Evaluation of polychlorotrifluoroethylene as sorbent material for on-line solid phase extraction systems: Determination of copper and lead by flame atomic absorption spectrometry in water samples

Analytica Chimica Acta 575 (2006) 126–132

Evaluation of polychlorotrifluoroethylene as sorbent material for on-line solid phase extraction systems: Determination of copper and lead by flame atomic absorption spectrometry in water samples Aristidis N. Anthemidis ∗ , Kallirroy-Ioanna G. Ioannou Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University, Thessaloniki 54124, Greece Received 17 March 2006; received in revised form 23 May 2006; accepted 23 May 2006 Available online 27 May 2006

Abstract Polychlorotrifluoroethylene (PCTFE) in the form of beads was applied, as packing material for flow injection on-line column preconcentration and separation systems coupled with flame atomic absorption spectrometry (FAAS). Its performance characteristics were evaluated for trace copper determination in environmental samples. The on-line formed complex of metal with diethyldithiophosphate (DDPA) was sorbed on the PCTFE surface. Isobutyl methyl ketone (IBMK) at a flow rate of 2.8 mL min−1 was used to elute the analyte complex directly into the nebulizer–burner system of spectrophotometer. The proposed sorbent material reveal, excellent chemical and mechanical resistance, fast adsorption kinetics permitting the use of high sample flow rates up to 15 mL min−1 without loss of retention efficiency. For copper determination, with 90 s preconcentration time the sample frequency was 30 h−1 , the enhancement factor was 250, which could be further improved by increasing the loading (preconcentration) time. The detection limit (3s) was cL = 0.07 ␮g L−1 , and the precision (R.S.D.) was 1.8%, at the 2.0 ␮g L−1 Cu(II) level. For lead determination, the detection limit was cL = 2.7 ␮g L−1 , and the precision (R.S.D.) 2.2%, at the 40.0 ␮g L−1 Pb(II) level. The accuracy of the developed method was evaluated by analyzing certified reference materials and by recovery measurements on spiked natural water samples. © 2006 Elsevier B.V. All rights reserved. Keywords: Copper; Polychlorotrifluoroethylene; On-line preconcentration; Atomic absorption spectrometry

1. Introduction Despite the sensitivity and selectivity of analytical techniques such as flame atomic absorption spectrometry (FAAS), there is a great necessity for the preconcentration of trace metals prior their determination, basically due to their low concentrations or the matrix interferences in aqueous samples. To improve the sensitivity and selectivity, preconcentration procedures such as liquid–liquid extraction, precipitation co-precipitation and solid phase extraction (SPE) are generally used before the detection. The above procedures operated in the batch mode, are timeconsuming and labor-intensive, require large sample/reagent volumes, and suffer from risks of contamination and analyte loss. In recent years, flow injection (FI) on-line sorption technique has shown great promise and become one of the most active research fields in automated analysis improving the performance

∗

Corresponding author. Tel.: +30 2310997826; fax: +30 2310997719. E-mail address: [email protected] (A.N. Anthemidis).

0003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2006.05.072

characteristics of the methods [1]. On-line SPE preconcentration and separation coupled with FAAS have shown to be powerful for determination of trace elements in a variety of matrices [2]. The nature and the properties of the sorbent materials are of prime importance for effective retention of metals [3]. The main requirements of a sorbent material are: fast and quantitative adsorption/elution; regeneration ability; high capacity; accessibility; the chemical and mechanical resistibility, to endure harsh conditions. Various packing materials for on-line column preconcentration have been used for the determination of copper and other heavy metals in various types of samples. Ion-exchange resins, Chelex-100 and resin 122 [4]; octadecyl functional groups bonded on silica gel, C18 [5–8]; modified silica gel [9,10]; polystyrene-divinylbenzen polymer (PS-DVB), Amberlite XAD-2 [11,12], XAD-4 [13], PS-DVB functionalized [14]; synthetic zeolite [15]; biopolymer chitosan [16]; alumina coated [17]; polyurethane foam, PUF [18]; Metalfix® Chelamine® [19] and polytetrafluoroethylene polymer, PTFE as turnings [20], as grafted fiber [21] or as tubing knotted reactor [22,23].

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Fig. 1. FI manifold and operation sequence for on-line copper FAAS determination: (a) preconcentration step; (b) elution step. DDPA, 0.2% (m/v) DDPA solution; FAAS, flame atomic absorption spectrometer; P1, P2, peristaltic pumps; V, injection valve; DB, displacement bottle; C, column packed with PCTFE beads; W, waste.

Polychlorotrifluoroethylene, PCTFE (commercial products: Neoflon or Kel-F) is a stable, nonreactive hydrophobic porous and solvent–resistance polymer, which has been widely used as molding material and rarely as packing material for reverse phase column chromatography [24]. Although, it has been reported that PCTFE can effectively adsorb some metal complexes and that the retained metal can be easily eluted with hydrochloric acid or methanol without any effect of swelling and shrinking [25], there is only one paper, which adopt it as sorbent material for off-line solid phase extraction preconcentration and determination of trace metals (Ag, Cd, Cu, Fe, Mn, Ni, and Zn) with flame atomic absorption spectrometry [26]. In the above paper the adsorption behaviour of water–soluble metal complexes with 8-quinolinol-5-sulfonic acid (SOx) compared with water–insoluble complexes of 8-quinolinol and 3phenyl-5-mercapto-1,3,4-thiazole-2-thion (Bismuthiol II) using an off-line PCTFE column. PCTFE powder (1.0 g of >100 mesh resin) was slurried in methanol and filled a glass column with a stopcock at the bottom. For the determination of metal with on-line solid phase extraction coupled to FAAS, the most efficient chelating agents are diethyl-dithiocarbamates (DDC) [5,6,22], ammonium pyrrolidine-dithiocarbamate (APDC) [15,18,20,23] and diethyldithiophosphate (DDPA) [7]. Bode and Arnswald [27] studied the complexation of ammonium diethyl-dithiophosphate with several metals and demonstrated its great ability to form stable complexes, even in strong acidic medium and its selectivity for cadmium, copper and lead, as it does not react with alkali, alkaline earth metals and others such as Mn, V, Ti, Co, Cr, Zn. In the present research the feasibility of the hydrophobic PCTFE-beads as column packing material for on-line preconcentration and separation of trace metals was investigated. To the best of our knowledge the application of PCTFE-beads as sorbent material in FI on-line systems was demonstrated for the

first time. All main factors affecting the adsorption and elution procedure were examined thoroughly. The performance characteristics of the present PCTFE-beads packed column were compared with those obtained by PTFE-beads and PTFE-turnings. The developed method was applied for on-line copper and lead determination in natural water samples and certified reference materials. 2. Experimental 2.1. Instrumentation A Perkin-Elmer, Norwalk, Connecticut, USA model 5100 PC flame atomic absorption spectrometer equipped with a deuterium arc background corrector was used as a detector. A copper hollow cathode lamp, operated at 15 mA and a lead electrodeless discharge lamp (EDL) operated at 10 mA, were used as light sources. The wavelength was set at 324.7 nm resonance line for copper and 283.3 nm for lead determination and the monochromator spectral bandpass at 0.7 nm. A time-constant of 0.2 s was used for peak height evaluation. The acetylene flow rate was slightly leaner than this recommended by the manufacture, to compensate for the effect of IBMK which serves as additional fuel. The resulting nebulizer free uptake rate was 5.6 mL min−1 . The flow injection system, which is shown schematically in Fig. 1, was coupled to the nebulizer system of the above spectrometer with a PTFE capillary as short as possible, (12 cm length, 0.35 mm i.d.), to minimize the dispersion. The manifold consisted of two peristaltic pumps (Watson Marlow, Cornwall, England model 205U/BA) and an injection valve two-position six-port (Labpro, Reodyne, USA). Tygon type peristaltic pump tubing was adopted to deliver the sample and reagent. A displacement bottle (Tecator, Hoganas, Sweden) was used to propel the IBMK, because this organic solvent is not compatible with

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Table 1 Operating sequence of the FI on-line solid phase preconcentration system for FI-FAAS determination of Cu(II) Step

Valve position

Pumps P1

1

Load

ON

2

Injection

OFF

Delivered medium

Flow rate (mL min−1 )

Sample 0.2% (m/v) DDPA IBMK

11.6 0.6 2.8

Time (s)

Operation

90

Preconcentration

30

Elution

P2 OF F ON

the peristaltic tubes. PTFE tubing of 0.5 mm i.d. was used for all connections. The length of tubing between the confluence point of sample with DDPA stream and the injection valve (Fig. 1a, port 6) was as short as possible (ca. 1 cm), because the PTFE material adsorbs the Cu(II)–DDPA complex, affecting the precision and the recovery of the method [20].

over 500 preconcentration cycles. For comparison purposes a proportionate a PTFE-turnings packed on-line column [20,28] was used in the developed on-line manifold. In addition the feasibility of using the PTFE-beads as sorbent material in the proposed preconcentration systems was examined. 2.4. Operating procedure

2.2. Reagents The polychlorotrifluoroethylene (PCTFE) powder (Aldrich, Steinheim, Germany) was sieved to obtain the fraction: 100–600 ␮m. For comparative studies polytetrafluoroethylene (PTFE) powder (Fluka Steinheim, Germany) was sieved to obtain the fraction > 100 ␮m, while PTFE-turnings were produced as described previously [20,28]. All chemicals were of analytical reagent grade and were provided by Merck (Darmstadt, Germany). Ultra-pure quality water was used throughout which was produced by a Milli-Q system (Millipore, Bedford, USA). Working standard solutions of copper(II) and lead(II) were prepared before use, by stepwise dilution of a 1000 mg L−1 Cu(II) and 1000 mg L−1 Pb(II), respectively stock standard solution (Merck) to the required ␮g L−1 levels. The chelating reagent, 0.2% (m/v) DDPA was prepared fresh daily by dissolving the appropriate amount of ammonium diethyl-dithiophosphate (Aldrich) in water. Isobutyl methyl ketone (IBMK) was used after saturation with water, without any other purification. Two standard reference materials were used in order to validate the accuracy of the developed method: NIST CRM 1643d (National Institute of Standard and Technology, Gaithersburg, MD, USA) containing trace elements in water and NRCC (National Research Council Canada, Canada) CASS-4 Nearshore Seawater Reference Material. Water samples were collected from Axios river, Prespa lake, and costal sea water from Northern Greece. All water samples were filtered through 0.45 ␮m membrane filters, acidified to ca. pH 1.5 with dilute HCl and stored at 4 ◦ C in acid-cleaned polyethylene bottles, in order to determine the “dissolved metal” fraction. 2.3. Preconcentration column The preconcentration column was made from a polyethylene syringe tube, with an effective length of 60 mm and inner diameter of 3.8 mm. An amount of 900 mg of PCTFE beads (100–600 ␮m) were firmly packed into the column, blocked by glass wool at both ends. The PCTFE beads were washed thoroughly by 2.0 mol L−1 HNO3 followed by ethanol and finally with water. The performance of the column was stable at least

The on-line manifold of the developed method is shown in Fig. 1. Details for the operational sequence of the solid phase preconcentration system are given in Table 1. In the preconcentration step (Fig. 1a), the injection valve V turned to the “load position” and pump P1 was activated. Sample or standard solution and DDPA one were merged together, while the on-line formed Cu(II)–DDPA complex retained onto the surface of the PCTFE-beads. During the elution step, the injection valve V turned to the “injection position”. IBMK is propelled through the column in order to elute the sorbed complex and transport directly into the nebulizer of the spectrometer. For minimum dispersion, the eluent IBMK flowed through the column in reverse direction than that of the sample/reagent. The peak height of the transient signal was proportional to copper concentration in the sample, and was used for all measurements. The recorded peaks were sharp and the baseline was stable. Five replicate measurements per sample were made in all instances. 3. Results and discussion 3.1. Optimization of chemical and manifold parameters Among the chemical variables, sample acidity, specified by pH of the sample solution, is the most critical parameter for effective on-line formation and retention of the metal–DDPA complex onto the sorbent material. The effect of the pH was studied in the range 0.1–6.3 by adjusting it (in the Cu(II) solution) with dilute nitric acid. The pH of the waste solution was similar to that of the sample, indicating that the introduction of the DDPA solution did not affect significantly the acidity. As it is shown in Fig. 2, the absorbance was maximum and practically stable in a wide pH window varied between 0.1 and 2.0, while over pH 2.2 the absorbance was decreased by increasing the pH. This fact enables the use of the method directly in many aqueous samples after common acid preservation, without any laborious precise pH adjustment. The decrease of the efficiency at higher and lower pH values is probably due to non-complete complex formation, and sorption, and not to any damage of the PCTFE sorbent material which has excellent chemical and mechanical resistance.

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Effect of pH on the absorbance of 15 ␮g L−1

Fig. 2. as in Table 1.

Cu(II). All other parameters

The effect of DDPA concentration was studied in the range 0.05–0.6% (m/v) at fixed 0.6 mL min−1 flow rate. The absorbance increased with DDPA concentration up to 0.1% (m/v), while for higher concentrations was almost stable. Therefore a 0.2% (m/v) DDPA solution in water was selected for further experiments. In on-line preconcentration systems with time-based sampling the sample flow rate determines the amount of sample to be processed in a given time. This rate is limited by flow resistance of the column and the reaction rate. The effect of the sample flow rate during the preconcentration step was studied in the range 2.9–14.8 mL min−1 . It was observed that the absorbance increased linearly with increase in sample flow rate up to at least 14.8 mL min−1 , implying that the kinetic of the complexation was very fast and the contact time for complete sorption was sufficient. This is a significant advantage over other on-line preconcentration column or knotted reactors. At 90 s preconcentration time a flow rate of 11.6 mL min−1 was selected for high sensitivity. The influence of sample loading time was investigated in the range from 15 to 180 s. The absorbance increased almost linearly within the studied range (Fig. 3), indicating that no partial

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Fig. 4. Effect of IBMK flow rate on the absorbance of 15 ␮g L−1 Cu(II). All other parameters as in Table 1.

leaching of the complex is observed as it is occurred in the systems that employ the tubing knotted reactor sorbents [29]. A 90 s loading time was chosen as a compromise between sensitivity and sampling frequency. Organic solvents like, ethanol, methanol and IBMK have been extensively used as effective eluents in FI on-line solid phase extraction preconcentration systems. However, IBMK is considered to be more convenient for FAAS, producing higher and sharpest signals. The gain is arising from the more intensive organic solvent effect on the flame atomization conditions and also from the analyte chelation decrease in dispersion because it is an immiscible solvent with aqueous phase. Thus, IBMK was adopted as eluent and the elution time was fixed at 30 s for complete elution. The effect of IBMK flow rate was studied within the range 1.7–5.4 mL min−1 . Maximum absorbance was achieved within the range 2.5–3.1 mL min−1 as it is presented in Fig. 4. Above 3.1 mL min−1 the absorbance decreases, mainly due to the higher dispersion, while at low flow rates the decrease was presumably produced from the high difference of elution flow rate from nebulizer uptake flow rate [28]. Thus, 2.8 mL min−1 IBMK flow rate was used in the proposed procedure. 3.2. Interference studies

Fig. 3. Effect of loading time on the absorbance of 10 ␮g L−1 Cu(II). All other parameters as in Table 1.

Although, it is well known that DDPA is a selective chelating agent for cadmium, copper and lead due to its great ability to form stable complexes in strong acidic medium [27], the effect of potential interferents occurring in environmental samples on the on-line determination of copper were tested using the optimized on-line preconcentration system. The recovery of 5 ␮g L−1 copper solution was tested with individual interferents added. Taking as criterion for interference the deviation of the recovery more than ±5%, the obtained results showed that, Al(III), Cd(II), Co(II), Cr(III), Fe(III), Mn(II), Pb(II) and Zn(II) are tolerated up to 10 mg L−1 while Hg(II) and Ni(II) are tolerated up to 1 and 5 mg L−1 , respectively. Moreover, the potential interferences from some common matrix cations such as Na(I), Ca(II), Mg(II) and Ba(II) were also investigated. They are tolerated at concentrations at least up to 1000 mg L−1 and NaCl up to 30 g L−1 .

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Table 2 Analytical performance of the FI on line solid phase extraction FAAS method for copper determination using a PCTFE-beads and a PTFE-turnings packed column with similar active surface Parameter

Column packed with PCTFE-beads

Column packed with PTFE-turnings

Preconcentration time (s) Sampling frequency (h−1 ) Enhancement factor Sample flow rate (mL min−1 ) Linear range (␮g L−1 ) Regression equation (n = 5) Correlation coefficient (r) Detection limit (3s) (␮g L−1 ) Precision (R.S.D., n = 10) (%)

90 30 250 11.6 0.24–20.0 A = 0.0126 [Cu] + 0.0024 0.9996 0.07 1.8 (2.0 ␮g L−1 )

90 30 300 12.0 0.21–15.0 A = 0.0164 [Cu] + 0.0032 0.9991 0.06 1.7 (2.0 ␮g L−1 )

3.3. Analytical performance Under the optimum conditions described above, the analytical performance characteristics of the proposed method are listed in Table 2. For 90 s preconcentration time the sampling frequency was 30 h−1 and the enhancement factor was 250 (comparing the slope of the proposed method with the slope of direct aspiration aqueous standard solution into FAAS). The linear range was varied in the range 0.24–20.0 ␮g L−1 . The detection limit calculated by 3s criterion and found to be cL = 0.07 ␮g L−1 . The precision, as relative standard deviation (R.S.D.), was sr = 1.8%, calculated from 10 replicate measurements at the 2.0 ␮g L−1 level of Cu(II). To evaluate the PCTFE-beads as sorbent material against PTFE-beads and PTFE-turnings under comparable conditions, on-line columns packed with the above three sorbent material were used with the proposed manifold for copper determination. The PTFE-beads column revealed serious clogging problems, which started to arise at high loading flow rate when non-polar organic solvent (IBMK) is used as eluent, as it is reported else-

where [30,31]. The non-polar organic solvent brings about a significant congestion of the hydrophobic soft PTFE-beads, resulting thus an extremely high back-pressure in the flow system. Conclusively, the packed column with PTFE-beads could not to be used together with eluent IBMK. Alternatively, HCl or HNO3 solution can be used as eluent missing thus the advantage of the IBMK in FAAS. The above limitations can be overcome using the PTFE-turnings packed column due to the inherent hardness, stability and robustness of the turnings. The obtained analytical performance data for PCTFE-beads and PTFE-turnings packed column, are compared in Table 2. Either PCTFE-beads or PTFE-turnings preconcentration columns appear similar analytical performance characteristics. No evidence of resistance was encountered under the experimental conditions used for both columns. It should be noticed that PCTFE beads have an additional advantage: it is commercial available from the suppliers, ready to use in hard and stable bead form, as packing material in the on-line preconcentration columns. In Table 3, the figures of merit of the present and other selected on-line solid phase extraction preconcentration FAAS

Table 3 Comparison of the characteristic data between recent published on-line solid phase extraction methods and the developed one for copper determination with FAAS Reference

Sorbent material

Reagent

Eluent

PT (s)

SC (mL)

f (h−1 )

cL (␮g L−1 )

sr (%)

EF

[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [20] [21] [22] [23] This work

Chelex-100 or 122 resin C18 C18 C18 C18 Nb2 O5 –SiO2 SiO2 -modified Amberlite XAD-2 (load.) Amberlite XAD-2 (functionalized) Amberlite XAD-4 (load.) PS-DVB (functionalized) Synth. Zeolite (Na-LTA) Chitosan (modified) Alumina (coated) Polyurethane PTFE-turnings PTFE-fiber (grafted) KR-PTFE KR-PTFE (MSP) PCTFE-beads

– DDC DDC DDPA Phenathroline – – –

HNO3 EtOH MeOH MeOH EtOH HNO3 HNO3 HCl HCl HCl–EtOH HCl IBMK HNO3 HNO3 IBMK IBMK HCl IBMK EtOH IBMK

100 20 30 20 30 120 90 180 180 3000 240 120 90 480 60 60 45 60 180 90

10 1.4 2.0 2.9 1.6 14 11.2 13.5 24 25 26.4 4.0 10.8 40 12 12 7.5 8.3 18 11.6

60 120 85 – 90 20 27 20 18 – 13 26 26 – 36 40 55 46 12 30

0.07 0.2 0.2 1.4 0.3 0.4 0.2 0.15 0.54 0.06 0.93 0.1 0.3 0.3 0.2 0.05 0.2 0.2 0.26 0.07

2.2 1.3 1.4 1.5 3.0 1.8 1.4 4.5 6.1 1.2 5.3 2.6 0.7 4.5 2.8 1.5 1.2 3.6 1.7 1.8

88 19 114 35 32 34 40 32 35 300 43 125 19 100 170 340 48 120 121 250

– APDC – – APDC APDC – DDC APDC DDPA

EF: enhancement factor; PT: preconcentration time; f: sampling frequency; cL : detection limit; sr : precision (relative standard deviation); SC: sample consumption.

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Table 4 Analytical results of copper determination in the certified reference materials and water samples (determination in “dissolved metal” fraction) Sample (␮g L−1 )

Certified value

Found

CASS-4 CRM 1643d

0.592 ± 0.055 20.5 ± 3.8

0.566 ± 0.023 19.9 ± 0.7

River water Lake water Coastal seawater a

Spiked

Founda

Recovery (%) 96 97

– 2.00 – 2.00 – 2.00

2.04 3.98 1.96 3.92 0.98 2.89

± ± ± ± ± ±

0.08 0.14 0.09 0.12 0.05 0.10

97 98 96

Mean value ± S.D. based on three replicate determinations.

methods are summarized for comparative purposes. The proposed method shows very good sensitivity and precision and has some good advantages over other on-line preconcentration methods reported in the literature. Only the method which uses the schema “APDC/PTFEturnings/IBMK” [20] shows better sensitivity and higher sampling frequency. The method with the schema “Amberlite/ HCl–EtOH” [13] has lower detection limit, but it needs extremely high loading/preconcentration time (3000 s). In order to evaluate the accuracy of the proposed method for copper determination certified reference materials: CASS-4 (Nearshore seawater) and CRM 1643d (Trace elements in water) were analyzed. As it is shown in Table 4, the found values were in good agreement with the certified ones and the calculated recovery (96–97%) was satisfactory. The proposed method was applied also to the analysis of local natural water samples (river, lake and sea water) and was validated by spiking the samples with known amounts of copper. The obtained results are presented in Table 4. The recoveries from spiked samples were varied in the range 96–98% and the relative standard deviation for copper determination in the examined samples varied in the range 3.1–5.1%. 3.4. Analytical performance of PCTFE-beads for lead determination The above described analytical procedure, keeping the optimum values of chemical and manifold parameters, was employed, in order to examine the ability of the PCTFE-beads for on-line preconcentration and determination of lead. The sample acidity was adjusted ca. pH 1.5, which is the recommended acidity for natural water sample preservation. Up to 60 s loading (preconcentration) time, the proposed material proved to be capable to retain quantitatively the analyte complex, and after this time a partial leaching of the Pb–DDPA complex is observable, in contrast to the behavior of Cu–DDPA one. This variation is resulted from the properties of the complexes and not from the sorbent material. The obtained regression equation was A = 0.0010 [Pb(II)] + 0.0023, the correlation coefficient was 0.9993, the limit of detection (3s) was 2.7 ␮g L−1 , the linear range was between 6.2 and 250 ␮g L−1 and the precision (R.S.D.) was 2.2%, at the 40.0 ␮g L−1 Pb(II) level. Finally, the recovery of lead was estimated by analyzing the certified reference material NIST CRM 1643d. The certified concentra-

tion was 18.15 ± 0.64 ␮g L−1 and the concentration found was 17.61 ± 0.85 ␮g L−1 (n = 3); thus, the calculated recovery was 97%. 4. Conclusions The practical applicability of PCTFE beads as a packing material for FI on-line sorption preconcentration coupled with FAAS for trace metal determination was successfully evaluated and demonstrated. The chemical inertness, the excellent swelling and shrinking resistance to organic non-polar eluents like IBMK, the hydrophobic nature and the fast kinetics of the proposed sorbent material make it very attractive in online column preconcentration systems. The packed column with PCTFE-beads entails the clear advantage of low hydrodynamic resistance allowing high sample loading flow rates for obtaining better preconcentration factors. The column is also easily reproducible without the need of pre-conditioning or activation steps. In comparison with PTFE-beads, the PCTFE-beads possess the advantage of IBMK utilization for FAAS determination, while in comparison with PTFE-turnings has the supremacy of the commercially availability. The proposed method proved to be simple, rapid and accurate for copper and lead determination with few interferences. References [1] Z. Fang, Flow Injection Atomic Absorption Spectrometry, John Wiley & Sons Ltd., West Sussex, England, 1995. [2] J. Ruzicka, A. Arndal, Anal. Chim. Acta 216 (1989) 243. [3] V. Camel, Spectrochim. Acta Part B 58 (2003) 1177. [4] Z. Fang, J. Ruzicka, E.H. Hansen, Anal. Chim. Acta 164 (1984) 23. [5] Z. Fang, T. Guo, B. Welz, Talanta 38 (1991) 613. [6] S. Xu, M. Sperling, B. Welz, Fresenius J. Anal. Chem. 344 (1992) 535. [7] R. Ma, W. van Mol, F. Adams, Anal. Chim. Acta 285 (1994) 33. [8] A. Ali, X. Yin, H. Shen, Y. Ye, X. Gu, Anal. Chim. Acta 392 (1999) 283. [9] E.L. da Silva, E.M. Ganzarolli, E. Carasek, Talanta 62 (2004) 727. [10] E.L. da Silva, A.O. Martins, A. Valentini, V.T. de Favere, E. Carasek, Talanta 64 (2004) 181. [11] S. Ferreira, V.A. Lemos, B.C. Moreira, A.C. Spinola Costa, R.E. Santelli, Anal. Chim. Acta 403 (2000) 259. [12] V.A. Lemos, P.X. Baliza, Talanta 76 (2005) 564. [13] M.C. Yebra, N. Carro, M.F. Enriquez, A. Moreno-Cid, A. Garcia, Analyst 126 (2001) 933. [14] R.J. Cassella, O.I.B. Magalhaes, M.T. Couto, E.L.S. Lima, M.A.F.S. Neves, F.M.B. Coutinho, Talanta 67 (2005) 121.

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