## 1 Introduction  
Aromatic plants are characterized by a particular aroma. This is the result of the production of an amazing diversity of volatile organic compounds (VOCs) by specialized secretory cells that are present in specific structures in these plants, called glandular trichomes [1, 2]. VOCs are characterized by low molecular mass (between 50 and 200 Da), a low boiling point and high vapor pressure. A consequence of their high volatility is a marked tendency to pass from a liquid state to a vapor state, which allows them to disperse into the biosphere and act over long distances [2]. Plants that produce VOCs, in particular aromatic plants, have become an important economic resource worldwide. Among these plants, rosmary is widely cultivated, mainly due to its essential oil, which has high commercial value [3]. Rosemary leaf essential oil is dominated by monoterpenes (>90%), hydrocarbons and oxygenated compounds. Among rosmary species, several chemotypes can be distinguished, based on the predominant constituents of the essential oil. Examples include: 1,8-cineole; 1,8-cineole/\(\alpha\)-pinene/cambror; myrcene; \(\alpha\)-pinene/verbeneo/bornyl acetate; and 1,8-cineole/borneol/p-cymene [4, 5]. [PERSON] et al. (2020) [6] showed important variation in volatile components in _R. officinalis_ var. albiflorus, with higher \(\alpha\)-pinene (37.5%), 1,8-cineole (15.69%), verbenone (6.61%) and camphene (4.64%) compared to _R. officinalis_ var. Gorizia, which yielded elevated 1,8-cineole (23.39%), \(\alpha\)-pinene (13.14%), camphor (13.02%) and camphene (6.54%). Furthermore, over 20 different types, varieties or cultivars can be distinguished as a function of morphological descriptors [4], such as the calyx, corolla, inflorescence and the presence of glandular trichomes [7], along with whether the plant is prostrate, its leaf size and flower characteristics [8; 9]. Given this diversity, the ability to easily discriminate between the great variety of rosemary plants would be a step forward, in order to process specific, high-quality products. Thus, in the following, we develop a simple method to measure VOCs from rosemary leaves, in order to classify these aromatic plants. The main analytical method used to measure VOCs is gas chromatography-mass spectrometry (GC-MS). It is widely used because of its high sensitivity, accuracy, reproducibility and overall robustness. However, disadvantages include the cost of equipment, the complexity of the operation and the long analysis time [10]. Depending on the aim of the analysis, a possible alternative is the use of a standalone photoionization detector (PID). These sensors are simple to use, inexpensive and the analysis only takes a few seconds. However, in turn, they also have some disadvantages. For example, a PID sensor can only measure a VOC concentration, and no information is provided about the chemical composition. In general, PID technology is designed to characterize the overall profile of a VOC mixture into a digital fingerprint, rather than quantify individual compounds [11]. However, the growing need for a rapid method to analyse VOC concentrations has stimulated interest in more widely instruments in terms of time, use and cost. The current literature contains reports from many studies that test the use of rapid VOC measurement instruments, in particular electronic noses [12]. To the best of our knowledge, no studies have investigated the use of a standalone PID to classify plants based on their VOC emissions. Unlike electronic noses, which use a pattern recognition algorithm to analyse several signals from an array of semi-selective gas sensors, we adopt a different approach. Specifically, we explore the response of a single PID to a wide range of VOCs, and the use of temporal data acquisition. The underlying idea in our approach is that signal kinetics in the time domain may be a function of the morphological (shape, dimension, histological classification) and physiological (essential oil composition) characteristics of rosemary plants.