Cloud point extraction flow injection–atomic absorption spectrometry for determination of lead in water samples Mir Mahdi Zahedi1, Nasser Dalali1*, Yadollah Yamini2 1
Department of Chemistry, Faculty of Science, Zanjan University, Zanjan, 45195-313, Iran; Department of Chemistry, Faculty of Science, Tarbiat Modarres University, Tehran, Iran
2
Received: November 4, 2008
Abstract A cloud point extraction-flow injection-atomic absorption spectrometry is developed for determination of lead (II) in water samples. The analyte is complexed with 2-guanidino benzimidazole at pH 8.5 and extracted by using the non-ionic surfactant, Triton X-114. The surfactant-rich phase containing the complex was collected in a mini-column packed with cotton wool and then eluted by passing an ethanolic solution of 0.2 M HNO3, into the nebulizer of flame atomic absorption spectrometer for analysis of lead content. The calibration curve is linear over the range of 6–600 µg L-1, with a detection limit (3s/m) of 2.0 µg L-1. The relative standard deviation for 6 replicate measurements of 100 µg L-1 Pb was 2.7 %. The method was successfully applied to the determination of lead in water and waste water samples. Keywords: Cloud point extraction, Flow injection, FAAS, Lead, 2-Guanidinobenzimidazole Résumé Nous avons développé un protocole extraction au point de trouble - analyse en continu - spectrométrie d’absorption atomique, pour la détermination du plomb (II) dans l’eau. Le métal a d’abord été complexé par 2guanidino benzimidazole à pH 8.5, avant d’être extrait par le surfactant non-ionique Triton X-114. La phase riche en surfactant contenant le complexe a été collectée dans une mini-colonne remplie de laine de coton et éluée par une solution éthanol de HNO3 0.2 M, directement dans le nébuliseur du spectromètre d’absorption *
Author to whom correspondence should be addressed: E-mail address:
[email protected] (Nasser Dalali)
Accepted (in revised form): February 3, 2009
atomique à flamme, pour l’analyse du contenu en plomb. La droite de calibration est linéaire sur la gamme 6–600 µg L-1, avec une limite de détection (3s/m) de 2.0 µg L-1. Un écart-type relatif de 2.7% a été obtenu sur 6 mesures répétées de 100 µg L-1 Pb. La méthode a été appliquée avec succès pour la détermination du plomb dans des échantillons d’eaux naturelle et de rejet. Introduction Lead is considered as a toxic element and chemical pollutant, which tends to be concentrated in environmental systems and humans. Absorption from the air in local environments and intake from the diet, are the main sources of human exposure. Lead can be accumulated in bone and in some soft tissues such as the kidney, liver and brain [1, 2]. Therefore its determination in environmental samples is of great interest for pollution control. Different methods for trace lead analysis have been employed such as X- ray fluorescence [3], activation analysis [4] and electrochemistry [5]. However, most of these methods require fairly sophisticated instrumentation. Lead can be determined directly by electro thermal atomic absorption spectrometry (ETAAS) or by inductively-coupled plasma mass spectrometry (ICPMS), with low detection limit [6, 7]. Flame atomic absorption spectrometry (FAAS) is a more accessible technique and has wide applications for determination of metal ions in solutions because of its availability in most laboratories. The technique is less subject to interferences than ETAAS or ICP-MS. However, its sensitivity is not usually sufficient for the low concentrations of Pb in water samples. Hence, preconcentration of the metal ion is required prior to its determination by FAAS. Liquid-liquid extraction [8,9], cation exchange techniques [10,11], sorption on different adsorbents Canadian Journal of Analytical Sciences and Spectroscopy
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Mir Mahdi Zahedi, Nasser Dalali and Yadollah Yamini
such as activated carbon [12,13], Amberlite XAD resins [14,15] and modified C-18 membrane disks [16] have been reported for this purpose. Methods involving flow injection (FI) [17] are preferred to those in batch, due to the advantages obtained in method automation. Many on-line systems have been developed using different analytical techniques such as precipitation [18, 19], co-precipitation [20] and adsorption on C-18 bonded silica [21, 22], Amberlite XAD-2 loaded with BTAC [23], and activated carbon [24]. Separations and preconcentrations based on cloud point extraction (CPE) in the micellar solutions using surfactants are finding important applications in analytical chemistry. The technique is based on the properties of non-ionic and zwitter-ionic surfactants in aqueous solutions to form micelles and become turbid when heated above a temperature known as cloud point temperature [25, 26]. At higher temperatures, two distinct phases are formed, one is surfactant-rich and the other contains small amount of surfactant equal to the critical micellar concentration (CMC) [27, 28]. The phase separation is attributed to the rapid increase in the aggregation number of micelles as a result of the increase of temperature [29]. The use of CPE methodology offers a convenient alternative to more conventional extraction systems for preconcentration of trace organic pollutants as well as metal ions. The advantages of the method include safety, rapidness, simplicity, high efficiency and relatively low cost [30-34]. Fernandez Lacspada et al. were the first to recognize the advantages of combining CPE with flow injection (FIA), where the cloud point methodology was performed off-line [35]. The on-line CPE/FIA system was reported by Fang et al. for the first time [36]. Since then, several on-line CPE works have been reported using UV/Vis spectrophotometric and chemiluminescence’s detection techniques [37, 38]. Flow injection atomic absorption spectrometry (FIAAS) systems offer several advantages of analytical nature. These include an improvement in nebulization efficiency in certain circumstances and the ability to easily adjust the signal magnitude. The measurements performed in FIAAS systems are less influenced by differences in the viscosity of samples or differences in matrix composition [39]. The most common sample pretreatment method used in FIAAS systems is on-line solid-phase extraction (SPE) [21-24]. We have recently reported the CPE-FIAAS method for trace determination of silver [40]. In continuation to our previous report [41], and by considering the advantages of CPE, flow injection methodology and the availability of FAAS, the present work is Volume 54, No. 1, 2009
aimed to investigate the cloud point extraction – FIAAS determination of lead with 2-guanidino benzimidazole (2-GBI) as chelating agent. No attempt has been cited for the use of 2-GBI as a complexing agent for on-line FI extraction/determination of metal ions. The proposed method was successfully applied for lead determination in water and waste water samples. Experimental Apparatus A Varian Model SpectrAA220 (Mulgrave, Vic., Australia) flame atomic absorption spectrometer equipped with a Deuterium lamp background corrector was used for the analysis of lead. A Pb hollow cathode lamp operating at 217 nm was used as the radiation source. The lamp current and slit width, were 6 mA and 0.3 nm respectively. The measurements were performed in an air-acetylene flame. The gas flow rate was adjusted in such a way to obtain the maximum absorption signal. A peristaltic pump (Alitea C-6 XV, Sweden) furnished with tygon tubes, was used to propel the solutions. The reaction coil was of PTFE tubing (i.d. 0.5 mm). The minicolumn consisted of a cylindrical glass tube (3 cm × 4 mm) packed with a suitable filtering material was used for collection of the surfactant-rich phase. A Rheodyne 5041 model four-way valve was used for loading/ injection steps. A Julabo MP5 thermostatic bath was operated to reach the cloud point temperature. pH measurements were accomplished with a Metrohm model 692 pH/Ion meter. Reagents and Chemicals Triton X-114 was obtained from Fluka and used as received. 2-guanidino benzimidazole (2-GBI) with analytical grade and stock standard solution of Pb (1000 mg L-1) were procured from Sigma-Aldrich and Merck. Working solutions were prepared by appropriate dilution of the standard solution. All other chemicals and metal salts which were used for the interference study were of highest purity available (Merck). Doubly distilled deionized water was used for preparation of solutions. A stock buffer solution (0.1 M) was prepared by dissolving appropriate amounts of potassium hydrogen phthalate and sodium tetraborate in water and then adjusting to desired pH values by adding dilute HCl or NaOH solution. The ethanolic solutions of HNO3 were prepared by appropriate dilution of concentrated (65 %w/w) nitric acid in ethanol.
Cloud point extraction flow injection-atomic absorption spectrometry for determination of lead in water Procedure The flow system is made of a peristaltic pump fitted with tygon tubes. Reaction coil (R) kept in a thermostatic bath at 40 ˚C, and a four-way valve in which the injection loop was replaced by a glass mini-column packed with cotton wool (CC) used to collect the surfactant-rich phase containing the complex (Figure 1). The system was operated in the time-based mode (where the sample flow rate and loading time governs the amount of sample loaded into the FI system), as follows: In the first stage (Figure 1a), the pH 8.5 buffered sample solution (sample) containing 1 to 600 μg L-1 of lead, 0.07% (v/v) Triton X-114, 1× 10-3 M 2-GBI and 2 × 10-3 M NaClO4 was loaded into FI manifold for 2 min at the rate of 2.5 mL min-1. The surfactant-rich and aqueous phases passed through the mini-column, where the surfactant-rich phase containing the complex was collected, while the aqueous phase was discharged (W). In the second stage (Figure 1b), the valve was switched on to the injection position for 15 s. The complex in the mini-column (CC) was eluted (in the reverse direction) with ethanolic solution of 0.2 M nitric acid (eluent) (8 mL min-1) into the nebulizer of FAAS. The absorption signals were processed in the peak height mode by using instrument software. The average values of peak height were obtained for three replicate experiments. Analysis of standard reference material 0.1 g of sewage sludge BCR-CRM No: 144 R was digested in 15 mL aqua regia and the solution was evaporated to dryness, the process was repeated twice.15 mL double distilled water was added to the residue and the resulting suspension was filtered through the nylon filter (0.45 μm pore size). The filter was washed with 15 mL distilled water, the filtrate and washings were collected
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in a 100 mL standard flask. The solution was neutralized with 0.5 M ammonia solution and diluted up to the mark with distilled water. Results and Discussion The influence of chemical variables such as pH of sample solution, concentrations of 2-GBI,Triton X-114 and counter ion (ClO4- gave better results than NO3and CH3COO-) on the preconcentration efficiency was investigated and optimized. The results of which are summarized in Table 1. Optimization of FI and preconcentration variables The home made mini-column was packed with different filtering materials in order to obtain phase separation by entrapping larger sized surfactant aggregates and allowing the smaller components in the aqueous medium to pass through. 10 mL aliquots of standard solutions containing 200 μg L-1 of lead and 1×10-3 M of 2-GBI buffered at pH 8.5 was passed through the column at the flow rate of 2 mL min-1 for 150 s.The retained complex was then eluted with ethanolic solution of 0.1 M HNO3 at the flow rate of 6 mL min-1for 15 s. Since cotton wool provided the best absorption signals and reproducibility, it was chosen as the filtering material for further evaluation of the performance of the system (Table 2). The lead elution from the CC column (packed with cotton wool) was studied by using pure ethanol and ethanol containing nitric acid of different concentrations (0.0-1 M) as eluting agent at the rate of 6 mL min-1. The complete elution was achieved with concentrations over 0.1 M, hence 0.2 M nitric acid was chosen for further studies (Figure 2). The column was washed with pH 8.5 buffer solution
Figure 1. Schematic diagram of the CPE/ FIAAS manifold for the preconcentration and determination of Pb (II). P: peristaltic pump; TB: thermostatic bath; R: reactor; V: valve; FAAS: flame atomic absorption spectrometer; C.C: collection column (3 cm ×4 mm); W: waste.Valve in the (a) loading step and (b) elution step. Canadian Journal of Analytical Sciences and Spectroscopy
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Table 1. Optimization of chemical variables.
Variable pH 2-GBI concentration (M) Triton X-114 concentration % (v/v) Counter ion concentration (M)
Studied Range 2–11 2×10-5–2×10-3 0.01–0.2 1×10-3–5×10-2
Optimum Value 8.5 1×10-3 0.07 2×10-3
Table 2. Effect of filtering material on the absorption signal and precision.
Filtering Material Glass wool Cotton wool Cigarette filter
Dry Amount 100 mg 100 mg 3 cm × 4 mm
Figure 2. Effect of ethanolic HNO3 concentration on the absorbance of Pb. Conditions: Initial concentration of Pb, 200 μg L-1; Column, 3 cm ×4 mm packed with 100 mg cotton; Sample flow rate, 2.5 mL min-1; Sample loading time, 150 s; Eluent flow rate, 6 mL min-1.
after each elution (before each loading) to recondition the column. In these conditions the average life time of the column was more than 50 cycles. The effect of sample flow rate on the preconcentration was studied in such a way that 5 mL of a solution containing 200 μg L-1 lead was pumped through the system at flow rates of 0.5 to 5 mL min-1. It was observed that the analytical signals decreased slightly by increasing the sample flow rate, which must be due to the incomplete reaction in the coil as well as insufficient detainment of the micellar phase on the mini-column. So the sample flow rate of 2.5 mL min-1 was selected for further studies. The performance of the flow system was investigated by varying the loading time from 30 to 240 s. The analytical signal increased with increase in loading time up to 120 Volume 54, No. 1, 2009
Absorbance 0.07 0.10 0.03
% R.S.D. (n = 4) 2.5 1.7 3.2
s, above which there was no significant change in signal (Figure 3). Hence, the loading time of 120 s was chosen. The length of column was another parameter optimized for its effect on the preconcentration efficiency. Columns with 0.5, 1, 2, 3, and 5 cm in length packed with cotton wool were tested. A 3 cm length column was chosen, since it gave the highest absorption signal. The shorter columns were inadequate for retaining the surfactant-rich phase, while the longer column decreased the signal due to the insufficient elution of the micellar aggregates. The amount of cotton wool in the mini-column is important for collecting the surfactant-rich phase; hence it was optimized for the selected column length by testing different amounts of dry cotton wool. The best results were obtained when ca.120 mg was used. Larger amounts of cotton wool led to the deterioration of the signal due to blockage of tubes and insufficient elution of the retained complex; when a smaller amount was used, collection was not efficient. The influence of elution rate was also investigated in the range of 2-10 mL min-1. Figure 4 shows that the absorption peak increased with increase in the flow rate and reached a maximum at flow rates in the range of 6-10 mL min-1. At flow rates below 6 mL min-1, the signal decreased considerably, resulting in broader peaks. This must be due to the incompatibility between elution and nebulization flow rates, which can cause a significant dispersion when eluent flow rate is smaller than nebulizer flow rate. Hence, a flow rate of 8 mL min-1 was selected for elution. The optimization results are summarized in Table 3. Interference study The effect of diverse ions on the determination of 400 μg L-1 lead was studied. The common ions at the usual concentrations found in natural water samples as well as those ions which may form complexes with 2-GBI, or
Cloud point extraction flow injection-atomic absorption spectrometry for determination of lead in water
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Figure 3. Effect of sample loading time on the absorbance of Pb. Conditions: Initial concentration of Pb, 200 μg L-1; Column, 3 cm × 4 mm packed with 100 mg cotton; Sample flow rate, 2.5 mL min-1; Eluent, HNO3 0.2 M in ethanol; Eluent flow rate, 6 mL min-1.
Figure 4. Effect of eluent flowrate on the absorbance of Pb. Conditions: Initial concentration of Pb, 200 μg L-1; Column, 3 cm × 4 mm packed with 120 mg cotton; Sample loading time and flow rate, 120 s and 2.5 mL min-1; Eluent, HNO3 0.2 M in ethanol. Table 3. Investigated parameters and selected values for the proposed method.
Parameter Sample flow rate (mL min-1) Eluent flow rate (mL min-1) Column length (cm) Cotton wool dry weight (mg) Reactor length (cm) Loading time (s)
Studied Range 0.5–5 2.0–10 0.5–5 50–200 25–150 30–240
Optimum Value 2.5 8 3 120 100 120
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Table 4. Effect of diverse ions on the determination of lead (0.4 μg mL-1).
Diverse Ions
Concentration (μg mL-1) 400 500 400 400 20 30 20 10 10 10 10 10 200 80 15 10
Na+ K+ Ca2+ Mg2+ Cr3+ Mn2+ Fe3+ Cu2+ Cd2+ Co2+ Ni2+ Zn2+ FClBrC2H3O2-
% Recovery of Pb ( ±SD for n=3) 101.0 (±1.7) 99.0 (±0.9) 103.0 (±0.8) 99.0 (±1.0) 98.0 (±1.6) 98.0 (±1.6) 99.0 (±1.2) 98.0 (±1.2) 98.0 (±1.6) 97.0 (±1.8) 98.8 (±1.5) 96.0 (±1.6) 99.0 (±1.2) 97.0 (±1.8) 97.0 (±1.4) 92.0 (±1.5)
Table 5. Determination and recovery of lead in water and waste water samples.
Sample a Tap water
Well water
River water
Synthetic water sample c (Na+, K+,Ca2+,Mg2+ 6 mg each cation;Ni2+,Cu2+,Cd2+ 0.1 mg each cation) Waste water
a
Added (μg L-1) -10.0 20.0 -10.0 20.0 -10.0 20.0 10.0 20.0
Found (μg L-1) (±SD for n=3) nd b 10.3 (±1.0) 20.5 (±0.5) 7.5 17.9 (±0.6) 26.9 (±0.2) 10.2 (±1.5) 20.0 (±1.2) 30.3 (±1.2) 10.2 (±1.1) 19.6 (±1.7)
% Recovery (±SD for n=3) -103.0 (±2.5) 102.5 (±2.2) -102.2 (±4.2) 97.8 (±0.8) -99.0 (±4.0) 100.3 (±3.0) 102.0 (±2.0) 98.0 (±2.6)
-10.0 20.0
102.0 (±1.3) 110.0 (±1.6) 123.0 (±1.5)
-98.2 (±3.6) 100.8 (±3.1)
water and waste water samples collected from Zanjan. not detected. c nitrate or chloride salts taken. b
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Cloud point extraction flow injection-atomic absorption spectrometry for determination of lead in water reduce the extraction of lead were tested. The results presented in Table 4 revealed that the recovery of lead was almost quantitative in the presence of diverse ions. Analytical performance of the method The calibration graph was linear over the range of 0.006-0.6 μg mL-1 for 2 min of preconcentration time. The calibration graph obtained under the optimal chemical and flow conditions was: A = 0.4827 ± 5.4×10-3 (μg mL-1) + 0.003 ± 1.4×10-3, (r = 0.9992), where A is absorbance. The precision for 6 replicate measurements at 0.1 μg mL-1 was 2.7 % relative standard deviation. The limit of detection (LOD), defined as CLOD = 3Sb m-1 [42], (where Sb is the standard deviation of the blank (n = 6), m is the slope of the calibration curve), was found to be 2.0 μg L-1 for 10 mL of sample solution. The calibration graph was also obtained with standard solutions of lead (in the concentration range of 1.0 - 14 μg mL-1) without preconcentration, as A = 0.0156 ± 0.51×10-3 (μg mL-1) – 0.006 ± 0.60×10-2, (r = 0.9990). The R.S.D % and LOD were 1.0 % (n = 6 at 10 μg mL-1 lead) and 120 μg L-1 (n = 6), respectively. The enhancement factor calculated as the ratio of the slopes of calibration curves obtained with and without preconcentration, was found to be 31. The sample throughput was 11 h-1 (mean time of analysis 5.5 min). Determination of lead in water and waste water samples In order to assess the applicability of the proposed method, it was applied to the determination of lead in a synthetic water sample, as well as three different water samples collected from Zanjan city and province. The method was also applied for a waste water sample collected from the Zinc processing plant in the Dandy industrial region of Zanjan. The samples were filtered through a 0.45 μm pore size nylon filter to remove any suspended particulate matter and were stored in the refrigerator (5˚C) prior to preconcentration. Then 10 mL of each sample was preconcentrated and determined by the proposed method. The validation of the method was performed by the recovery experiments for spiked samples (Table 5), as well as by analysis of a standard reference material, BCR-CRM 144 R with a Pb content of 96.0 mg kg-1. The content of Pb found by present method in this CRM was 93.2 ± 3.0 mg kg-1 (t-test at P = 0.05, n = 3). The results obtained are in good agreement with added and certified amounts of lead ions.
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Conclusion The results presented demonstrate the suitability of 2-GBI as a complexing reagent for on-line CPE/preconcentration of lead. Triton X-114 surfactant has a low CPT (23˚C), hence, its application provided reproducible signals by avoiding the bubble formation in the FI system, which otherwise usually occurs when using Triton X-100 surfactant (CPT = 68˚C). The prevention of this requires the reduction of CPT by applying the salting out agents [43]. The developed FIAAS system provided good results in terms of accuracy and precision for rapid determination of lead in water samples. The FI system with a single pump in this method offered a mean time of analysis of 5.5 min, a sample throuput of 11 h-1and detection limit of 2.0 μg L-1. Acknowledgements The authors thank the Management and Programming Organization of Iran for financial support under Project No. 31303384. References 1. E.I. Underwood, Trace Elements in Human and Animal nutrition, Academic press, London, 1971. 2. C.D. Klassen, M.D. Amdur and J. Dull,; Casarett and Dulls Toxicology, third ed., Macmillan, New York, 1986. 3. Y. Yamamoto, Y. Nishino and K. Ueda, Talanta, 32, 662, (1985). 4. N. Debruker, K. Strijchmans and C. Vandecasteele, Anal. Chim. Acta, 195, 323 (1987). 5. M.M. Palrecha, R. Purthasarathy and M.S. Das, Anal. Chim. Acta, 194, 299 (1987). 6. C.N. Ferrarello, M.M. Bayon, J.J.G. Alonso and A. Sanz-Medel, Anal. Chim. Acta, 429, 227 (2001). 7. K. Ndungu, S. Hibdon and A.R. Flegal, Talanta, 64, 258 (2004). 8. P.L. Malvankar and V.M. Shinde, Analyst, 116, 1081 (1991). 9. P.S. More and A.D. Sawant, Anal. Lett., 27, 1737 (1994). 10. S.Y. Bae, X. Zeng and G.M. Murray, J. Anal. At. Spectrom., 13, 1177 (1998). 11. N. Prakashi, G. Casanady, R.A. Michaelis and G. Knapp, Mikrochim. Acta, 3, 257 (1989). 12. M. Soylak, I. Narin and M. Dogan, Anal. Lett., 30, 2801 (1997). Canadian Journal of Analytical Sciences and Spectroscopy
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