Article pubs.acs.org/JAFC
Use of Pollen Solid-Phase Extraction for the Determination of transResveratrol in Peanut Oils Qian Lu,†,§ Qin Zhao,‡,§ Qiong-Wei Yu,† and Yu-Qi Feng*,† †
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, China ‡ Department of Preventive Medicine, School of Medicine, Wuhan University of Science and Technology, Wuhan 430074, China S Supporting Information *
ABSTRACT: In this study, a simple and convenient method for the determination of trans-resveratrol (TRA) in peanut oils based on pollen grain solid-phase extraction (SPE) was developed. Pollen grains were used as normal-phase SPE sorbent to separate TRA from peanut oils for the first time. As a naturally occurring material, pollen grains exhibited an excellent adsorption capacity for polyphenolic compounds due to their particular functional structures such as hydroxyl groups, saturated and unsaturated aliphatic chains with aromatics. Their stable compositions as well as adequate particle size (30−40 μm) also make them suitable for SPE. Several parameters influencing extraction performance were investigated. Coupled with high-performance liquid chromatography-ultraviolet detection (HPLC-UV), a green purification method for fast determination of TRA in peanut oils using pollen grain cartridges as sorbents was established. The linearity range of the proposed method was 10−2500 ng·g−1 with a satisfactory correlation coefficient (r2) of 0.9999. The limit of detection (LOD) for TRA in peanut oils was 2.7 ng·g−1, and the recoveries in spiked oil samples were from 70.2% to 98.4% with the relative standard deviations (RSDs) less than 4.9% (intraday) and 5.2% (interday). This method was successfully applied to the analysis of TRA in several peanut oils with different brands from local market as well as other kinds of vegetable oils. KEYWORDS: pollen grains, trans-resveratrol, peanut oils, normal-phase solid-phase extraction, HPLC-UV
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INTRODUCTION Resveratrol (3,5,4′-trihydroxystilbene) is a phytoalexin, which is produced by a wide variety of plants including grapes, blueberries, knotweed, pistachio nuts, and peanuts.1 Due to its benefits to human health, such as antioxidative effect,2 analgesic effect,3 cardiovascular protective effect,4 cancer prevention,5 and antiaging property,6 resveratrol has been widely studied over the past few decades. Resveratrol exists in two isomeric forms, trans- and cis-isomers, and trans-resveratrol (TRA) appears predominantly and has been proved to be more biologically active.7 Peanut oil as one of the main vegetable oils in human diet contains a well-balanced fatty acid profile and antioxidants, which provide protection against harmful substances especially free radicals.8 Some scientific data showed that the antioxidative effect of peanut oil is due to the presence of phenolic compounds like TRA.9 However, much attention was focused on the content of TRA in peanut skins,10 peanut hulls,11 peanut roots,12 and peanut leaves13 in recent years, and analysis of TRA in peanut oils was less reported.14 The content of TRA in peanut oil is much lower than that in peanut kernels or peanut skins, and the high-fat matrix cause the main difficulties for the analysis of trace amounts of TRA in oil matrices. Therefore, an effective sample pretreatment method for TRA separation and enrichment before instrument analysis is needed. Very recently, Zhao et al. employed a mixedmode SPE cartridge purchased from Waters Oasis WCX for direct separation of TRA from soybean and peanut oils.15 The method was effective, and high recoveries were achieved. However, in their experiment, the cartridge was used only one © 2015 American Chemical Society
time, and the price of this commercial cartridge was expensive for routine analysis. Ma et al. utilized homemade hydrophilic multiwalled carbon nanotubes (HMWCNTs) as sorbents for magnetic solid-phase extraction of TRA in vegetable oils.16 The sample pretreatment procedure was simple and rapid, whereas the preparation of HMWCNTs involved large amount of concentrated acid (H2SO4/HNO3), which was harmful to the manipulator and violates several rules of green analytical chemistry such as the application of less toxic reagents, generating as less waste as possible, increasing safety for operators, among others.17,18 Therefore, the development of new selective sorbents obtained from renewable source for TRA separation was still desirable. Pollen grains are naturally occurring material from plants and are the carriers of plant’s genetic substance needed for pollination.19 Exine, the outer pollen wall, consists of an extremely stable and complex biopolymer known as sporopollenin, which is highly resistant to chemical attack and high temperature.20 Because of the diverse chemical composition of exine, pollen grains have been used in drug deliveries,21,22 in novel materials as templates,23,24 as carbon microspheres,25 in protein chromatography,26 and during removal of organic pollutants.27 The particle size of pollen grains is 30−40 μm, and the Fourier transform infrared (FT-IR) spectrum revealed the existence of numerous hydroxyl groups on the surface of pine Received: Revised: Accepted: Published: 4771
December 9, 2014 April 26, 2015 April 27, 2015 April 27, 2015 DOI: 10.1021/jf505938w J. Agric. Food Chem. 2015, 63, 4771−4776
Article
Journal of Agricultural and Food Chemistry
Figure 1. Schematic diagram for the extraction of TRA by NP-SPE. Oil (2.00 ± 0.01 g) was accurately weighed into a 10 mL volumetric flask. It was diluted to the required volume with n-hexane and shaken for 1 min. Then, 2 mL of the sample solution was loaded onto a 300 mg pollen grain SPE cartridge (sorbent materials were packed into a 3 mL polypropylene syringe and retained by two polyethylene frits), which was sequentially preconditioned with 3 mL of acetone and 2 mL of n-hexane for activation (in Figure 1). After the cartridge was washed with 3 mL of n-hexane/2-propanol (80/20, v/v), 1.5 mL of ethanol was used for elution, and the eluate was collected into a centrifuge tube. The collected fraction was evaporated to dryness under a mild nitrogen stream at 35 °C. The residue was dissolved in 200 μL of 2propanol, and 10 μL of the solution was injected into the HPLC system for analysis. Analytical Conditions. The HPLC apparatus (Shimadzu, Kyoto, Japan) used in this study consisted of binary LC-20AT pumps, a DGU-20A3 degasser, a SPD-20A ultraviolet detector, a SIL-20A autosampler, and a CTO-20AC column oven. The chromatographic data were acquired by Shimadzu LC Solution (Kyoto, Japan). The HPLC separation was accomplished on a Hisep C18-T column (150 mm × 4.6 mm i.d., 5 μm), which was obtained from Weltech Co., Ltd. (Wuhan, China). The column temperature was set at 40 °C and the injection volume was 10 μL. A mixture of acetonitrile/0.2% acetic acid in water (30/70, v/v) was used as mobile phase at a flow rate of 1 mL·min−1 in isocratic mode. The detection wavelength was set at 306 nm, and the analysis was completed within 15 min.
pollen grains. Therefore, pollen grains are a promising normalphase SPE (NP-SPE) sorbent. Recently, we reported a green method for determination of 16 plant growth regulators in fruits and vegetables by using pollen grains as SPE sorbent.28 Herein, we expand this natural sorbent to the extraction of TRA from peanut and other vegetable oils. When nonpolar solvent was used for sampling and washing, TRA in peanut oils was retained on the pollen grains as a result of the hydrogenbonding interaction, whereas the matrix of peanut oils such as neutral and nonpolar lipids were removed. Parameters that affect the extraction efficiency were investigated and discussed in detail. By coupling with HPLC-UV analysis, a rapid, simple, and convenient method for the determination of TRA in peanut oils was established.
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EXPERIMENTAL PROCEDURES
Chemicals and Materials. Pine pollen (Pinus massoniana Lamb.) was obtained from Anhui Guzhitang Edible Fungus Co., Ltd. (Anhui, China). Before use, pollen grains were placed in a thimble holder inside a Soxhlet extractor and then refluxed in methanol for 24 h to remove pigments and fats. The resultant pollen grains were dried under reduced pressure at 60 °C for 12 h. Methanol, ethanol, acetonitrile, acetone, n-hexane, and 2-propanol were purchased from Sinopharm Chemical Reagent (Shanghai, China). 2-Propanol (HPLC grade) and acetonitrile (HPLC grade) were obtained from Tedia Company Inc. (Fairfield, OH, U.S.A.). Purified water was obtained using a Millipore Milli-Q apparatus (Bedford, MA, U.S.A.). trans-Resveratrol (TRA, 99%) was purchased from Aladdin (Shanghai, China). TRA stock solution was prepared in 2-propanol (HPLC grade) at a concentration of 50 μg·mL−1 and stored at −20 °C in darkness. With the stock solution, the sample solution was spiked to the desired concentration for the following experiments. Sample Preparation. Oil samples were prepared by spiking TRA at a known concentration (25 ng·g−1) to study the SPE performance under different conditions. Various kinds of peanut oils were purchased from local markets in Wuhan (China) and stored at room temperature. One refined soybean oil sample which was detected to be free of TRA was used as blank matrix for calibration and validation purposes. Oil sample (2.0 g) was directly spiked with TRA. After mixing uniformly, the sample was diluted to 10 mL with n-hexane. Extraction Procedure. SPE was performed on a Supelco 12-port model SPE Vacuum Manifold (Bellefonte, PA, U.S.A.).
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RESULTS AND DISCUSSION Optimization of SPE Conditions. Several parameters that might affect the extraction efficiency of SPE were investigated, including sorbent amount, washing volume, and the type and volume of desorption solvent. The oily solution spiked with TRA at the concentration of 5 ng·mL−1 was used to study the SPE performance under different conditions, and a total of three replicates were performed in all optimization experiment to obtain a mean value. The extraction efficiency was calculated as the ratio of the peak area obtained in redissolved solution (2propanol) to the standard solution (2-propanol) spiked with TRA at the concentration of 50 ng·mL−1. Effect of Sorbent Amount. The amount of sorbent is an important factor influencing the extraction efficiency in SPE. Therefore, the cartridges packed with different pollen amounts from 100 to 500 mg were evaluated. The extraction efficiency increased as the amount of sorbents increased from 100 to 300 mg, and then reached a plateau as the sorbent amount further 4772
DOI: 10.1021/jf505938w J. Agric. Food Chem. 2015, 63, 4771−4776
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Journal of Agricultural and Food Chemistry
Figure 2. Influence of several parameters on the extraction efficiency: (a) sorbent amount, (b) washing solvent volume, (c) desorption solvent, and (d) desorption volume.
increased (Figure 2a). Consequently, the cartridge of 300 mg was employed in the subsequent experiment. Effect of Washing Volume. The removal of lipid which was adsorbed on pollen grain SPE cartridge is essential not only to minimize its maintenance in the chromatographic system but also to obtain low detection limits. In this work, n-hexane/2propanol (80/20, v/v) was used as washing solution, and its volume on TRA extraction efficiency was investigated. The result showed that the extraction efficiency of TRA decreased when the volume was higher than 3.5 mL (Figure 2b). Therefore, 3 mL of washing solvent was adopted to avoid significant loss of TRA in the washing step. Effect of Desorption Solvent. Four different kinds of polar solvents (methanol, ethanol, acetonitrile, and acetone) were investigated as desorption solvent. The adsorption of TRA onto pollen grains behaved as typical normal phase chromatography, and the extraction efficiency increased when more polar solvents were used. The hydroxyl-rich solvents (methanol and ethanol) showed higher extraction efficiency (Figure 2c), indicating that TRA was retained based on the hydrogen bonding interactions. Ethanol was selected as eluent for further experiments due to its low-toxin content. Effect of Desorption Volume. Desorption volumes ranging from 0.5 to 3.0 mL were investigated. The results showed that extraction efficiency for TRA reached a plateau with 1.0 mL of ethanol (Figure 2d), viz., 1.0 mL of desorption solvent was enough to elute TRA. To ensure complete desorption of TRA, 1.5 mL of ethanol was employed in the following experiment. Reusability of Pollen SPE Cartridge. The reusability of pollen grain cartridge was evaluated by repeating the extraction procedure with the same SPE cartridge. The results showed that pine pollen grains were highly stable and reusable under
the analysis conditions, with RSD of TRA recovery less than 5.0% in 10 cycles of repeated experiments (Table S1), which further demonstrated the known robust nature of the sporopollenin materials. The ability to regenerate and reuse pollen grains using common organic solvents to extract TRA from peanut oils is of value because it will reduce the overall cost and confirm its environmentally friendly merit. Comparison with Other NP-SPE Sorbents. In order to compare the extraction efficiency of TRA by this novel NP-SPE sorbent with some traditional NP-SPE sorbents, 300 mg of silica gel (∼50 μm) or aluminum oxide (neutral, ∼100 μm) was loaded in the cartridges separately. Then, three different NPSPE sorbents were compared following the same extraction protocol as in Figure 1. Each sorbent was performed in triplicate and the recoveries as well as RSD values were recorded in Table 1. When comparing sorbents purification properties, the sampling and washing effluent were collected together in a 10 mL centrifuge tube and then evaporated under a mild nitrogen stream at 35 °C to get rid of the organic solvent. By weighing the residues, we can conclude that 90% of oil matrix components were eluted out by these sorbents, which Table 1. Comparison of Purification and Extraction Efficiencies among Different NP-SPE Sorbentsa
a
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sorbent
pollen grains
aluminum oxide
silica gel
silica gel
silica gel
silica gel
amount (mg) oil removal (g) recovery% RSD%
300 0.34 103.2 7.2
300 0.32 0 /
300 0.36 4.2 6.9
400 / 5.3 4.9
500 / 5.8 2.5
600 / 7.4 4.7
The total oil sampling amount was 0.40 g. DOI: 10.1021/jf505938w J. Agric. Food Chem. 2015, 63, 4771−4776
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Journal of Agricultural and Food Chemistry
which indicated that the oil matrix can be effectively removed by pollen SPE extraction. Method Validation. Experiments with regard to linearity, limit of detection (LOD), limit of quantification (LOQ), and reproducibility were performed to validate the proposed method under optimized conditions. The linearity was studied using a blank soybean oil sample spiked with TRA at 10 different concentrations ranging from 10 to 2500 ng·g−1 (10, 15, 20, 25, 50, 100, 250, 500, 1000, and 2500 ng·g−1). In the construction of the calibration curve, triplicate measurements of each concentration level of the calibration samples were performed, and the calibration curve was obtained by plotting mean peak area versus concentrations (Figure S1). The linear range, regression data, LOD, and LOQ from the validation are listed in Table 2 (raw data in Table S3). A good linearity was obtained with r2 up to 0.9999 for the target TRA. A blank soybean oil sample spiked with TRA at 10 ng·g−1 gave a peak response with signal-to-noise of ∼11.2 (triplicate experiments with the SD of 1.0). The LOD for TRA, calculated at a signalto-noise of 3, was ∼2.7 ng·g−1 and the LOQ, calculated at a signal-to-noise of 10, was ∼8.9 ng·g−1. The reproducibility of the method was investigated by determining the intra- and interday precisions. The intra- and interday relative standard deviations (RSDs) were calculated with blank soybean oil spiked with TRA at three different concentrations (25, 100, and 250 ng·g−1). Six parallel extractions of a sample solution over 1 day provided the intraday RSDs, and interday RSDs were determined by extracting sample solutions that had been independently prepared for three continuous days. The RSD and recovery values were recorded in Table 3 (raw data in Table S4). The results showed that the finally adopted experimental protocol for the analysis of TRA is characterized by significantly high recovery values (min 93.7%) as well as low RSD values (max 5.2%). This pollen SPE method shows high accuracy (as recovery%) and well precision (as RSD%) of detection. Analysis of Real Samples. Ten peanut oil samples with different brands and another five kinds of vegetable oils were analyzed to demonstrate the applicability of the method. All samples obtained were analyzed in three replicates, and the detailed results are outlined in Table 4. The recoveries were obtained by comparing the amount of TRA calculated from the calibration curve with the corresponding spiking amount. As shown in Table 4, the recoveries of the target TRA from various real samples were in the range from 70.2 to 98.4% with RSDs less than 8.7%, indicating the good applicability of the present method in different kinds of vegetable oils. In addition, six peanut oil samples were found to contain a certain amount of TRA with the concentration ranging from 10.9 to 16.9 ng·g−1. Therefore, TRA could be potential biomarker constituents for
demonstrated that pollen grains have the similar cleanup efficiency for the samples in high-fat matrix like silica gel or aluminum oxide. When comparing retention properties of TRA on sorbents, eluate from these three sorbents was evaporated to dryness under a nitrogen stream and redissolved for HPLC analysis. The result showed that aluminum oxide has no retention for TRA, whereas the retention on silica gel was weak. The extraction efficiency did not increase significantly as the amount of silica gel sorbents increased from 300 to 600 mg, which may be ascribed to the fact that silica gel sorbents lack aromatic groups possessed by pollen grains.28 This result further proved that the selective extraction of TRA from peanut oils by pollen grains is based on the hydrogen-bond and π−π interaction. Matrix Effect of Pollen SPE. The matrix effect of the proposed method was evaluated by comparing the assay result obtained with or without oil matrix. As shown in Figure 3b−d,
Figure 3. HPLC chromatogram of the following: (a) the standard solution (2-propanol) spiked with TRA at the concentration of 50 ng· mL−1; (b) a n-hexane sample spiked with TRA at the concentration of 5 ng·mL−1; (c) a blank soybean oil sample; (d) a blank soybean oil solution spiked with TRA at the concentration of 5 ng·mL−1; (e) peanut oil sample 6; (f) peanut oil solution 6 spiked with TRA at the concentration of 10 ng·mL−1.
the chromatograms of the spiked n-hexane sample (5 ng·mL−1) and the oily solution spiked with TRA at the concentration of 5 ng·mL−1 were practically the same, which demonstrated that the assay result is not influenced by the presence of oil matrix. Besides, we compared three recoveries of TRA, with one in the standard solution (2-propanol, 50 ng·mL−1) and the other two in blank soybean oil sample spiked before (5 ng·mL−1) and after (50 ng·mL−1) pollen SPE extraction. Those three recoveries were nearly the same (RSD < 4.9%, Table S2),
Table 2. Linear Range, Regression Data, Limit of Detection (LOD), Limit of Quantification (LOQ) for the Determination of TRA in Oil Samples
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Table 3. Method Precisions (Intra- and Interday) and Recoveries at Three Different Concentrations for the Determination of TRA in Oil Samples intraday assay (n = 6)
interday assay (n = 3)
low (25 ng·g−1)
medium (100 ng·g−1)
high (250 ng·g−1)
low (25 ng·g−1)
medium (100 ng·g−1)
high (250 ng·g−1)
4.9 93.7
3.1 95.2
2.7 100.9
2.9 95.0
5.2 98.0
3.6 102.7
RSD% recovery%
Table 4. Concentrations (concn, ng·g−1), Accuracy (recovery%), and Precision (RSD%, n = 3) in Real Samplesa
a
peanut oil 1
peanut oil 2
peanut oil 3
concn (ng·g−1, RSD%, n = 3) found (ng·g−1, RSD%, n = 3) recovery%
16.6 (4.3) 63.8 (5.8) 94.3 peanut oil 6
16.9 (2.4) 63.2 (0.7) 92.7 peanut oil 7
11.0 (0.9) 56.3 (2.8) 90.5 peanut oil 8
peanut oil 4
concn (ng·g−1, RSD%, n = 3) found (ng·g−1, RSD%, n = 3) recovery%
14.2 (1.6) 53.8 (0.2) 79.1 soybean oil
N.d. 40.7 (0.1) 81.4 corn oil
N.d. 41.3 (2.5) 82.6 rapeseed oil
N.d. 46.0 (7.1) 92.1 rice bran oil
N.d. 44.3 (5.3) 88.6 flax seed oil
concn (ng·g−1, RSD%, n = 3) found (ng·g−1, RSD%, n = 3) recovery%
N.d. 49.2 (2.3) 98.4
N.d 48.0 (1.3) 96.0
N.d. 47.1 (5.1) 94.2
N.d. 44.2 (8.7) 88.5
N.d. 45.2 (7.6) 90.5
11.0 (0.8) 48.8 (0.2) 75.7 peanut oil 9
peanut oil 5 10.9 (1.8) 46.0 (2.2) 70.2 peanut oil 10
The concentrations of the spiked TRA was 50 ng·g−1. N.d., not detected.
Table 5. Comparison of Sample Preparation Procedures among Different Methods sample pretreatment (sorbents) MSPE (h-MWCNT-MNPs) SPE (Waters Oasis WCX) SPE (pollen grains)
determination
LOD (ng·g−1)
recovery% (RSD%)
advantages and drawbacks
ref
LC−MS/MS
0.60
90.0−110.0 (4.0−17.5)
16
LC−MS/MS
0.12
84.6−103.1 (1.3−2.4)
HPLC-UV
2.70
70.2−94.3 (0.2−5.8)
simple and rapid but sorbent preparation is tedious and not environmental friendly applicable and effective but the commercial sorbent is expensive and cannot be recycled simple, effective, natural sorbent, environmental friendly, regenerable and low cost
15 this work
purification of other polar analytes due to its good adsorption properties and low cost.
quality assessment of peanut oils adulterated with other vegetable oils. Figure 3e showed the HPLC chromatogram of TRA detected in peanut oil sample 6. The results confirmed the feasibility of the proposed method for determination of TRA in real samples. A comparative study of our developed pollen grains NP-SPE method for TRA analysis in peanut oils to previously reported methods was performed, and the results are presented in Table 5. Comparing to those two sorbents, these natural pollen sorbents obtained from a renewable source avoid cumbersome material synthesis steps, which fully meet the needs of green analytical chemistry. Besides, this pollen SPE cartridge can be reused for at least 10 times, which will reduce the overall cost for routine analysis. The results demonstrate that using pollen grains as constituent of SPE sorbents directly is simple, effective, eco-friendly, and inexpensive. Although the LOD in this developed methodology is not as good as other methods using LC-MS/MS for determination, the HPLC-UV detection possesses several merits like reduced cost, lower power consumption, and easier maintainability. For the next work, we will try to improve the detection sensitivity, and the related study will be conducted in our following experiment. In conclusion, using pollen grains as a novel normal-phase solid-phase extraction sorbent, coupled with HPLC-UV detection, was demonstrated to be an environmentally friendly and effective method for the determination of TRA in peanut oils. Oil samples after pretreatment can be directly injected for HPLC analysis. The results show that these natural materials have the potential to be applied for the separation and
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ASSOCIATED CONTENT
S Supporting Information *
recycling experiment data (Table S1), matrix effect experiment data (Table S2), data for calibration, LOD and LOQ (Table S3), intraday and interday assays (Table S4), calibration curve (Figure S1). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ jf505938w.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +86-27-68755595. Fax: +86-27-68755595. Author Contributions §
These authors contributed equally to this work (Q.L. and Q.Z.). Funding
This work was supported by the National Key Technologies R&D Program (2012BAK08B03) and the National Natural Science Foundation of China (21475098, 91217309). Notes
The authors declare no competing financial interest. 4775
DOI: 10.1021/jf505938w J. Agric. Food Chem. 2015, 63, 4771−4776
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ACKNOWLEDGMENTS The authors wish to thank the lab members (Zheng Zhang, Hao-Bo Zheng and Li-Jing Mao) for critiques and advice.
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DOI: 10.1021/jf505938w J. Agric. Food Chem. 2015, 63, 4771−4776
