Using bamboo charcoal as solid-phase extraction ... - Springer Link

Anal Bioanal Chem (2008) 390:1671–1676 DOI 10.1007/s00216-008-1859-5

ORIGINAL PAPER

Using bamboo charcoal as solid-phase extraction adsorbent for the ultratrace-level determination of perfluorooctanoic acid in water samples by high-performance liquid chromatography–mass spectrometry Ru-Song Zhao & Xia Wang & Xu Wang & Jin-Ming Lin & Jin-Peng Yuan & Li-Zong Chen

Received: 26 September 2007 / Revised: 15 December 2007 / Accepted: 9 January 2008 / Published online: 7 February 2008 # Springer-Verlag 2008

Abstract In recent years, bamboo charcoal, a new kind of material with special microporous and biological characteristics, has attracted great attention in many application fields. In this paper, the potential of bamboo charcoal to act as a solid-phase extraction (SPE) adsorbent for the enrichment of the environmental pollutant perfluorooctanoic acid, which is one of the newest types of persistent organic pollutants in the environment, has been investigated. Important factors that may influence the enrichment efficiency—such as the eluent and its volume, the flow rate of the sample, the pH of the sample and the sample volume—were investigated and optimized in detail. Under the optimum conditions, the limit of detection for PFOA was 0.2 ng L−1. The experimental results indicated that this approach gives good linearity (R2 =0.9995) over the range 1–1000 ng L−1 and good reproducibility, with a relative standard deviation of 4.0% (n=5). The proposed method has been applied to the analysis of real water samples, and satisfactory results were obtained. The average spiked recoveries were in the range 79.5∼118.3 %. All of the results indicate that the proposed method could be used for the determination of PFOA at ultratrace levels in water samples. R.-S. Zhao (*) : X. Wang : J.-P. Yuan : L.-Z. Chen Key laboratory for applied technology of sophisticated analytical instruments of Shandong province, Analysis and Test Center, Shandong Academy of Sciences, Jinan, Shandong 250014, China e-mail: [email protected] R.-S. Zhao : X. Wang : J.-M. Lin (*) Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China e-mail: [email protected]

Keywords Perfluorooctanoic acid . Bamboo charcoal . Solid-phase extraction . High-performance liquid chromatography–mass spectrometry

Introduction Perfluorinated surfactants have emerged as priority environmental contaminants due to recent reports of their detection in environmental and biological matrices as well as concerns over their persistence and toxicity [1]. Perfluorooctanoic acid (PFOA) is just one of several compounds that possess the unique chemistry of fluorine, which allows them to repel both water and lipids [2]. There are a number of possible sources of PFOA in the aquatic environment, including firefighting foams, the combustion of fluoropolymers such as Teflon, precursor compounds such as fluorotelomer alcohols, and releases from production facilities [1–8]. This compound has attracted attention due to its extreme stability under a variety of environmental conditions and its widespread distribution in various environmental and biotic matrices [1, 2, 9–11]. Because environmental concentrations of PFOA in water samples usually range from the ng/L to low μg/L levels, a rapid, simple and sensitive sample pretreatment method is urgently required. Chromatographic techniques, including gas chromatography (GC) and high-performance liquid chromatography (HPLC), have long been the most important methods for determining PFOA. Gas chromatographic methods were developed based on the reaction of PFOA with derivatizing reagents to form methyl esters for chromatographic separation and detection, because it is a compound with strong polarity [12, 13]. More recently, because high-performance

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liquid chromatography/mass spectrometry (HPLC/MS) does not need prior derivatization, it has been shown to be superior to GC/MS, and is a frequently reported technique for the measurement of PFOA [1, 9–11]. The commonly used sample preparation methods for the chromatographic determination of PFOA include liquid– liquid extraction (LLE) [12], solid-phase extraction (SPE) [10], and ultrasonic solvent extraction (USE) [11]. Among the above mentioned sample pretreatment methods, solidphase extraction is the most common technique for environmental water sample pretreatment because it has many obvious advantages compared with LLE and USE, such as high recovery, high preconcentration factors, low consumption of organic solvents, simplicity, easy automation and operation, and so on [14, 15]. In the SPE procedure, the choice of adsorbent is the most important factor for obtaining high analyte enrichment efficiency [14]. Several types of solid-phase extraction adsorbents, such as cetyltrimethylammonium bromide-coated silica and sodium dodecyl sulfate (SDS)-coated alumina [15], C18-bonded silica [16] and activated carbons [16], have been used as SPE adsorbents for the pretreatment of PFOA in water samples. Bamboo charcoal, or bamboo carbon, has attracted great attention in recent years because of its special microporous and biological characteristics [17]. It has been applied for many roles in many fields, such as to supply negative ions, to emit far-infrared rays, to prevent oxidation, to remove microbes from water, as a humidity regulator and a rich source of minerals, and so on [18]. The process behind the production of bamboo charcoal-making is relatively straightforward. Pieces of bamboo taken from plants that are at least five years old are burned inside an oven at temperatures of over 800 °C [18]. Bamboo charcoal burnt at high temperatures exhibits properties that are vastly different from the bamboo plant itself, including a high density and a porous structure [19]. Figure 1 shows a picture of bamboo charcoal under Electron Probe X-ray

Fig. 1 Bamboo charcoal under Electron Probe X-ray Microanalyzer

Anal Bioanal Chem (2008) 390:1671–1676

Microanalyzer. Compared with wood charcoal, bamboo charcoal has about four times more cavities, three times more mineral content and a fourfold better absorption rate. In terms of surface area, bamboo charcoal (300 m2/gram) is ten times greater than wood charcoal (30 m2/gram) [18]. The extremely large surface area and the unique microporous structure make bamboo charcoal a promising adsorbent material. Xiao et al. studied the adsorption of the dye reactive brilliant red X-3B using bamboo charcoal as adsorbent [20]; Zhang and Wang found that bamboo charcoal is a good absorptive material for removing lead (II) ions and fluoride ions from water solution [21, 22]. Ye et al. also studied the adsorption performance of bamboo charcoal for phenol in aqueous solution [23]. All of the applications of bamboo charcoal mentioned above mainly focus on the removal of environmental pollutants from water samples, and not on the enrichment and determination of pollutants at trace and ultratrace levels in water samples. At the same time, they also reflect the fact that bamboo charcoal may have great potential as an efficient solid-phase extraction adsorbent for some pollutants in environmental waters. To our knowledge, there are no reports on the enrichment and determination of some pollutants in aqueous samples using bamboo charcoal as a solid-phase extraction adsorbent. In the present paper, the main goal was to investigate the enrichment potential of bamboo charcoal as a solid-phase extraction adsorbent for the determination of PFOA in water samples. The important factors affecting the performance of SPE such as the eluent and its volume, the flow rate of the sample, the pH of the sample, and the sample volume have been studied and optimized in detail.

Experimental Reagents and materials HPLC-grade methanol and acetonitrile were purchased from Tedia Company Inc. (Fairfield, OH, USA). Dichloromethane and hexane (95%) used for organic residue analysis were ultra resi-analyzed grade and purchased from Mallinckrodt Baker Inc. (Phillipsburg, NJ, USA). PFOA (96%) standard was purchased from Aldrich Chemical Co. (New Jersey, USA). A standard stock solution (2000 mg L−1) of PFOA was prepared by dissolving 0.020 g of the compound in 10 ml methanol. The solution was further diluted to 2.0 mg L−1 with methanol and stored at 4 °C. Fresh working solutions were prepared daily by appropriate dilution of the stock solutions with purified water. Bamboo charcoal (Quzhou Minxin Charcoal Company, Zhejiang, China), sold as an indoor air freshener, was purchased from a local market. Before using it to develop


the SPE method, it was triturated and sieved through a 80 mesh sieve, and then dried at 80 °C for 2 h. Apparatus A bamboo charcoal-packed cartridge was prepared by modifying an Agilent AccuBond SPE ENV PS DVB cartridge (1000 mg, 6 mL, polypropylene) [14], which was purchased from Agilent Technologies (Palo Alto, CA, USA). The PS DVB packing was removed, and 1.0 g of bamboo charcoal was packed into the cartridge. The polypropylene upper frit was reset at the upper end of the cartridge to hold the bamboo charcoal packing in place. Then the outlet tip of the cartridge was connected to a SHBIII vacuum pump (Great Wall Scientific and Trade Co. Ltd., Zhengzhou, Henan), and the inlet end of cartridge was connected to a PTFE suction tube, the other end of which was inserted into the sample solution. In order to reduce interferences from organic contaminants, the entire SPE assembly was washed with 50 mL methanol and enough purified water before it was first used. The high-performance liquid chromatography–mass spectrometry (HPLC/MS) equipment used was an Agilent 1100 LC system, including an electrospray ionization mass spectrometer (ESI/MS), a quaternary pump, a column thermostat and an automatic sample injector with a 100-μL loop. A personal computer equipped with the Agilent ChemStation program for HPLC was used to process the chromatographic data. A 250 mm×4.6 mm I.D. Agilent Zorbax Eclipse XDB-C18 column (made in the USA, 5-μm particles) was used to analyze the PFOA. HPLC/MS determination The chromatographic separation was performed with a mobile phase consisting of 50% acetonitrile and 50% water with 0.1% acetic acid (V/V) and 0.2% ammonium acetate (W/V). The flow rate of the mobile phase was controlled at 0.5 mL min−1. Under these chromatographic conditions, one HPLC run can be finished within 10 min for PFOA. MS conditions were maintained as follows: drying gas 10 L/min; neb. pressure 30 psi; capcur. 39 nA; quad. temperature 99 °C; highvac. 1.1×10−5 Tor; roughvac −3.3 Tor. PFOA analysis was carried out in “negative ionization mode” and quantification was performed under SIM conditions (m/z 413). Solid-phase extraction The bamboo charcoal-packed cartridge was pretreated by washing with 10 mL methanol and 10 mL purified water prior to each extraction procedure. Then 200 mL of a purified water sample spiked with PFOA was passed

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through the pretreated cartridge at the maximum flow rate. After the sample had passed through, the cartridge was washed with 10 mL of purified water to remove coadsorbed matrix materials from the cartridge. Then the bamboo charcoal column was dried by negative pressure for 20 min. Subsequently, the PFOA retained on the SPE cartridge was eluted with an optimal volume of methanol of 12 mL and the resulting eluate was blown to 1.0 mL with a gentle N2 flow at 40 °C. Finally, the extract was analyzed by HPLC/MS with an injection of 100 μL. Water samples In this experiment, two real-world environmental water samples of tap water and rainwater were used to evaluate the developed method. The tap water sample was taken from a water tap after it had been flowing for 10 min in our laboratory. The rainwater sample was collected in Jinan, Shandong Province, in July 18, 2007. Prior to analysis, the environmental water samples were filtered through 0.45 μm micropore membranes and stored in brown glass bottles at 4 °C.

Results and discussion Optimization of the SPE procedure In general, several parameters such as the eluent and its volume, the flow rate of the sample, the pH of the sample, and the sample volume have obvious influences on the enrichment performance of SPE. To evaluate the enrichment potential of bamboo charcoal as a SPE sorbent for PFOA, these parameters were optimized and investigated in detail. Selection of the eluent The eluent is one of the most important factors in the sample preconcentration procedure because it is directly related to the desorption efficiency of target compounds on the bamboo charcoal sorbents. In this experiment, four solvents differing in polarity—methanol, acetonitrile, dichloromethane and hexane—were tested in order to discern the most effective eluent. The extraction experiments were examined by monitoring the differences in peak area for the four solvents under the following conditions: volume of sample, 200 ml; concentration of PFOA in sample, 100 ng L−1; pH of sample, 7; flow rate of sample, 5 ml min−1; volume of eluent, 12 ml. Experimental data from the trials indicated that methanol and acetonitrile have almost the same eluent efficiency, and they gave the best desorption efficiencies. The main reason for this is that PFOA is a compound with strong polarity, and the polarities

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Anal Bioanal Chem (2008) 390:1671–1676 120 100

Recovery (%)

of the four solvents take the following order: methanol (6.60) >acetonitrile (6.20) >dichloromethane (3.40) >hexane (0.06); thus, according to the principle of “like dissolves like,” methanol and acetonitrile should have the most effective influence. However, using acetonitrile as the eluent results in poor separation from interferences in the chromatographic analysis. Therefore, methanol was chosen as the eluent in subsequent work.

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Influence of the eluent volume The amount of eluent also plays an important role in the process of eluting target compounds from the SPE column. In order to obtain the most suitable eluent volume, a series of experiments was designed and performed based on changing the volume of the eluent (methanol) from 4 to 12 mL. During the experiments, another 10 mL of methanol and 10 mL of purified water was passed through the bamboo charcoalpacked cartridge before the next SPE extraction in order to remove possible PFOA residues. The influence of the eluent volume was studied by using 200 ml of purified water (pH 7.0) spiked with 100 ng L−1 PFOA, and the flow rate of the sample was adjusted to 5 ml min−1. The recoveries obtained for PFOA with different elution volumes are shown in Fig. 2. According to Fig. 2, the recoveries increased as the volumes of methanol increased from 4 to 10 ml. For elution volumes of more than 10 ml, however, the recoveries remain constant. Therefore, in all subsequent optimized experiments, 12 ml of methanol were used. Influence of flow rate of the sample The flow rate of the sample is another factor that affects both the retention of analytes on the SPE cartridge and the loading time [14]. In order to save analytical time and obtain satisfactory results, the flow rate of the sample was investigated and optimized in the range of 2∼5 mL min−1.

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Flow rate (mL/min) Fig. 3 Influence of the flow rate on the recovery of PFOA. Conditions: volume of sample, 200 ml; concentration of PFOA in sample, 100 ng L−1; pH of sample, 7; eluent was 12 ml methanol

Because of the limits on instrumental conditions, the highest flow rate of the sample tested was 5 mL min−1; flow rates of more than 5 mL min−1 were not considered. In the experiments, the extraction conditions were maintained as follows: volume of samples, 200 ml; concentration of sample, 100 ng L−1; pH of sample, 7; 12 ml of eluent (methanol). The experimental results are shown in Fig. 3. From Fig. 3, it was found that for a flow rate of 2∼5 mL min−1 the recoveries were between 92.9 and 120.5% and were almost constant, which satisfies our analytical needs. Based on the above considerations, a sample flow rate of 5 mL min−1 was used in subsequent experiments for PFOA. Influence of pH of the sample The pH is a crucial parameter in the SPE preconcentration procedure, and it was investigated over the range of 3.0∼11.0. The experimental results are shown in Fig. 4. From Fig. 4, it is obvious that when the sample pH was equal to or lower than 7.0, the recoveries of PFOA were more than 80%. Recoveries of PFOA decreased with increasing pH, and when the working solution pH was equal 120

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Fig. 2 Influence of the eluent volume on the recovery of PFOA. Conditions: volume of sample, 200 ml; concentration of PFOA in sample, 100 ng L−1; pH of sample, 7; flow rate of sample, 5 ml min−1; eluent, methanol

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pH Fig. 4 Influence of the pH on the recovery of PFOA. Conditions: volume of sample, 200 ml; concentration of PFOA in sample, 100 ng L−1; eluent was 12 ml of methanol, flow rate of sample, 5 ml min−1


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of eluent, 12 ml. The sample volume was changed from 200 to 1000 mL. The results are shown in Fig. 5, and it was clear that almost no variation in recovery occurred when the sample volume ranged from 200 to 1000 mL; all recoveries were close to 100%. However, the sample volume can not only affect the retentions of the targeted compounds, but it can also control the time of analysis. Since it does not seem practical to wait for long periods, the sample volume should be just large enough to satisfy the demands of the PFOA analysis. Thus, the sample volume was fixed at 200 mL in the following experiments.

Fig. 5 Influence of the sample volume on the recovery of PFOA. Conditions: concentration of PFOA in sample, 100 ng L−1; eluent was 12 ml of methanol, flow rate of sample, 5 ml min−1; pH of sample, 3

Linear range, limit of detection, and repeatability

to 11.0 the recovery of PFOA decreased to 10.2%. A possible reason for this is that the pH of the working solution affects the forms of PFOA that exist in aqueous samples; under acidic conditions, PFOA mainly exists as the unionized acid form, while under basic conditions, PFOA is mainly ionized, which will further improve its solubility in the water samples. These results demonstrated that PFOA can be effectively adsorbed onto a bamboo charcoal-packed cartridge under acidic conditions, especially under strongly acidic conditions. However, considering the chromatographic conditions applied, the working solution was adjusted to pH 3.0, and solutions with pH