Identification of volatiles in headspace emitted from Mahonia japonica flowers

Journal of Essential Oil Research: JEOR, Jul/Aug 2003 by MacTavish, Hazel S, Picone, Joanne M, Clery, Robin A

Received: September 2000

Revised: March 2001

Accepted: May 2001

Abstract

Volatiles emitted into the headspace above flowering Mahonia japonica (Bealei group) (Fort.) Carr. plants were trapped using two methods: PDMS/carbotrap with thermal desorption and Porapak Q with solvent desorption, followed by GC/MS analysis. The complete GC/MS analysis of floral volatiles of M. japonica is presented for the first time, showing the predominance of benzaldehyde (4.1%), benzyl alcohol (14.0%), (E)-[beta]-ocimene (57.0%) and citronellol (6.7%).

Key Word Index

Mahonia japonica, Berberidaceae, headspace volatiles, flower volatiles, benzyl alcohol, (E)-[beta]-ocimene.

Introduction

The genus Mahonia (Berberidaceae) contains approximately 70 species of evergreen shrubs native to East Asia, North and Central America (1). A number of species of Mahonia have been extensively studied for their pharmacologically active components: the roots of M. aquifolium contain alkaloids with relaxant, antioxidant and antifungal properties (2-5). Mahonia are also grown for their floral fragrance; however, quantitative analysis of this is limited. The concrete and absolute of which species were examined by Toleva et al., (6) for their value to perfumery, but no analytical data was recorded. Mahonia japonica is a popular garden shrub with a Lily-of-the-Valley-like scent (Muguet; Convalaria majalis) described in perfumery terms as floral, indolic and rosy. It flowers in winter, producing bell-shaped yellow flowers borne on horizontal racemes of approximately 20 cm length.

In this study, the volatiles emitted into the headspace above M. japonica flowers were identified for the first time, using two adsorption systems: PDMS/carbotrap with thermal desorption and Porapak Q with solvent desorption, both combined with GC/MS analysis. Throughout this work, appropriate controls and method development were included.

Experimental

Mahonia japonica plants selected for their floral scent were purchased from a local plant center in January and acclimated to glasshouse conditions without additional lighting or temperature control. For sampling, a 2 L flanged glass flask was used to enclose two flowering racemes (laterals), both with unopened buds present at the apex and senescent flowers at the base. Teflon tubes were inserted through the basal hole to a region near the tip of the flowering raceme. For sampling, the tubes were attached to preconditioned Optic thermal desorption traps packed with 5 mg granular polydimethylsiloxane (PDMS) and 2 mg carbotrap (Supelco, Dorset, UK) and thence to suction pumps at 50 mL/min for 30 min. Between sampling, air was drawn through the flask at the same rate so that the temperature and humidity were stable and little/no accumulation of volatiles occurred. The traps were analyzed by direct thermal desorption at 220[degrees]C using the Optic injection system (Atas UK, Cambridge, England) onto the GC-FID and GC/MS systems. Further samples were absorbed onto traps packed with 100 mg Porapak Q, during 3 h at 50 mL/min, then desorbed in the opposite direction to previous airflow with two washes of 400 [mu]L diethyl ether. The recovered solvent from each wash was analyzed by on-column injection on the GC system. The efficiency of desorption of sample from the traps was tested by spiking each type of trap with a synthetic mixture of compounds found in headspace from M. japonica. The results obtained by thermal and solvent desorption were found to be virtually identical with the direct injection of the test mixture.

GC analysis: A Hewlett Packard 6890 GC equipped with a FID, on-column injector, autosampler, and an HP5 (5% diphenyl, 95% dimethyl-polysiloxane) column 25 m x 0.2 mm, film thickness 0.33 [mu]m, was used (Cat. No. 19091J-102). Carrier gas : H^sub 2^ at 2 mL/min; oven temperature program: 50[degrees]C initially, then 10.2[degrees]C/min to 280[degrees]C for 7.5 min, making a 30 min total run time. Injector temperature: 250[degrees]C, detector temperature: 280[degrees]C. Injection volume: 0.5 [mu]L. The results are quoted as relative proportion of total peak area (% RPA) from the FID chromatogram.

GC/MS system: Compounds were identified by GC/MS (MS database, Quest International) and confirmed by retention times of pure reference compounds. A Varian 3400 GC equipped with an OPTIC injector and coupled to a Finnigan ITS40 ion trap mass spectrometer was used. The column was an HP ultra2 (5% diphenyl, 95% dimethyl-polysiloxane) 50 m x 0.2 mm, with film thickness 0.33 [mu]m (Cat. No. 19091B-105). Carrier gas: He at 1.6 mL/min; oven temperature program: 50[degrees]-270[degrees]C at 2[degrees]C/min. Injector temperature: 250[degrees]C; injection volume: 0.5 [mu]L.

Results and Discussion

The complete list of identified volatiles in the headspace above M. japonica flowers is presented in Table I. The predominance of benzaldehyde, benzyl alcohol, (Z)- and (E)-[beta]-ocimene, linalool, citronellol and indole in the headspace was noted, with indole having the greatest overall impact on the scent due to its low odor perception threshold. The benzyl alcohol and citronellol contribute rosy and muguet notes to the overall fragrance. An apparent difference in the flower scent was observed between the morning and the evening and this has been the subject of a more detailed study (7).