioactive ion sources because of the continuous reactant ion production without the requirement of regular maintenance by the operator [5]. By collision of the emitted electrons with the drift or carrier gas molecules, a complex gas phase reaction cascade is initiated and leads to proton-water clusters (H + (H 2 O) n ) as reactant ions in positive mode due to trace levels of water in the surrounding gas atmosphere.…”

“…Although monoterpenes are known as substances with comparatively low proton affinities [7], radioactive ionization-based IMS is a suitable approach for the qualitative and quantitative analysis in highly concentrated samples [1,2]. In previous studies, we already demonstrated the advantages of headspace GC-IMS with 3 H ionization (HS-GC-IMS) for the identification of monoterpenes in the volatilome of hop samples for (food) quality control as well as for the quantitation of monoterpene alcohols in cosmetic oils [2,3]. Furthermore, Rodriguez-Maecker et al (2017) used HS-GC-IMS for the qualitative analysis of terpenes in essential oils [1].…”

“…Furthermore, Rodriguez-Maecker et al (2017) used HS-GC-IMS for the qualitative analysis of terpenes in essential oils [ 1 ]. It is already known that several isomeric monoterpenes feature identical monomer ions in 3 H-IMS, while they can be differentiated by the number and abundance of their dimer ions [ 1 , 3 ].…”