Fabrication of Fe3O4 Nanoparticle-coalesced Hydroxylated Multi ...

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May 10, 2015 - toxicological research, clinical study, forensic analysis, fair play in sports ... A magnetic carbon nanomaterial for Fe3O4-modified hydroxylated ...



2015 © The Japan Society for Analytical Chemistry

Fabrication of Fe3O4 Nanoparticle-coalesced Hydroxylated Multiwalled Carbon Nanotubes for the Analysis of Strychnine in Human Serum Zufei FENG, Yuehong XU, Shuguang WEI, Bao ZHANG, Fanglin GUAN, and Shengbin LI† College of Forensic & Medicine, Xi’an Jiaotong University, No. 76 Yanta West Street, Xi’an 710061, P. R. China

A magnetic carbon nanomaterial for Fe3O4-modified hydroxylated multi-walled carbon nanotubes (Fe3O4-MWCNTs-OH) was prepared by the aggregating effect of Fe3O4 nanoparticles on MWCNTs-OH, and this material was combined with high-performance liquid chromatography (HPLC)/photodiode array detector (PAD) to determine strychnine in human serum samples. Some important parameters that could influence the extraction efficiency of strychnine were optimized, including the extraction time, amounts of Fe3O4-MWCNTs-OH, pH of sample solution, desorption solvent and desorption time. Under optimal conditions, the recoveries of spiked serum samples were between 98.3 and 102.7%, and the relative standard deviations (RSDs) ranged from 0.9 to 5.3%. The correlation coefficient was 0.9997. The LODs and LOQs of strychnine were 6.2 and 20.5 ng mL–1, at signal-to-noise ratios of 3 and 10, respectively. These experimental results showed that the proposed method is feasible for the analysis of strychnine in serum samples. Keywords Fe3O4 nanoparticles, hydroxylated multi-walled carbon nanotubes, strychnine, serum samples, forensic toxicological analysis, traditional Chinese medicine (Received December 4, 2014; Accepted February 25, 2015; Published May 10, 2015)

Introduction Strychnos nuxvomica Linn (family Loganiaceae) is a mediumsized tree that grows in open areas and is widely distributed throughout India and Southeast Asia.1 It is a traditional Chinese medicine that has been used in clinical practice for thousands of years.2–4 This plant contains alkaloids in the seeds that can stimulate the central nervous system and make the sensory organs more sensitive when ingested.2 For this reason, the alkaloids have also been included on the doping list of prohibited substances by the Medical Commission of the International Olympic Committee.5 Strychnine is one of these alkaloids, and it is used here as a representative alkaloid. At low doses (such as a 10 mg daily dose of strychnine), it is often used to treat nervous system diseases and vomiting, as well as arthritic and traumatic pains.3 However, strychnine is highly toxic, and the margin between therapeutic and toxic doses is very narrow. In antiquity, people discovered the lethal properties of Nuxvomica and used it as poison for arrow heads and for suicide. This virulent poison affects the central nervous system and causes acute muscle spasms. Normally, death occurs due to respiratory failure or exhaustion caused by spasms and convulsions.6,7 Strychnine is a cumulative poison, and non-fatal doses consumed over long periods can also be fatal. As reported in the literature, the lethal dose of strychnine for adults is between 50 and 100 mg, and the lethal dose for children is approximately 5 – 10 mg.8 Therefore, to ensure its safe and legal use, it is To whom correspondence should be addressed. E-mail: [email protected]

necessary to establish a simple, direct and sensitive technique to monitor trace levels of strychnine in the biofluids used for toxicological research, clinical study, forensic analysis, fair play in sports and drug abuse. Sample pretreatment plays an important role in analyzing trace levels of analyte, especially analytes in complex matrices.9,10 Carbon nanomaterials are an innovative type of solid-phase sorbent used in sample pretreatment; they include graphene, single-walled carbon nanotubes, multi-walled carbon nanotubes (MWCNTs), activated carbon, glassy carbon, fullerenes, carbon nanohorns, and carbon nanocones/disks.11 Among these materials, carbon nanotubes and graphene are considered to be the most widely used due to their typical characteristics, such as high surface-area-to-weight ratio, good physical and chemical stability, and low cost.12–14 MWCNTs exhibit excellent affinity to compounds containing polycyclic conjugated systems, which is beneficial for the formation of π–π Furthermore, stacking interactions between the CNTs.14 hydroxyl and carboxyl groups on the surfaces of MWCNTs can serve as chelating sites to form hydrogen bonds. MWCNTs can also be modified covalently or noncovalently with functional groups to meet the requirements of the extraction of specific analytes. MWCNTs have been employed in the extraction of a variety of compounds, including phenolic compounds,14,16 pharmaceuticals, pesticides,17 and inorganic ions.18 To avoid the step of separating MWCNTs from a ground substance, the reports mentioned above packed the MWCNTs into a cartridge. However, the entire process consumed a substantial amount of organic solvents and took a long time to desorb the target analytes from the sorbents. Thus, magnetic carrier technology has been introduced to overcome these drawbacks and to


Fig. 1 The chemical structure of strychnine.

improve the extraction efficiency of MWCNTs. Robinson and coworkers reported that magnetic carrier technology could eliminate tedious steps such as centrifugation and filtration during the sample pretreatment process, as first reported in 1973.19 Due to their hydrophilic characteristics, MWCNTs are easy to disperse into a sample solution. As a result, the MWCNTs can accelerate the mass transfer of target analytes from a sample solution onto themselves and thus enhance extraction efficiency. When extraction is finished, MWCNTs carrying the target analyte can be collected from the sample solution under an external magnetic field, which is enabled by the magnetic action of magnetic nanoparticles (MNPs; Fe3O4 or γ-Fe2O3) as carriers on the MWCNTs. Several studies have reported the preparation of magnetic MWCNTs. Researchers have attached MNPs onto MWCNT surfaces by linking molecules or via electrostatic self-assembly. Sol-gel technology has also been used to composite MNPs and hydroxylated MWCNTs. The most promising method is the in situ mode, which is particularly suitable for the synthesis of MNP sorbents and carboxylated MWCNTs.11 Because of these advantages, magnetic MWCNT hybrids have been used in various fields, including biological analysis, food, and environmental water analysis.20–22 However, the use of magnetic MWCNTs in forensic toxicological analysis has seldom been reported. In the present study, a simple method of chemical coprecipitation was used to fabricate Fe3O4-enclosed hydroxylated MWCNTs (Fe3O4-MWCNTs-OH). During the synthesis, Fe3O4 nanoparticles were coalesced onto MWCNTsOH by means of their aggregating effects. The synthesized functional magnetic nanomaterials possessed superparamagnetic characteristics, which indicated that the aggregation phenomenon would not occur in cases of repeated use. The synthesized sorbents combined with HPLC/PAD have been successfully applied to the analysis of strychnine in human serum samples. The extraction and desorption conditions were optimized for the target analyte. It was found that the use of Fe3O4-MWCNTsOH nanoparticles within a complicated matrix is feasible. Thus, Fe3O4-MWCNTs-OH is a promising and novel solid-phase extraction (SPE) material for sample pretreatment in forensic toxicological analysis.

Experimental Reagents and materials A strychnine standard was purchased from Meilun Biology Technology Co., Ltd. (Dalian, China) (Fig. 1). Hydroxylated MWCNTs (o.d., 30 – 50 nm; –OH content, 1.06 wt%; length, ~20 μm; purity > 95%) were purchased from Chengdu Organic

ANALYTICAL SCIENCES MAY 2015, VOL. 31 Chemical Co., Ltd. (Chengdu, China). Acetonitrile and methanol (HPLC grade) were supplied by Guanghua Sci-Tech Co., Ltd. (Guangzhou, China). Ultrapure water was prepared by an ultrapurification system. Acetone, triethylamine, sodium chloride, phosphoric acid, ammonium hydroxide, and sodium hydroxide were of analytical grade and were purchased from Guanghua Sci-Tech Co., Ltd. (Guangzhou, China). Ferric chloride (FeCl3·6H2O) and ferrous chloride (FeCl2·4H2O) were purchased from Fuchen Chemical Reagents Factory (Tianjin, China). Serum samples were collected from healthy volunteers and provided by the Forensic Center of Xi’an Jiaotong University. This study was reviewed and approved by the Laboratory Human/Animal Care Committee, Xi’an Jiaotong University. All volunteers received an informed consent statement, and signed an informed consent form (ICF). The ICFs have been documented by the Laboratory Human/Animal Care Committee. All clinical investigations were conducted according to the principles expressed in the Declaration of Helsinki. Instrumentation The HPLC analysis was performed in a WATERS Series (WATERS Technologies, Milford, MA) LC system, which was equipped with an e2695 Alliance Quaternary Pump, a 2998 Photodiode Array Detector (PAD), an Alliance Col Heater column oven and an automatic sampler. The system was controlled by an Empower 2 Personal Single System. Chromatographic separation was performed with a Sino-Chrom ODS-AP column (5 μm, 230 mm × 4.6 mm) (Dalian Elite Analytical Instruments Co., Ltd., Dalian, China). The mobile phase consisted of 0.1% formic acid aqueous solution containing 0.05% triethylamine (A) and water (B) in a ratio of 28:72 (v/v) with a flow rate of 1.0 mL min–1. The detection wavelength and column temperature were set at 260 nm and 28° C, respectively. The flow rate was 1.0 mL min–1, and the loading volume was 20 μL. The pH measurements were performed with a Model FE20 Plus pH meter (Mettler-Toledo, Shanghai, China) equipped with InLab® Micro pH combination electrodes (Mettler-Toledo, Switzerland). The PALL ultrapure water systems were supplied by ELGA LabWater Instrument Co., Ltd. (Bucks, UK). X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Advance (XRD, Bruker, Germany). Transmission electron micrographs were obtained from a Hitachi H-7650 transmission electron microscope (TEM, Hitachi, Japan). The magnetic properties were measured by a Lake Shore 7307 vibrating sample magnetometer (VSM) (LakeShore, USA). Preparation of Fe3O4 MNPs and Fe3O4-MWCNT-OH The Fe3O4 MNPs were synthesized by a slightly modified chemical co-precipitation method in which 5.11 g FeCl3·6H2O and 1.83 g FeCl2·4H2O were dissolved in 80 mL of deoxygenated water. The solution was constantly stirred in a 250 mL threenecked flask. When the temperature increased to 80° C, 60 mL of 5% ammonium hydroxide solution was added dropwise, and the mixture was stirred vigorously for 60 min. The entire reaction process was performed under nitrogen gas protection. When the reaction finished, Fe3O4 MNPs were collected with a magnet and washed several times with deionized water. The MWCNTs-OH coalesced with Fe3O4 MNPs and was synthesized under the same reaction conditions. The additional amounts of FeCl3·6H2O, FeCl2·4H2O and MWCNTs-OH were adjusted to 0.1257, 0.0470 and 0.2041 g, respectively. After the reaction, the black product precipitate was separated from the reaction medium via centrifugation at 2000 rpm and washed with deionized water several times until the pH level of the



Fig. 2 The experimental process of the Fe3O4-MWCNTs-OH SPE method.

washings became neutral. Preparation of standard solution A standard stock solution of strychnine (410 μg mL–1) was prepared in methanol and then diluted into a series of concentrations. The solution was stored at 4° C. The standard solution of 400 ng mL–1 was considered to be the working solution. Extraction procedures The extraction procedures were performed as follows (Fig. 2): Preparation of sample solution. An accurate volume of 1 mL of serum was placed into a 1.5 mL Eppendorf tube. Then, 20 μL of working solution was seriatim spiked into the serum. Extraction. Exactly 0.14 mL of Fe3O4-MWCNTs-OH suspension (24.1 mg mL–1) was added to the sample solution. The mixture was shaken gently for 30 s and then kept undisturbed for 30 min to achieve the adsorption equilibrium of the target analyte between Fe3O4-MWCNTs-OH and the sample solution. Separation. A magnet was deposited outside the bottom of the Eppendorf tube for 30 s to separate Fe3O4-MWCNTs-OH sorbents from the sample solution. Then, the supernatant was decanted. Desorption. The target analyte was eluted from the Fe3O4MWCNTs-OH sorbents with 0.1 mL of desorption solvent for 20 min, repeated twice. Then, the combined eluents were transferred to a 1.5 mL Eppendorf tube. Finally, the desorption solution was concentrated to dryness under nitrogen protection. Analysis. The residue in the Eppendorf tube was redissolved with 50 μL of acetonitrile, 20 μL of which was injected into an HPLC system for analysis.

Results and Discussion Characterization of the Fe3O4-MWCNTs-OH and Fe3O4 MNPs A TEM image (Fig. 3B) of the Fe3O4-MWCNTs-OH shows that the MWCNTs-OH is coalesced by Fe3O4 nanoparticles due to the aggregation of Fe3O4 nanoparticles during the reaction. The mean diameters of the Fe3O4 nanoparticles were mainly distributed between 10 and 20 nm (Fig. 3). The magnetic properties of the Fe3O4-MWCNTs-OH sorbents and Fe3O4 MNPs were characterized by VSM. Figure 4 indicates that the maximum saturation magnetization (Ms) values of the Fe3O4 MNPs and Fe3O4-MWCNTs-OH sorbents were 67.34 and 4.88 emu g–1, respectively. An external magnet can separate the sorbent from the solution rapidly under these Ms values. The Ms value decreased because of the relatively low amounts of Fe3O4 MNPs loading on MWCNTs-OH. The results show that Fe3O4-MWCNTs-OH sorbents and Fe3O4 MNPs displayed typical superparamagnetic behaviors, which avoided the aggregation. Figure 5 shows the XRD patterns of the Fe3O4-MWCNTs-OH sorbents and pure Fe3O4 MNPs. Six characteristic peaks for Fe3O4, recognized by their indices (220), (311), (400), (422), (511) and (440), and two characteristic peaks for MWCNTsOH, identified by their indices (002) and (110), were obtained from the graph of Fe3O4-MWCNTs-OH sorbents. These peaks are consistent with the database JCPDS file (PDF No. 65-3107). These results prove that the Fe3O4-MWCNTs-OH sorbents are composed of Fe3O4 and MWCNTs-OH. Optimization of the extraction process To achieve the best extraction efficiency of the target analyte, several conditions were optimized, including the amount of Fe3O4-MWCNTs-OH, pH value, extraction time, desorption solvent and desorption time.


Fig. 3 TEM image of pure Fe3O4 nanoparicles (A) and Fe3O4MWCNTs-OH sorbents (B).


Fig. 4 VSM magnetization curves of pure Fe3O4 (A) and Fe3O4MWCNTs-OH sorbents (B).

Effect of the amount of Fe3O4-MWCNTs-OH The amount of Fe3O4-MWCNT-OH could directly affect the extraction efficiency of the target analytes. Consequently, amounts of 0.48, 1.45, 2.41, 3.38, 4.35 and 4.83 mg were investigated. Figure 6A shows that the extraction efficiency of the target analyte increased with the addition of 0.48 – 4.83 mg of Fe3O4-MWCNTs-OH. However, increased amounts from 3.38 to 4.83 mg did not significantly affect the extraction efficiency of the target analyte. Extraction attained a maximum plateau value at 3.38 mg. As a result, 3.38 mg was chosen as the optimal amount of Fe3O4-MWCNTs-OH. Effect of the pH level of the sample solution In solid-phase extraction, mass transfer is promoted by optimal pH conditions in the sample solution. As analytes are absorbed by MWCNTs via the main interactions, such as hydrophobic and π–π interactions, the pH value of the sample solution should suppress analyte ionization to keep them in molecular form. Considering that strychnine is a basic compound, a base such as sodium hydroxide should be added to raise the pH of the sample solution above 7. In the present study, the extraction was performed and investigated under different pH conditions ranging from 7 to 14. As shown in Fig. 6B, the extraction efficiency of the target analyte increased when the pH level was increased. This variation with pH is due to the increase in the hydrophobic effects of the target analyte. As a result, the maximum extraction efficiency value was obtained at pH 14,

Fig. 5 XRD patterns for the pure Fe3O4 (A), Fe3O4-MWCNTs-OH sorbents (B) and pure MWCNTs-OH (C).

which was used for the subsequent experiments. Effect of extraction time The extraction time plays an important role in the extraction efficiency of the target analyte. It takes several minutes to reach the adsorption equilibrium of the target analyte between the Fe3O4-MWCNTs-OH sorbents and the sample solution. As a



Fig. 6 The optimization of the extraction process. Blank serum sample, 0.1 mL; aqueous solution, 1.0 mL; spiked working solution, 20 μL; desorption solvent, 0.1 mL acetonitrile, twice; desorption time, 20 min. (A) extraction time, 30 min; (B) Fe3O4-MWCNTs-OH amount, 3.38 mg; (C) effect of solution pH; (D) effect of desorption solvent; (E) effect of desorption time.

result, the adsorption process achieves its maximum extraction efficiency. Thus, extraction times within 5 – 60 min were investigated (Fig. 6C). The maximum extraction efficiency was achieved at 30 min. However, from 30 to 60 min, the extraction efficiency could not significantly increase, indicating that the adsorption had attained equilibrium. Thus, 30 min was sufficient to complete the extraction process. Effects of the desorption solvent and desorption time To elute the target analytes that adsorbed onto the Fe3O4MWCNTs-OH sorbents, methanol, acetone, and acetonitrile were optimized as the common organic elution solvents. Figure 6D shows that acetonitrile had the best desorption ability for target analytes. Then, the elution time was optimized. The results indicate that a two-time elution process was

sufficient with 0.1 mL of acetonitrile. To obtain the desorption profile of the target analyte, 5, 10, 20, 30, 45, and 60 min for desorption were evaluated. Figure 6E shows that 20 min was enough to accomplish one desorption process. Therefore, the desorption time was set at 20 min. Optimization of analytical parameters Under optimal conditions, a series of experimental parameters, including linear range, correlation coefficients, precision, limits of detection (LODs), and limits of quantification (LOQs), were optimized (Table 1). The linear range of the calibration curve was 20.5 – 410 ng mL–1. The regression coefficient (r2) of strychnine was 0.9997, and the LODs and LOQs were the concentrations corresponding to signal-to-noise ratios of 3 and 10, respectively. The LODs and LOQs of strychnine were 6.2



Table 1 Performance parameters of the method


Regression equation



y = 39486x – 27.758

0.999 7


Liner range/ ng mL–1

LODs/ ng mL–1

LOQs/ ng mL–1

20.5 – 410



RSD, % Intra-day (n = 5)

Inter-day (n = 15)



Table 2 Accuracy of the method (relative recoveries with their RSDs) for sample solution spiked at different concentrations (n = 5) Analyte strychnine

Added/ ng mL–1

Found/ ng mL–1

Recovery, %a

RSD, %

41 205 820

42.1 197.3 806

102.7 96.2 98.3

5.3 1.4 0.9

a. [(Found-base)/added]× 100.

Fig. 8 HPLC chromatogram of the two sorbents’ desorption solution: (A)-modified CNTs; (B)-pure CNTs (blank serum sample, 0.1 mL; aqueous solution, 1.0 mL; spiked working solution, 20 μL; extraction time, 30 min; sorbents, 3.38 mg; pH = 14; desorption solvent, 0.1 mL acetonitrile, twice; desorption time, 20 min).

Fig. 7 Chromatogram of serum samples spiked with strychnine working solution extracted by Fe3O4-MWCNTs-OH sorbents at optimum conditions (A), blank serum sample (B), and strychnine working solution (C).

and 20.5 ng mL–1, respectively. The precision of the proposed method was evaluated by measuring intra- and inter-day relative standard deviations (RSDs) at concentrations of 410 ng mL–1. The intra- and inter-day precisions were 3.48 and 3.65%, respectively. These results indicated that the new analytical method was reliable and sensitive. Analysis of real biological samples Under the optimized conditions, the feasibility of the proposed method was validated using a recovery test. The accuracy of the proposed procedure was also evaluated. Strychnine was spiked into blank serum sample solutions at three concentrations to determine the extraction efficiency of the proposed method, as well as the effects of the serum matrix on the analysis of the target analyte. As listed in Table 2, the recoveries were between 98.3 and 102.7%, with RSDs ranging from 0.9 to 5.3%. The satisfactory recoveries indicated that Fe3O4-MWCNTs-OH sorbents are feasible for the analysis of strychnine in serum samples. Furthermore, no matrix interference was found in the chromatogram (Fig. 7).

Evaluation on extraction efficiency of pure MWCNTs and modified MWCNTs In order to determine that whether the modification reaction affected the extraction efficiency of MWCNTs, a controlled experiment was carried out. Pure MWCNTs and modified MWCNTs were used as the sorbent under the optimized condition, respectively. Figure 8 shows the HPLC results of the two sorbents’ desorption solution. The areas of the strychnine peaks are 628 for MWCNTs and 612 for modified MWCNTs. The very similar results demonstrate that the modification had no significant effect on the adsorption efficiency of MWCNTs. This result is easy to understand. The modification is performed to introduce the Fe3O4 onto MWCNTs’ surface by means of the chemical coprecipitation method. Only a few negligible hydrogen bonding sites are shielded. So the modification will not affect the function of MWCNTs. On the other hand, Fig. 5 shows that the presence of crystal planes with cubic crystal structures proves that the magnetic nanoparticles and MWCNTs were composed of Fe3O4. The crystalline structure of the Fe3O4 nanoparticles and MWCNTs was essentially maintained. As a result, the modification resulted in no significant damage to the structure and function of both Fe3O4 and MWCNTs. Comparison of published methods to determine the analytes In recent years, some studies have reported methods to determine strychnine. They can be divided according to two respects. One is the modification of the extraction method. As reported by Song et al.,23 a novel method for the determination of strychnine and brucine used CNTs-HF and result in ideal LOQs and recovery rate. Compared with that study, the current work



Table 3 Comparison of some methods used for determination of strychnine No.


Extraction method


LOD/ng mL–1

Recovery, %

Total time/min


1 2 3 4 5

Seeds Blood Serum Urine Serum



4.6 × 10–7 M 6.83 9.1 0.7 6.2

— 54 62.4 94.28 99.06

>60 >50 — 95 70

27 28 24 23 This one

has some advantages mentioned below. First, Song’s method included several laborious procedures. The complete process needs at least 95 min. Another significant drawback is that the CNTs-HF is non-renewable. To eliminate the possible carryover effect, fresh CNTs–HF should be used in each experiment. Compared with CNTs-HF, the current reported material can be collected by the extra magnetic field, and the centrifugal or filtering process can be eliminated. Moreover, it is recyclable. Second, the aggregation of CNTs is a serious problem that would affect the efficiency of extraction. Song’s study investigated surfactant factors that would affect the dispersing of CNTs. However, in this current research, the magnetic CNTs would not aggregate due to its superparamagnetism. When the extra magnetic field exists, the Fe3O4-CNTs will be collected easily; when without extra magnetism, they are well distributed in the solvent. Third, although the toxicity of 1-octanol is low, it is still irritating to the skin and eyes. This is also an existing problem. The current study just uses conventional solvent. Another study24 reported a method that simultaneously determined 13 plant alkaloids in a human specimen by HPLCPDA. This method focused on chromatographic separation and a quantitative methodology. The SPE procedure used a commercial column (CHROMABOND C18ec cartridges, 3 mL, Macherey, Nagel, Germany). This method is suitable for the simultaneous enrichment of the reported 13 alkaloids. However, the current article focuses on strychnine. The SPE material is magnetically modified. As a result, the material can be collected by the extra magnetic field, and the centrifugal or filtering process can be eliminated. So this method will save a lot of time in the entire experiment. This material also has another significant merit — it is recyclable. The other aspect of recent studies on the determination of strychnine focused on to the detector. Some studies reported several MS methods to detect aim compounds.25,26 So the current study sought an easy applicable alternative (PDA) to the sophisticated HPLC-tandem MS or GC-MS method in view of the limited financial resources of many laboratories. Table 3 describes the methods used to determine strychnine in several matrices in recent years. In comparison, even though the LOD is not the best, this novel method still has the advantages of improved simplicity, good recovery, low cost, recyclable material and feasible conversion into green analytical techniques.

Conclusions A magnetic nanomaterial consisting of Fe3O4-coalesced hydroxylated MWCNTs was fabricated by aggregating Fe3O4 around MWCNTs-OH. The resulting material was successfully applied for the analysis of strychnine in human serum samples. Under the optimized conditions, the use of Fe3O4-MWCNTsOH sorbents achieved the best affinity to the target analyte. This proposed method also has the advantages of rapidity, simplicity, ease of operation, and environmental friendliness.

Good recoveries and precisions were obtained, which indicated that Fe3O4-MWCNTs-OH sorbents have considerable potential in the pretreatment of biological samples. Therefore, Fe3O4MWCNTs-OH sorbents are potential materials for SPE in the sample pretreatment process in forensic toxicological analysis.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 81302619) and China Postdoctoral Science Foundation (No. 2013M532057).

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