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

Rebecca Brendel,Philipp Weller,Sascha Rohn

doi:10.1007/s00216-021-03306-7

Abstract

The ion mobility spectra of the isomeric monoterpenes α-pinene, β-pinene, myrcene, and limonene in drift tube ion mobility spectrometry (IMS) with 3H radioactive ionization are highly similar and difficult to distinguish. The aim of this work was to enhance the selectivity of IMS by the addition of nitrogen monoxide (NO) as dopant and to investigate the underlying changes in ion formation responsible for the modified ion signals observed in the ion mobility spectra. Even though 3H-based-IMS systems have been used in hyphenation with gas chromatography (GC) for profiling of volatile organic compounds (VOCs), the investigation of ion formation still remains challenging and was exemplified by the investigated monoterpenes. Nonetheless, the formation of monomeric, dimeric, and trimeric ion clusters could be tentatively confirmed by a mass-to-mobility correlation and the highly similar pattern of ion signals in the monomer region was attributed to isomerization mechanisms potentially occurring after proton transfer reactions. The addition of NO as dopant could finally lead to the formation of additional product ions and increased the selectivity of IMS for the investigated monoterpenes as confirmed by principal component analysis (PCA). The discrimination of monoterpenes in the volatile profile is highly relevant in the quality control of hops and was given as the example for application. The results indicate that additional product ions were obtained by the formation of NO+ adduct ions, next to hydride abstraction, charge transfer, or fragmentation reactions. This approach can potentially leverage selectivity issues in VOC profiling of complex matrices, such as food matrices or raw materials in combination with chemometric pattern recognition techniques.Graphical abstract

Highlights

Ion mobility spectrometry (IMS) becomes increasingly important for the analysis of volatile organic compounds (VOCs) in a number of application fields
The volatile profile of a pelletised hop sample of the hop cultivar Citra was analysed by HS-gas chromatography (GC)-IMS and the monoterpenes α-pinene, β-pinene, myrcene, and limonene were identified by comparison of the retention times and the ion signal pattern with a reference standard mixture
Without distinct knowledge of the actual underlying ion structure, the signals were divided into monomer, dimer, and trimer signals based on the monomer-dimer distribution observable along the retention time axis, which is typical for IMS data with radioactive ionization [2, 12, 14, 15]

Summary

Introduction

Ion mobility spectrometry (IMS) becomes increasingly important for the analysis of VOCs in a number of application fields. 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]. 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+(H2O)n) as reactant ions in positive mode due to trace levels of water in the surrounding gas atmosphere. Analytes entering the ionization region are primarily ionized by proton transfer reactions with the reactant ions to protonated monomer ion clusters (MH+(H2O)n-x). When the analyte concentration further increases, an additional analyte molecule can attach to the monomer, leading to proton-bound dimer ion clusters (M2H+(H2O)n-x) [6]. The situation is reversed—the number of dimer ions decreases, while the monomer ions increase

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