ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2GG9, том 64, № 3, с. 3G3-3G8
ОРИГИНАЛЬНЫЕ СТАТЬИ
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A SIMPLE, FAST, SOLVENT-FREE METHOD FOR THE DETERMINATION OF VOLATILE COMPOUNDS IN Magnolia grandiflora Linn
© 2009 S. B. Wanga, R. M. Mua, X. R. Wanga, S. X. Liua, X. L. Yuanb, Z. Q. Fana
aDepartment of Environmental Science & Engineering, Fudan University, Shanghai 200433, China bInstitute of Environmental Research, Shandong University, Jinan 250100, China. Received 30.09.2007
Magnolia grandiflora Linn belonging to the Magnoliaceae family has been used to treat hypertension for many years in China. Based on microwave-assisted extraction (MAE) and headspace solid-phase microextraction (HS-SPME) followed by gas chromatography-mass spectrometry (GC-MS), the volatile compounds of Magnolia grandiflora L. were determined. Results indicated that the optimum conditions for the determination of the volatile compounds in Magnolia grandiflora L. were achieved with fiber coating of carboxen/polidimethylsiloxane, microwave power of 700 W and irradiation time of 4 min. Under the optimal conditions, for the first time, 48 volatile compounds were separated and identified from the fresh leaves of Magnolia grandiflora L. The highest content component of 48 compounds was Y-elemene (15.67%). Relative standard deviation (RSD) values less than 11% shows that the present method has good precision. The experimental results demonstrate that MAE-HS-SPME followed by GC-MS is a simple, time-saving solvent-free method, and it is a potential analytic tool for the determination of the volatile compounds in plant materials.
Magnolia grandiflora L., which is native in North America, is a large evergreen tree of Magnoliaceae family. It is most often found in full sunlight to partial shade. It thrives in "loose, moist, fertile, acid soil" and can be grown in soils with a high salt content. In China, it is planted in the southern parts as an ornamental and officinal tree. In many urban areas where other species hardly survive, Magnolia grandiflora L. can grow because of its resistance to damage by sulfur dioxide. As a pharmaceutical, the leave of Magnolia grandiflora L. has been applied to the treatment of hypertension for many years. Many volatile compounds with bioactivity are present in leaves Understanding the chemical composition of volatile compounds in the leaves of Magnolia grandiflora L. is an essential step in using it more scientifically. However, a limited work has been done up to now dealing with the chemical compositions of volatile compounds in the leaves of Magnolia grandiflora L.
Gas chromatography-mass spectrometry (GC-MS) is an important technique for the determination of the volatile compounds of plants. However, the method of the sample treatment is also very important when the volatile compounds are to be studied. Solid-phase microextraction (SPME), introduced by Pawliszyn's group in 1990, is a relatively new sampling and concentration technique [1]. It is a simple, rapid, sensitive and solvent-free technique and has been widely adopted for the determination of chemical components of plant essential oils [2-12]. Microwave-assisted extraction (MAE) of organic pollutants was first introduced in 1986 by Ganzler et al. [13]. Since that time, MAE technique has also been widespread applied to the analysis of volatile compounds. Its main virtue is the re-
duction of extraction and organic solvent [14-18]. MAE also can improve the efficiency of extraction because it employs the ability of some liquids or solids to transform electromagnetic energy into heat and reduce the extraction time. At the same time, the solvent-free headspace solidphase microextraction (HS-SPME) technique is also developed for further extraction and concentration of volatile compounds. Therefore, MAE procedure followed by HS-SPME and GC-MS simplifies the conventional extraction procedures and combines the sample preparation and GC-MS analysis into one step.
In this contribution, for the first time, MAE procedure followed by HS-SPME and GC-MS was developed for the fast determination of volatile compounds in Magnolia grandiflora L. The experimental parameters were studied and the method precision was investigated.
EXPERIMENTAL
Plant samples, SPME fibers and microwave oven.
Fresh leaves of Magnolia grandiflora L. were collected from Shanghai, China. After being ground to fine powder, the samples of Magnolia grandiflora L. (2.0 g) were used for MAE-HS-SPME.
The fiber coatings including 65 |m polydimethylsilox-ane/divinylbenzene (PDMS/DVB), 100 |m polydimethyl-siloxane (PDMS), 30 |m divinyibenzene/carboxen/PDMS (DVB/CAR/PDMS) and 75 |m carboxen/polydimethylsi-loxane (CAR/PDMS) were purchased from Supelco (Bellefonte, PA, USA). The microwave oven with a maximum delivered power of 700 W was purchased from Haier (Qingdao, China).
304 WANG и др.
SPME fiber
Microwave oven Glass bottle Plant material
Fig. 1. Home-made MAE-HS-SPME apparatus.
The procedure of MAE-HS-SPME. The home made MAE-HS-SPME apparatus used in this study is illustrated in Fig. 1. 2.0 g of leaves of Magnolia grandiflora L. were ground to fine powder and introduced into a 25 ml glass bottle. Then the bottle was put into the microwave oven where the plant samples were heated by a microwave at the power of 200, 400 and 700 W for 2-6 min respectively, and a condenser with a continuous flow of freezing water was used to condense the vapors, so that the water could take part in the extraction process repeatedly. Then, at the heating process, the volatile compounds were extracted from Magnolia grandiflora L. by fiber coating.
To obtain the optimal MAE-HS-SPME experimental conditions, the parameters including different fiber coatings, microwave power and irradiation time were evaluated. At first, four fibers of PDMS/DVB, PDMS, DVB/CAR/PDMS and CAR/PDMS were tested individually, with the following microwave parameters: power of 400 W and irradiation time of 2.0 min. Next, the microwave power (200, 400 and 700 W) and irradiation time (2, 4 and 6 min) were also studied. All the volatile compounds absorbed on the SPME fiber were desorbed at GC injector (250°C for 3 min), and then analyzed by GC-MS, respectively.
GC-MS analysis. An HP 6890 GC system, coupled with a HP MD5973 quadrupole mass spectrometer, was used to analyze volatile compounds. The extracted compounds were separated on an HP-5MS capillary column (30 m length x 0.25 mm I.D., 0.25 ^m film thickness). Splitless injection was employed. The column oven temperature was programmed to rise from an initial tempera-
ture of 40°C (3 min) to 160°C at 8°C per min, then to 300°C at 15°C per min. The injection temperature and ion source temperature were 250 and 230°C, respectively. Helium was used as the carrier gas with a flow rate of 1 ml/min. The ionizing energy was 70 eV. All data were obtained by collecting the full-scan mass spectra within the scan range 40-350 amu. Compounds were identified using the Wiley 6.0 (Wiley, New York, NY, USA) Mass Spectral Library.
The precision of MAE-HS-SPME. The method precision was studied by five replicate analyses of the volatile compounds in Magnolia grandiflora L. by MAE-HS-SPME at the optimum conditions. The precision was expressed by relative standard deviation (RSD, %). The peak areas of the volatile compounds in the leaves of Magnolia grandiflora L. obtained by replicate analyses were used for calculation of their RSD values.
RESULUTS AND DISCUSSION
Optimization of MAE-HS-SPME parameters.
Since the volatile compounds are always present at very low concentrations, sample extraction is a key technique to determine the volatile compounds in the leaves of Magnolia grandiflora L. According to the proposed method, the volatile compounds were extracted and further concentrated from leaves of Magnolia grandiflora L. using MAE-HS-SPME technique. Three experimental parameters including fiber coating, microwave power and irradiation time, which can evidently affect the extraction efficiency, were systematically studied.
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Fig. 2. GC-MS total ion chromatograms of Magnolia grandiflora L. obtained by usinq different fibers of PDMS/DVB, PDMS, DVB/CAR/PDMS and CAR/PDMS, respectively.
At first, the optimum fiber coating was studied. The samples were introduced into a 25 mL bottle, which had been washed twice by distilled water, dry distilled under microwave irradiation (400 W, 2 min) and simultaneously headspace extracted and concentrated by the four different fibers of PDMS/DVB, PDMS, DVB/CAR/PDMS and CAR/PDMS, respectively. Fig. 2 is the total ion chromatograms of volatile compounds in the leaves of Magnolia grandiflora L. obtained by MAE-HS-SPME with four different fiber coatings. The comparison among the four types of fiber coating used in this study showed different GC responses. Obviously, the CAR/PDMS fiber coating had much better extraction efficiencies than the others. Therefore, the CAR/PDMS was chosen as the optimal fiber coating and used for the further experiments.
Then microwave power and irradiation time have been studied because they are also two key parameters for HS-SPME. In order to obtain higher amounts of volatiles and the highest number of individual volatile compounds, the samples of Magnolia grandiflora L. were extracted at different microwave power (200, 400 and 700 W) and different irradiation time (2, 4 and 6 min). Then the analytes were further extracted and concentrated by a CAR/PDMS fiber, followed by analyzing with GC-MS. Sums of peak area obtained at different microwave power and irradiation time are shown in Fig. 3. Sums of peak area were achieved at the highest level at the microwave power of 700 W and
irradiation time of 4 min, respectively. Obviously, the best extracti
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