Constituents of Some Essential Oil Bearing Plants from Vietnam

Essential oils obtained from hydrodistillation of three medicinal plants grown in Vietnam were analyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC-MS). The monoterpene hydrocarbons, α-pinene (50.2%), β-pinene (23.6%) and limonene (5.3%) were the most abundant constituents of the rhizome oil of Zingiber collinsii Mood & Theilade (Zingiberaceae). The main compounds of the stem oil of Croton kongensis Gagnep., (Euphorbiaceae) were benzyl benzoate (12.7%), β-selinene (9.8%), bulnesol (8.0%) and 5,6,7,8-tetrahydroquinoxaline (7.4%). The leaf oil of Goniothalamus albiflous Ban., consisted mainly of α-pinene (26.2%), caryophyllene oxide (10.6%) and 1,8-cineole (9.7%). The composition of the oils of Zingiber collinsii and Croton kongensis was being reported for the first time.


Introduction
In this paper, we report on the volatile constituents identified from the three medicinal plants, as part of our continued interest on the chemical analysis of the flora of Vietnam [1] [2].Zingiber collinsii Mood & Theilade (Family Zingiberacea), a deciduous species, was recently collected in Vietnam and introduced by Mark Collins [3].Naturally dormant in winter, leaves have silver streaks on green above and maroon beneath.Cones are dull orange and form at the base of the plant.It is the most beautiful ginger in cultivation.The foliage is spectacular with silver bands on the leaves and basal flowers in clusters of orange to red that look like brightly colored hot pokers [4].Literature information is scanty on its volatile and non-volatile constituents as well as the biological potential of this plant.
Croton kongensis Gagnep., (family Euphorbiaceae), is known in Vietnam as Cù đèn cửu long.The leaves are used for medicinal purposes.It is frequently used in folk medicine for dysmenorrhoea.The leaves are used in Indo-China for various stomach disorders including ulcers, and a decoction is externally applied for furuncles and impetigo [5].Crude extract of the plant was previously reported to have exhibited antimalarial and antimycobacterial activities [5].Several metabolites including antimycobacterial and antimalarial diterpenes [6]- [9] and phenolic compounds [9] have been characterized from this plant.The volatile constituent has not been any subject of literature discussion.

Plants Collection
Rhizomes of Z. collinsii and the stem of C. kongensis were collected from Pu Mat National Park, Nghe An Province in July 2012 and September 2011 respectively.The leaves of G. albiflorus were collected from Bạch Mã National Park, Thừa Thiên-Huế Province, Vietnam, in August 2011.Voucher specimens were coded DND 278, HDT 288 and DND 807 respectively.All specimens were deposited at the Botany Museum Vinh University, Vietnam.

Isolation of Essential Oils
0.5 Kg of air-dried of each plant sample was shredded and their oils were obtained by hydrodistillation for 3h at normal pressure, according to the Vietnamese Pharmacopoeia [12].

Gas Chromatography (GC) Analysis of the Oils
Gas chromatography (GC) analysis was performed on Agilent Technologies HP 6890 Plus Gas chromatograph equipped with an FID and fitted with HP-5MS column (30 m × 0.25 mm, film thickness 0.25 µm, Agilent Technology).The analytical conditions were: carrier gas H 2 (2 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 µL.Inlet pressure was 6.1 kPa.The relative amounts of individual components were calculated based on the GC peak area (FID response) without using correction factors.

Gas Chromatography-Mass Spectrometry (GC-MS) Analysis of the Oils
An Agilent Technologies HP 6890N Plus Chromatograph fitted with a fused silica capillary column HP-5 MS column (30 m × 0.25 mm, film thickness 0.25 µm) and interface with a mass spectrometer HP 5973 MSD was used for the GC/MS analysis, under the same condition for GC analysis.The conditions were the same as described above with He (1 mL/min) as carrier gas.The MS conditions were as follows: ionization voltage 70eV; emission current 40 mA; acquisitions scan mass range of 35 -350 amu at a sampling rate of 1.0 scan/s.The MS fragmentation patterns was checked with those of other essential oils of known composition patterns with Wiley (Wiley 9 th Version), NIST 08 Libraries (on ChemStation HP), with those in the literature and also with standard substances.

Identification of Constituents
The identification of constituents was performed on the basis of retention indices (RI) determined with reference to the homologous series of n-alkanes, under identical experimental conditions, co-injection with standards (Sigma-Aldrich, St. Louis, MO, USA) or known essential oil constituents, MS library search (NIST 08 and Wiley 9 th Version), and by comparing with MS literature data [13] [14].

Table 1 .
Compounds identified from the studied samples.
a Compounds identified by RI from column, co-injection, literature MS pattern and literature retention indices, except where stated; b Retention indices on HP-5 MS capillary column; c Literature retention indices; -not identified and not found in Literature; d correct isomer not identified; Z.c = Zingiber collinsii; C.k = Croton kongensii; G.a = Goniothalamus albiflorus.