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Polyurethane foam as a sorbent for continuous flow analysis: Preconcentration and spectrophotometric determination of zinc in biological materials
Analytica Chimica Acta 366 (1998) 263±269
Polyurethane foam as a sorbent for continuous ¯ow analysis: Preconcentration and spectrophotometric determination of zinc in biological materials Djane Santiago de Jesusa, Ricardo Jorgensen Cassellab,e, SeÂrgio Luis Costa Ferreirac, Antonio Celso Spinolla Costac, Marcelo Souza de Carvalhod, Ricardo Erthal Santellie,* a
Federal of Bahia Center of Technological Education, Salvador/BA, Brazil b Federal School of Technical Chemistry, Rio de Janeiro/RJ, Brazil c Analytical Chemistry Department, Chemistry Institute, Federal of Bahia University, Salvador/BA, 40170-290, Brazil d Nuclear Engineer Institute ± IEN-CNEN, Rio de Janeiro/RJ, Brazil e Geochemistry Department, Federal Fluminense University, NiteroÂi/RJ, 24020-007, Brazil Received 12 September 1997; received in revised form 5 January 1998; accepted 14 January 1998
Abstract A simple and accurate procedure was developed to preconcentrate and determine zinc in biological matrices. A polyurethane foam (PUF) minicolumn was used for the ®rst time for on-line preconcentration. This system was applied to preconcentrate zinc from aqueous solutions which is sorbed as zinc±thiocyanate complex. Then, the complex was eluted by water and zinc was determined by spectrophotometry using 4-(2-pyridylazo)-resorcinol. The method was validated by the analysis of several biological certi®ed reference materials. The detection limit was 0.9 ng mlÿ1 and the RSD was 1.2% for 1 min preconcentration time. Generally, concentration of Fe(III), Al(III), Mo(VI), Ni(II), Mn(II), Cu(II), and Co(II) found in biological matrices did not interfere. Dynamic ranges of concentration from 20 to 100 ng mlÿ1 (1 min preconcentration time) and from 5 to 20 ng mlÿ1 (5 min preconcentration time) achieve a sample throughput of 20 and 8 samples per hour and a preconcentration factor of 8 and 32, respectively. # 1998 Elsevier Science B.V. Keywords: Polyurethane foam; Zinc preconcentration; Continuous ¯ow analysis; Biological samples
1. Introduction Solid-phase extraction (SPE) is a very important preconcentration technique in trace metal determination. The importance of polyurethane foam (PUF) in
*Corresponding author. Tel.: 0055 21 620 1313; fax: 0055 21 620 7025. 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00111-1
separation and preconcentration of metals has increased in the last few years. PUF has been used as a solid sorbent to separate and preconcentrate a wide variety of inorganic and organic compounds from different media. It was ®rstly proposed by Bowen [1] in 1970. Braun [2±5] and Palagyi [6] have published some reviews about the use of PUF in separation and preconcentration procedures applied to several analytical systems. PUF can be directly used
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without previous pretreatment [7±9]. On the other hand, several chelating agents and liquid ion exchangers have been immobilized on PUF and used as a coadjuvant in separate procedures [10±13]. A number of possible sorption mechanisms on PUF have also been researched [14,15]. PUF±metal±thiocyanate system has been studied in column or batch process. Moody et al. [16] have reported on the sorption of Co(II), Fe(III), Zn(II), Cd(II) and Mn(II) from potassium thiocyanate solution by using polyether type PUF. Hamon et al. [15] have published a detailed investigation about Co(II) sorption from thiocyanate medium using PUF cubes (1.3 cm3) in a squeezing procedure. Carvalho et al. [17] have used the PUF±thiocyanate system in a batch procedure to separate Co(II) from Fe(III) for the spectrophotometric determination of Co in steel materials using small particles of PUF. Hamza et al. [18] have used PUF treated with ditizone. They have described a quantitative method to collect Zn(II) and Bi(III) from aqueous media and applied it to their semi-quantitative determinations. Recently, Jesus et al. [19] have used the PUF±thiocyanate system to separate zinc traces from cadmium matrices and zinc has been determined by spectrophotometry. Flow-injection analysis (FIA) has improved several applications in all branches of analytical chemistry. Due to its inherent characteristics, FIA has been used to automate batch analytical procedures with some advantages, such as high analytical throughput, precision, accuracy coupled to low reagents and sample consumption [20±22]. Effective preconcentration can be especially carried out in ¯ow-injection systems and this methodology has been discussed in several reviews [23±25]. There is now some literature on FIA mainly about several preconcentrating procedures, such as C-18 immobilized on silica gel [26,27], activated carbon [28], precipitation [29,30], functionalized cellulose [31] and fullerenes [32,33]. This paper is the ®rst report on the use of PUF minicolumns in continuous ¯ow systems. The effectiveness of this sorbent was studied using zinc preconcentration in a thiocyanate medium under ¯ow conditions and zinc was determined by spectrophotometry using 4-(2-piridylazo)-resorcinol as chromogenic reagent. The procedure described can easily be employed to analyze biological materials.
2. Experimental 2.1. Apparatus A Micronal B-380 spectrophotometer equipped with a Hellma ¯ow cell (178-010-OS) set at 496 nm coupled to a Chessell x±y recorder was used along with an Ismatec peristaltic pump model IPC-12 furnished with Tygon tubes to propel all solutions, a Rheodyne 5041 injection valve to select the preconcentration/elution steps, and a microwave oven SPEX CDS7000 with PFA sealed tubes to dissolve the samples. All connections were made using ®ttings, unions and tees made of plastic and PEEK materials. The manifold was built up with PTFE tube of 0.5 mm bore. 2.2. Reagents and solutions Milli-Q (Millipore) water was used to prepare all solutions. All reagents were of analytical grade. A zinc(II) stock solution containing 1000 mg lÿ1 was prepared by dissolving 2.4696 g of dried ZnSO4 in water containing 10 ml of concentrated nitric acid and dilution up to 1000 ml. The analytical solutions were prepared daily by successive dilutions from the stock solution, maintained at pH 3.00.2. A reagent containing 1 mol lÿ1 potassium thiocyanate and 0.5% sodium citrate was prepared daily by dissolving 24.30 g of KSCN and 1.25 g of Na3C6H5O72H2O in water, adjusting pH at 3.00.1 and dilution with water to 250 ml. A 0.01% 4-(2-pyridylazo)-resorcinol monosodium salt monohydrate ± PAR solution was prepared by dissolving 0.1 g in 800 ml of water and adding 61.83 g of boric acid and 20 g of sodium hydroxide. The pH was adjusted at 10 and diluting with water to 1000 ml. This solution must be kept still for at least two days and if any solid appear (boric acid) ®ltration must be done. This solution is stable for at least one week. Polyurethane foam (PUF) open cell polyether-type was obtained from the commercial product (Vulcan of Brazil ± VCON 202, 42% resilience and 10±12 cells/ linear cm). It was broken into small particles in a blender with demineralized water [17,19]. A minicolumn was packed by using ca. 100 mg of foam in a small plastic tube of 3.0 cm length3 mm i.d. The
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Fig. 1. Schematic diagram of the flow system employed for zinc preconcentration on PUF minicolumn and its spectrophotometric determination. Pperistaltic pump; Ssample (6.7 ml minÿ1); R1thiocyanatecitrate reagent (0.43 ml minÿ1); Ewater (2.0 ml minÿ1); R2PAR reagent (0.48 ml minÿ1); RCreactor coil (50 cm); Vfour-way valve; Ddetector at 496 nm; Wwaste. Four-way valve in the (a) preconcentration and (b) elution positions.
sorption capacity of the PUF used was 0.016 mmol Zn gÿ1 determined by sorption isotherm [19]. 2.3. Flow-injection system A diagram of the ¯ow system is shown in Fig. 1. In this system, a sample solution pumped at 6.7 ml minÿ1 merges with a thiocyanate solution stream at a ¯ow rate of 0.43 ml minÿ1 and percolates through the polyurethane foam minicolumn. Then, the zinc±thiocyanate complex is retained and the remained solution goes to the waste. The preconcentration step was done by using the time-based technique. Switching the injection valve, a stream of water ¯owing at 2.0 ml minÿ1 displaces the zinc retained. The eluate merges with a PAR solution at 0.48 ml minÿ1 and the absorbance of the resultant solution was monitored at 496 nm. The peak heights were always used to make all calculations. In this work, 1 min preconcentration time and 2 min elution time were used. After the elution, the PUF minicolumn was prepared to perform a new preconcentration cycle. 2.4. Sample preparation Several certi®ed reference materials (CRMs) were analyzed, such as copepoda (MA-A-1/TM), ®sh
tissue (MA-B-3/TM), ®sh ¯esh (MA-A-2/TM), and tuna homogenate (IAEA-350) from International Atomic Energy Agency (IAEA), Monaco and rice ¯our-unpolished sample Nos. 10(a±c) from National Institute of Environmental Studies (NIES), Japan. These samples after drying overnight at 11058C were weighted into a PFA liner and 8 ml of concentrated nitric acid was added. The sample quantities were taken according to the zinc sample content (ranging from 0.1 to 0.7 g). After standing overnight in contact with nitric acid, 2 ml of 30% hydrogen peroxide were added and the liners were closed to dissolve the samples. The microwave oven was controlled by its software according to the preselected `®sh method' in order not to exceed an internal liner pressure of 180 psig. Two cycles (of 25 min of microwave power) were performed (10 min ramp and 15 min dwell, and the temperature limit was 1908C). After each cycle the liners were opened to release gases. The obtained solutions were colorless and perfectly clear. The ®nal volume was diluting with water to 250 ml after adjusting the pH at 3.00.2 with ammonium hydroxide. The solution samples were stored in polypropylene ¯asks and analyzed according to the procedure described. At least, one blank solution was prepared for each sample to compensate the reagent contamination.
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3. Results and discussion 3.1. Flow-system optimization In order to improve the performance of the preconcentrating system, both chemical and ¯ow variables were studied using the ¯ow system shown in Fig. 1. Although in batch procedure the pH of sample was not found as an important factor to preconcentrate zinc from thiocyanate medium [19], this variable was carefully studied. Highest signals were obtained in the range of pH from 1.3 to 3.9. Below pH 1.3, signals were constant but blank signals were higher due to the change of color of the chromogenic reagent. When the pH was higher than 3.9, signals decreased abruptly and, therefore, the pH chosen was at 3.00.2. Thiocyanate concentration was tested ranging from 0.05 to 2.0 mol lÿ1. From 0.05 mol lÿ1, the signal increased and the best signals were obtained above 0.8 mol lÿ1. Thus, a thiocyanate concentration of 1.0 mol lÿ1 was selected. PAR content was tested ranging from 0.00050 to 0.040%. The results obtained show that signals were constant for PAR concentration higher than 0.0025%. The PAR concentration chosen was 0.01%. According to the batch methodology [19] water was used as eluent. It was possible to desorb zinc from polyurethane foam minicolumn by displacing the equilibrium. The effectiveness of other possible eluents were investigated. Nitric acid concentrations at 110ÿ3, 110ÿ4 and 110ÿ5 mol lÿ1 were used without enhancing ef®ciency. It was expected that more concentrated acids would be more ef®cient. However, it could not be used because the absorbance of PAR reagent increased. Ethanol±water solutions were also tested at 5, 10 and 20%. Ethanol solutions did not improve the elution and a noise appeared due to the Schieliren effect. Acetone±water mixtures (1 : 5 and 1 : 2) were investigated without success and the noise effect described was also observed. Variations of the sample ¯ow rate were studied ranging from 2.0 to 12.8 ml minÿ1. The best preconcentration response was obtained at a ¯ow rate of 6.7 ml minÿ1. However, the best absorbance signal/ sample volume was observed at 3.0 ml minÿ1, although the signal at this ¯ow rate was lower than that obtained at 6.7 ml minÿ1. Thus, 6.7 ml minÿ1
¯ow rate was selected to increase the sample throughput and enhance the sensitivity. Sample ¯ow rates higher than 6.7 ml minÿ1 were not enough to retain zinc and thus the signals decreased. Thiocyanate ¯ow rate was investigated in the range from 0.28 to 1.69 ml minÿ1. The best results were observed above 0.43 ml minÿ1. Therefore, this ¯ow rate was selected for subsequent experiments in order to maintain low sample dilution. PAR ¯ow rate was selected at 0.48 ml minÿ1 and it was not changed because higher ¯ow rates made spectrophotometer adjustment to the baseline troublesome. Eluent ¯ow rate was tested ranging from 1.69 to 5.8 ml minÿ1. It was observed that 2.0 ml minÿ1 was suitable to elute zinc without losing sensitivity. Displacing the retainable zinc was not so fast and it was controlled by the slow kinetic desorption. Moreover, the time required to decrease the thiocyanate concentration inside the column is not so low. Therefore, elution was not a fast step. Higher ¯ow rates only improved the dispersion and made the signals somewhat lower. The reactor coil length was studied in the 50±200 cm range, but such variation did not in¯uence the analytical signal and for that reason the smaller reactor coil was selected. The length of minicolumns and the amount of PUF were studied after preparing three columns of 1 cm (15 mg), 3 cm (50 mg), and 3 cm (100 mg). Fig. 2 shows the analytical curves obtained by these minicolumns. The minicolumn, 1 cm in length, was enough to retain at least 0.54 mg of Zn under ¯ow conditions (preconcentrated from analytical solutions containing 80 ng mlÿ1 pumped at 6.7 ml minÿ1 for 1 min). However, minicolumn of 3 cm in length was used to guarantee safety in all experiments. The use of PUF minicolumns under ¯ow conditions is attractive due to the very low overpressure observed, unlike other sorbents used in ¯ow systems, such as chelating agents and fullerenes. 3.2. Interference study Several interferent cations, specially which form thiocyanate complexes and/or react with PAR, mainly Fe(III), Cu(II), Co(II), Mn(II), Ni(II), Cd(II), Al(III) and Mo(VI) were used in order to verify the procedure selectivity. On using a mixed solution containing thiocyanate and sodium citrate, several interferences can be masked. However, when the sodium citrate
D.S. de Jesus et al. / Analytica Chimica Acta 366 (1998) 263±269
267
Fig. 2. Analytical curves obtained with different PUF minicolumns.
concentration increases, the zinc signal decreases. Thus, a 0.5% sodium citrate concentration was used in order to guarantee the best sensitivity and selectivity. The maximum ratios zinc : interferent tested were: Fe(III) up to 1 : 200; Mo(VI) up to 1 : 12; Mn(II), Cu(II), and Co(II) up to 1 : 5. At such levels no interference was observed. Their ratios were higher than those found in the samples analyzed. Al(III) and Ni(II) did not interfere because they do not react with thiocyanate. In contrast with batch experiments [19], Cd(II) was the most relevant interfering specie but it did not observed any problem due to the very low Cd concentration in the samples analyzed. 3.3. Analytical features The ¯ow system using 1 min preconcentration time shows linear in the range of concentration from 20 to 100 ng mlÿ1 (in Fig. 1). It may be represented by the equation A0.0030.00507 [Zn (ng mlÿ1)], and correlation coef®cient r0.999. The detection limit estimated as three times the standard deviation was
0.9 ng mlÿ1. The RSD was calculated by taking 12 measurements of 20 ng mlÿ1 and it was better than 1.2%. Precision study was also carried out with two real samples. The coef®cient of variation (CV) of four copepoda and four rice ¯our samples were 7 and 2%, respectively. The analytical sample throughput was 20 samples per hour. Analytical curves in the concentration range 5±20 ng mlÿ1 were also made using 5 min preconcentration time (A0.0130.0193 [Zn(ng mlÿ1)]), r0.999. The detection limit was 0.6 ng mlÿ1. The preconcentration factors were calculated by comparing the analytical curve slopes obtained from preconcentration procedure with the ones obtained from the static method using the same volumes and concentrations of the reactants. Using 1 min preconcentration time, the preconcentration factor was ca. 8. However, more sensitivity can be achieved by using higher preconcentration times. The preconcentration factor for 5 min preconcentration time was 32. The concentration ef®ciency (CE) [22] for 1 min preconcentration time was 2.67 and for 5 min preconcentra-
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Table 1 Results obtained for the certified reference materials analyzed. Data in mg gÿ1 Sample
Results obtained with FIA±PUF methodology
Certified values a
Rice flour, unpolished, low Cd level, No. 10a ± NIES Rice flour, unpolished, medium Cd level, No. 10b ± NIES Rice flour, unpolished, high Cd level, No. 10c ± NIES Copepoda, MA-A-1/TM ± IAEA Fish flesh, MA-A-2/TM ± IAEA Fish tissue, MA-B-3/TM ± IAEA Tuna homogenate, IAEA 350
25.81.8 21.80.2 23.10.3 1784 36.60.3 110.90.5 18.30.4
25.20.8 22.30.9 23.10.8 156±160 32±34 106.4±111.9 16.6±18.5
a
Data from Refs. [34,35].
Table 2 Precision of the FIA±PUF methodology for zinc preconcentration and determination in biological samples analyzed in four replicates Sample
Results obtained with FIA±PUF methodology (mg gÿ1)
Rice flour, unpolished, low Cd level, No. 10a ± NIES
28.0 26.4 25.0 23.9
Copepoda, MA-A-1/TM ± IAEA
182.9 179.4 176.2 174.6
tion time it was 4.67. Such numbers are not so good due to the long elution time required. 3.4. Applications The ¯ow system described, including PUF minicolumns, was good enough to preconcentrate and determine zinc in biological matrices. Several certi®ed reference materials were analyzed and the obtained results are shown in Table 1. It can be seen that all the results are in agreement with those certi®ed except for copepoda sample. Table 2 shows the concentration values for two real samples analyzed for four times and their coef®cient of variation. 4. Conclusions This paper is the ®rst report on the use of PUF minicolumns in continuous ¯ow systems. The ¯ow system is a simple, rapid, and inexpensive method to
Mean and standard deviation (mg gÿ1)
Coefficient of variation (%)
25.81.8
7
178.33.6
2
preconcentrate and determine zinc in biological samples. The selective procedure is enough to analyze biological material with precision and accuracy in sample solutions containing more than 5 ng Zn mlÿ1. The analytical potentiality of PUF minicolumns under continuous ¯ow conditions can be implemented for metals separation and preconcentration. Besides, PUF minicolumns can be useful using more selective and sensitive detection techniques such as AAS or ICP±AES for multielement determination. Acknowledgements The authors would like to thank Conselho Nacional de Desenvolvimento Cientõ®co e TecnoloÂgico (CNPq), Financiadora de Estudos e Projetos (FINEP) and ComissaÄo Nacional de Energia Nuclear (CNEN) for grants and fellowships.
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References [1] H.J.M. Bowen, J. Chem. Soc. (1970) 1082. [2] T. Braun, J.D. Navratil, A.B. Farag, Polyurethane foam sorbents in separation science, CRC Press, Boca Raton, Fl, 1985. [3] T. Braun, A.B. Farag, Talanta 22 (1975) 699. [4] T. Braun, A.B. Farag, Anal. Chim. Acta 99 (1978) 1. [5] T. Braun, A.B. Farag, Anal. Chim. Acta 73 (1974) 301. [6] S. PalaÂgyi, T. Braun, Separation and Preconcentration of Trace Elements and Inorganic Species on Solid Polyurethane Foam Sorbents, in: Z.B. Alfassi, C.M. Wai (Eds.), Preconcentration techniques for trace elements, CRC Press, Boca Raton, Fl, 1992. [7] D. Wildhagen, V. Krivan, Anal. Chim. Acta 274 (1993) 257. [8] S. Beltyukova, G. Balamtsarashvili, Talanta 42 (1995) 1833. [9] S.V. Beltyukova, N.A. Nazarenko, S.V. Tsygankova, Analyst 120 (1995) 1693. [10] A. Alexandrova, S. Arpadjan, Anal. Chim. Acta 307 (1995) 71. [11] S. Katragadda, H.D. Gesser, A. Chow, Talanta 42 (1995) 725. [12] L. Vuchkova, S. Arpadjan, Talanta 43 (1996) 479. [13] M.S. El-Shahiwi, R.S. Al-Mehrezi, Talanta 44 (1997) 483. [14] S. PalaÂgyi, T. Braun, J. Radioanal. Nucl. Chem. Articles 163 (1992) 69. [15] R.F. Hamon, A.S. Khan, A. Chow, Talanta 29 (1982) 313. [16] G.J. Moody, J.D.R. Thomas, M.A. Yarmo, Anal. Proc. 20 (1983) 132. [17] M.S. Carvalho, I.C.S. Fraga, K.C. Mateus Neto, E.Q. Silva Filho, Talanta 43 (1996) 1675. [18] A. Hamza, A.B. Farag, A. Amierh, Anal. Sci. 6 (1990) 889.
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[19] D.S. de Jesus, M.S. Carvalho, A.C.S. da Costa, S.L.C. Ferreira, Talanta, submitted. [20] J. Ruzicka, E.H. Hansen, Flow injection analysis, 2nd ed., John Wiley and Sons, New York, 1988. [21] M. ValcaÂrcel, M.D. Luque de Castro, Flow injection analysis ± Principles and applications, Ellis Horwood Limited, Chichester, 1987. [22] Z. Fang, Flow injection atomic absorption spectrometry, John Wiley and Sons, Chichester, 1995. [23] Z. Fang, S. Xu, S. Zhang, Anal. Chim. Acta 200 (1987) 35. [24] Z. Fang, Spectrochim. Acta Ver. 14 (1991) 235. [25] M. Trojanowicz, E. Olbrych-Sleszyniska, Chem. Anal. 37 (1992) 111. [26] J. Ruzicka, A. Arndal, Anal. Chim. Acta 216 (1989) 243. [27] R. Lima, K.C. Leandro, R.E. Santelli, Talanta 43 (1996) 983. [28] R.E. Santelli, M. Gallego, M. ValcaÂrcel, Talanta 41 (1994) 817. [29] R.E. Santelli, M. Gallego, M. ValcaÂrcel, Anal. Chem. 61 (1989) 1427. [30] P.C.S. Mendes, R.E. Santelli, M. Gallego, M. ValcaÂrcel, J. Anal. At. Spectrom. 9 (1994) 663. [31] N. Naghmush, K. Pyrzynska, M. Trojanowicz, Talanta 42 (1995) 851. [32] M. Gallego, Y.P. de PenÄa, M. ValcaÂrcel, Anal. Chem. 66 (1994) 4074. [33] Y.P. de PenÄa, M. Gallego, M. ValcaÂrcel, J. Anal. At. Spectrom. 12 (1997) 453. [34] International Atomic Energy Agency ± IAEA, Analytical Quality Control Services, Reference Materials Booklet, 1991. [35] National Institute for Environmental Studies ± NIES, Japan Environment Agency, NIES Certified Reference Material, 1989.
Polyurethane foam as a sorbent for continuous ¯ow analysis: Preconcentration and spectrophotometric determination of zinc in biological materials Djane Santiago de Jesusa, Ricardo Jorgensen Cassellab,e, SeÂrgio Luis Costa Ferreirac, Antonio Celso Spinolla Costac, Marcelo Souza de Carvalhod, Ricardo Erthal Santellie,* a
Federal of Bahia Center of Technological Education, Salvador/BA, Brazil b Federal School of Technical Chemistry, Rio de Janeiro/RJ, Brazil c Analytical Chemistry Department, Chemistry Institute, Federal of Bahia University, Salvador/BA, 40170-290, Brazil d Nuclear Engineer Institute ± IEN-CNEN, Rio de Janeiro/RJ, Brazil e Geochemistry Department, Federal Fluminense University, NiteroÂi/RJ, 24020-007, Brazil Received 12 September 1997; received in revised form 5 January 1998; accepted 14 January 1998
Abstract A simple and accurate procedure was developed to preconcentrate and determine zinc in biological matrices. A polyurethane foam (PUF) minicolumn was used for the ®rst time for on-line preconcentration. This system was applied to preconcentrate zinc from aqueous solutions which is sorbed as zinc±thiocyanate complex. Then, the complex was eluted by water and zinc was determined by spectrophotometry using 4-(2-pyridylazo)-resorcinol. The method was validated by the analysis of several biological certi®ed reference materials. The detection limit was 0.9 ng mlÿ1 and the RSD was 1.2% for 1 min preconcentration time. Generally, concentration of Fe(III), Al(III), Mo(VI), Ni(II), Mn(II), Cu(II), and Co(II) found in biological matrices did not interfere. Dynamic ranges of concentration from 20 to 100 ng mlÿ1 (1 min preconcentration time) and from 5 to 20 ng mlÿ1 (5 min preconcentration time) achieve a sample throughput of 20 and 8 samples per hour and a preconcentration factor of 8 and 32, respectively. # 1998 Elsevier Science B.V. Keywords: Polyurethane foam; Zinc preconcentration; Continuous ¯ow analysis; Biological samples
1. Introduction Solid-phase extraction (SPE) is a very important preconcentration technique in trace metal determination. The importance of polyurethane foam (PUF) in
*Corresponding author. Tel.: 0055 21 620 1313; fax: 0055 21 620 7025. 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00111-1
separation and preconcentration of metals has increased in the last few years. PUF has been used as a solid sorbent to separate and preconcentrate a wide variety of inorganic and organic compounds from different media. It was ®rstly proposed by Bowen [1] in 1970. Braun [2±5] and Palagyi [6] have published some reviews about the use of PUF in separation and preconcentration procedures applied to several analytical systems. PUF can be directly used
264
D.S. de Jesus et al. / Analytica Chimica Acta 366 (1998) 263±269
without previous pretreatment [7±9]. On the other hand, several chelating agents and liquid ion exchangers have been immobilized on PUF and used as a coadjuvant in separate procedures [10±13]. A number of possible sorption mechanisms on PUF have also been researched [14,15]. PUF±metal±thiocyanate system has been studied in column or batch process. Moody et al. [16] have reported on the sorption of Co(II), Fe(III), Zn(II), Cd(II) and Mn(II) from potassium thiocyanate solution by using polyether type PUF. Hamon et al. [15] have published a detailed investigation about Co(II) sorption from thiocyanate medium using PUF cubes (1.3 cm3) in a squeezing procedure. Carvalho et al. [17] have used the PUF±thiocyanate system in a batch procedure to separate Co(II) from Fe(III) for the spectrophotometric determination of Co in steel materials using small particles of PUF. Hamza et al. [18] have used PUF treated with ditizone. They have described a quantitative method to collect Zn(II) and Bi(III) from aqueous media and applied it to their semi-quantitative determinations. Recently, Jesus et al. [19] have used the PUF±thiocyanate system to separate zinc traces from cadmium matrices and zinc has been determined by spectrophotometry. Flow-injection analysis (FIA) has improved several applications in all branches of analytical chemistry. Due to its inherent characteristics, FIA has been used to automate batch analytical procedures with some advantages, such as high analytical throughput, precision, accuracy coupled to low reagents and sample consumption [20±22]. Effective preconcentration can be especially carried out in ¯ow-injection systems and this methodology has been discussed in several reviews [23±25]. There is now some literature on FIA mainly about several preconcentrating procedures, such as C-18 immobilized on silica gel [26,27], activated carbon [28], precipitation [29,30], functionalized cellulose [31] and fullerenes [32,33]. This paper is the ®rst report on the use of PUF minicolumns in continuous ¯ow systems. The effectiveness of this sorbent was studied using zinc preconcentration in a thiocyanate medium under ¯ow conditions and zinc was determined by spectrophotometry using 4-(2-piridylazo)-resorcinol as chromogenic reagent. The procedure described can easily be employed to analyze biological materials.
2. Experimental 2.1. Apparatus A Micronal B-380 spectrophotometer equipped with a Hellma ¯ow cell (178-010-OS) set at 496 nm coupled to a Chessell x±y recorder was used along with an Ismatec peristaltic pump model IPC-12 furnished with Tygon tubes to propel all solutions, a Rheodyne 5041 injection valve to select the preconcentration/elution steps, and a microwave oven SPEX CDS7000 with PFA sealed tubes to dissolve the samples. All connections were made using ®ttings, unions and tees made of plastic and PEEK materials. The manifold was built up with PTFE tube of 0.5 mm bore. 2.2. Reagents and solutions Milli-Q (Millipore) water was used to prepare all solutions. All reagents were of analytical grade. A zinc(II) stock solution containing 1000 mg lÿ1 was prepared by dissolving 2.4696 g of dried ZnSO4 in water containing 10 ml of concentrated nitric acid and dilution up to 1000 ml. The analytical solutions were prepared daily by successive dilutions from the stock solution, maintained at pH 3.00.2. A reagent containing 1 mol lÿ1 potassium thiocyanate and 0.5% sodium citrate was prepared daily by dissolving 24.30 g of KSCN and 1.25 g of Na3C6H5O72H2O in water, adjusting pH at 3.00.1 and dilution with water to 250 ml. A 0.01% 4-(2-pyridylazo)-resorcinol monosodium salt monohydrate ± PAR solution was prepared by dissolving 0.1 g in 800 ml of water and adding 61.83 g of boric acid and 20 g of sodium hydroxide. The pH was adjusted at 10 and diluting with water to 1000 ml. This solution must be kept still for at least two days and if any solid appear (boric acid) ®ltration must be done. This solution is stable for at least one week. Polyurethane foam (PUF) open cell polyether-type was obtained from the commercial product (Vulcan of Brazil ± VCON 202, 42% resilience and 10±12 cells/ linear cm). It was broken into small particles in a blender with demineralized water [17,19]. A minicolumn was packed by using ca. 100 mg of foam in a small plastic tube of 3.0 cm length3 mm i.d. The
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Fig. 1. Schematic diagram of the flow system employed for zinc preconcentration on PUF minicolumn and its spectrophotometric determination. Pperistaltic pump; Ssample (6.7 ml minÿ1); R1thiocyanatecitrate reagent (0.43 ml minÿ1); Ewater (2.0 ml minÿ1); R2PAR reagent (0.48 ml minÿ1); RCreactor coil (50 cm); Vfour-way valve; Ddetector at 496 nm; Wwaste. Four-way valve in the (a) preconcentration and (b) elution positions.
sorption capacity of the PUF used was 0.016 mmol Zn gÿ1 determined by sorption isotherm [19]. 2.3. Flow-injection system A diagram of the ¯ow system is shown in Fig. 1. In this system, a sample solution pumped at 6.7 ml minÿ1 merges with a thiocyanate solution stream at a ¯ow rate of 0.43 ml minÿ1 and percolates through the polyurethane foam minicolumn. Then, the zinc±thiocyanate complex is retained and the remained solution goes to the waste. The preconcentration step was done by using the time-based technique. Switching the injection valve, a stream of water ¯owing at 2.0 ml minÿ1 displaces the zinc retained. The eluate merges with a PAR solution at 0.48 ml minÿ1 and the absorbance of the resultant solution was monitored at 496 nm. The peak heights were always used to make all calculations. In this work, 1 min preconcentration time and 2 min elution time were used. After the elution, the PUF minicolumn was prepared to perform a new preconcentration cycle. 2.4. Sample preparation Several certi®ed reference materials (CRMs) were analyzed, such as copepoda (MA-A-1/TM), ®sh
tissue (MA-B-3/TM), ®sh ¯esh (MA-A-2/TM), and tuna homogenate (IAEA-350) from International Atomic Energy Agency (IAEA), Monaco and rice ¯our-unpolished sample Nos. 10(a±c) from National Institute of Environmental Studies (NIES), Japan. These samples after drying overnight at 11058C were weighted into a PFA liner and 8 ml of concentrated nitric acid was added. The sample quantities were taken according to the zinc sample content (ranging from 0.1 to 0.7 g). After standing overnight in contact with nitric acid, 2 ml of 30% hydrogen peroxide were added and the liners were closed to dissolve the samples. The microwave oven was controlled by its software according to the preselected `®sh method' in order not to exceed an internal liner pressure of 180 psig. Two cycles (of 25 min of microwave power) were performed (10 min ramp and 15 min dwell, and the temperature limit was 1908C). After each cycle the liners were opened to release gases. The obtained solutions were colorless and perfectly clear. The ®nal volume was diluting with water to 250 ml after adjusting the pH at 3.00.2 with ammonium hydroxide. The solution samples were stored in polypropylene ¯asks and analyzed according to the procedure described. At least, one blank solution was prepared for each sample to compensate the reagent contamination.
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3. Results and discussion 3.1. Flow-system optimization In order to improve the performance of the preconcentrating system, both chemical and ¯ow variables were studied using the ¯ow system shown in Fig. 1. Although in batch procedure the pH of sample was not found as an important factor to preconcentrate zinc from thiocyanate medium [19], this variable was carefully studied. Highest signals were obtained in the range of pH from 1.3 to 3.9. Below pH 1.3, signals were constant but blank signals were higher due to the change of color of the chromogenic reagent. When the pH was higher than 3.9, signals decreased abruptly and, therefore, the pH chosen was at 3.00.2. Thiocyanate concentration was tested ranging from 0.05 to 2.0 mol lÿ1. From 0.05 mol lÿ1, the signal increased and the best signals were obtained above 0.8 mol lÿ1. Thus, a thiocyanate concentration of 1.0 mol lÿ1 was selected. PAR content was tested ranging from 0.00050 to 0.040%. The results obtained show that signals were constant for PAR concentration higher than 0.0025%. The PAR concentration chosen was 0.01%. According to the batch methodology [19] water was used as eluent. It was possible to desorb zinc from polyurethane foam minicolumn by displacing the equilibrium. The effectiveness of other possible eluents were investigated. Nitric acid concentrations at 110ÿ3, 110ÿ4 and 110ÿ5 mol lÿ1 were used without enhancing ef®ciency. It was expected that more concentrated acids would be more ef®cient. However, it could not be used because the absorbance of PAR reagent increased. Ethanol±water solutions were also tested at 5, 10 and 20%. Ethanol solutions did not improve the elution and a noise appeared due to the Schieliren effect. Acetone±water mixtures (1 : 5 and 1 : 2) were investigated without success and the noise effect described was also observed. Variations of the sample ¯ow rate were studied ranging from 2.0 to 12.8 ml minÿ1. The best preconcentration response was obtained at a ¯ow rate of 6.7 ml minÿ1. However, the best absorbance signal/ sample volume was observed at 3.0 ml minÿ1, although the signal at this ¯ow rate was lower than that obtained at 6.7 ml minÿ1. Thus, 6.7 ml minÿ1
¯ow rate was selected to increase the sample throughput and enhance the sensitivity. Sample ¯ow rates higher than 6.7 ml minÿ1 were not enough to retain zinc and thus the signals decreased. Thiocyanate ¯ow rate was investigated in the range from 0.28 to 1.69 ml minÿ1. The best results were observed above 0.43 ml minÿ1. Therefore, this ¯ow rate was selected for subsequent experiments in order to maintain low sample dilution. PAR ¯ow rate was selected at 0.48 ml minÿ1 and it was not changed because higher ¯ow rates made spectrophotometer adjustment to the baseline troublesome. Eluent ¯ow rate was tested ranging from 1.69 to 5.8 ml minÿ1. It was observed that 2.0 ml minÿ1 was suitable to elute zinc without losing sensitivity. Displacing the retainable zinc was not so fast and it was controlled by the slow kinetic desorption. Moreover, the time required to decrease the thiocyanate concentration inside the column is not so low. Therefore, elution was not a fast step. Higher ¯ow rates only improved the dispersion and made the signals somewhat lower. The reactor coil length was studied in the 50±200 cm range, but such variation did not in¯uence the analytical signal and for that reason the smaller reactor coil was selected. The length of minicolumns and the amount of PUF were studied after preparing three columns of 1 cm (15 mg), 3 cm (50 mg), and 3 cm (100 mg). Fig. 2 shows the analytical curves obtained by these minicolumns. The minicolumn, 1 cm in length, was enough to retain at least 0.54 mg of Zn under ¯ow conditions (preconcentrated from analytical solutions containing 80 ng mlÿ1 pumped at 6.7 ml minÿ1 for 1 min). However, minicolumn of 3 cm in length was used to guarantee safety in all experiments. The use of PUF minicolumns under ¯ow conditions is attractive due to the very low overpressure observed, unlike other sorbents used in ¯ow systems, such as chelating agents and fullerenes. 3.2. Interference study Several interferent cations, specially which form thiocyanate complexes and/or react with PAR, mainly Fe(III), Cu(II), Co(II), Mn(II), Ni(II), Cd(II), Al(III) and Mo(VI) were used in order to verify the procedure selectivity. On using a mixed solution containing thiocyanate and sodium citrate, several interferences can be masked. However, when the sodium citrate
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267
Fig. 2. Analytical curves obtained with different PUF minicolumns.
concentration increases, the zinc signal decreases. Thus, a 0.5% sodium citrate concentration was used in order to guarantee the best sensitivity and selectivity. The maximum ratios zinc : interferent tested were: Fe(III) up to 1 : 200; Mo(VI) up to 1 : 12; Mn(II), Cu(II), and Co(II) up to 1 : 5. At such levels no interference was observed. Their ratios were higher than those found in the samples analyzed. Al(III) and Ni(II) did not interfere because they do not react with thiocyanate. In contrast with batch experiments [19], Cd(II) was the most relevant interfering specie but it did not observed any problem due to the very low Cd concentration in the samples analyzed. 3.3. Analytical features The ¯ow system using 1 min preconcentration time shows linear in the range of concentration from 20 to 100 ng mlÿ1 (in Fig. 1). It may be represented by the equation A0.0030.00507 [Zn (ng mlÿ1)], and correlation coef®cient r0.999. The detection limit estimated as three times the standard deviation was
0.9 ng mlÿ1. The RSD was calculated by taking 12 measurements of 20 ng mlÿ1 and it was better than 1.2%. Precision study was also carried out with two real samples. The coef®cient of variation (CV) of four copepoda and four rice ¯our samples were 7 and 2%, respectively. The analytical sample throughput was 20 samples per hour. Analytical curves in the concentration range 5±20 ng mlÿ1 were also made using 5 min preconcentration time (A0.0130.0193 [Zn(ng mlÿ1)]), r0.999. The detection limit was 0.6 ng mlÿ1. The preconcentration factors were calculated by comparing the analytical curve slopes obtained from preconcentration procedure with the ones obtained from the static method using the same volumes and concentrations of the reactants. Using 1 min preconcentration time, the preconcentration factor was ca. 8. However, more sensitivity can be achieved by using higher preconcentration times. The preconcentration factor for 5 min preconcentration time was 32. The concentration ef®ciency (CE) [22] for 1 min preconcentration time was 2.67 and for 5 min preconcentra-
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Table 1 Results obtained for the certified reference materials analyzed. Data in mg gÿ1 Sample
Results obtained with FIA±PUF methodology
Certified values a
Rice flour, unpolished, low Cd level, No. 10a ± NIES Rice flour, unpolished, medium Cd level, No. 10b ± NIES Rice flour, unpolished, high Cd level, No. 10c ± NIES Copepoda, MA-A-1/TM ± IAEA Fish flesh, MA-A-2/TM ± IAEA Fish tissue, MA-B-3/TM ± IAEA Tuna homogenate, IAEA 350
25.81.8 21.80.2 23.10.3 1784 36.60.3 110.90.5 18.30.4
25.20.8 22.30.9 23.10.8 156±160 32±34 106.4±111.9 16.6±18.5
a
Data from Refs. [34,35].
Table 2 Precision of the FIA±PUF methodology for zinc preconcentration and determination in biological samples analyzed in four replicates Sample
Results obtained with FIA±PUF methodology (mg gÿ1)
Rice flour, unpolished, low Cd level, No. 10a ± NIES
28.0 26.4 25.0 23.9
Copepoda, MA-A-1/TM ± IAEA
182.9 179.4 176.2 174.6
tion time it was 4.67. Such numbers are not so good due to the long elution time required. 3.4. Applications The ¯ow system described, including PUF minicolumns, was good enough to preconcentrate and determine zinc in biological matrices. Several certi®ed reference materials were analyzed and the obtained results are shown in Table 1. It can be seen that all the results are in agreement with those certi®ed except for copepoda sample. Table 2 shows the concentration values for two real samples analyzed for four times and their coef®cient of variation. 4. Conclusions This paper is the ®rst report on the use of PUF minicolumns in continuous ¯ow systems. The ¯ow system is a simple, rapid, and inexpensive method to
Mean and standard deviation (mg gÿ1)
Coefficient of variation (%)
25.81.8
7
178.33.6
2
preconcentrate and determine zinc in biological samples. The selective procedure is enough to analyze biological material with precision and accuracy in sample solutions containing more than 5 ng Zn mlÿ1. The analytical potentiality of PUF minicolumns under continuous ¯ow conditions can be implemented for metals separation and preconcentration. Besides, PUF minicolumns can be useful using more selective and sensitive detection techniques such as AAS or ICP±AES for multielement determination. Acknowledgements The authors would like to thank Conselho Nacional de Desenvolvimento Cientõ®co e TecnoloÂgico (CNPq), Financiadora de Estudos e Projetos (FINEP) and ComissaÄo Nacional de Energia Nuclear (CNEN) for grants and fellowships.
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