## 1 Introduction
Alternatively, phenolic compounds are usually divided into two groups: phenolic acids including phenolic esters (i) and flavonoids (ii) [15]. As observed also for other minor chemical compounds, the composition of polyphenols in honey depends on many factors: the botanic source, seasonal and environmental factors, geographic origin, and storage conditions [5; 8; 16]. Typical analytical methods used to determine polyphenols in honey are based on a two-step approach [17]: isolation and extraction of phenolic compounds from the honey matrix (i.e., by solid-phase extraction) followed by the identification of chemical compounds by chromatographic analysis (i.e., HPLC) with various different detectors (i.e. diode-array detector - DAD, fluorescence and MS detectors) [14; 15; 16; 17; 18; 19; 20]. The determination of polyphenols' content in honey of different botanic origin has also been associated to the antioxidant activities of honey in several countries, as reported for Turkish [18], Greek [19], and Italian [20] honey samples. During the years, several destructive analytical methods were developed to characterize the chemical composition of honey, to discriminate honey samples of different botanic and geographic origin, and to detect adulterations with sugar or fake honey. However, these classical analytical methods have several limitations [5; 8]: they are usually expensive, time consuming, and the extraction of minor compounds from the raw matrix may be critical for their quantification. For these reasons, non-destructive approaches, based on infrared (IR), Raman, ultraviolet-visible (UV-Vis), and near infrared (NIR) spectroscopic techniques [21; 22], as well as several fluorescence spectroscopic methods [23; 24; 25; 26; 27; 28] were developed to investigate honey samples and to detect eventual adulterations in an easier and faster way. On the other hand, fluorescence spectroscopy has become quite popular in recent years to study food matrices and for quality assessment of food processes and conservation [29]. This is related to the high sensitivity and selectivity of fluorescence spectroscopy, which are able to detect very small concentrations of fluorophores [30], and to the natural presence of several kinds of fluorescent molecules in food [23; 24; 25; 26; 27; 28; 29; 31; 32; 33; 34]. In particular, fluorescence properties of honey have been related mainly to the presence of free amino acids, enzymes and proteins, flavonoids and phenolic acids, vitamins, and Maillard reaction products [23; 24; 25; 26; 27; 28; 29; 31; 35; 36; 37; 38; 39; 40; 41]. Some of the fluorescent compounds naturally found in honey (i.e., tryptophan, tyrosine and phenylalanine, among free amino acids [31]; and 2'-methoxyacetophenone, among organic ketophenone derivatives; and 3-phenyllactic acid, produced by the metabolism of phenylalanine [26]) have also been identified as biomarkers of honey of different botanic and geographic origins. Since honey is a viscous, opaque and multi-fluorophoric matrix, most fluorescence-based methods use the reflectance or front-face fluorescence (FFF) approach [23; 24; 25; 27; 35; 36; 37; 38; 39; 40; 41]. Right-angle fluorescence (RAF) spectroscopy, however, has been recently applied for discriminating among different botanic origins in Croatian honey samples, previously solubilized in buffer solutions and methanol solutions as well [28]. The first FFF studies of honey [23; 24; 36] put in evidence the specificity of the spectral profiles (i.e., emission, excitation, and synchronous spectra) of honey samples with different floral origin. In particular, synchronous spectra, obtained from the simultaneous scan of excitation and emission wavelengths (\(\lambda_{\text{ex}}\), \(\lambda_{\text{em}}\)), maintaining a constant wavelength interval (\(\Delta\lambda\)) between them [30], demonstrated to be useful to study multicomponent fluorescent systems being very sensitive to both the presence of specific fluorophores and their relative concentrations [23].