## 1 Introduction  
Propolis is a naturally occurring material mainly consisting of vegetal resins mixed with beeswax. This material is used as a building element for covering internal walls of hives, as its antioxidant and antimicrobial properties are able to maintain an aseptic area inside the hive [1, 2]. Because of its widespread spectrum of bioactivities, propolis is increasingly used for manufacturing nutraceuticalal products, which are perceived as natural medicines. The chemical composition of propolis is quite variable, thus its standardization and quality control are essential. Literature about the quality control of propolis has highlighted the importance of knowing the botanical origin [3] and determining toxic contaminants and potential allergenic agents, mainly caffeic acid esters [4]. Poplar-type propolis is so far the most commercialized one [5] and extensive studies in terms of biological activity are available. This promoted the design of various formulations based on propolis, such as hydroalcoholic solutions,lotions, ointments, lip balms, cosme, shampoos, conditioners, and toothpastes [6]. The beneficial bioactivity reported for poplar-type propolis is mainly ascribed to a major component, the caffeic acid phenethyl ester (CAPE) [1; 5; 7; 8]. However, this compound and other analogous are reported as very strong allergenic agents as they may induce contact dermatitis when applied to skin or mucous membranes [6; 9; 10; 11; 12; 13; 14]. From 1.2% to 6.6% of people were reported to be allergic to propolis [14], and among all the described about 50% are related to cosme and 25% to contact allergies in occupational exposures [6; 9; 10; 15]. Indeed, caffeates were described as the main contact allergens of propolis in middle Europe, where resins excreted by poplar trees (_Populus_ spp.) are the preferred resources for bees [11], whilst resins from birch (_Betula_ spp.) are selected by bees in colder temperate climates [5]. In the hot arid regions of north-western Argentina, such as the Del Monte desert, the exudates from native resinous shrubs are the preferred choice for bees [16]. In this context, _Larrea_ spp. and _Zuccapnia punctata Cav._ are the prevailing species [17]. The occurrence of various sensitizing effects related to poplar-type propolis use clearly compels preliminary chemical analysis before its direct use. Recently, studies on north-western Argentinean propolis (NAP) became more exhaustive in terms of description of chemical and biological properties. This increased interest is due to its introduction into the national Food Code, justified by the beneficial effects, mainly associated to its high phenolic content [16; 18; 19]. To date only the vegetal species _Z. punctata_ has been reliably related with NAPs collected in Santa Maria and Amaicha del Valle (\"Calchaqui Region\") [20; 21]. Although some new caffeates were identified in _Z. punctata_[21], a chemical analysis aimed to identify sensitizing agents in propolis has been never carried out. Liquid chromatography-UV (LC-UV) analysis is commonly used for quality control of standardized propolis and plants [22]. This technique, anyway, is not suitable for the selective identification of non-target compounds present as minor components in propolis likely coming from different botanical sources. Recently, plant extracts and related foodstuffs have been characterized by LC-high resolution mass spectrometry (LC-HRMS) [23; 24] by means of accurate (<5 ppm) measurements of both pseudo-molecular and fragment ions, allowing very robust assignments when standards are not available. A recent paper highlighted that the different chemical profile in terms of caffeinates and hydroxycinnamates, produced by various extraction methods and evidenced with LC-MS analysis, affects the allergenic or pharmacologic potential of the poplar propolis [25]. In this context, a peculiar source-type propolis with low allergen content can be a direct alternative to a propolis requiring an industrial processing tailored to optimize its use and quality control.