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Chemical Constituents of the Root Essential Oils of Zingiber rubens Roxb., and Zingiber zerumbet (L.) Smith

Zingiber rubens Roxb., and Zingiber zerumbet (L.) Smith were collected respectively from Nghe An, Province and Ha Tinh, Province, Vietnam. The root essential oils were obtained by water distillation and analyzed by Gas chromatography (GC) and Gas chromatography coupled with Mass spectrometry (GC-MS). The most abundant components of Z. rubens were ()-citral (30.1%), camphene (9.7%), β-phellandrene (7.5%) and 1,8-cineole (7.0%) and zingiberene (5.3%). The main oil constituents of Z. zerumbet were ()-citral (26.1%), camphene (16.3%), sabinene (14.6%), zingiberene (7.2%) and lavandulyl acetate (6.7%). This species has low zerumbone (1.2%) content.


Introduction
In continuation of our research into the volatile oils of Vietnamese flora as they are made available [1], we report herein the constituents of the root oils of Zingiber rubens Roxb., and Zingiber zerumbet (L.) Smith. Zingiber Miller is a genus of the big family of plant, Zingiberaceae. It has about 150 species distributed in tropical rain forest and in much of the Southeast Asia, China, India and throughout the Islands in the Pacific [2]. In Vietnam, the genus is diverse with about 10 endemic species. They contained essential oils which are used as medicinal drugs, spices and raw materials for industry [3]. The plants are also used for ornamental purposes [4]. The chemistry of volatile compounds of Zingiber species have been studied by various authors. For example, the major content of Z. zerumbet, zerumbone, varied according to geographical location [5]. Several other terpenoid compounds have been isolated and described from these species.

Plant Materials
Mature roots from Z. rubens were collected from Nghe An Province while Z. zerumbet was obtained from Ha Tinh, Vietnam, in 2010. Voucher specimens DND 211 and DND 212 respectively were deposited at the Botany Museum, Vinh University, Vietnam.

Extraction of the Volatile Oils
Aliquots of air dried and pulverized samples (0.5 kg each) were subjected to water distillation for 3 h at normal pressure, according to the Vietnamese Pharmacopoeia [6]. The yields of the oils were 0.40% and 0.35% (v/w) respectively for Z. rubens and Z. zerumbet, on a dry weight basis. The chemical composition of the oils is summarized in Table 1.

Gas Chromatography (GC)
About 15 mg of each oil sample, which was dried with anhydrous sodium sulfate, was dissolved in 1mL of hexane (for spectroscopy or chromatography). GC analysis was performed on Agilent Technologies HP 6890 Plus Gas chromatograph equipped with a FID and fitted with HP-Wax and HP-5MS columns (both 30 m × 0.25 mm, film thickness 0.25 m, Agilent Technology). The analytical conditions were: carrier gas H 2 (10 mL/min), injector temperature (PTV) 250˚C, detector temperature 260˚C, column temperature programmed 60˚C (2 min hold) to 220˚C (10 min hold) at 4˚C/min. Samples were injected by splitting and the split ratio was 10:1. The volume injected was 1.0 L. Inlet pressure was 6.1 kPa.
It is well known that the root oil from a plant could be different in chemical analysis of oils from other parts of the same plant [1]. This may have been responsible for the observed compositional variations within the oils of Z. zerumbet. ()-Citral, camphene and sabinene were not previously described to be of significant compound of the oils from the leaves and rhizomes of Z. zerumbet. While, ()-citral was known to be a significant compound of Z. officinale from Central African Republic [15], camphene occurred in high proportion in the oil of Z. officinale of Vietnam origin [16].