An improved flow system for spectrophotometric determination of anions exploiting multicommutation and multidetection

Transcription

1 Analytica Chimica Acta 438 (2001) An improved flow system for spectrophotometric determination of anions exploiting multicommutation and multidetection Fábio R.P. Rocha a,b, Patrícia B. Martelli a, Boaventura F. Reis a, a Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, P.O. Box 96, Piracicaba , Brazil b Departamento de Química, Universidade Federal de São Carlos, P.O. Box 676, São Carlos , Brazil Received 11 July 2000; received in revised form 20 October 2000; accepted 23 October 2000 Abstract A multicommutated flow system is proposed for the determination of anions in water samples. The flow set up was assembled with a set of computer-controlled three-way solenoid valves in order to manage the addition of different reagents by binary sampling. An optical-fiber CCD-array spectrophotometer with a tungsten-halogen lamp was employed for multidetection. Water samples were used as carrier and the chromogenic reagents were intermittently added, allowing the sequential determination of nitrate, nitrite, chloride and phosphate with or without in-line concentration by ion exchange. In-line concentration of the analytes was performed during the signal measurements of the other species. In this way, a 180 s loading time was implemented without impairing the sampling rate (estimated as 50 determinations per hour). Under the proposed conditions, the procedure can be used for samples containing gl 1 N-NO 2, mg l 1 N-NO 3, mg l 1 Cl, and mg l 1 P-PO 3 4. Detection limits were estimated as 6 gl 1 N-NO 2,40 gl 1 N-NO 3, 400 gl 1 Cl and 30 gl 1 P-PO 3 4 at 99.7% confidence level. Coefficients of variation were estimated (n = 20) as 1.6, 2.2, 2.3 and 1.5% for nitrite, nitrate, chloride and phosphate, respectively. The reagent consumption was reduced from 3- to 40-fold and from 20- to 760-fold regarding the conventional FIA systems and batch procedures, respectively. Results for river water samples agreed with those obtained by single-analyte FIA procedures at the 95% confidence level Elsevier Science B.V. All rights reserved. Keywords: Flow-injection spectrophotometry; Anions determination; Water; Multicommutation; Multidetection; In-line concentration 1. Introduction The determination of anionic species in waters is often required for environmental monitoring. This can be successfully performed by means of flow analysis [1] and often systems for determination of a single species are used. In this sense, attractive approaches exploiting reagent injection have been proposed to maximize sensitivity and to reduce the reagent consumption [2]. In addition, sequential determinations can be easily implemented by changing the Corresponding author. Fax: address: reis@cena.usp.br (B.F. Reis). reagents introduced into the sample-carrier stream [3]. The increase in the required sample volume is not a hindrance for this kind of sample. The intermittent addition of different reagents can be easily achieved by building up the flow systems with discrete commutation devices (multicommutation approach [4]). Thus, dynamic manifolds can be designed, which can be reconfigured to perform sequential determinations, changing the reaction conditions [5] or introducing different chromogenic reagents [6 8]. Despite the potentiality for multidetermination, multicommutated flow systems have been usually exploited for determination of just two species through measurements at the same wavelength [5 7] /01/$ see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (00)

2 12 F.R.P. Rocha et al. / Analytica Chimica Acta 438 (2001) Spectrophotometric detection at several wavelengths (multidetection) has been carried out with fast scanning detectors (such as array spectrophotometers) or by employing several flow-through photometric modules [9]. Multidetection has been used in flow analysis mainly to perform multidetermination but it is also useful to compensate spurious signals caused by refractive index gradients [10,11] and color or turbidity of the samples [11]. In this work, it is presented a flow system for sequential determination of anions in water, exploiting reagent additions in a continuous-flowing sample. The feasibility of this strategy is demonstrated for the determination of nitrite, nitrate, chloride and phosphate in natural waters. The active structure of the flow manifold is exploited for the sequential addition of reagents and to perform in-line concentration simultaneously with the measurement cycles. Multidetection with a fiber-optic spectrophotometer based on charge coupled devices (CCD) was employed to detect the reaction products at the absorption maxima and to compensate drawbacks caused by refractive index effect. 2. Experimental 2.1. Apparatus The flow network was assembled with computercontrolled three-way solenoid valves (NResearch- 161T031), mixing coils of 0.8 mm i.d. polyethylene tubing and Perspex joint points. A peristaltic pump (Ismatec, IPC 4) equipped with Tygon tubes was employed to propel the carrier and the eluent solutions. A Pentium 166 MHz microcomputer equipped with an electronic interface (Advantech Corp.-PCL-711S) was employed to switch the commutation devices. Control signals were generated at TTL pattern and a previously described electronic interface [4] was employed to generate the required electric potential and current (12 V and ca. 100 ma). Data acquisition in the range nm was performed through a CCD-array spectrophotometer (Ocean Optics) by means of an Ocean Optics Dynamic Link Library interface package. Optical fibers were used to link the tungsten-halogen radiation source to the detection system through the flow cell (80 l, 10 mm optical path). The control of the devices, data acquisition and processing were made through a software wrote in Visual Basic Reagents and solutions All solutions were prepared with distillated/ deionized water and analytical grade reagents. Single analyte 1.00 g l 1 stock solutions were prepared from NaNO 3, NaNO 2, NaCl and KH 2 PO 4. Nitrate, chloride and phosphate salts were dried at 110 C for 2 h before weighting. Nitrite stock solution was standardized with potassium permanganate. Reference solutions within the ranges mg l 1 N-NO 3, gl 1 N-NO 2, mg l 1 Cl and mg l 1 P-PO 3 4 were prepared by appropriated dilutions. For nitrate determination, a buffer solution (B S ) was prepared by dissolving 6.20 g (NH 4 ) 2 SO 4, 1.00 g Na 2 B 4 O 7 10H 2 O and 0.10 g Na 2 EDTA in 50 ml water. Chromogenic reagent for nitrite determination (R 1 ) was a solution containing 2.00% (m/v) sulfanilamide, 0.10% (m/v) N-(1-naphthyl)ethylenediamine dihydrochloride (NED) and 0.5 mol l 1 HClO 4. The reducing column (C R ) was a glass tube (10 cm long, 3 mm i.d.) filled with copperized cadmium fillings prepared as previously described [12] and plugged with glass-wool. The color forming reagent for chloride determination (R 2 ) was prepared dissolving by heating 0.03 g Hg(SCN) 2 in 40 ml water. After cooling, the solution was mixed with 1.60 g Fe(NO 3 ) 3 9H 2 O dissolved in 5 ml of 2.0 mol l 1 HNO 3. The volume was completed to 50 ml. Solutions 1.00%(m/v) ammonium molybdate with 0.25 mol l 1 HClO 4 (R 3 ) and 1.00% (m/v) ascorbic acid (R 4 ) were employed for phosphate determination. The eluent (E) was a 0.05 mol l 1 HNO 3 solution. For in-line analyte concentration, a 15 mm long column (4 mm i.d.) was filled with anion-exchange resin (AG 1-X8, mesh) in the chloride form. A 0.05 mol l 1 HNO 3 solution was pumped through the column at 2.5 ml min 1 for 2 min in order to convert the resin to the nitrate form Flow diagram and procedure Optimization of the reaction conditions for the determination of each species was carried out with the system outlined in Fig. 1. Sample or reference

3 F.R.P. Rocha et al. / Analytica Chimica Acta 438 (2001) Fig. 1. Flow diagram of the system employed for optimization of the reaction conditions for determination of nitrite, nitrate, chloride and phosphate in waters: S sample; S L buffer or eluent solutions; R a,r b reagents; C column with anion exchange resin (AG1-X8) or copperized cadmium fillings; V i three-way solenoid valves; B 1,B 2 coiled reactors; D CCD array spectrophotometer and W waste. solutions (S) were employed as carrier and pumped at 2.5 ml min 1. The other solutions were introduced by gravity with flow rates of ca. 3 ml min 1 [8]. The chromogenic reagents (R a and R b ) were introduced through the valves V 4 and V 5 and the buffer or eluent solutions (S L ) through the valve V 2. Concentrations of the reagents, volumetric fractions, and the number of sampling cycles were changed in order to maximize sensitivity. The flow diagram showed in Fig. 2, operated according to the valve switching course specified in Table 1, was employed for sequential determination of nitrite, nitrate plus nitrite, chloride and phosphate in waters. Sample solutions (S) were employed as carrier and the chromogenic reagents (R i ) were intermittently added by binary sampling [4]. During part of the measurement intervals (steps 4, 11, 15 and 20), the sample solution was pumped through the anion exchange column (C P ) for in-line concentration for ca. 180 s (45 s in each step). This strategy was evaluated for determination of low concentrations of phosphate. Sample (S) and eluent (E) solutions were pumped at 2.5 ml min 1 and the other solutions (B S, R 1 R 4 ) were added by gravity. For the addition of the reagents, the valve V 1 was switched on simultaneously with the valves used to control the addition of each solution (V 3, V 6 V 9 ). Thus, the sample solution was recycled and suitable aliquots of the corresponding reagent were introduced in the analytical path. A sampling cycle consists of a sample aliquot intercalated between reagent aliquots [4,6]. Perturbations caused by Schlieren effect [10,12] were compensated by dual-wavelength spectrophotometry [10]. Transient signals were measured simultaneously at the absorption maximum of the products (λ 1 ) and at reference wavelengths (λ 2 ), in which Fig. 2. Flow diagram of the system for sequential determination of anions: S sample; B S buffer solution; E eluent solution; R i chromogenic reagents; C P column with anion exchange resin (AG1-X8); C R column with copperized cadmium fillings; V i three-way solenoid valves; B 1,B 2 coiled reactors (50 and 150 cm); D CCD array spectrophotometer and W waste.

4 14 F.R.P. Rocha et al. / Analytica Chimica Acta 438 (2001) Table 1 Valves switching course for the sequential determination of anions with the flow system showed in Fig. 2 a Step V 1 V 2 V 3 V 4 V 5 V 6 V 7 V 8 V 9 t (s) Description Nitrite 1 b Sampling R 1 2 b Sampling S Signal measurement Signal measurement and in-line concentration Nitrate + nitrite 5 b Sampling B S 6 b Sampling S 7 b Sampling R 1 8 b Sampling S Signal measurement signal measurement and in-line concentration Chloride 11 c Sampling R 2 12 c Sampling S Signal measurement Signal measurement in-line concentration Phosphate d 15 c Sampling R 3 16 c Sampling S 17 c Sampling R Signal measurement Signal measurement and in-line concentration Phosphate e Sampling R Elution Sampling R Signal measurement a Numbers 0 and 1 represent valves switched off or on, respectively. b Two sampling cycles. c Six sampling cycles. d Without in-line concentration. e In-line concentration for 180 s. radiation absorption by the products is negligible. The difference between both signals was employed as the analytical signal [10]. For nitrite determination, aliquots of the Griess reagent (R 1 ) were sandwiched between sample aliquots in two sampling cycles (Table 1, steps 1 and 2). The determination of nitrate plus nitrite was performed after ph adjustment by inserting small aliquots of the buffer solution (B S ) between sample aliquots (Table 1, steps 5 and 6). The buffered sample zone was pumped through the reducing column C R, in which nitrate was converted to nitrite. The chromogenic reagent R 1 was introduced to the effluent of the column by binary sampling (Table 1, steps 7 and 8). The product of the diazo-coupling reaction was measured at 540 nm (λ 1 ) with reference at 800 nm (λ 2 ). Chromogenic reagents for chloride (R 2 ) and phosphate (R 3 and R 4 ) were added to the sample-stream (Table 1, steps and 15 17, respectively). Six sampling cycles were employed for the determination of both species. Signals for chloride were measured at λ 1 = 455 and λ 2 = 800 nm. The difference between signals measured at λ 1 = 716 and λ 2 = 450 nm was employed as analytical signal for phosphate determination. After measuring the signals, the ions retained in the resin column C P were eluted by the solution

5 F.R.P. Rocha et al. / Analytica Chimica Acta 438 (2001) E. The reagents for phosphate determination (R 3 and R 4 ) were added to the effluent of the column (Table 1, steps 20 22). Water samples were collected near Piracicaba in polyethylene bottles and analyzed in the same day by the proposed procedure and by FIA systems designed for single-analyte determination. Before analysis, the samples were filtered through 45 m cellulose acetate membranes and pumped through columns containing C 18 -bonded silica (Sep-Pak, Waters, USA) at 2.0 ml min 1 to remove the excess of dissolved organic matter. 3. Results and discussion 3.1. Flow system and chemical variables In the proposed system, water samples were employed as carrier and the reagent solutions were added by binary sampling [4]. Systems in which reagents are introduced in the sample stream show the advantage of the maximization of the sensitivity, due to the inversion of the dispersion effects of the analyte [2]. In conventional FIA systems, the sample dispersion in the carrier causes the decrease of the analyte concentration in the sample zone. Thus, the analyte concentration is inversely proportional to the sample dispersion. On the other hand, in systems with addition of reagents, the analyte concentration in the reagent zone (initially equal to zero) increases with the sample dispersion. Low-volume aliquots of concentrated reagents were introduced in the sample stream in order to maximize the sensitivity. Binary sampling was employed to facilitate the mixing between sample and reagents providing the formation of the products. The introduction of the reagent aliquots was performed by exploiting the gravitational propulsion of the solutions in order to improve the precision [8], eliminating the effect caused by pulsation of the peristaltic pump. This was achieved by placing the solution vessels at 1.6 m higher than the waste, attaining flow rates of 3 ml min 1. The carrier and the eluent solutions were propelled by the peristaltic pump, in view of the increase of the backpressure caused by the copperized cadmium column and by the ion exchange column. Salts and acids containing chloride and phosphate were not employed in order to avoid contamination in the determinations of these species. Preferentially, reagents containing sulfate or perchlorate were employed. However, elution and in-line concentration of phosphate in the anion exchange column (C P ) were affected (analytical signal reduced to 30 50%) when sulfuric or perchloric acids were used as eluent. Regarding chloride determination, a 50% higher sensitivity was observed when iron(iii) nitrate was used instead of iron(iii) sulfate. This occurs due to side reactions involving iron(iii) and sulfate, which reduce the amount of free iron(iii) to react with thiocyanate [13]. Sensitivity similar to the observed employing the iron(iii) nitrate reagent was achieved with a solution containing equimolar amount of iron(iii) perchlorate. However, blank signals caused by the absorption of the reagent at the measurement wavelength were higher. On the other hand, eluent (nitric acid) and the reagent for chloride determination (iron(iii) nitrate and nitric acid) did not affect nitrate determination because the solutions did not pass through the reduction column (C R ). Thus, nitric acid was employed as eluent and iron(iii) nitrate and nitric acid were used in the reagent for chloride, without causing contamination troubles. In the reagent for chloride determination, the complex [Fe(SCN)] 2+ is formed due to the equilibria of dissociation of the Hg(SCN) 2 and complexation of thiocyanate by iron(iii) [13]. The formation of this species, which increases the blank signal, can be minimized be reducing the temperature of the solutions containing Hg(SCN) 2 and iron(iii) before mixing. The volume and the volumetric fraction of each reagent and the number of sampling cycles were changed in order to maximize the sensitivity, and minimize the Schlieren effect and the reagent consumption. Best results were achieved by employing the time intervals specified in Table Compensation of Schlieren effect In the spectrophotometer employed, optical fibers with 0.5 mm slit were used to transport the radiation from the light source to the flow cell and from this device to the detector array. The light beam was focused by lenses placed at the support of the flow cell (inlet and outlet) in order to minimize light scattering. Spectrophotometric measurements with this

6 16 F.R.P. Rocha et al. / Analytica Chimica Acta 438 (2001) configuration are strongly affected by refractive index gradients (Schlieren effect). Differences of refractive indexes can generate liquid interfaces which can act as mirrors or lenses that can scatter the radiation through the optical path of the flow cell [10]. In the proposed system, the insertion of reagent aliquots with chemical and physical characteristics different from those of the carrier favors the formation of refractive index gradients. This effect was compensated by exploiting the dual wavelength strategy. With the multi-channel spectrophotometer, measurements were simultaneously performed at the absorption maxima (λ 1 ) and at the wavelengths in which the absorption by the products is negligible (λ 2 ). According to the absorption spectra of the products, the selected wavelengths were: λ 1 = 540 and λ 2 = 800 nm, for nitrate and nitrite, λ 1 = 455 and λ 2 = 800 nm, for chloride and λ 1 = 716 and λ 2 = 450 nm, for phosphate. Multidetection at these wavelengths was exploited to maximize the sensitivity and to reduce the Schlieren effect. As showed in Fig. 3, the difference between the absorbance measured at λ 1 and λ 2 allowed compensating the effect of refractive index gradients Analytical features and sample analysis Under the conditions recommended in Table 1, linear responses were obtained within 30 and 300 gl 1 N-NO 2 (A = C, r = 0.999); 0.10 and 1.0 mg l 1 N-NO 3 (A = C, r = 0.999); 1.0 and 10 mg l 1 Cl (A = C, r = 0.996) or 0.5 and 2.5 mg l 1 P-PO 4 3 (A = C, r = 0.999). Detection limits at the 99.7% confidence level were estimated according to IUPAC recommendations [14] as 6 gl 1 N-NO 2,40 gl 1 N-NO 3, 400 gl 1 Cl and 360 gl 1 P-PO 4 3. Coefficients of variation of peak heights were estimated as 1.6; 2.2; 2.3 and 1.5% for nitrite, nitrate, chloride and phosphate, respectively, through 20-fold processing of a solution containing 150 gl 1 N-NO 2, 500 gl 1 N-NO 3, 6 mg l 1 Cl and 1.5 mg l 1 P-PO 4 3. With the system operated according to the valve switching course showed in Table 1, the sampling rate was estimated as 50 determinations per hour. As described in Table 1, the control software of the flow system allowed implementing in-line analyte Fig. 3. Compensation of refractive index effect by means of multidetection (a) wavelength of maximum absorption of the products; (b) reference wavelength and (c) difference between the signals. Transient signals were obtained using distilled water as carrier stream, with the system operated according to the valves switching course showed in Table 1. concentration simultaneously with the measurements of nitrite, nitrate, chloride and phosphate. Sample stream was pumped through the anion exchange column (C P ) during part of the measurement cycles. Thus, sample waste was avoided and in-line concentration (with loading time of 180 s) was performed without affecting the sampling rate. After sample processing for direct determination of the four species, the anions retained at column C P could be eluted to determine a species with concentration lower than the detection limit achieved by direct determination. This was evaluated for in-line phosphate concentration, resulting in a 7.7-fold increase in sensitivity. Linear

7 F.R.P. Rocha et al. / Analytica Chimica Acta 438 (2001) Table 2 Analytical features of different procedures for determination of the analytes Species Analytical parameter Proposed method Confluent system [17 19] SIA [20,21] Batch method [15,16] Nitrite Sampling rate (h 1 ) Detection limit ( gl 1 N-NO 2 ) Linear range ( gl 1 N-NO 2 ) Coefficient of variation (%) < Effluent volume per determination (ml) Nitrate Sampling rate (h 1 ) Detection limit ( gl 1 N-NO 3 ) Linear range (mg l 1 N-NO 3 ) Coefficient of variation (%) < Effluent volume per determination (ml) Phosphate Sampling rate (h 1 ) Detection limit ( gl 1 P-PO 3 4 ) Linear range (mg l 1 P-PO 3 4 ) < Coefficient of variation (%) Effluent volume per determination (ml) Chloride Sampling rate (h 1 ) Detection limit ( gl 1 Cl ) Linear range (mg l 1 Cl ) <2.5 Coefficient of variation (%) Effluent volume per determination (ml) response was observed within 0.05 and 1.0 mg l 1 P-PO 4 3 (A = C, r = 0.999). The detection limit was estimated as 30 gl 1 P-PO 4 3 at the 99.7% confidence level and the coefficient of variation was 2.7% (n = 20). In Table 2, the analytical features of the proposed system are compared with those presented for batch procedures [15,16] and for flow systems with con- tinuous addition of solutions by confluence [17 19] or with sequential injections [20,21]. The reagent amounts employed per determination in each procedure is presented in Table 3. The binary sampling strategy allowed reducing the reagent consumption from 3- to 40-fold and from 20- to 760-fold regarding the conventional FIA systems and batch procedures, respectively. The reagent consumption in the Table 3 Comparison of the reagent amounts consumed by the proposed method, by flow systems with continuous addition of reagents [17 19] or sequential injections [20,21] and by batch procedures [15,16] Reagent Amount of reagent (mg per determination) Proposed method Confluent systems SIA Batch methods Sulfanilamide NED (NH 4 ) 2 SO NH 4 Cl Na 2 B 4 O 7 10H 2 O Na 2 EDTA Hg(SCN) (NH 4 ) 6 Mo 7 O 24 4H 2 O Ascorbic acid SnCl 2 2H 2 O 0.38

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9 F.R.P. Rocha et al. / Analytica Chimica Acta 438 (2001) sequential injection procedures was slightly lower than the observed in the proposed system. Some drawbacks were observed during the analysis of nitrate and phosphate in river water samples. The reduction efficiency of the copperized cadmium column was continuously lessened when samples with high organic matter content were processed, affecting both the precision and the accuracy for nitrate determination. Moreover, the organic compounds were irreversibly retained in the ion-exchange column, hindering the retention of phosphate. These drawbacks were successfully overcame by pumping the sample through the C 18 -bonded silica column before analysis, in order to remove the dissolved organic matter. After this pretreatment, six water samples were analyzed by the proposed method and by FIA systems proposed for the determination of each species independently. As showed in Table 4, the results agreed at the 95% confidence level. 4. Conclusions The proposed flow system can be employed for sequential determination of anions in waters. The strategy was demonstrated for the determination of nitrite, nitrate, chloride and phosphate, but other species can be determined by changing the reagents. Multicommutation was exploited for the intermittent addition of reagents and to perform in-line concentration simultaneously with the measurement cycles. In-field monitoring is also possible due to the reliability and the minimization of both the reagent consumption and waste generation. The computer-controlled flow setup can be easily automated by inserting a feedback system to decide which species should be quantified in the effluent of the anion exchange column. This could be implemented by comparing the signals obtained by direct sample processing with threshold values (signals corresponding to the detection limits of the direct procedures). Multidetection was exploited for the measurement of the products at the absorption maxima and to compensate spurious signals caused by refractive index gradients. Measurements at different wavelengths can also be exploited for detecting inaccuracy sources. In the presence of interfering species, the results probably would not be in agreement, because it is not expected that the interferences manifest themselves in the same way at different wavelengths. Acknowledgements Prof. Elias A.G. Zagatto is thanked for critical comments. Grants and financial support from the Brazilian agencies FAPESP (97/ ), CNPq, CAPES and FINEP/PRONEX ( ) are also appreciated. References [1] K.N. Andrew, N.J. Blundell, D. Price, P.J. Worsfold, Anal. Chem. 54 (1994) 916A. [2] K.S. Johnson, R.L. Petty, Anal. Chem. 54 (1982) [3] A. Ríos, M.D. Luque de Castro, M. Valcárcel, Analyst 110 (1985) 277. [4] B.F. Reis, M.F. Giné, E.A.G. Zagatto, J.L.F.C. Lima, R.A.S. Lapa, Anal. Chim. Acta 293 (1994) 129. [5] C.C. Oliveira, R.P. Sartini, B.F. Reis, E.A.G. Zagatto, Anal. Chim. Acta 332 (1996) 173. [6] P.B. Martelli, B.F. Reis, E.A.M. Kronka, H. Bergamin F o, M. Korn, E.A.G. Zagatto, J.L.F.C. Lima, A.N. Araújo, Anal. Chim. Acta 308 (1995) 397. [7] E.A.M. Kronka, B.F. Reis, M. Korn, H. Bergamin F o, Anal. Chim. Acta 334 (1996) 287. [8] F.R.P. Rocha, B.F. Reis, Anal. Chim. Acta 409 (2000) 227. [9] A. Rios, M.D. Luque de Castro, M. Valcárcel, Anal. Chem. 57 (1985) [10] E.A.G. Zagatto, M.A.Z. Arruda, A.O. Jacintho, I.L. Mattos, Anal. Chim. Acta 234 (1990) 153. [11] H. Liu, P.K. Dasgupta, Anal. Chim. Acta 289 (1994) 347. [12] F.R.P. Rocha, J.A. Nóbrega, Talanta 45 (1997) 265. [13] F.R.P. Rocha, J.A. Nóbrega, J. Braz. Chem. Soc. 8 (1997) 625. [14] Analytical Methods Committee, Analyst 112 (1987) 199. [15] A.D. Eaton, L.S. Clesceri, A.E. Greenberg, Standard Methods for the Examination of Water and Wastewater, 19th Edition, American Public Health Association, Washington, [16] T.M. Florence, Anal. Chim. Acta 54 (1971) 373. [17] M.F. Giné, H. Bergamin F o, E.A.G. Zagatto, B.F. Reis, Anal. Chim. Acta 114 (1980) 191. [18] F.J. Krug, L.C.R. Pessenda, E.A.G. Zagatto, A.O. Jacintho, B.F. Reis, Anal. Chim. Acta 130 (1981) 409. [19] A.O. Jacintho, E.A.M. Kronka, E.A.G. Zagatto, M.A.Z. Arruda, J.R. Ferreira, J. Flow Injection Anal. 6 (1989) 19. [20] A. Cerdà, M.T. Oms, R. Forteza, V. Cerdà, Anal. Chim. Acta 371 (1998) 63. [21] A. Muñoz, F. Mas Torres, J.M. Estela, V. Cerdà, Anal. Chim. Acta 350 (1997) 21.