Retention index database for identification of general green leaf volatiles in plants by coupled capillary gas chromatography−mass spectrometry

Abstract

A series of ubiquitously occurring saturated and monounsaturated six-carbon aldehydes, alcohols and esters thereof is summarised as ‘green leaf volatiles’ (GLVs). The present study gives a comprehensive data collection of retention indices of 35 GLVs on commonly used non-polar DB-5, mid-polar DB-1701, and polar DB-Wax stationary phases. Seventeen commercially not available compounds were synthesised. Thus, the present study allows reliable identification of most known GLV in natural plant volatile samples. Applications revealed the presence of several seldom reported GLVs in headspace samples of mechanically damaged plant leaves of Carpinus betulus and Fagus sylvatica.

Introduction

Following mechanical damage, green plant tissue releases a characteristic odour known as ‘green leaf odour’. Investigation of the involved processes revealed that this odour is due to a series of saturated and monounsaturated six-carbon aldehydes, i.e., hexanal, (E)-2-hexenal, (E)- and (Z)-3-hexenal and the corresponding alcohols which are formed by enzymatic degradation of unsaturated fatty acids [1], [2]. Esterification of most abundant alcohols (Z)-3-hexenol, (E)-2-hexenol, and 1-hexanol with common metabolic carboxylic acids additionally leads to the formation of green leaf esters. For some of these so-called ‘green leaf volatiles’ (GLVs) antimicrobial activity has been shown suggesting that these compounds might serve as wound disinfectants [3]. Several GLVs have been shown to contribute significantly to the flavour of many plant-derived foods [4], [5], [6], [7] and, thus, are commonly used by the flavour industry as aroma chemicals in flavourings and fragrances [8]. On the other hand, several studies have investigated the behaviour-modifying properties of GLVs towards phytophagous arthropods. GLVs may, e.g., attract herbivores to their host plant [3], [9], [10], synergistically enhance the attractiveness of insect pheromones [11], inhibit the response of insects to their sex pheromone [12], or may even be directly used by insects to locate sexual mates [13]. Thus, scientists from different disciplines are interested in this class of compounds. Headspace enrichment and coupled capillary gas chromatography–mass spectrometry (GC–MS) is normally used for chemical analysis of GLVs [10], [14], [15], [16], [17], [18], [19]. However, several GLVs are not commercially available as synthetic reference compounds and, therefore, identification of less common GLVs is sometimes based merely on comparison of mass spectra with library spectra [17], [18]. This status of identification is not satisfying because several GLVs occur as configurational isomers with different sensory properties [20] but very similar mass spectra. For identification of monoterpenes, a class of compounds where similar problems have to be faced, it can be helpful to involve retention data from the literature [21], because modern capillary columns are manufactured with a high degree of reproducibility. It has been shown that retention indices of many aroma chemicals on comparable stationary phases can be estimated at different laboratories with standard deviations of far less than 1% [22]. This paper presents a data collection of retention indices of 35 GLVs on non-polar, medium polar, and polar stationary phases and describes easy methods for the synthesis of commercially not available GLVs to allow optimisation of GLVs identification in natural plant samples. Additionally some applications are given to demonstrate practical relevance of the data.