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Natural cotton fibers as adsorbent for solid-phase extraction of polycyclic aromatic hydrocarbons in water samples† Jianping Wang, Shengquan Liu, Chunyan Chen, Ying Zou, Huiping Hu, Qingyun Cai* and Shouzhuo Yao* A natural material, cotton fiber, has been applied as a solid-phase extraction (SPE) adsorbent for sample preparation for the analysis of polycyclic aromatic hydrocarbons (PAH) in water samples using highperformance liquid chromatography. The cotton fiber was used directly without any chemical modifications, which avoided a complex synthesis process and consumption of a large volume of organic solvent. The conditions affecting the extraction efficiency were optimized to achieve high detection sensitivity, and included elution solvent, ultrasonic elution time, extraction time, sample volume, salt concentration and organic modifier addition. Under the optimal conditions, the detection limits for seven PAH compounds could reach up to 0.1–2.0 ng L1. The method accuracy was evaluated using recovery measurements in standard spiked samples and good recoveries of 70.69–110.04% with relative standard deviations of less than 10% have been achieved. Consequently, the method developed
Received 26th January 2014 Accepted 7th May 2014
was successfully applied for determining PAH in environmental samples: snow water, metal-fabrication factory wastewater and Xiangjiang River water, with PAH contents ranging from 13.2 to 83.1 ng L1.
DOI: 10.1039/c4an00195h
Therefore, using cotton fiber as a new SPE adsorbent, was easy to prepare, had a low cost and great
www.rsc.org/analyst
reusability, and this implies it is a promising method for sample preparation.
1. Introduction Polycyclic aromatic hydrocarbons (PAH) are formed originally during the incomplete combustion of hydrocarbons and are commonly released into the environment during fossil fuel combustion, oil renement, and industrial and municipal discharges.1 PAH are recalcitrant and have high carcinogenicity and mutagenicity, especially aer they are concentrated by the biological food chain.2 They can induce oxidative stress and oxidative DNA damage by metabolic activation and the generation of reactive oxygen species, so they have become an appreciable environmental concern around the world.3–5 Various analytical methods have been developed and applied for monitoring PAH in the natural environment, such as highperformance liquid chromatography with uorescence detection (HPLC-FLD)6,7 and gas chromatography-mass spectrometry,8,9 because the sample matrices were complicated and because trace amounts of the analytes existed in the environment, sample clean-up and preconcentration became necessary
State Key Laboratory of Chemo/Biosensing & Chemometrics, College of Chemistry & Chemical Engineering, Hunan University, Changsha 410082, China. E-mail: szyao@ hnu.edu.cn; [email protected]; Tel: +86-731-88821968 † Electronic supplementary 10.1039/c4an00195h
information
(ESI)
This journal is © The Royal Society of Chemistry 2014
available.
See
DOI:
to improve the detection sensitivity and selectivity, and to adapt the methods for the analytical instruments used. In the last decades, a variety of extraction methods have been developed for preparing environmental samples, including solid-phase micro-extraction (SPME),10,11 liquid-phase microextraction,12,13 dispersive liquid–liquid micro-extraction,14 microwave-assisted headspace solid-phase micro-extraction (MA-HS-SPME),15,16 and solid-phase extraction (SPE).17,18 SPE has the advantages of high recovery, short extraction time, high enrichment factor, low consumption of organic solvents, and ease of automation and operation. However, adsorbents with superior properties play a key role in SPE for enhancing the enrichment efficiency and analytical performance. Nowadays, multiple materials have been synthesized and utilized as SPE sorbents, including multi-walled carbon nanotubes,19,20 graphene,21,22 magnetic nanoparticles,23,24 and metal–organic frameworks.25 However, the synthesis process of such materials is rather complex and oen consumes large amounts of organic solvents and time. Therefore, new materials with a simple preparation process are still highly desirable. Natural ber is a very important material and widely used in many applications. For example, hemp ber is applied to adsorb heavy metal ions from aqueous solutions.26 Choi et al. reported that cotton, milkweed, and kenaf have great sorption capacities for oil with sorption properties 1.5–3 times better than that of polypropylene bers.27,28 As a type of natural ber,
Analyst, 2014, 139, 3593–3599 | 3593
View Article Online
Published on 07 May 2014. Downloaded by Universiteit Utrecht on 26/10/2014 23:37:45.
Analyst
Scheme 1
Paper
Structural formula of cellulose and the SPE procedure.
cotton ber mainly consists of cellulose which is a polysaccharide composed mainly of carbon, oxygen, and hydrogen (Scheme 1). Jonker and Zhang have reported that cellulose exhibited the ability to adsorb PAH from water with cellulose– water partition coefficients ranging from 3.80 to 5.64.29,30 Liu et al. used cotton as a sorbent for an on-line precolumn used to enrich PAH from water,31 however, only four kinds of PAH have been investigated with high detection limits and PAH had not been detected in the real sample. Takagai et al. applied blue cotton, which is cotton modied with a blue pigment, copper(II) phthalocyanine trisulfonate, as a sorbent to enrich PAH.32 In the present work, natural cotton ber was used as an SPE adsorbent to enrich PAH from real water samples. The green material was easy to obtain, did not require preparation and chemical modication, and was convenient for usage in PAH adsorption and desorption using only acetonitrile. Under optimal conditions, highly sensitive detection for seven PAH was achieved, and to the best of our knowledge, it is the rst time for such a material to be successfully used for real environmental sample analysis. Thus, use of natural cotton ber proves it is promising as an application for sample enrichment and analysis.
2.
Experimental
2.1. Chemicals and materials The certied reference standards of uorine (Flu), anthrance (Ant), uoranthene (FlA), pyrene (Pyr), benzo[a]anthracene (BaA), benzo [b]uoranthene (BbF) and benzo[k]uoranthene (BkF) were purchased from Acros Organics (NJ, USA). Stock solutions of these PAH at concentrations of 100 mg mL1 (Flu, Ant, Pyr), 1000 mg mL1 (FlA, BaA) and 50 mg mL1 (BbF, BkF) were prepared in methanol. To achieve a response at the same level in HPLC-FLD detection, the standard mixture of PAH was composed of Flu (1.5 mg mL1), Ant (4 mg mL1), FlA (8 mg mL1), Pyr (4 mg mL1), BaA (1.5 mg mL1), BbF (1.5 mg mL1) and BkF (0.4 mg mL1) which were diluted from their stock solutions with methanol. n-Hexane and dichloromethane were purchased from Sinopharm Chemical Reagent (Shanghai, China), while HPLC grade methanol and acetonitrile were purchased from Merck (Darmstadt, Germany).
3594 | Analyst, 2014, 139, 3593–3599
The cotton ber was purchased from Zhenxiang Labor Protection Product Company (Changsha, China). Titanium wire was obtained from Lihua Non-ferrous Metals Co., Ltd (Baoji, China). Ultrapure water was prepared using the Milli-Q system (Millipore, Bedford, USA). Working solutions were freshly prepared by diluting the mixed standard solution with methanol to the required concentrations. All standards and working solutions were stored at 4 C. 2.2. Instrumentation HPLC analysis of PAH was performed with a Shimadzu (Tokyo, Japan) LC-20AT liquid chromatograph equipped with a Shimadzu RF-10AXL uorescence detector and a Shimadzu CTO10ASVP column oven. A Diamonsil® C18 column (250 mm 4.6 mm, particle size 5 mm; Dikma Technologies, China) was used for separation. Ultrasonic wash and elution were performed with a B5500S-DTH ultrasonic machine (Branson, USA). 2.3. Water sample collection The water samples selected for the investigation included a river water sample collected from the Xiangjiang River (Changsha, China), a snow water sample collected from the Yuelu mountain (Changsha, China) and factory wastewater collected from a metal-fabrication factory (Changsha, China). Before the experiments, all the water samples were ltered through 0.22 mm micropore membranes and then stored at 4 C in a refrigerator. 2.4. Preparation of cotton bers The cotton bers were cut into sections of 5 cm (55 2 mg, B: 1.80 mm) with scissors and one end was xed at a titanium wire as shown in Scheme 1. To increase the contact area between the ber and the sample to improve the enrichment efficiency, the cotton bers were unravelled until each cotton was separated as shown in Scheme 1, and they were then mixed ultrasonically with acetonitrile for about 10 min at room temperature. 2.5. SPE procedure The SPE procedure is shown in Scheme 1. Briey, the extraction process was performed in a 50 mL glass vial containing 50 mL of
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
Analyst
sample solution. A cotton ber was directly immersed in the sample solution for 1 h under constant stirring at room temperature. Aer extraction, the extracted PAH were eluted ultrasonically from the cotton bers with 3 2 mL of acetonitrile. The eluate was collected and dried to about 200 mL under a gentle nitrogen stream, and then reconstituted to 500 mL with acetonitrile. Finally, 20 mL of this solution was injected directly into the HPLC system for analysis.
Published on 07 May 2014. Downloaded by Universiteit Utrecht on 26/10/2014 23:37:45.
2.6. HPLC-FLD analysis Determination of PAH was carried out on a HPLC-FLD system. Chromatographic analysis was performed using a Diamonsil® C18 column maintained at 30 C. The mobile phase consisted of water (solvent A) and acetonitrile (solvent B) and the ow rate was 1 mL min1. The gradient elution program was set as follows: 75% B for 15 min, then B was increased to 90% over 3 min, and maintained from 18 min to 26 min; nally, B was decreased to 75% at 27 min and then maintained for 5 min to equilibrate the column. The time program for the uorescence detection is given in Table S1.†
3.
Results and discussion
3.1. Optimization of the SPE procedure In order to achieve accurate and sensitive chromatographic quantication of the trace PAH in the samples, the optimum conditions for SPE using cotton bers were investigated. Several conditions affecting the extraction efficiency were optimized, including elution of the solvent, ultrasonic elution time, extraction time, sample volume, volume of organic modier, and salt concentration. Optimization experiments were performed using a standard aqueous solution of PAH containing 0.75 mg mL1 Flu, 2 mg mL1 Ant, 4 mg mL1 FlA, 2 mg mL1 Pyr, 0.75 mg mL1 BaA, 0.75 mg mL1 BbF, and 0.2 mg mL1 BkF to ensure the same level of responses to each compound. 3.1.1. Optimization of desorption conditions. As far as the SPE method is concerned, analyte desorption from the adsorbent can signicantly affect the sensitivity of the analyte extraction. Thus, proper elution of the solvent plays a key role in the process. Because of the properties of PAH, methanol, dichloromethane, n-hexane and acetonitrile were selected as possible solvents in this experiment. As shown in Fig. 1, n-hexane is a poor eluent for PAHs. Dichloromethane yields the highest recovery for BaA, whereas acetonitrile is preferable for recovery of Flu, Ant, FlA, Pyr, BbF and BkF. Therefore, acetonitrile was chosen as the elution solvent and was used for further studies. Meanwhile, the elution efficiency also relies on the volume of the elution solvent. As shown in Fig. S1,† for most of the PAH, the recovery increased as the eluent volume increased from 2 mL to 6 mL, which remains nearly unchanged if the volume is further increased from 6 mL to 8 mL. To get high recoveries by as far as possible less solvent, 6 mL (2 mL for each time) of acetonitrile was selected for desorption. In order to resolve any possible carryover problem and avoid the loss of PAH, the ultrasonic desorption time was further
This journal is © The Royal Society of Chemistry 2014
Effect of different eluting solvents on SPE performance. Extraction conditions: sample volume, 50 mL; extraction time: 60 min; eluent volume: 2 mL; ultrasonic desorption time: 10 min.
Fig. 1
optimized. The process of desorption was carried out in an ultrasonic mode with desorption times of 1, 3, 5 and 10 min. The results shown in Fig. S2† prove that the peak areas of PAH increased as the desorption time increased from 1 min to 5 min, but remained unchanged as the desorption time was increased further. Thus, 5 min was sufficient to achieve maximum desorption. Mosier et al. reported that Trypan blue, a polyaromatic, planar molecule was irreversibly adsorbed to cotton cellulose at temperatures of
Published on 07 May 2014. Downloaded by Universiteit Utrecht on 26/10/2014 23:37:45.
PAPER
View Journal | View Issue
Cite this: Analyst, 2014, 139, 3593
Natural cotton fibers as adsorbent for solid-phase extraction of polycyclic aromatic hydrocarbons in water samples† Jianping Wang, Shengquan Liu, Chunyan Chen, Ying Zou, Huiping Hu, Qingyun Cai* and Shouzhuo Yao* A natural material, cotton fiber, has been applied as a solid-phase extraction (SPE) adsorbent for sample preparation for the analysis of polycyclic aromatic hydrocarbons (PAH) in water samples using highperformance liquid chromatography. The cotton fiber was used directly without any chemical modifications, which avoided a complex synthesis process and consumption of a large volume of organic solvent. The conditions affecting the extraction efficiency were optimized to achieve high detection sensitivity, and included elution solvent, ultrasonic elution time, extraction time, sample volume, salt concentration and organic modifier addition. Under the optimal conditions, the detection limits for seven PAH compounds could reach up to 0.1–2.0 ng L1. The method accuracy was evaluated using recovery measurements in standard spiked samples and good recoveries of 70.69–110.04% with relative standard deviations of less than 10% have been achieved. Consequently, the method developed
Received 26th January 2014 Accepted 7th May 2014
was successfully applied for determining PAH in environmental samples: snow water, metal-fabrication factory wastewater and Xiangjiang River water, with PAH contents ranging from 13.2 to 83.1 ng L1.
DOI: 10.1039/c4an00195h
Therefore, using cotton fiber as a new SPE adsorbent, was easy to prepare, had a low cost and great
www.rsc.org/analyst
reusability, and this implies it is a promising method for sample preparation.
1. Introduction Polycyclic aromatic hydrocarbons (PAH) are formed originally during the incomplete combustion of hydrocarbons and are commonly released into the environment during fossil fuel combustion, oil renement, and industrial and municipal discharges.1 PAH are recalcitrant and have high carcinogenicity and mutagenicity, especially aer they are concentrated by the biological food chain.2 They can induce oxidative stress and oxidative DNA damage by metabolic activation and the generation of reactive oxygen species, so they have become an appreciable environmental concern around the world.3–5 Various analytical methods have been developed and applied for monitoring PAH in the natural environment, such as highperformance liquid chromatography with uorescence detection (HPLC-FLD)6,7 and gas chromatography-mass spectrometry,8,9 because the sample matrices were complicated and because trace amounts of the analytes existed in the environment, sample clean-up and preconcentration became necessary
State Key Laboratory of Chemo/Biosensing & Chemometrics, College of Chemistry & Chemical Engineering, Hunan University, Changsha 410082, China. E-mail: szyao@ hnu.edu.cn; [email protected]; Tel: +86-731-88821968 † Electronic supplementary 10.1039/c4an00195h
information
(ESI)
This journal is © The Royal Society of Chemistry 2014
available.
See
DOI:
to improve the detection sensitivity and selectivity, and to adapt the methods for the analytical instruments used. In the last decades, a variety of extraction methods have been developed for preparing environmental samples, including solid-phase micro-extraction (SPME),10,11 liquid-phase microextraction,12,13 dispersive liquid–liquid micro-extraction,14 microwave-assisted headspace solid-phase micro-extraction (MA-HS-SPME),15,16 and solid-phase extraction (SPE).17,18 SPE has the advantages of high recovery, short extraction time, high enrichment factor, low consumption of organic solvents, and ease of automation and operation. However, adsorbents with superior properties play a key role in SPE for enhancing the enrichment efficiency and analytical performance. Nowadays, multiple materials have been synthesized and utilized as SPE sorbents, including multi-walled carbon nanotubes,19,20 graphene,21,22 magnetic nanoparticles,23,24 and metal–organic frameworks.25 However, the synthesis process of such materials is rather complex and oen consumes large amounts of organic solvents and time. Therefore, new materials with a simple preparation process are still highly desirable. Natural ber is a very important material and widely used in many applications. For example, hemp ber is applied to adsorb heavy metal ions from aqueous solutions.26 Choi et al. reported that cotton, milkweed, and kenaf have great sorption capacities for oil with sorption properties 1.5–3 times better than that of polypropylene bers.27,28 As a type of natural ber,
Analyst, 2014, 139, 3593–3599 | 3593
View Article Online
Published on 07 May 2014. Downloaded by Universiteit Utrecht on 26/10/2014 23:37:45.
Analyst
Scheme 1
Paper
Structural formula of cellulose and the SPE procedure.
cotton ber mainly consists of cellulose which is a polysaccharide composed mainly of carbon, oxygen, and hydrogen (Scheme 1). Jonker and Zhang have reported that cellulose exhibited the ability to adsorb PAH from water with cellulose– water partition coefficients ranging from 3.80 to 5.64.29,30 Liu et al. used cotton as a sorbent for an on-line precolumn used to enrich PAH from water,31 however, only four kinds of PAH have been investigated with high detection limits and PAH had not been detected in the real sample. Takagai et al. applied blue cotton, which is cotton modied with a blue pigment, copper(II) phthalocyanine trisulfonate, as a sorbent to enrich PAH.32 In the present work, natural cotton ber was used as an SPE adsorbent to enrich PAH from real water samples. The green material was easy to obtain, did not require preparation and chemical modication, and was convenient for usage in PAH adsorption and desorption using only acetonitrile. Under optimal conditions, highly sensitive detection for seven PAH was achieved, and to the best of our knowledge, it is the rst time for such a material to be successfully used for real environmental sample analysis. Thus, use of natural cotton ber proves it is promising as an application for sample enrichment and analysis.
2.
Experimental
2.1. Chemicals and materials The certied reference standards of uorine (Flu), anthrance (Ant), uoranthene (FlA), pyrene (Pyr), benzo[a]anthracene (BaA), benzo [b]uoranthene (BbF) and benzo[k]uoranthene (BkF) were purchased from Acros Organics (NJ, USA). Stock solutions of these PAH at concentrations of 100 mg mL1 (Flu, Ant, Pyr), 1000 mg mL1 (FlA, BaA) and 50 mg mL1 (BbF, BkF) were prepared in methanol. To achieve a response at the same level in HPLC-FLD detection, the standard mixture of PAH was composed of Flu (1.5 mg mL1), Ant (4 mg mL1), FlA (8 mg mL1), Pyr (4 mg mL1), BaA (1.5 mg mL1), BbF (1.5 mg mL1) and BkF (0.4 mg mL1) which were diluted from their stock solutions with methanol. n-Hexane and dichloromethane were purchased from Sinopharm Chemical Reagent (Shanghai, China), while HPLC grade methanol and acetonitrile were purchased from Merck (Darmstadt, Germany).
3594 | Analyst, 2014, 139, 3593–3599
The cotton ber was purchased from Zhenxiang Labor Protection Product Company (Changsha, China). Titanium wire was obtained from Lihua Non-ferrous Metals Co., Ltd (Baoji, China). Ultrapure water was prepared using the Milli-Q system (Millipore, Bedford, USA). Working solutions were freshly prepared by diluting the mixed standard solution with methanol to the required concentrations. All standards and working solutions were stored at 4 C. 2.2. Instrumentation HPLC analysis of PAH was performed with a Shimadzu (Tokyo, Japan) LC-20AT liquid chromatograph equipped with a Shimadzu RF-10AXL uorescence detector and a Shimadzu CTO10ASVP column oven. A Diamonsil® C18 column (250 mm 4.6 mm, particle size 5 mm; Dikma Technologies, China) was used for separation. Ultrasonic wash and elution were performed with a B5500S-DTH ultrasonic machine (Branson, USA). 2.3. Water sample collection The water samples selected for the investigation included a river water sample collected from the Xiangjiang River (Changsha, China), a snow water sample collected from the Yuelu mountain (Changsha, China) and factory wastewater collected from a metal-fabrication factory (Changsha, China). Before the experiments, all the water samples were ltered through 0.22 mm micropore membranes and then stored at 4 C in a refrigerator. 2.4. Preparation of cotton bers The cotton bers were cut into sections of 5 cm (55 2 mg, B: 1.80 mm) with scissors and one end was xed at a titanium wire as shown in Scheme 1. To increase the contact area between the ber and the sample to improve the enrichment efficiency, the cotton bers were unravelled until each cotton was separated as shown in Scheme 1, and they were then mixed ultrasonically with acetonitrile for about 10 min at room temperature. 2.5. SPE procedure The SPE procedure is shown in Scheme 1. Briey, the extraction process was performed in a 50 mL glass vial containing 50 mL of
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
Analyst
sample solution. A cotton ber was directly immersed in the sample solution for 1 h under constant stirring at room temperature. Aer extraction, the extracted PAH were eluted ultrasonically from the cotton bers with 3 2 mL of acetonitrile. The eluate was collected and dried to about 200 mL under a gentle nitrogen stream, and then reconstituted to 500 mL with acetonitrile. Finally, 20 mL of this solution was injected directly into the HPLC system for analysis.
Published on 07 May 2014. Downloaded by Universiteit Utrecht on 26/10/2014 23:37:45.
2.6. HPLC-FLD analysis Determination of PAH was carried out on a HPLC-FLD system. Chromatographic analysis was performed using a Diamonsil® C18 column maintained at 30 C. The mobile phase consisted of water (solvent A) and acetonitrile (solvent B) and the ow rate was 1 mL min1. The gradient elution program was set as follows: 75% B for 15 min, then B was increased to 90% over 3 min, and maintained from 18 min to 26 min; nally, B was decreased to 75% at 27 min and then maintained for 5 min to equilibrate the column. The time program for the uorescence detection is given in Table S1.†
3.
Results and discussion
3.1. Optimization of the SPE procedure In order to achieve accurate and sensitive chromatographic quantication of the trace PAH in the samples, the optimum conditions for SPE using cotton bers were investigated. Several conditions affecting the extraction efficiency were optimized, including elution of the solvent, ultrasonic elution time, extraction time, sample volume, volume of organic modier, and salt concentration. Optimization experiments were performed using a standard aqueous solution of PAH containing 0.75 mg mL1 Flu, 2 mg mL1 Ant, 4 mg mL1 FlA, 2 mg mL1 Pyr, 0.75 mg mL1 BaA, 0.75 mg mL1 BbF, and 0.2 mg mL1 BkF to ensure the same level of responses to each compound. 3.1.1. Optimization of desorption conditions. As far as the SPE method is concerned, analyte desorption from the adsorbent can signicantly affect the sensitivity of the analyte extraction. Thus, proper elution of the solvent plays a key role in the process. Because of the properties of PAH, methanol, dichloromethane, n-hexane and acetonitrile were selected as possible solvents in this experiment. As shown in Fig. 1, n-hexane is a poor eluent for PAHs. Dichloromethane yields the highest recovery for BaA, whereas acetonitrile is preferable for recovery of Flu, Ant, FlA, Pyr, BbF and BkF. Therefore, acetonitrile was chosen as the elution solvent and was used for further studies. Meanwhile, the elution efficiency also relies on the volume of the elution solvent. As shown in Fig. S1,† for most of the PAH, the recovery increased as the eluent volume increased from 2 mL to 6 mL, which remains nearly unchanged if the volume is further increased from 6 mL to 8 mL. To get high recoveries by as far as possible less solvent, 6 mL (2 mL for each time) of acetonitrile was selected for desorption. In order to resolve any possible carryover problem and avoid the loss of PAH, the ultrasonic desorption time was further
This journal is © The Royal Society of Chemistry 2014
Effect of different eluting solvents on SPE performance. Extraction conditions: sample volume, 50 mL; extraction time: 60 min; eluent volume: 2 mL; ultrasonic desorption time: 10 min.
Fig. 1
optimized. The process of desorption was carried out in an ultrasonic mode with desorption times of 1, 3, 5 and 10 min. The results shown in Fig. S2† prove that the peak areas of PAH increased as the desorption time increased from 1 min to 5 min, but remained unchanged as the desorption time was increased further. Thus, 5 min was sufficient to achieve maximum desorption. Mosier et al. reported that Trypan blue, a polyaromatic, planar molecule was irreversibly adsorbed to cotton cellulose at temperatures of
