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Ultra-trace determination of lead in water and food samples by using ionic liquid-based single drop microextraction-electrothermal atomic absorption spectrometry
Analytica Chimica Acta 644 (2009) 48–52
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Ultra-trace determination of lead in water and food samples by using ionic liquid-based single drop microextraction-electrothermal atomic absorption spectrometry Jamshid L. Manzoori ∗ , Mohammad Amjadi, Jafar Abulhassani Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 9 February 2009 Received in revised form 15 April 2009 Accepted 18 April 2009 Available online 3 May 2009 Keywords: Single drop microextraction Ionic liquid Lead Electrothermal atomic absorption spectrometry
a b s t r a c t An improved single drop microextraction procedure was developed for the preconcentration of lead prior to its determination by electrothermal atomic absorption spectrometry. Ionic liquid, 1-butyl-3methylimidazolium hexafluorophosphate [C4 MIM][PF6 ], was used as an alternative to volatile organic solvents for extraction. Lead was complexed with ammonium pyrroldinedithiocarbamate (APDC) and extracted into a 7-L ionic liquid drop. The extracted complex was directly injected into the graphite furnace. Several variables affecting microextraction efficiency and ETAAS signal, such as pyrolysis and atomization temperature, pH, APDC concentration, extraction time, drop volume and stirring rate were investigated and optimized. In the optimum experimental conditions, the limit of detection (3s) and the enhancement factor were 0.015 g L−1 and 76, respectively. The relative standard deviation (RSD) for five replicate determinations of 0.2 g L−1 Pb was 5.2%. The developed method was validated by the analysis of certified reference materials and applied successfully to the determination of lead in several real samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Lead is a naturally occurring heavy metal found in the small amounts in earth’s crust. However, nowadays the most important source of lead in our environment is anthropogenic activities such as burning fossil fuels, mining and various manufacturing. Lead is a toxic element and can affect almost every organ or system in human body. The main target for lead toxicity is nervous system. It also increases blood pressure and causes weakness in fingers, wrists and ankles. Moreover, exposure to high level of lead can severely damage kidneys and brain. The International Agency for Research on Cancer (IARC) has determined that inorganic lead is probably carcinogenic to human [1]. Due to these adverse effects, monitoring of lead in environmental, biological and food samples even at ultratrace level is very important. Electrothermal atomic absorption spectrometry (ETAAS) is a powerful and well-established technique for this purpose [2]. But the direct determination of lead at very low concentrations is often difficult because of insufficient sensitivity of this technique as well as the matrix interferences occurring in real samples. For this reason, a preliminary separation and preconcentration step are often required.
∗ Corresponding author. E-mail address: [email protected] (J.L. Manzoori). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.04.029
Liquid–liquid extraction is one of the most widely used techniques for separation/preconcentration of lead due to its simplicity and convenience but its main disadvantage is the requirement for relatively large amounts of toxic organic solvents [3]. Liquid-phase microextraction (LPME) methodologies have recently been developed to overcome this limitation [4–6]. Single drop microextraction (SDME) [7,8] is a mode of LPME with several advantages such as significant reduction in the amount of organic solvent used, simplicity, cost-effectiveness and high sample throughput. Moreover, SDME combines extraction, preconcentration and sample introduction in one step [8]. In recent years, some reports on the use of SDME for preconcentration of lead prior to detection by ETAAS or ETV-ICPMS have been appeared [9–12]. In these procedures, however, very toxic solvents such as benzene, toluene or chloroform have been used as extraction phase. Room temperature ionic liquids (RTILs) have recently attracted special interest as environment-friendly solvents to replace traditional volatile organic solvents in various area of chemistry. They are salts that are liquid over a wide temperature range including room temperature and result from combination of organic cations with various anions. RTILs have some unique physicochemical properties such as negligible vapor pressure, non-flammability as well as good extractability for various organic compounds and metal ions, which make them very useful for LLE and LPME [13,14]. Several reports have been appeared in which RTILs have successfully been utilized for extraction of metal ions as chelate [15–18]. Also
J.L. Manzoori et al. / Analytica Chimica Acta 644 (2009) 48–52
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there are some reports on the use of these solvents for preconcentration of metal ions prior to detection with ETAAS, in which a back-extraction step is required before sample introduction to the graphite furnace [19,20]. The attempt of our research group has currently focused on the application of RTILs in SDME of metal ions with direct injection into the graphite furnace [21]. In the present work, the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [C4 MIM][PF6 ], was employed as a solvent for SDME of lead as ammonium pyrroldinedithiocarbamate (APDC) complex. The extracted complex was directly injected into the graphite furnace. Furthermore, the sensitivity of the method was improved by increasing the volume of suspended drop by attaching a plastic tube to the tip of syringe needle and creating grooves in the inner surface of the tube. 2. Experimental 2.1. Apparatus A Shimadzu (Kyoto, Japan) Model AA-670G atomic absorption spectrometer equipped with a GFA-4A graphite furnace atomizer and deuterium lamp background correction was employed. A lead hollow cathode lamp (Hamamatsu photonics K.K., Japan) was used as the radiation source. The operating conditions of the hollow cathode lamp were those recommended by the manufacturer. Pyrolytically coated graphite tubes were used throughout. Argon 99.999% (Roham gas Co. Tehran, Iran), with 1.5 L min−1 flow rate, was used as a protective and purge gas. The detailed graphite furnace temperature program used for the determination of lead is shown in Table 1. A 10 L microsyringe (Hamilton) was employed to introduce 7 L ionic liquid extracting phase to the solution and to inject it into the graphite furnace. However, It is not possible to suspend a microdrop of [C4 MIM][PF6 ] from the tip of a bare needle because it is easily released. In order to avoid this problem, the tip of microsyringe was inserted into a 3 mm long plastic tube with about 0.6 mm i.d. and 1.5 mm o.d. as proposed by Liu et al. [22]. Furthermore, in order to increase contact area, several grooves were created in the inner surface of the plastic tube by a hot copper wire. In this way, the probability of drop detachment is decreased and microdrops with volumes up to 7 L can be suspended at the tip of syringe with good stability. Fig. 1 shows the schematic representation of SDME setup. A Metrohm model 654 pH meter was used for pH measurements.
Fig. 1. Schematic representation of SDME system. (1) Stirring bar; (2) sample solution; (3) IL microdrop; (4) plastic tube; (5) septum; (6) microsyringe; (7) grooves inside the plastic tube.
A stock standard solution of lead was prepared by dissolving an appropriate amount of Pb(NO3 )2 (Merck) into a 100 mL flask and diluting to the mark with distilled water. The working solutions of lead were made by suitable dilution of the stock solution with doubly distilled water. 1% (w/v) solutions of ammonium pyrrolidinedithiocarbamate (APDC) were prepared by dissolving appropriate amount of APDC (Fluka) in water and used as chelating agent.
2.2. Standard solutions and reagents All chemicals used were of analytical-reagent grade and all solutions were prepared with doubly distilled deionized water (obtained from Ghazi Serum Co., Tabriz, Iran). 1-Butyl-3-methylimidazolium hexafluorophosphate was purchased from Merck (Darmstadt, Germany) and used as obtained.
Table 1 Optimum ETAAS operating conditions for the determination of lead. Wavelength (nm) Lamp current (mA) Spectral bandpass (nm) Background Correction Drying temperature (◦ C) Pyrolysis temperature (◦ C) Atomization temperature (◦ C) Cleaning temperature (◦ C) Argon purge gas flow rate (L min−1 ) Determination mode
217 7 0.3 Deuterium 120 (ramp 20 s) 400 (hold 40 s) 1200 (hold 3 s, gas stop) 2000 (hold 2 s) 1.5 Peak height
2.3. Sample preparation Two standard reference materials were used for validation of the method; (1) NIST SRM 1643e (trace elements in water): suitable aliquot of this sample was diluted 50 fold and its pH was adjusted to 3 with 1.0 mol L−1 HCl and NaOH solutions. (2) NIST SRM 1549 (non-fat milk powder): an accurately measured amount of the sample was placed in a 100 mL beaker and 30 mL concentrated HNO3 and 8 mL concentrated HClO4 was added, covered with a watch glass. The beaker was heated on an oil bath of 100 ◦ C for 45 min. Then the heating process was continued for 45 min at 150 ◦ C. The watch glass was removed and the acid evaporated to dryness at 200 ◦ C [23]. The white residue obtained was dissolved in about 50 mL distilled water and after adjusting its pH to 3 with 1.0 mol L−1 HCl and NaOH, the solution was diluted to the mark in a 100 mL volumetric flask.
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2.4. SDME procedure 1.75 mL of sample was added into a 5 mL capacity vial with a PTFE septum and a magnetic bar. Then 250 L 1% APDC solution was added. The vial was placed on a magnetic stirrer with a stirring rate of 1100 rpm. A 7 L volume of [C4 MIM][PF6 ] was drawn into the microsyringe and the syringe was fixed above the vial with a clamp. After introducing the needle through the septum, the needle tip was immersed into the sample solution and the microdrop was exposed. The microdrop was left for 7 min under constant stirring, and after extraction it was aspirated back into the microsyringe and inserted manually into the graphite furnace. The temperature program in Table 1 followed each injection. Calibration was performed using aqueous calibration solutions submitted to the same SDME procedure described above. Before each extraction, microsyringe was rinsed with ethanol to avoid formation of air bubbles and the carryover of compounds between extractions. 3. Results and discussion 3.1. Optimization of ETAAS conditions In order to decreases the possibility of chemical interference and reduces the magnitude of the background signal, the pyrolysis and atomization temperatures should be optimized. Here, these parameters were studied using 0.2 g L−1 Pb solutions submitted to the SDME procedure. It was found that at the pyrolysis temperature of 400 ◦ C, the maximum absorbance was achieved. At lower pyrolysis temperature, the background signal was high, which is probably due to the vaporization of excess APDC and/or ionic liquid itself at the atomization step. This causes a significant signal suppression, which resulted in the low absorbance values for low pyrolysis temperatures. Increasing pyrolysis temperature above 400 ◦ C leads to loss of analyte and hence decreases analytical signal. Therefore, 400 ◦ C was selected as the optimized pyrolysis temperature for the determination of lead. The effect of pyrolysis time on the absorbance of Pb was also investigated. The results showed that the absorbance was increased with increasing pyrolysis time up to 40 s and no appreciable improvements were observed for longer times. As a result, a pyrolysis time of 40 s was chosen. The atomization temperature was similarly optimized. According to the results, the signal was reached a maximum at about 1200 ◦ C, and then decreased with the further increasing of temperature. So, the atomization temperature of 1200 ◦ C was selected for the further experiments. Since atomization time had little effect on atomic signal, an atomization time of 4 s was selected for atomization of lead. 3.2. Optimization of SDME conditions Extraction of metal ions into RTILs may be accomplished by using a neutral or anionic ligand. In the former case ion-pair formation between ionic complex and the constituent ions of RTILs is believed to be responsible for extraction. However, the use of anionic ligands to form neutral or low-charged metal complexes seems to be preferred in the extraction using RTILs [16]. In this work APDC was chosen as a widely used chelating agent for the microextraction of lead since it can form extractable complex with lead in acidic conditions [24]. The effect of pH on the formation and extraction of Pb–APDC was studied within the range of 1–6. The absorbance signal for lead is relatively constant in the range of 1–3 and diminishes at higher pHs. Therefore, a pH value of 3.0 was selected for further studies. This pH was adjusted by using 1.0 mol L−1 HCl and NaOH solutions.
The effect of APDC concentration on the extraction efficiency of lead was also investigated. The results indicated that the absorbance signal increased with increasing APDC concentrations from 0.015% to 0.1%, as a result of the high extraction efficiency of the Pb–APDC complex to [C4 MIM][PF6 ] microdrop. For APDC concentrations above 0.1%, the absorbance remained unchanged. The value of 0.125% was chosen as the optimum amount of APDC. One of the important factors in SDME, which affects the extraction efficiency, is the volume of the microdrop. It is well known that the extraction efficiency is dependant on the volume ratio of two phases. It has been shown that in the case of microextraction, the amount of analyte extracted by the organic drop is linearly proportional to the drop volume [25]. However, increasing the drop volume in SDME usually result in the release of the microdrop. It has been shown that the stability of the organic drop depends on upward floating force, downward gravity and adhesion forces [26]. In our case since the density of ionic liquid-phase is greater than that of aqueous solution, the balance between gravity and adhesion force determines the stability of microdrop at the tip of needle. Thus, in order to enhance the adhesion force of the microdrop, a plastic tube was attached to the tip of syringe needle and furthermore, the inner surface of the tube was made rough by grooving as described in Section 2.1. In this way, larger volumes of IL can be suspended at tip of needle and also higher stirring rates can be tolerated. The effect of [C4 MIM][PF6 ] drop size on the absorbance signal of lead, investigated in the range of 1–8 L. The results showed that the signal enhanced nearly linearly with the increasing of the microdrop volume. However, when volumes larger than 7 L are used, the microdrop becomes unstable and is easily released from the tip of the syringe needle. So, this volume was selected as appropriate drop size. Another main parameter in SDME is stirring rate, which affects the speed of extraction. The observed rate constant for SDME according to Jeannot and Cantwell [7] is given by: k=
Ai ¯ Vo +1 ˇo K Vo Va
in which Ai is the interfacial area of contact between the ionic liq¯ o is the overall mass transfer coefficient. uid and water phases and ˇ The latter parameter is inversely dependant on the thickness of the interfacial layer surrounding the solvent droplet, which in turn, can be controlled by stirring rate. The effect of sample stirring on the absorbance signal of lead was investigated, which indicated that agitation of the sample greatly improves extraction efficiency. However, a steep increase in the stirring rate may lead to the release of the microdrop from the tip of the syringe needle. Increasing stirring rate can also cause a reduction of [C4 MIM][PF6 ] microdrop volume because of enhancing its dissolution. As a compromise between drop stability and higher diffusion of the analyte, a stirring rate of 1000 rpm was selected in this work. In general, mass transfer is a time-dependent process and the maximum absorbance signal is attained when the system is at equilibrium. However, as long as extraction condition is reproducible, complete equilibrium needs not to attain to obtain accurate and precise analysis. The effect of extraction time has been studied by varying the exposure time of the microdrop to the aqueous solution from 2 to 12 min and the results are shown in Fig. 2. As could be seen, the analytical signal increased with the increase of extraction time. In order to achieve a higher sample throughput, the extraction time of 7 min was selected for all subsequent works. 3.3. Analytical figures of merit In the optimum conditions, a calibration graph was constructed for lead by preconcentrating seven standard solutions according to
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Table 3 Comparison of the proposed method with other preconcentration-ETAAS methods.
Fig. 2. Effect of extraction time on the absorbance of lead. Conditions: lead, 0.2 g L−1 ; pH 3.0; APDC 0.125%; stirring rate 1100 rpm; microdrop volume 7 L.
the procedure under experimental. The linear concentration range was 0.025–0.80 g L−1 with a correlation coefficient 0.9988. The calibration function was Y = 0.2610 (±0.0056)C + 0.0035 (±0.0023), where C is the concentration of lead in g L−1 . The limit of detection (3s) was found to be 0.015 g L−1 , while the corresponding value without preconcentration was 0.66 g L−1 . In order to study the precision of the proposed method a series of six solutions containing 0.2 g L−1 lead were measured at the same day. The relative standard deviation (RSD) was 5.2%. The enhancement factor defined as the slope ratio of two calibration curves with microextraction and without microextraction was 76.
Preconcentration technique
Linear range (g L−1 )
EFa
LOD (g L−1 )
Sample volume (mL)
References
SDME SDME SFODME DLLME DLLME CPE SPE IL-LLE IL-SDME
0–40 1–15 4–30b 0.05–1.0 0.1–20 0.08–30 – 0.001–0.1 0.025–0.8
16 52 500 150 78 50 21 200 76
0.025 0.2 0.9b 0.02 0.039 0.08 0.007 0.001 0.015
2 2 10 5 5 10 3 1000 1.75
[12] [10] [27] [28] [29] [30] [31] [19] This work
SDME: single drop microextraction, SFODME: solidified floating organic drop microextraction, DLLME: dispersive liquid–liquid microectraction, CPE: cloud point extraction, SPE: solid phase extraction, IL-LLE: ionic liquid-based liquid–liquid extraction. a Enhancement or enrichment factor. b ng L−1 .
in water, and NIST SRM 1549 non-fat milk powder. The certified amount of lead in SRM 1643e is 19.63 ± 0.21 g L−1 and in SRM 1549 is 0.019 ± 0.003 g g−1 . The obtained values by using the proposed SDME–ETAAS method were 18.58 ± 0.03 g L−1 and 0.020 ± 0.004 g g−1 , respectively, which are in good agreements with the certified concentrations. It can be concluded that the proposed method is accurate and free from systematic errors. This method was also applied to various real water samples and satisfactory results were obtained. 4. Conclusion
3.4. Study of interferences In order to demonstrate the selectivity of the developed microextraction method for the determination of lead, the effect of alkali, alkaline earth and several heavy metals that are common elements in environmental and food samples have been investigated. Different amounts of ions were added to the test solutions containing 0.2 g L−1 of lead and then followed according to general procedure. An ion was considered to interfere when its presence produced a variation of more than 5% in the absorbance of the sample. The results are shown in Table 2. It can be seen that commonly encountered concomitant ions such as alkali and alkaline earth elements do not interfere at high concentrations. Whereas, some of the transition metal ions can interfere at ratios higher than 50 fold. However, in most of real samples the concentration ratio of these ions to lead is much lower than these values. As shown below, these results permit the application of the proposed system for interference-free determination of ultra-trace lead in water and food samples. 3.5. Analysis of real samples In order to verify the accuracy of the proposed procedure, the method was applied to the determination of lead in two standard reference materials, NIST SRM 1643e Trace elements Table 2 Tolerance limits of interfering ions in the determination of 0.2 g L−1 Pb. Coexisting ions
Foreign ion to analyte ratio
Na+ , K+ , Ca2+ , Mg2+ , NO3 − , Cl− Li+ , SO4 2− , CH3 COO− , CO3 2− , F− Ag+ , I- , Al3+ , Ba2+ Cr3+ , Ni2+ , Co2+ , As3+ , Bi3+ Mn2+ , V5+ , Cd2+ , Fe3+ , Zn2+ , Cu2+ , Fe2+
50,000a 10,000 5000 100 50
a
Maximum ratio tested.
A novel SDME–ETAAS method based on the use of RTILs as extraction solvent has been proposed for the determination of lead in water and food samples. The method was proved to be simple, selective, fast and environment-friendly. It was successfully applied to monitor low concentrations of lead in real water samples with good accuracy and precision. Comparison of analytical features of this method with those of some other preconcentrationETAAS techniques (Table 3) indicates that the linear range and LOD of the proposed method are better than or comparable with most of other methods. Solidified floating organic drop microextraction has somewhat extraordinary characteristics, but it should be mentioned that this technique is relatively time-consuming and has very limited linear range. The data in Table 3 also reveals that our proposed method requires smallest volume of sample while having comparable and good enhancement factor. References [1] Toxicological Profile for Lead, U.S. ATSDR, 2000 http://www.atsdr.cdc.gov/ toxprofiles/tp13.html. [2] ASTM D 3558-96: Standard Test Methods for Lead in Water, 2002. [3] M.d.G.A. Korn, J.B. de Andrade, D.S. de Jesus, V.A. Lemos, M.L.S.F. Banderia, W.N.L. dos Santos, M.A. Bezerra, F.A.C. Amorim, A.S. Souza, S.L.C. Ferreira, Talanta 69 (2006) 16. [4] M.A. Jeannot, F.F. Cantwell, Anal. Chem. 68 (1996) 2236. [5] Y. He, H.K. Lee, Anal. Chem. 69 (1997) 4634. [6] F. Pena-Pereira, I. Lavilla, C. Bendicho, Spectrochim. Acta B 64 (2009) 1. [7] M.A. Jeannot, F.F. Cantwell, Anal. Chem. 69 (1997) 235. [8] L. Xu, C. Basheer, H.K. Lee, J. Chromatogr. A 1152 (2007) 184. [9] L. Li, B. Hu, L. Xia, Z. Jiang, Talanta 70 (2006) 468. [10] H.F. Maltez, D.L. Borges, E. Carasek, B. Welz, A.J. Curtius, Talanta 74 (2008) 800. [11] H. Jiang, B. Hu, Microchim. Acta 161 (2008) 101. [12] P. Liang, R. Liu, J. Cao, Microchim. Acta 160 (2008) 135. [13] J.-F. Liu, J.A. Jönsson, G.-b. Jiang, Trend. Anal. Chem. 24 (2005) 20. [14] S. Pandey, Anal. Chim. Acta 556 (2006) 38. [15] G.-T. Wei, Z. Yang, C.-J. Chen, Anal. Chim. Acta 488 (2003) 183. [16] N. Hirayama, M. Deguchi, H. Kawasumi, T. Honjo, Talanta 65 (2005) 255. [17] Z. Li, Q. Wei, R. Yuan, X. Zhou, H. Liu, H. Shan, Q. Song, Talanta 71 (2007) 68. [18] Z. Li, N. Lu, X. Zhou, H. Liu, Q. Song, J. Pharm. Biomed. Anal. 43 (2007) 1609. [19] H. Shan, Z. Li, M. Li, Microchim. Acta 159 (2007) 95.
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[20] Z. Li, Q. Peng, H. Shan, J. AOAC Int. 90 (2007) 1191. [21] J.L. Manzoori, M. Amjadi, J. Abulhasani, Talanta 77 (2009) 1539. [22] J.-F. Liu, Y.-G. Chi, G.-B. Jiang, C. Tai, J.-F. Peng, J.-T. Hu, J. Chromatogr. A 1026 (2004) 143. [23] M.H. Pournaghi-Azar, M.R. Ramazani, Electroanalysis 14 (2002) 1203. [24] K.L. Cheng, K. Ueno, T. Imamura, Handbook of Organic Analytical Reagents, CRC Press, Boca Raton, 1982, p. 397. [25] L. Hou, H.K. Lee, J. Chromatogr. A 976 (2002) 377.
[26] C. Ye, Q. Zhou, X. Wang, J. Chromatogr. A 1139 (2007) 7. [27] S. Dadfarnia, A.M. Salmanzadeh, A.M. Haji Shabani, Anal. Chim. Acta 623 (2008) 163. [28] M.T. Naseri, M.R.M. Hosseini, Y. Asadi, A. Kiani, Talanta 75 (2008) 56. [29] P. Liang, H. Sang, Anal. Biochem. 380 (2008) 21. [30] J. Chen, S. Xiao, X. Wu, K. Fang, W. Liu, Talanta 67 (2005) 992. [31] S. Xiangguang, W. Meijia, Z. Yihua, Z. Jianhua, Z. Hanqi, J. Qinhan, Talanta 59 (2003) 989.
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Ultra-trace determination of lead in water and food samples by using ionic liquid-based single drop microextraction-electrothermal atomic absorption spectrometry Jamshid L. Manzoori ∗ , Mohammad Amjadi, Jafar Abulhassani Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 9 February 2009 Received in revised form 15 April 2009 Accepted 18 April 2009 Available online 3 May 2009 Keywords: Single drop microextraction Ionic liquid Lead Electrothermal atomic absorption spectrometry
a b s t r a c t An improved single drop microextraction procedure was developed for the preconcentration of lead prior to its determination by electrothermal atomic absorption spectrometry. Ionic liquid, 1-butyl-3methylimidazolium hexafluorophosphate [C4 MIM][PF6 ], was used as an alternative to volatile organic solvents for extraction. Lead was complexed with ammonium pyrroldinedithiocarbamate (APDC) and extracted into a 7-L ionic liquid drop. The extracted complex was directly injected into the graphite furnace. Several variables affecting microextraction efficiency and ETAAS signal, such as pyrolysis and atomization temperature, pH, APDC concentration, extraction time, drop volume and stirring rate were investigated and optimized. In the optimum experimental conditions, the limit of detection (3s) and the enhancement factor were 0.015 g L−1 and 76, respectively. The relative standard deviation (RSD) for five replicate determinations of 0.2 g L−1 Pb was 5.2%. The developed method was validated by the analysis of certified reference materials and applied successfully to the determination of lead in several real samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Lead is a naturally occurring heavy metal found in the small amounts in earth’s crust. However, nowadays the most important source of lead in our environment is anthropogenic activities such as burning fossil fuels, mining and various manufacturing. Lead is a toxic element and can affect almost every organ or system in human body. The main target for lead toxicity is nervous system. It also increases blood pressure and causes weakness in fingers, wrists and ankles. Moreover, exposure to high level of lead can severely damage kidneys and brain. The International Agency for Research on Cancer (IARC) has determined that inorganic lead is probably carcinogenic to human [1]. Due to these adverse effects, monitoring of lead in environmental, biological and food samples even at ultratrace level is very important. Electrothermal atomic absorption spectrometry (ETAAS) is a powerful and well-established technique for this purpose [2]. But the direct determination of lead at very low concentrations is often difficult because of insufficient sensitivity of this technique as well as the matrix interferences occurring in real samples. For this reason, a preliminary separation and preconcentration step are often required.
∗ Corresponding author. E-mail address: [email protected] (J.L. Manzoori). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.04.029
Liquid–liquid extraction is one of the most widely used techniques for separation/preconcentration of lead due to its simplicity and convenience but its main disadvantage is the requirement for relatively large amounts of toxic organic solvents [3]. Liquid-phase microextraction (LPME) methodologies have recently been developed to overcome this limitation [4–6]. Single drop microextraction (SDME) [7,8] is a mode of LPME with several advantages such as significant reduction in the amount of organic solvent used, simplicity, cost-effectiveness and high sample throughput. Moreover, SDME combines extraction, preconcentration and sample introduction in one step [8]. In recent years, some reports on the use of SDME for preconcentration of lead prior to detection by ETAAS or ETV-ICPMS have been appeared [9–12]. In these procedures, however, very toxic solvents such as benzene, toluene or chloroform have been used as extraction phase. Room temperature ionic liquids (RTILs) have recently attracted special interest as environment-friendly solvents to replace traditional volatile organic solvents in various area of chemistry. They are salts that are liquid over a wide temperature range including room temperature and result from combination of organic cations with various anions. RTILs have some unique physicochemical properties such as negligible vapor pressure, non-flammability as well as good extractability for various organic compounds and metal ions, which make them very useful for LLE and LPME [13,14]. Several reports have been appeared in which RTILs have successfully been utilized for extraction of metal ions as chelate [15–18]. Also
J.L. Manzoori et al. / Analytica Chimica Acta 644 (2009) 48–52
49
there are some reports on the use of these solvents for preconcentration of metal ions prior to detection with ETAAS, in which a back-extraction step is required before sample introduction to the graphite furnace [19,20]. The attempt of our research group has currently focused on the application of RTILs in SDME of metal ions with direct injection into the graphite furnace [21]. In the present work, the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [C4 MIM][PF6 ], was employed as a solvent for SDME of lead as ammonium pyrroldinedithiocarbamate (APDC) complex. The extracted complex was directly injected into the graphite furnace. Furthermore, the sensitivity of the method was improved by increasing the volume of suspended drop by attaching a plastic tube to the tip of syringe needle and creating grooves in the inner surface of the tube. 2. Experimental 2.1. Apparatus A Shimadzu (Kyoto, Japan) Model AA-670G atomic absorption spectrometer equipped with a GFA-4A graphite furnace atomizer and deuterium lamp background correction was employed. A lead hollow cathode lamp (Hamamatsu photonics K.K., Japan) was used as the radiation source. The operating conditions of the hollow cathode lamp were those recommended by the manufacturer. Pyrolytically coated graphite tubes were used throughout. Argon 99.999% (Roham gas Co. Tehran, Iran), with 1.5 L min−1 flow rate, was used as a protective and purge gas. The detailed graphite furnace temperature program used for the determination of lead is shown in Table 1. A 10 L microsyringe (Hamilton) was employed to introduce 7 L ionic liquid extracting phase to the solution and to inject it into the graphite furnace. However, It is not possible to suspend a microdrop of [C4 MIM][PF6 ] from the tip of a bare needle because it is easily released. In order to avoid this problem, the tip of microsyringe was inserted into a 3 mm long plastic tube with about 0.6 mm i.d. and 1.5 mm o.d. as proposed by Liu et al. [22]. Furthermore, in order to increase contact area, several grooves were created in the inner surface of the plastic tube by a hot copper wire. In this way, the probability of drop detachment is decreased and microdrops with volumes up to 7 L can be suspended at the tip of syringe with good stability. Fig. 1 shows the schematic representation of SDME setup. A Metrohm model 654 pH meter was used for pH measurements.
Fig. 1. Schematic representation of SDME system. (1) Stirring bar; (2) sample solution; (3) IL microdrop; (4) plastic tube; (5) septum; (6) microsyringe; (7) grooves inside the plastic tube.
A stock standard solution of lead was prepared by dissolving an appropriate amount of Pb(NO3 )2 (Merck) into a 100 mL flask and diluting to the mark with distilled water. The working solutions of lead were made by suitable dilution of the stock solution with doubly distilled water. 1% (w/v) solutions of ammonium pyrrolidinedithiocarbamate (APDC) were prepared by dissolving appropriate amount of APDC (Fluka) in water and used as chelating agent.
2.2. Standard solutions and reagents All chemicals used were of analytical-reagent grade and all solutions were prepared with doubly distilled deionized water (obtained from Ghazi Serum Co., Tabriz, Iran). 1-Butyl-3-methylimidazolium hexafluorophosphate was purchased from Merck (Darmstadt, Germany) and used as obtained.
Table 1 Optimum ETAAS operating conditions for the determination of lead. Wavelength (nm) Lamp current (mA) Spectral bandpass (nm) Background Correction Drying temperature (◦ C) Pyrolysis temperature (◦ C) Atomization temperature (◦ C) Cleaning temperature (◦ C) Argon purge gas flow rate (L min−1 ) Determination mode
217 7 0.3 Deuterium 120 (ramp 20 s) 400 (hold 40 s) 1200 (hold 3 s, gas stop) 2000 (hold 2 s) 1.5 Peak height
2.3. Sample preparation Two standard reference materials were used for validation of the method; (1) NIST SRM 1643e (trace elements in water): suitable aliquot of this sample was diluted 50 fold and its pH was adjusted to 3 with 1.0 mol L−1 HCl and NaOH solutions. (2) NIST SRM 1549 (non-fat milk powder): an accurately measured amount of the sample was placed in a 100 mL beaker and 30 mL concentrated HNO3 and 8 mL concentrated HClO4 was added, covered with a watch glass. The beaker was heated on an oil bath of 100 ◦ C for 45 min. Then the heating process was continued for 45 min at 150 ◦ C. The watch glass was removed and the acid evaporated to dryness at 200 ◦ C [23]. The white residue obtained was dissolved in about 50 mL distilled water and after adjusting its pH to 3 with 1.0 mol L−1 HCl and NaOH, the solution was diluted to the mark in a 100 mL volumetric flask.
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2.4. SDME procedure 1.75 mL of sample was added into a 5 mL capacity vial with a PTFE septum and a magnetic bar. Then 250 L 1% APDC solution was added. The vial was placed on a magnetic stirrer with a stirring rate of 1100 rpm. A 7 L volume of [C4 MIM][PF6 ] was drawn into the microsyringe and the syringe was fixed above the vial with a clamp. After introducing the needle through the septum, the needle tip was immersed into the sample solution and the microdrop was exposed. The microdrop was left for 7 min under constant stirring, and after extraction it was aspirated back into the microsyringe and inserted manually into the graphite furnace. The temperature program in Table 1 followed each injection. Calibration was performed using aqueous calibration solutions submitted to the same SDME procedure described above. Before each extraction, microsyringe was rinsed with ethanol to avoid formation of air bubbles and the carryover of compounds between extractions. 3. Results and discussion 3.1. Optimization of ETAAS conditions In order to decreases the possibility of chemical interference and reduces the magnitude of the background signal, the pyrolysis and atomization temperatures should be optimized. Here, these parameters were studied using 0.2 g L−1 Pb solutions submitted to the SDME procedure. It was found that at the pyrolysis temperature of 400 ◦ C, the maximum absorbance was achieved. At lower pyrolysis temperature, the background signal was high, which is probably due to the vaporization of excess APDC and/or ionic liquid itself at the atomization step. This causes a significant signal suppression, which resulted in the low absorbance values for low pyrolysis temperatures. Increasing pyrolysis temperature above 400 ◦ C leads to loss of analyte and hence decreases analytical signal. Therefore, 400 ◦ C was selected as the optimized pyrolysis temperature for the determination of lead. The effect of pyrolysis time on the absorbance of Pb was also investigated. The results showed that the absorbance was increased with increasing pyrolysis time up to 40 s and no appreciable improvements were observed for longer times. As a result, a pyrolysis time of 40 s was chosen. The atomization temperature was similarly optimized. According to the results, the signal was reached a maximum at about 1200 ◦ C, and then decreased with the further increasing of temperature. So, the atomization temperature of 1200 ◦ C was selected for the further experiments. Since atomization time had little effect on atomic signal, an atomization time of 4 s was selected for atomization of lead. 3.2. Optimization of SDME conditions Extraction of metal ions into RTILs may be accomplished by using a neutral or anionic ligand. In the former case ion-pair formation between ionic complex and the constituent ions of RTILs is believed to be responsible for extraction. However, the use of anionic ligands to form neutral or low-charged metal complexes seems to be preferred in the extraction using RTILs [16]. In this work APDC was chosen as a widely used chelating agent for the microextraction of lead since it can form extractable complex with lead in acidic conditions [24]. The effect of pH on the formation and extraction of Pb–APDC was studied within the range of 1–6. The absorbance signal for lead is relatively constant in the range of 1–3 and diminishes at higher pHs. Therefore, a pH value of 3.0 was selected for further studies. This pH was adjusted by using 1.0 mol L−1 HCl and NaOH solutions.
The effect of APDC concentration on the extraction efficiency of lead was also investigated. The results indicated that the absorbance signal increased with increasing APDC concentrations from 0.015% to 0.1%, as a result of the high extraction efficiency of the Pb–APDC complex to [C4 MIM][PF6 ] microdrop. For APDC concentrations above 0.1%, the absorbance remained unchanged. The value of 0.125% was chosen as the optimum amount of APDC. One of the important factors in SDME, which affects the extraction efficiency, is the volume of the microdrop. It is well known that the extraction efficiency is dependant on the volume ratio of two phases. It has been shown that in the case of microextraction, the amount of analyte extracted by the organic drop is linearly proportional to the drop volume [25]. However, increasing the drop volume in SDME usually result in the release of the microdrop. It has been shown that the stability of the organic drop depends on upward floating force, downward gravity and adhesion forces [26]. In our case since the density of ionic liquid-phase is greater than that of aqueous solution, the balance between gravity and adhesion force determines the stability of microdrop at the tip of needle. Thus, in order to enhance the adhesion force of the microdrop, a plastic tube was attached to the tip of syringe needle and furthermore, the inner surface of the tube was made rough by grooving as described in Section 2.1. In this way, larger volumes of IL can be suspended at tip of needle and also higher stirring rates can be tolerated. The effect of [C4 MIM][PF6 ] drop size on the absorbance signal of lead, investigated in the range of 1–8 L. The results showed that the signal enhanced nearly linearly with the increasing of the microdrop volume. However, when volumes larger than 7 L are used, the microdrop becomes unstable and is easily released from the tip of the syringe needle. So, this volume was selected as appropriate drop size. Another main parameter in SDME is stirring rate, which affects the speed of extraction. The observed rate constant for SDME according to Jeannot and Cantwell [7] is given by: k=
Ai ¯ Vo +1 ˇo K Vo Va
in which Ai is the interfacial area of contact between the ionic liq¯ o is the overall mass transfer coefficient. uid and water phases and ˇ The latter parameter is inversely dependant on the thickness of the interfacial layer surrounding the solvent droplet, which in turn, can be controlled by stirring rate. The effect of sample stirring on the absorbance signal of lead was investigated, which indicated that agitation of the sample greatly improves extraction efficiency. However, a steep increase in the stirring rate may lead to the release of the microdrop from the tip of the syringe needle. Increasing stirring rate can also cause a reduction of [C4 MIM][PF6 ] microdrop volume because of enhancing its dissolution. As a compromise between drop stability and higher diffusion of the analyte, a stirring rate of 1000 rpm was selected in this work. In general, mass transfer is a time-dependent process and the maximum absorbance signal is attained when the system is at equilibrium. However, as long as extraction condition is reproducible, complete equilibrium needs not to attain to obtain accurate and precise analysis. The effect of extraction time has been studied by varying the exposure time of the microdrop to the aqueous solution from 2 to 12 min and the results are shown in Fig. 2. As could be seen, the analytical signal increased with the increase of extraction time. In order to achieve a higher sample throughput, the extraction time of 7 min was selected for all subsequent works. 3.3. Analytical figures of merit In the optimum conditions, a calibration graph was constructed for lead by preconcentrating seven standard solutions according to
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Table 3 Comparison of the proposed method with other preconcentration-ETAAS methods.
Fig. 2. Effect of extraction time on the absorbance of lead. Conditions: lead, 0.2 g L−1 ; pH 3.0; APDC 0.125%; stirring rate 1100 rpm; microdrop volume 7 L.
the procedure under experimental. The linear concentration range was 0.025–0.80 g L−1 with a correlation coefficient 0.9988. The calibration function was Y = 0.2610 (±0.0056)C + 0.0035 (±0.0023), where C is the concentration of lead in g L−1 . The limit of detection (3s) was found to be 0.015 g L−1 , while the corresponding value without preconcentration was 0.66 g L−1 . In order to study the precision of the proposed method a series of six solutions containing 0.2 g L−1 lead were measured at the same day. The relative standard deviation (RSD) was 5.2%. The enhancement factor defined as the slope ratio of two calibration curves with microextraction and without microextraction was 76.
Preconcentration technique
Linear range (g L−1 )
EFa
LOD (g L−1 )
Sample volume (mL)
References
SDME SDME SFODME DLLME DLLME CPE SPE IL-LLE IL-SDME
0–40 1–15 4–30b 0.05–1.0 0.1–20 0.08–30 – 0.001–0.1 0.025–0.8
16 52 500 150 78 50 21 200 76
0.025 0.2 0.9b 0.02 0.039 0.08 0.007 0.001 0.015
2 2 10 5 5 10 3 1000 1.75
[12] [10] [27] [28] [29] [30] [31] [19] This work
SDME: single drop microextraction, SFODME: solidified floating organic drop microextraction, DLLME: dispersive liquid–liquid microectraction, CPE: cloud point extraction, SPE: solid phase extraction, IL-LLE: ionic liquid-based liquid–liquid extraction. a Enhancement or enrichment factor. b ng L−1 .
in water, and NIST SRM 1549 non-fat milk powder. The certified amount of lead in SRM 1643e is 19.63 ± 0.21 g L−1 and in SRM 1549 is 0.019 ± 0.003 g g−1 . The obtained values by using the proposed SDME–ETAAS method were 18.58 ± 0.03 g L−1 and 0.020 ± 0.004 g g−1 , respectively, which are in good agreements with the certified concentrations. It can be concluded that the proposed method is accurate and free from systematic errors. This method was also applied to various real water samples and satisfactory results were obtained. 4. Conclusion
3.4. Study of interferences In order to demonstrate the selectivity of the developed microextraction method for the determination of lead, the effect of alkali, alkaline earth and several heavy metals that are common elements in environmental and food samples have been investigated. Different amounts of ions were added to the test solutions containing 0.2 g L−1 of lead and then followed according to general procedure. An ion was considered to interfere when its presence produced a variation of more than 5% in the absorbance of the sample. The results are shown in Table 2. It can be seen that commonly encountered concomitant ions such as alkali and alkaline earth elements do not interfere at high concentrations. Whereas, some of the transition metal ions can interfere at ratios higher than 50 fold. However, in most of real samples the concentration ratio of these ions to lead is much lower than these values. As shown below, these results permit the application of the proposed system for interference-free determination of ultra-trace lead in water and food samples. 3.5. Analysis of real samples In order to verify the accuracy of the proposed procedure, the method was applied to the determination of lead in two standard reference materials, NIST SRM 1643e Trace elements Table 2 Tolerance limits of interfering ions in the determination of 0.2 g L−1 Pb. Coexisting ions
Foreign ion to analyte ratio
Na+ , K+ , Ca2+ , Mg2+ , NO3 − , Cl− Li+ , SO4 2− , CH3 COO− , CO3 2− , F− Ag+ , I- , Al3+ , Ba2+ Cr3+ , Ni2+ , Co2+ , As3+ , Bi3+ Mn2+ , V5+ , Cd2+ , Fe3+ , Zn2+ , Cu2+ , Fe2+
50,000a 10,000 5000 100 50
a
Maximum ratio tested.
A novel SDME–ETAAS method based on the use of RTILs as extraction solvent has been proposed for the determination of lead in water and food samples. The method was proved to be simple, selective, fast and environment-friendly. It was successfully applied to monitor low concentrations of lead in real water samples with good accuracy and precision. Comparison of analytical features of this method with those of some other preconcentrationETAAS techniques (Table 3) indicates that the linear range and LOD of the proposed method are better than or comparable with most of other methods. Solidified floating organic drop microextraction has somewhat extraordinary characteristics, but it should be mentioned that this technique is relatively time-consuming and has very limited linear range. The data in Table 3 also reveals that our proposed method requires smallest volume of sample while having comparable and good enhancement factor. References [1] Toxicological Profile for Lead, U.S. ATSDR, 2000 http://www.atsdr.cdc.gov/ toxprofiles/tp13.html. [2] ASTM D 3558-96: Standard Test Methods for Lead in Water, 2002. [3] M.d.G.A. Korn, J.B. de Andrade, D.S. de Jesus, V.A. Lemos, M.L.S.F. Banderia, W.N.L. dos Santos, M.A. Bezerra, F.A.C. Amorim, A.S. Souza, S.L.C. Ferreira, Talanta 69 (2006) 16. [4] M.A. Jeannot, F.F. Cantwell, Anal. Chem. 68 (1996) 2236. [5] Y. He, H.K. Lee, Anal. Chem. 69 (1997) 4634. [6] F. Pena-Pereira, I. Lavilla, C. Bendicho, Spectrochim. Acta B 64 (2009) 1. [7] M.A. Jeannot, F.F. Cantwell, Anal. Chem. 69 (1997) 235. [8] L. Xu, C. Basheer, H.K. Lee, J. Chromatogr. A 1152 (2007) 184. [9] L. Li, B. Hu, L. Xia, Z. Jiang, Talanta 70 (2006) 468. [10] H.F. Maltez, D.L. Borges, E. Carasek, B. Welz, A.J. Curtius, Talanta 74 (2008) 800. [11] H. Jiang, B. Hu, Microchim. Acta 161 (2008) 101. [12] P. Liang, R. Liu, J. Cao, Microchim. Acta 160 (2008) 135. [13] J.-F. Liu, J.A. Jönsson, G.-b. Jiang, Trend. Anal. Chem. 24 (2005) 20. [14] S. Pandey, Anal. Chim. Acta 556 (2006) 38. [15] G.-T. Wei, Z. Yang, C.-J. Chen, Anal. Chim. Acta 488 (2003) 183. [16] N. Hirayama, M. Deguchi, H. Kawasumi, T. Honjo, Talanta 65 (2005) 255. [17] Z. Li, Q. Wei, R. Yuan, X. Zhou, H. Liu, H. Shan, Q. Song, Talanta 71 (2007) 68. [18] Z. Li, N. Lu, X. Zhou, H. Liu, Q. Song, J. Pharm. Biomed. Anal. 43 (2007) 1609. [19] H. Shan, Z. Li, M. Li, Microchim. Acta 159 (2007) 95.
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