## 1 Introduction
Volatile fraction of high quality EVOO, which is responsible for its characteristic so-called _green_ and _fruity_ flavor, consists mainly of C5 and C6 volatiles (aldehydes, ketones, alcohols, and esters) generated enzymatically in the so-called lipoxygenase (LOX) pathway and other subsequent bioprocesses during olive processing. LOX-derived compounds are accompanied by those from other chemical classes, such as hydrocarbons, terpen, benzenoids, etc. with mostly unknown or minor sensory relevance [12; 14; 21; 22; 23]. Besides being among the most important contributors to EVOO antioxidant activity, phenols, especially secoriidoids which are the most abundant, are responsible for the characteristic EVOO bitterness and pungency [12; 13; 14; 24; 25]. Olive oil phenols are formed mainly by cleavage of their glycosides by hydrolytic enzymes during olive fruit processing and their concentrations are further affected by oxidative degradation catalyzed by polyphenoxidases and peroxididases [26; 27]. The activity of the mentioned enzymes responsible for the formation of both volatile compounds and phenols is strongly genetically predetermined [28; 29], which makes these compounds a logical choice for potential varietal markers in EVOO varietal characterization and differentiation studies. Croatia is the latest country that joined EU in 2013, and some of the most recently registered PDOs are Croatian [30]. Despite relatively small quantities produced in relation to the leading olive oil producing countries, such as Spain, Italy, Greece etc. [31], EVOOs from Croatia are emerging rapidly on the global market and are much appreciated. For example, Croatian EVOOs are often among those awarded with the highest prizes at relevant international competitions, while Istria, one of the most important olive growing and EVOO producing regions in Croatia, has been represented in the first and leading global EVOO guide _Flos Olei_ by the largest number of EVOOs among all the regions for the last four years in a row (2015-2018). The olive plantations in Croatia have high genetic diversity, including many native varieties which concentrate close to their area of origin and show a limited geographical dispersion [32]. For this reason, Croatian EVOOs protected by various PDOs owe a significant part of their typicity to the varietal origin of the olives, which certainly becomes most pronounced in the case of monovarietal EVOO. In spite of that, and despite existing reports on the chemical and sensory characteristics of Croatian monovarietal EVOO [33; 34; 35; 36; 37; 38; 39; 40], the potential of Croatian native olive varieties to produce diverse and specific EVOO has not been investigated enough to be adequately exploited in designing more unique and robust PDOs. The main aim of this study was to investigate the inter-varietal diversity of typical volatile and phenolic profiles of Croatian monovarietal EVOOs by gas chromatography-ion trap mass spectrometry (GC-IT-MS) and ultra-performance liquid chromatography with diode array detection (UPLC-DAD), respectively. The approach was tested for the characterization and differentiation of EVOOs made from native varieties grown in the two most important olive growing regions in Croatia, Istria and Dalmatia, with each monovarietal EVOO represented by a heterogeneous sample group in terms of geographical microlocations, growing conditions, harvest date, olive processing technology, and EVOO finalization and storage parameters. It was expected that the results obtained would be useful for improving the understanding of the origins of the typical sensory characteristics of the investigated Croatian monovarietal EVOOs. However, the main premise was that the instrumental techniques utilized would be effective in tracing robust chemical markers among the investigated compounds despite the aforementioned sample heterogeneity, able to provide complementary information about varietal origin to that obtained by sensory analysis.