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Cigarette filter as sorbent for on-line coupling of solid-phase extraction to high-performance liquid chromatography for determination of polycyclic aromatic hydrocarbons in water
Journal of Chromatography A, 1103 (2006) 9–14
Cigarette filter as sorbent for on-line coupling of solid-phase extraction to high-performance liquid chromatography for determination of polycyclic aromatic hydrocarbons in water Hua-Ding Liang b , De-Man Han a,b , Xiu-Ping Yan a,∗ a
Key Laboratory of Functional Polymer Materials, The Ministry of Education of China, Research Centre for Analytical Sciences, Department of Chemistry, Nankai University, Tianjin 300071, China b Department of Chemistry, Taizhou University, Linhai 317000, China Received 19 July 2005; received in revised form 29 October 2005; accepted 1 November 2005 Available online 23 November 2005
Abstract An on-line solid-phase extraction (SPE) protocol using the cigarette filter as sorbent coupled with high-performance liquid chromatography (HPLC) was developed for simultaneous determination of trace naphthalene (NAPH), phenanthrene (PHEN), anthracene (ANT), fluoranthene (FLU), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP), and benzo(ghi)perylene (BghiP) in water samples. To on-line interface solid-phase extraction to HPLC, a preconcentration column packed with the cigarette filter was used to replace a conventional sample loop on the injector valve of the HPLC for on-line solid-phase extraction. The sample solution was loaded and the analytes were then preconcentrated onto the preconcentration column. The collected analytes were subsequently eluted with a mobile phase of methanol–water (95:5). HPLC with a photodiode array detector was used for their separation and detection. The detection limits (S/N = 3) for preconcentrating 42 mL of sample solution ranged from 0.9 to 58.6 ng L−1 at a sample throughput of 2 samples h−1 . The enhancement factors were in the range of 409–1710. The developed method was applied to the determination of trace NAPH, PHEN, ANT, FLU, BbF, BkF, BaP and BghiP in local river water samples. The recoveries of PAHs spiked in real water samples ranged from 87 to 115%. The precisions for nine replicate measurements of a standard mixture (NAPH: 4.0 g L−1 , PHEN: 0.40 g L−1 , ANT: 0.40 g L−1 , FLU: 2.0 g L−1 , BbF: 1.6 g L−1 , BkF: 2.0 g L−1 , BaP: 2.0 g L−1 , BghiP: 1.7 g L−1 ) were in the range of 1.2–5.1%. © 2005 Elsevier B.V. All rights reserved. Keywords: Solid-phase extraction; Cigarette filter; High-performance liquid chromatography; PAHs
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are well-known environmental pollutants at low concentrations and are included in the European Union and US Environmental Protection Agency (EPA) priority pollutant list due to their mutagenic and carcinogenic properties [1–3]. They are generated by incomplete combustion of organic materials arising in part from natural combustion such as forest fires and volcanic eruptions [4]. Anthropogenic sources such as industrial production, transportation and waste incineration generate significant levels of PAHs
∗
Corresponding author. Fax: +86 22 23506075. E-mail address: [email protected] (X.-P. Yan).
0021-9673/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2005.11.003
[5,6]. The determination of small amounts of organic substances, especially toxic ones, in water is necessary for environmental monitoring and for industrial process control [7,8]. This problem is often hard to solve even using modern instrumental analysis methods, such as chromatography or mass-spectrometry. High-performance liquid chromatography (HPLC) is one of the most versatile methods for determination of organic compounds in various sample matrices. HPLC offers several advantages in PAHs analysis including good resolution for separation of isomers; sufficient specificity of UV and fluorescence detector; possible estimation of molecular sizes of PAHs on the basis of the retention time using reversed phase column; possibility to determine compounds with high molecular mass; analysis usually carried out at ambient temperature without risk of thermal decomposition of analytes [9].
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H.-D. Liang et al. / J. Chromatogr. A 1103 (2006) 9–14
However, its sensitivity is often insufficient for direct determination of maximum allowable concentrations of toxic substances, such as PAHs, pesticides, etc. in natural and potable water. HPLC analysis of complex matrices requires preliminary separation of organic macrocomponents. As a consequence, separation and preconcentration techniques are frequently applied prior to the analysis for improving the detection limits and selectivity. The development of automated on-line techniques, which include both preconcentration and analysis, is a promising trend in modern analytical chemistry [10,11]. General methods for preconcentration of organic substances from aqueous solutions were: liquid–liquid extraction, solidphase extraction (SPE), membrane extraction, electrochemical deposition, etc. One of the most practically feasible and suitable for automation is SPE, which does not require sophisticated equipment and provides high preconcentration efficiency [12,13]. It is well known that the cigarette filter can efficiently adsorb many poisonous organic compounds and, hence, alleviate the poisonous effect on smokers. The cigarette filter material has been successfully used as the sorbent for the preconcentration and separation of the MeHg-DDTC and Hg-APDC chelates [14,15]. The aim of this work was to develop a simple, costeffective and sensitive methodology for the determination of PAHs at low ng L−1 levels in water without the need for time-consuming sample treatment by on-line coupling SPE using cigarette filter as sorbent to HPLC with UV detection. The design of the on-line SPE preconcentration system for HPLC, and the potential factors affecting the SPE and subsequent HPLC separation of the analytes were described and discussed in detail. The developed methodology was applied to determine trace naphthalene (NAPH), phenanthrene (PHEN), anthracene (ANT), fluoranthene (FLU), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP), and benzo(ghi)perylene (BghiP) in local river water samples. 2. Experimental 2.1. Apparatus The chromatographic system consisted of a Waters model 600 HPLC pump and a Waters 2996 Photodiode Array Detector (Milford, MA, USA). All separations were achieved on an analytical reversed-phase column (Symmetry-C18 5 m, 4.6 mm i.d. × 25 cm long, Waters, USA) with a mobile flow rate of 1.0 mL min−1 under isocratic conditions at room temperature. The Empower Software was used to acquire and process spectral and chromatographic data. The diode array detector was operated between 210 and 400 nm. A Model FIA-3100 flow injection system (Vital Instruments Co. Ltd, Beijing, China) was used for the on-line solid-phase extraction preconcentration. Tygon pump tubes were used for delivering the sample solution. Small-bore (0.5 mm i.d.) PTFE tubings were adapted for all connections, which were kept at the shortest possible length to minimize the dead volume.
2.2. Materials and reagents All reagents were of the highest available purity and at least of analytical grade. Doubly deionized water (DDW, 18 M cm−1 ) obtained from a WaterPro water system (Labconco Corporation, Kansas City, MO, USA) was used throughout. The stock standard solutions of NAPH (2.0 mg L−1 ), PHEN (0.20 mg L−1 ), ANT (0.20 mg L−1 ), FLU (1.0 mg L−1 ), BbF (0.78 mg L−1 ), BkF (1.0 mg L−1 ), BaP (1.0 mg L−1 ), and BghiP (0.85 mg L−1 ) were purchased from the National Research Center for Certified Reference Materials (Beijing, China) and stored in the dark at 4 ◦ C. Working solutions were prepared from the standard stock solutions by stepwise dilution just before use. A preconcentration column (1.0 cm × 8 mm i.d.) packed with 70 mg of the cigarette filter (Baisha brand, Tianjin, China) was used for the solid-phase extraction preconcentration of PAHs in water. Best chromatographic resolution for the separation of PAHs was obtained with a mixture of methanol (Concord Technology Co. Ltd. Tianjin, China) and water (95:5). The mobile phase was filtered through 0.45 m filter and degassed prior to use. 2.3. Samples River water samples were collected locally. The samples were filtered through 0.45 m Supor filters, stored in precleaned glass bottles (thoroughly washed with detergents, water, methanol, and doubly deionized water, and dried before use), and analyzed immediately after sampling. Water samples were adjusted to pH = 2.5–10.0 with HCl or NaOH to insure the efficient solidphase extraction of the analytes. 2.4. Procedures for the on-line SPE preconcentration and HPLC separation A schematic diagram for the on-line SPE preconcentration coupled to HPLC for determination of trace PAHs in water is shown in Fig. 1. First, the sample solution was introduced onto the preconcentration column packed with the cigarette filter at a flow rate of 4.2 mL min−1 for 10 min while the HPLC injector valve was in the load position so that the PAHs were preconcentrated onto the cigarette filter packed preconcentration column whereas the unwanted water went to waste (W) (Fig. 1A). Second, the analytes adsorbed on the preconcentration column were eluted in the backflush mode with the HPLC mobile phase at a flow rate of 1.0 mL min−1 into the chromatographic separation column for 2 min by switching HPLC valve from “load” to “inject” position (Fig. 1B). As such, the sample band in the preconcentration column was compressed into a narrow band before entering the analytical column and the band broaden effect was reduced. Third, the HPLC injector valve was turned to the “load” position for next sample preconcentration while the analytes were separated in the chromatographic separation column to improve sample throughput. In this way, a complete cycle of the on-line SPE preconcentration and HPLC separation of the PAHs lasted 30 min. The peak areas were calculated at their respective characteristic absorbance wavelength (220.4 nm
H.-D. Liang et al. / J. Chromatogr. A 1103 (2006) 9–14
11
Fig. 2. Effect of sample loading flow rate on the peak areas of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ), and BghiP (1.7 g L−1 ) for 10 min preconcentration.
Based on the above results, we selected the following conditions for the on-line solid-phase extraction preconcentration of PAHs: sample loading time = 10 min, sample loading flow rate = 4.2 mL min−1 , sample pH = 2.5–10.0.
Fig. 1. Schematic diagram of the on-line solid-phase extraction preconcentration coupled with HPLC. HPLC injector valve position: (A) load and (B) inject.
for NAPH, 251.0 nm for PHEN, 251.0 nm for ANT, 286.0 nm for FLU, 255.7 nm for BbF, 306.0 nm for BkF, 295.5 nm for BaP, and 299.5 nm for BghiP) and used for data evaluation. 3. Results and discussion 3.1. Factors affecting the on-line SPE preconcentration of PAHs The effect of sample pH on the adsorption of the PAHs was studied in the pH range of 2.5–10.0. No significant variation in the chromatographic peak areas of the PAHs was observed in the pH range examined. The influence of sample loading flow rate on the adsorption preconcentration of the PAHs was investigated with a mixture of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ), and BghiP (1.7 g L−1 ) for 10 min preconcentration. As shown in Fig. 2, the chromatographic peak areas of the PAHs increased almost linearly with sample loading flow rate, from 1.5 to 5.6 mL min−1 . Fig. 3 shows the effect of sample loading time on the adsorption of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ), and BghiP (1.7 g L−1 ) at a sample flow rate of 4.2 mL min−1 . The chromatographic peak areas of the PAHs increased almost linearly as the sample loading time increased up to at least 15 min.
3.2. Desorption of the adsorbed PAHs from the cigarette filter-packed column For simplicity, the optimum HPLC mobile phase (methanol: water = 95:5) was used for the desorption of the adsorbed PAHs from the cigarette filter-packed column. The time required for quantitative desorption of the adsorbed PAHs from the SPE column was evaluated in order to determine when the HPLC injector valve should turn to the “load” position for next online solid-phase extraction during the HPLC separation of the analytes. It was found that the chromatographic peak areas of the PAHs increased remarkably as the desorption time increased
Fig. 3. Effect of sample loading time on the peak areas of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ) and BghiP (1.7 g L−1 ) at a sample flow rate of 4.2 mL min−1 .
H.-D. Liang et al. / J. Chromatogr. A 1103 (2006) 9–14
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Table 1 Characteristic data of the on-line SPE-HPLC for determination of PAHs using cigarette filter as sorbent Analyte
Linear range (g L−1 )
Correlation coefficient
Detection limit (ng L−1 )
Enrichment factor
RSDa (n = 9) (%)
NAPH PHEN ANT FLU BbF BkF BaP BghiP
0.02–53 0.01–5 0.01–5 0.1–27 0.08–16 0.1–16 0.1–16 0.4–34
0.9995 0.9996 0.9986 0.9995 0.9996 0.9995 0.9989 0.9940
1.6 2.1 0.9 7.7 17.1 18.8 31.2 58.6
1103 1500 1710 1372 768 733 803 409
1.2 2.0 2.3 2.1 3.6 3.1 5.1 3.4
a Using a mixture of NAPH: 4.0 g L−1 , PHEN: 0.40 g L−1 , ANT: 0.40 g L−1 , FLU: 2.0 g L−1 , BbF: 1.6 g L−1 , BkF: 2.0 g L−1 , BaP: 2.0 g L−1 , BghiP: 1.7 g L−1 .
Table 2 Comparison of the cigarette filter sorbent with C18 and XAD-4 for on-line SPE-HPLC determination of PAHs under the same conditions in Table 1 Analyte
NAPH PHEN ANT FLU BbF BkF BaP BghiP
Detection limit (ng L−1 )
Enrichment factor
RSD (%)
Cigarette filter
C18
XAD-4
Cigarette filter
C18
XAD-4
Cigarette filter
C18
XAD-4
1.6 2.1 0.9 7.7 17.1 18.8 31.2 58.6
4.1 3.1 2.1 12.9 38.6 50.6 36.3 245
14.3 9.9 8.6 62.7 41.1 42.6 101 217.4
1103 1500 1710 1372 768 733 803 409
1152 352 833 1001 656 330 383 276
402 424 175 328 329 379 349 286
1.2 2.0 2.3 2.1 3.6 3.1 5.1 3.4
14.1 4.2 4.8 3.2 9.0 2.2 3.6 7.7
9.5 9.7 8.5 7.3 6.8 8.2 6.9 5.7
from 0.5 to 1.5 min, then leveled off in the range of 1.5–10.0 min. Accordingly, 2.0 min desorption was selected to ensure the complete stripping of the adsorbed PAHs from the SPE column. Once the adsorbed PAHs was quantitatively stripped from the SPE column, the HPLC injector valve turned to the “load” position for next preconcentration so that the current HPLC separation and the next preconcentration proceeded in parallel. 3.3. HPLC separation The mobile phase was optimized to obtain baseline separation of the eight PAHs as short as possible. Various ratios of methanol to water (i.e. methanol/water = 100:0; 95:5; 90:10; 80:20) were tested as the mobile phase. When the higher ratio of methanol was used, the eight PAHs cannot be baseline separated. With the decrease of the methanol content, the resolution became better
while the separation time became longer. The eight PAHs could be baseline separated within 25 min with the mobile phase of methanol/water = 95:5. So we selected methanol/water = 95:5 as the mobile phase for HPLC separation of the eight PAHs. 3.4. Analytical performance The analytical characteristic data of the developed on-line solid-phase extraction preconcentration coupled with HPLC for the determination of PAHs using cigarette filter as sorbent were summarized in Table 1. The precisions (peak area) for nine replicate measurements of a standard mixture (NAPH: 4.0 g L−1 , PHEN: 0.40 g L−1 , ANT: 0.40 g L−1 , FLU: 2.0 g L−1 , BbF: 1.6 g L−1 , BkF: 2.0 g L−1 , BaP: 2.0 g L−1 , BghiP: 1.7 g L−1 ) were in the range of 1.2–5.1%. With the consumption of 42 mL sample solution, the enrichment factors ranged from 409 to 1710 in comparison with direct injection of 20 L
Table 3 Analytical results for the PAHs in water samples Sample
Concentration determined (mean ± σ, n = 3) (g L−1 ) NAPH
River water 1 River water 2 River water 3 River water 4 n.d.: not detected.
2.3 2.0 0.042 0.056
± ± ± ±
PHEN 0.2 0.1 0.005 0.002
0.26 0.30 0.014 0.012
± ± ± ±
0.01 0.03 0.002 0.001
ANT
FLU
BbF
BkF
BaP
BghiP
0.014 ± 0.004 n.d. 0.025 ± 0.003 0.024 ± 0.002
n.d. n.d. 0.033 ± 0.002 0.031 ± 0.002
0.10 ± 0.01 0.09 ± 0.01 n.d. n.d.
0.13 ± 0.01 n.d. n.d. n.d.
0.13 ± 0.01 0.015 ± 0.003 n.d. 0.021 ± 0.003
n.d n.d n.d. n.d.
H.-D. Liang et al. / J. Chromatogr. A 1103 (2006) 9–14
sample solution. The detection limits (S/N = 3) of NAPH, PHEN, ANT, FLU, BbF, BkF, BaP, and BghiP were 1.6, 2.1, 0.9, 7.7, 17.1, 18.8, 31.2 and 58.6 ng L−1 , respectively. For comparison, the detection limits, enrichment factors and precisions obtained using the cigarette filter, C18 and AmberliteTM XAD-4 as sorbent for on-line SPE-HPLC determination of PAHs under the same experimental conditions are summarized in Table 2. Generally speaking, the cigarette filer gave lower detection limits, higher enrichment factors and better precisions than C18 and AmberliteTM XAD-4. To evaluate the usefulness of the developed method, local environmental water samples were collected, and analyzed for trace PAHs by the developed method. Typical chromatograms are shown in Fig. 4. The analytical results and recovery date are
13
Table 4 Recoveries of PAHs in river water 1 Analyte
Spiked (g L−1 )
NAPH
1.0 2.0
95.5 93.2
PHEN
0.10 0.20
101.1 105.5
ANT
0.10 0.20
105.2 99.0
FLU
0.50 1.0
99.2 87.1
BbF
0.40 0.80
109.9 101.1
BkF
0.50 1.0
94.4 111.8
BaP
0.5 1.0
94.1 115.5
BghiP
0.42 0.85
109.7 115.2
Recovery (%)
summarized in Tables 3 and 4, respectively. The recoveries of PAHs ranged from 87 to 115%. 4. Conclusions The results in this work have demonstrated that the feasibility of the cigarette filter sorbent for on-line SPE coupled with HPLC-UV for the determination of trace PAHs in environmental water samples. The developed methodology is simple, sensitive and cost-effective. The cigarette filter possesses great potential for its application in the field of PAHs analytical chemistry due to its good adsorption properties and low cost. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 20437020) and the National Basic Research Program of China (No. 2003CB415001). References
Fig. 4. Chromatograms of (a) direct injection of 20 L of a standard solution of NPH (2000 g L−1 ), PHE (200 g L−1 ), ANT (200 g L−1 ), FLU (1000 g L−1 ), BbF (780 g L−1 ), BkF (1000 g L−1 ), BaP (1000 g L−1 ), BghiP (850 g L−1 ); (b) after on-line SPE of the standard solution of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ), BghiP (1.7 g L−1 ). 1: NPH; 2: PHE; 3: ANT; 4: FLU; 5: BbF; 6: BkF; 7: BaP; 8: BghiP.
[1] M.J. Nieva-Cano, S. Rubio-Barroso, M.J. Santos-Delgado, Analyst 126 (2001) 1326. [2] E. Manoli, C. Samara, Trends Anal. Chem. 18 (1999) 417. [3] P.J. Tsai, H.Y. Shieh, W.J. Lee, S.O. Lai, J. Hazard. Mater. A91 (2002) 25. [4] N. Grova, C. Feidt, C. Crepineau, C. Laurent, P.E. Lafargue, A. Hachimi, G. Rychen, J. Agric. Food Chem. 50 (2002) 4640. [5] S.O. Baek, R.A. Field, M.E. Goldstone, P.W. Kirk, J.N. Lester, R.A. Perry, Water Air Soil Pollut. 60 (1991) 279. [6] H.H. Yang, W.J. Lee, S.J. Chen, S.O. Lai, J. Hazard. Mater. 60 (1998) 159. [7] J.H. Yu, C.H. Zhu, J. Environ. Monitor. China 13 (1997) 20. [8] R.B. Jia, J. Environ. Monitor. China 15 (1999) 40. [9] K. Tamakawa, T. Kato, M. Oba, in: L. Nollet (Ed.), Handbook of Food Analysis, Marcel Dekker, New York, 1996, p. 1641.
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[10] E. Pocurull, C. Aguilar, F. Borrull, R.M. Marc´e, J. Chromatogr. A 818 (1998) 85. [11] Z.-L. Fang, Anal. Chim. Acta 400 (1999) 233. [12] L.A. Berrueta, B. Gallo, F. Vicente, Chromatographia 40 (1995) 474.
[13] O. Lubov, S. Mikhail, T. Grigoriy, S. Oleg, Z. Yury, Anal. Chim. Acta 538 (2005) 35. [14] X.-P. Yan, Y. Li, Y. Jiang, Anal. Chem. 75 (2003) 2251. [15] L.-M. Dong, X.-P. Yan, Y. Li, Y. Jiang, S.-W. Wang, D.-Q. Jiang, J. Chromatogr. A 1036 (2004) 119.
Cigarette filter as sorbent for on-line coupling of solid-phase extraction to high-performance liquid chromatography for determination of polycyclic aromatic hydrocarbons in water Hua-Ding Liang b , De-Man Han a,b , Xiu-Ping Yan a,∗ a
Key Laboratory of Functional Polymer Materials, The Ministry of Education of China, Research Centre for Analytical Sciences, Department of Chemistry, Nankai University, Tianjin 300071, China b Department of Chemistry, Taizhou University, Linhai 317000, China Received 19 July 2005; received in revised form 29 October 2005; accepted 1 November 2005 Available online 23 November 2005
Abstract An on-line solid-phase extraction (SPE) protocol using the cigarette filter as sorbent coupled with high-performance liquid chromatography (HPLC) was developed for simultaneous determination of trace naphthalene (NAPH), phenanthrene (PHEN), anthracene (ANT), fluoranthene (FLU), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP), and benzo(ghi)perylene (BghiP) in water samples. To on-line interface solid-phase extraction to HPLC, a preconcentration column packed with the cigarette filter was used to replace a conventional sample loop on the injector valve of the HPLC for on-line solid-phase extraction. The sample solution was loaded and the analytes were then preconcentrated onto the preconcentration column. The collected analytes were subsequently eluted with a mobile phase of methanol–water (95:5). HPLC with a photodiode array detector was used for their separation and detection. The detection limits (S/N = 3) for preconcentrating 42 mL of sample solution ranged from 0.9 to 58.6 ng L−1 at a sample throughput of 2 samples h−1 . The enhancement factors were in the range of 409–1710. The developed method was applied to the determination of trace NAPH, PHEN, ANT, FLU, BbF, BkF, BaP and BghiP in local river water samples. The recoveries of PAHs spiked in real water samples ranged from 87 to 115%. The precisions for nine replicate measurements of a standard mixture (NAPH: 4.0 g L−1 , PHEN: 0.40 g L−1 , ANT: 0.40 g L−1 , FLU: 2.0 g L−1 , BbF: 1.6 g L−1 , BkF: 2.0 g L−1 , BaP: 2.0 g L−1 , BghiP: 1.7 g L−1 ) were in the range of 1.2–5.1%. © 2005 Elsevier B.V. All rights reserved. Keywords: Solid-phase extraction; Cigarette filter; High-performance liquid chromatography; PAHs
1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are well-known environmental pollutants at low concentrations and are included in the European Union and US Environmental Protection Agency (EPA) priority pollutant list due to their mutagenic and carcinogenic properties [1–3]. They are generated by incomplete combustion of organic materials arising in part from natural combustion such as forest fires and volcanic eruptions [4]. Anthropogenic sources such as industrial production, transportation and waste incineration generate significant levels of PAHs
∗
Corresponding author. Fax: +86 22 23506075. E-mail address: [email protected] (X.-P. Yan).
0021-9673/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2005.11.003
[5,6]. The determination of small amounts of organic substances, especially toxic ones, in water is necessary for environmental monitoring and for industrial process control [7,8]. This problem is often hard to solve even using modern instrumental analysis methods, such as chromatography or mass-spectrometry. High-performance liquid chromatography (HPLC) is one of the most versatile methods for determination of organic compounds in various sample matrices. HPLC offers several advantages in PAHs analysis including good resolution for separation of isomers; sufficient specificity of UV and fluorescence detector; possible estimation of molecular sizes of PAHs on the basis of the retention time using reversed phase column; possibility to determine compounds with high molecular mass; analysis usually carried out at ambient temperature without risk of thermal decomposition of analytes [9].
10
H.-D. Liang et al. / J. Chromatogr. A 1103 (2006) 9–14
However, its sensitivity is often insufficient for direct determination of maximum allowable concentrations of toxic substances, such as PAHs, pesticides, etc. in natural and potable water. HPLC analysis of complex matrices requires preliminary separation of organic macrocomponents. As a consequence, separation and preconcentration techniques are frequently applied prior to the analysis for improving the detection limits and selectivity. The development of automated on-line techniques, which include both preconcentration and analysis, is a promising trend in modern analytical chemistry [10,11]. General methods for preconcentration of organic substances from aqueous solutions were: liquid–liquid extraction, solidphase extraction (SPE), membrane extraction, electrochemical deposition, etc. One of the most practically feasible and suitable for automation is SPE, which does not require sophisticated equipment and provides high preconcentration efficiency [12,13]. It is well known that the cigarette filter can efficiently adsorb many poisonous organic compounds and, hence, alleviate the poisonous effect on smokers. The cigarette filter material has been successfully used as the sorbent for the preconcentration and separation of the MeHg-DDTC and Hg-APDC chelates [14,15]. The aim of this work was to develop a simple, costeffective and sensitive methodology for the determination of PAHs at low ng L−1 levels in water without the need for time-consuming sample treatment by on-line coupling SPE using cigarette filter as sorbent to HPLC with UV detection. The design of the on-line SPE preconcentration system for HPLC, and the potential factors affecting the SPE and subsequent HPLC separation of the analytes were described and discussed in detail. The developed methodology was applied to determine trace naphthalene (NAPH), phenanthrene (PHEN), anthracene (ANT), fluoranthene (FLU), benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP), and benzo(ghi)perylene (BghiP) in local river water samples. 2. Experimental 2.1. Apparatus The chromatographic system consisted of a Waters model 600 HPLC pump and a Waters 2996 Photodiode Array Detector (Milford, MA, USA). All separations were achieved on an analytical reversed-phase column (Symmetry-C18 5 m, 4.6 mm i.d. × 25 cm long, Waters, USA) with a mobile flow rate of 1.0 mL min−1 under isocratic conditions at room temperature. The Empower Software was used to acquire and process spectral and chromatographic data. The diode array detector was operated between 210 and 400 nm. A Model FIA-3100 flow injection system (Vital Instruments Co. Ltd, Beijing, China) was used for the on-line solid-phase extraction preconcentration. Tygon pump tubes were used for delivering the sample solution. Small-bore (0.5 mm i.d.) PTFE tubings were adapted for all connections, which were kept at the shortest possible length to minimize the dead volume.
2.2. Materials and reagents All reagents were of the highest available purity and at least of analytical grade. Doubly deionized water (DDW, 18 M cm−1 ) obtained from a WaterPro water system (Labconco Corporation, Kansas City, MO, USA) was used throughout. The stock standard solutions of NAPH (2.0 mg L−1 ), PHEN (0.20 mg L−1 ), ANT (0.20 mg L−1 ), FLU (1.0 mg L−1 ), BbF (0.78 mg L−1 ), BkF (1.0 mg L−1 ), BaP (1.0 mg L−1 ), and BghiP (0.85 mg L−1 ) were purchased from the National Research Center for Certified Reference Materials (Beijing, China) and stored in the dark at 4 ◦ C. Working solutions were prepared from the standard stock solutions by stepwise dilution just before use. A preconcentration column (1.0 cm × 8 mm i.d.) packed with 70 mg of the cigarette filter (Baisha brand, Tianjin, China) was used for the solid-phase extraction preconcentration of PAHs in water. Best chromatographic resolution for the separation of PAHs was obtained with a mixture of methanol (Concord Technology Co. Ltd. Tianjin, China) and water (95:5). The mobile phase was filtered through 0.45 m filter and degassed prior to use. 2.3. Samples River water samples were collected locally. The samples were filtered through 0.45 m Supor filters, stored in precleaned glass bottles (thoroughly washed with detergents, water, methanol, and doubly deionized water, and dried before use), and analyzed immediately after sampling. Water samples were adjusted to pH = 2.5–10.0 with HCl or NaOH to insure the efficient solidphase extraction of the analytes. 2.4. Procedures for the on-line SPE preconcentration and HPLC separation A schematic diagram for the on-line SPE preconcentration coupled to HPLC for determination of trace PAHs in water is shown in Fig. 1. First, the sample solution was introduced onto the preconcentration column packed with the cigarette filter at a flow rate of 4.2 mL min−1 for 10 min while the HPLC injector valve was in the load position so that the PAHs were preconcentrated onto the cigarette filter packed preconcentration column whereas the unwanted water went to waste (W) (Fig. 1A). Second, the analytes adsorbed on the preconcentration column were eluted in the backflush mode with the HPLC mobile phase at a flow rate of 1.0 mL min−1 into the chromatographic separation column for 2 min by switching HPLC valve from “load” to “inject” position (Fig. 1B). As such, the sample band in the preconcentration column was compressed into a narrow band before entering the analytical column and the band broaden effect was reduced. Third, the HPLC injector valve was turned to the “load” position for next sample preconcentration while the analytes were separated in the chromatographic separation column to improve sample throughput. In this way, a complete cycle of the on-line SPE preconcentration and HPLC separation of the PAHs lasted 30 min. The peak areas were calculated at their respective characteristic absorbance wavelength (220.4 nm
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Fig. 2. Effect of sample loading flow rate on the peak areas of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ), and BghiP (1.7 g L−1 ) for 10 min preconcentration.
Based on the above results, we selected the following conditions for the on-line solid-phase extraction preconcentration of PAHs: sample loading time = 10 min, sample loading flow rate = 4.2 mL min−1 , sample pH = 2.5–10.0.
Fig. 1. Schematic diagram of the on-line solid-phase extraction preconcentration coupled with HPLC. HPLC injector valve position: (A) load and (B) inject.
for NAPH, 251.0 nm for PHEN, 251.0 nm for ANT, 286.0 nm for FLU, 255.7 nm for BbF, 306.0 nm for BkF, 295.5 nm for BaP, and 299.5 nm for BghiP) and used for data evaluation. 3. Results and discussion 3.1. Factors affecting the on-line SPE preconcentration of PAHs The effect of sample pH on the adsorption of the PAHs was studied in the pH range of 2.5–10.0. No significant variation in the chromatographic peak areas of the PAHs was observed in the pH range examined. The influence of sample loading flow rate on the adsorption preconcentration of the PAHs was investigated with a mixture of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ), and BghiP (1.7 g L−1 ) for 10 min preconcentration. As shown in Fig. 2, the chromatographic peak areas of the PAHs increased almost linearly with sample loading flow rate, from 1.5 to 5.6 mL min−1 . Fig. 3 shows the effect of sample loading time on the adsorption of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ), and BghiP (1.7 g L−1 ) at a sample flow rate of 4.2 mL min−1 . The chromatographic peak areas of the PAHs increased almost linearly as the sample loading time increased up to at least 15 min.
3.2. Desorption of the adsorbed PAHs from the cigarette filter-packed column For simplicity, the optimum HPLC mobile phase (methanol: water = 95:5) was used for the desorption of the adsorbed PAHs from the cigarette filter-packed column. The time required for quantitative desorption of the adsorbed PAHs from the SPE column was evaluated in order to determine when the HPLC injector valve should turn to the “load” position for next online solid-phase extraction during the HPLC separation of the analytes. It was found that the chromatographic peak areas of the PAHs increased remarkably as the desorption time increased
Fig. 3. Effect of sample loading time on the peak areas of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ) and BghiP (1.7 g L−1 ) at a sample flow rate of 4.2 mL min−1 .
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Table 1 Characteristic data of the on-line SPE-HPLC for determination of PAHs using cigarette filter as sorbent Analyte
Linear range (g L−1 )
Correlation coefficient
Detection limit (ng L−1 )
Enrichment factor
RSDa (n = 9) (%)
NAPH PHEN ANT FLU BbF BkF BaP BghiP
0.02–53 0.01–5 0.01–5 0.1–27 0.08–16 0.1–16 0.1–16 0.4–34
0.9995 0.9996 0.9986 0.9995 0.9996 0.9995 0.9989 0.9940
1.6 2.1 0.9 7.7 17.1 18.8 31.2 58.6
1103 1500 1710 1372 768 733 803 409
1.2 2.0 2.3 2.1 3.6 3.1 5.1 3.4
a Using a mixture of NAPH: 4.0 g L−1 , PHEN: 0.40 g L−1 , ANT: 0.40 g L−1 , FLU: 2.0 g L−1 , BbF: 1.6 g L−1 , BkF: 2.0 g L−1 , BaP: 2.0 g L−1 , BghiP: 1.7 g L−1 .
Table 2 Comparison of the cigarette filter sorbent with C18 and XAD-4 for on-line SPE-HPLC determination of PAHs under the same conditions in Table 1 Analyte
NAPH PHEN ANT FLU BbF BkF BaP BghiP
Detection limit (ng L−1 )
Enrichment factor
RSD (%)
Cigarette filter
C18
XAD-4
Cigarette filter
C18
XAD-4
Cigarette filter
C18
XAD-4
1.6 2.1 0.9 7.7 17.1 18.8 31.2 58.6
4.1 3.1 2.1 12.9 38.6 50.6 36.3 245
14.3 9.9 8.6 62.7 41.1 42.6 101 217.4
1103 1500 1710 1372 768 733 803 409
1152 352 833 1001 656 330 383 276
402 424 175 328 329 379 349 286
1.2 2.0 2.3 2.1 3.6 3.1 5.1 3.4
14.1 4.2 4.8 3.2 9.0 2.2 3.6 7.7
9.5 9.7 8.5 7.3 6.8 8.2 6.9 5.7
from 0.5 to 1.5 min, then leveled off in the range of 1.5–10.0 min. Accordingly, 2.0 min desorption was selected to ensure the complete stripping of the adsorbed PAHs from the SPE column. Once the adsorbed PAHs was quantitatively stripped from the SPE column, the HPLC injector valve turned to the “load” position for next preconcentration so that the current HPLC separation and the next preconcentration proceeded in parallel. 3.3. HPLC separation The mobile phase was optimized to obtain baseline separation of the eight PAHs as short as possible. Various ratios of methanol to water (i.e. methanol/water = 100:0; 95:5; 90:10; 80:20) were tested as the mobile phase. When the higher ratio of methanol was used, the eight PAHs cannot be baseline separated. With the decrease of the methanol content, the resolution became better
while the separation time became longer. The eight PAHs could be baseline separated within 25 min with the mobile phase of methanol/water = 95:5. So we selected methanol/water = 95:5 as the mobile phase for HPLC separation of the eight PAHs. 3.4. Analytical performance The analytical characteristic data of the developed on-line solid-phase extraction preconcentration coupled with HPLC for the determination of PAHs using cigarette filter as sorbent were summarized in Table 1. The precisions (peak area) for nine replicate measurements of a standard mixture (NAPH: 4.0 g L−1 , PHEN: 0.40 g L−1 , ANT: 0.40 g L−1 , FLU: 2.0 g L−1 , BbF: 1.6 g L−1 , BkF: 2.0 g L−1 , BaP: 2.0 g L−1 , BghiP: 1.7 g L−1 ) were in the range of 1.2–5.1%. With the consumption of 42 mL sample solution, the enrichment factors ranged from 409 to 1710 in comparison with direct injection of 20 L
Table 3 Analytical results for the PAHs in water samples Sample
Concentration determined (mean ± σ, n = 3) (g L−1 ) NAPH
River water 1 River water 2 River water 3 River water 4 n.d.: not detected.
2.3 2.0 0.042 0.056
± ± ± ±
PHEN 0.2 0.1 0.005 0.002
0.26 0.30 0.014 0.012
± ± ± ±
0.01 0.03 0.002 0.001
ANT
FLU
BbF
BkF
BaP
BghiP
0.014 ± 0.004 n.d. 0.025 ± 0.003 0.024 ± 0.002
n.d. n.d. 0.033 ± 0.002 0.031 ± 0.002
0.10 ± 0.01 0.09 ± 0.01 n.d. n.d.
0.13 ± 0.01 n.d. n.d. n.d.
0.13 ± 0.01 0.015 ± 0.003 n.d. 0.021 ± 0.003
n.d n.d n.d. n.d.
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sample solution. The detection limits (S/N = 3) of NAPH, PHEN, ANT, FLU, BbF, BkF, BaP, and BghiP were 1.6, 2.1, 0.9, 7.7, 17.1, 18.8, 31.2 and 58.6 ng L−1 , respectively. For comparison, the detection limits, enrichment factors and precisions obtained using the cigarette filter, C18 and AmberliteTM XAD-4 as sorbent for on-line SPE-HPLC determination of PAHs under the same experimental conditions are summarized in Table 2. Generally speaking, the cigarette filer gave lower detection limits, higher enrichment factors and better precisions than C18 and AmberliteTM XAD-4. To evaluate the usefulness of the developed method, local environmental water samples were collected, and analyzed for trace PAHs by the developed method. Typical chromatograms are shown in Fig. 4. The analytical results and recovery date are
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Table 4 Recoveries of PAHs in river water 1 Analyte
Spiked (g L−1 )
NAPH
1.0 2.0
95.5 93.2
PHEN
0.10 0.20
101.1 105.5
ANT
0.10 0.20
105.2 99.0
FLU
0.50 1.0
99.2 87.1
BbF
0.40 0.80
109.9 101.1
BkF
0.50 1.0
94.4 111.8
BaP
0.5 1.0
94.1 115.5
BghiP
0.42 0.85
109.7 115.2
Recovery (%)
summarized in Tables 3 and 4, respectively. The recoveries of PAHs ranged from 87 to 115%. 4. Conclusions The results in this work have demonstrated that the feasibility of the cigarette filter sorbent for on-line SPE coupled with HPLC-UV for the determination of trace PAHs in environmental water samples. The developed methodology is simple, sensitive and cost-effective. The cigarette filter possesses great potential for its application in the field of PAHs analytical chemistry due to its good adsorption properties and low cost. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 20437020) and the National Basic Research Program of China (No. 2003CB415001). References
Fig. 4. Chromatograms of (a) direct injection of 20 L of a standard solution of NPH (2000 g L−1 ), PHE (200 g L−1 ), ANT (200 g L−1 ), FLU (1000 g L−1 ), BbF (780 g L−1 ), BkF (1000 g L−1 ), BaP (1000 g L−1 ), BghiP (850 g L−1 ); (b) after on-line SPE of the standard solution of NPH (4.0 g L−1 ), PHE (0.40 g L−1 ), ANT (0.40 g L−1 ), FLU (2.0 g L−1 ), BbF (1.6 g L−1 ), BkF (2.0 g L−1 ), BaP (2.0 g L−1 ), BghiP (1.7 g L−1 ). 1: NPH; 2: PHE; 3: ANT; 4: FLU; 5: BbF; 6: BkF; 7: BaP; 8: BghiP.
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