Headspace Solid-Phase Microextraction and Ultrasonic Extraction with the Solvent Sequences in Chemical Profiling of Allium ursinum L. Honey

A volatile profile of ramson (wild garlic, Allium ursinum L.) honey was investigated by headspace solid-phase microextraction (HS-SPME) and ultrasonic solvent extraction (USE) followed by gas chromatography and mass spectrometry (GC-FID/GC-MS) analyses. The headspace was dominated by linalool derivatives: cis- and trans-linalool oxides (25.3%; 9.2%), hotrienol (12.7%), and linalool (5.8%). Besides direct extraction with dichloromethane and pentane/diethyl ether mixture (1:2, v/v), two solvent sequences (I: pentane → diethyl ether; II: pentane → pentane/diethyl ether (1:2, v/v) → dichloromethane) were applied. Striking differences were noted among the obtained chemical profiles. The extracts with diethyl ether contained hydroquinone (25.8–36.8%) and 4-hydroxybenzoic acid (11.6–16.6%) as the major compounds, while (E)-4-(r-1′,t-2′,c-4′-trihydroxy-2′,6′,6′-trimethylcyclohexyl)but-3-en-2-one predominated in dichloromethane extracts (18.3–49.1%). Therefore, combination of different solvents was crucial for the comprehensive investigation of volatile organic compounds in this honey type. This particular magastigmane was previously reported only in thymus honey and hydroquinone in vipers bugloss honey, while a combination of the mentioned predominant compounds is unique for A. ursinum honey.


Introduction
Ramson (wild garlic, Allium ursinum L.) is a perennial plant, widely distributed in Europe. Phytochemical investigations of this plant revealed the presence of S-alk(en)yl-L-cysteine-sulfoxides (methiin, alliin, isoalliin, propiin, and ethiin) and their degradation products ((poly)sulfides, dithiins, or ajoenes) [1,2]. Apart from the abovementioned, various sulphur compounds have also been detected as constituents of its essential oil, e.g., disulfides, trisulfides, and tetrasulfides [3,4]. A. ursinum has been also reported to be a rich source of phenolic compounds (up to 27.9 g GAE (gallic acid equivalent)/ 100 g) [5]. Similar to organosulfur compounds, it was found to contain steroidal saponins that are also commonly found in the Allium genus [1,6]. Other identified constituents of interest include lectins, polysaccharides, and fatty acids [1]. A great number of in vitro and in vivo experiments showed that A. ursinum is a plant with antimicrobial, cytotoxic, antioxidant, and cardio-protective effects [1,7].
A. ursinum provides excellent spring bee pasture with a good nectar flow [8,9]. Allium species tend to secrete highly concentrated nectar, and the daily nectar production of A. ursinum ranged from 0.1 to 3.8 µL per flower, with sugar concentrations of 25% to 50%. However, the floral nectar volume and concentration varies in different populations of A. ursinum which can be also strongly affected by the varying conditions in different natural habitats. Nevertheless, the honey cannot be produced on a regular basis and its production is limited [8].
In continuation of the chemical fingerprinting of different unifloral honey types in search of specific or nonspecific chemical markers of botanical origin, the focus of this work was on not yet investigated volatile organic compounds (VOCs) of Allium ursinum L. honey of Croatian origin (a very rare sample). Headspace solid-phase microextraction (HS-SPME) followed by gas chromatography and mass spectrometry (GC-FID/GC-MS) analysis was applied to investigate its headspace chemical profile. To complement the honey profiling with data on less volatile organic compounds, ultrasonic solvent extraction (USE) was applied with solvents of different polarities, and the obtained extracts were analysed by GC-FID/GC-MS.
At the time of blooming, A. ursinum plants emit a strong garlic odour that can also be smelled in the nectar and in front of the beehives. However, it has been reported that the odour of the corresponding ripe honey is different, with a pleasant, particular aroma [8]. Therefore, significant differences among the chemical profiles obtained from A. ursinum honey VOCs and the corresponding plant VOCs were expected. To investigate in detail the headspace, volatile, and semi-volatile compounds from A. ursinum honey, up to-date complementary methodologies were applied: headspace solid-phase microextraction (HS-SPME) and ultrasonic solvent extraction (USE) followed by GC-FID/GC-MS analyses. Striking differences were found among the chemical profiles obtained by those methods and the plant VOCs.
In comparison with HS-SPME (Tables 1 and 2), only a few compounds were similar, while linalool and its derivatives were found with significantly lower abundance in the extracts than in the headspace. Epoxidation of linalool gives 6,7-epoxylinalool, which undergoes further reactions to form linalool oxides, while hotrienol is derived from hydroxylated linalool derivatives [11]. Higher abundance of linalool, cis-, and trans-linalool oxide were found in the headspace of Coriandrum sativum L. [14] and Citrus spp. [13,15,16] honey types. Regarding the extract chemical profiles, no major similarity was found among the profiles of other honey types. A combination of predominant compounds (E)-4-(r-1 ,t-2 ,c-4 -trihydroxy-3 ,6 ,6 -trimethylcyclohexyl)-but-3-en-2-one, hydroquinone, methyl syringate, and 4-hydroxybenzoic acid is unique to A. ursinum honey. 1,4-Dihydroxybenzene was proposed as a floral marker compound for vipers bugloss (Echium vulgare L.) honey [17]. High proportions of benzoic acid and its derivatives were found in Salix spp. honeydew extractives [18], but with a minor percentage of 4-hydroxybenzoic acid. The latter was found abundant by HPLC in buckwheat (Fagopyrum esculentum L.) honey [19]. (E)-4-(r-1 , t-2 ,c-4 -trihydroxy-2 ,6 ,6 -trimethylcyclohexyl)but-3-en-2-one contains a megastigmane structure. Structurally, megastigmanes are C 13 -carbon skeleton compounds, which are commonly classified as C 13 -norisoprenoids, also assumed to be apocarotenoides. Megastigmanes possess a unique basic skeleton with a six-membered ring with a double bond within the ring system, followed by methyl and dimethyl substitutions and an attached four membered chain with a double bond in the trans-mode [20]. The biosynthesis of this compound can be envisaged as proceeding via the alkene with a double bond within the ring system and via one or both of the epoxides [20]. Although a wide variety of degraded carotenoid-like substances have been identified from different honey types [13], this appears to be a rare situation where a trihydroxy ketone has been found. In fact, it was previously isolated and characterized by X-ray crystallographic analysis as a dominant substance from the ether extracts of New Zealand thyme honey [21]. Its recorded MS spectra were m/z 224 (6%), 141 (9)

Materials and Methods
A rare and representative Allium ursinum L. honey sample was collected from a professional beekeeper in Croatia (more unifloral samples were not available). The sample was stored in a hermetically closed glass bottle at 4 • C until the volatiles were isolated. Melissopalynological analysis was performed according to the International Commission for Bee Botany [23]. Microscopical examination was carried out on a Hund H 500 light microscope (Helmut Hund GmbH, Wetzlar, Germany) attached to a digital camera (Motic m 1000, Motic Deutschland GmbH, Wetzlar, Germany) and coupled to an image analysis system (Motic Images Plus software, Motic Deutschland GmbH) for the morphometry of pollen grains.

Headspace Solid-Phase Microextraction (HS-SPME)
The headspace solid-phase extraction (HS-SPME) was performed using a manual SPME holder using polydimethylsiloxane/divinylbenzene (PDMS/DVB) fiber that was conditioned prior to the usage according to Supelco (Bellefonte, PA, USA) instructions. The honey/saturated water solution (5 mL, 1:1 (v/v); saturated with NaCl) was placed in a 15-mL glass vial and hermetically sealed with polytetrafluorethylene (PTFE)/silicone septa. The vial was maintained in a water bath at 60 • C during equilibration (15 min) and HS-SPME (45 min) under constant stirring (1000 rpm) with a magnetic stirrer, and the sample was kept below the water level of the water bath. After sampling, the SPME fiber was withdrawn into the needle, removed from the vial, and inserted into the injector (250 • C) of the GC-FID and GC-MS for 6 min, where the extracted volatiles were thermally desorbed directly to the GC column. The experiment was performed in triplicate.

Ultrasonic Solvent Extraction (USE)
Ultrasound-assisted solvent microextraction (USE) was performed in an ultrasound cleaning bath (Clean 01, MRC Scientific Instruments, London, UK) by the indirect sonication mode at a frequency of 37 kHz at 25 ± 3 • C. The advantage of using USE is the isolation of volatile and semi-volatile as well as water-soluble organic compounds without the application of heat. Different solvents were used for USE: a mixture of pentane/diethyl ether, 1:2 (v/v), dichloromethane, pentane, and diethyl ether. The mixture and dichloromethane were separately used for the extractions. A previously developed USE method was modified with the solvent sequences that were applied for the honey extraction. Sequence I consisted of the extraction with pentane followed by the extraction with diethyl ether (pentane → diethyl ether). Sequence II consisted of pentane extraction followed by the extraction with pentane:diethyl ether 1:2 (v/v) and afterwards with dichloromethane (pentane → pentane:diethyl ether 1:2 (v/v) → dichloromethane). For each extraction, 40 grams of the honey was dissolved in distilled water (22 mL) in a 100-mL flask. Magnesium sulfate (1.5 g) was added and vortexed (10 min). The solvent volume was 20 mL and the sonication was applied for 30 min. After the sonication, the organic layer was separated by centrifugation and filtered over anhydrous MgSO 4 . The aqueous layer was returned to the flask and another batch of the same extraction solvent was added and extracted for 30 min. The organic layer was separated in the same way as the previous layer and filtered over anhydrous MgSO 4 , and the aqueous layer was sonicated a third time for 30 min with another batch of the extraction solvent. Combined organic extracts were concentrated to 0.2 mL by distillation with a Vigreaux column, and 1 µL was used for GC-FID/GC-MS analyses. The experiments were performed in triplicate.

GC-FID and GC-MS Analyses
The GC-FID analyses were conducted with an Agilent Technologies (Palo Alto, CA, USA) gas chromatograph model 7890A equipped with a flame ionization detector (FID) and a HP-5MS capillary column (5% phenyl-methylpolysiloxane, Agilent J and W, Santa Clara, CA, USA). The GC conditions were described previously [13,18]. In brief, the oven temperature was programmed isothermal at 70 • C for 2 min, increasing from 70-200 • C at 3 • C·min −1 , and held isothermally at 200 • C for 15 min; the carrier gas was He (1.0 mL·min −1 ); and the total run time was 65 min.
The GC-MS analyses were conducted with an Agilent Technologies (Palo Alto, CA, USA) gas chromatograph model 7820A equipped with a mass selective detector (MSD) model 5977E (Agilent Technologies) and a HP-5MS capillary column, under the same conditions as those described for the GC-FID analysis. The MSD (EI mode) was operated at 70 eV, and the mass range was 30-300 amu, as previously reported [13].
The identification was based on the comparison of VOC retention indices (RI), determined relative to the retention times of a homologous series of n-alkanes (C 9 -C 25 ), with those reported in the literature and their mass spectra with authentic compounds available in our laboratories or those listed in Wiley 9 (Wiley, New York, NY, USA) and NIST 14 (D-Gaithersburg) mass spectral libraries. The percentage composition of the samples was computed from the GC peak areas using the normalization method (without correction factors). The average component percentages in Tables 1 and 2 were calculated from duplicate GC-FID and GC-MS analyses.

Conclusions
The unusual chemical profile of A. ursinum honey was investigated and described for the first time. The headspace was dominated by linalool and its derivatives, which is not specific. The extracts showed remarkable variabilities according to the solvents applied, which is important to point out since the use of only one solvent could lead to incomplete results for A. ursinum honey. Namely, the extracts obtained with diethyl ether as the solvent contained 1,4-benzenediol and 4-hydroxybenzoic acid as the major compounds, while (E)-4-(r-1 ,t-2 ,c-4 -trihydroxy-2 ,6 ,6 -trimethylcyclohexyl)but-3-en-2-one predominated in the dichloromethane extracts. The applied sequence of solvents enabled the fractionation of the compounds according to polarity, and sequence II was useful for the concentration and possible isolation of (E)-4-(r-1 ,t-2 ,c-4 -trihydroxy-2 ,6 ,6 -trimethylcyclohexyl)but-3-en-2-one. More samples should be investigated to confirm these compounds as characteristic of this honey type.