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An electrochemical flow-cell for permanent modification of graphite tube with palladium for mercury determination by electrothermal atomic absorption spectrometry
Spectrochimica Acta Part B 57 (2002) 769–778
An electrochemical flow-cell for permanent modification of graphite tube with palladium for mercury determination by electrothermal atomic absorption spectrometry Ruben G.M. Morenoa, Elisabeth de Oliveiraa, Jairo J. Pedrottib, Pedro V. Oliveiraa,* b
a ´ ˜ Paulo, CEP 05508-900, C.P. 26077, Sao ˜ Paulo, SP, Brazil Instituto de Quımica, Universidade de Sao ´ ˆ ´ Departamento de Quımica, Faculdade de Ciencias Biologicas, Exatas e Experimentais, Universidade Presbiteriana Mackenzie, ˜ Paulo, SP, Brazil CEP 01239-900, Sao
Received 2 August 2001; accepted 31 December 2001
Abstract An electrochemical procedure for palladium deposition on the inner surface of pyrolytic graphite-coated tubes for permanent chemical modification and a cold vapor generation system for the pre-concentration and determination of mercury trace levels in rain, potable, and non-potable water and lake sediment by electrothermal atomic absorption spectrometry is proposed. A tubular electrochemical flow-cell was assembled on the original geometry of the graphite tube, which operated as the working electrode. A stainless steel tube, positioned downstream from the working electrode, was used as the auxiliary electrode. The applied potential was measured against a micro AgyAgCl(sat) reference electrode inserted in the auxiliary electrode. Palladium solution in acetate buffer (100 mmol ly1, pHs4.8), flowing at 0.5 ml miny1 for 60 min was used to perform the electrodeposition. A homemade cold vapor generation system composed of a peristaltic pump, an injector–commutator, a flow meter and a disposable polyethylene gas– liquid separator flask (approx. 4.0 ml volume) were used. Volumes of 1.0 ml of reagent (2.0% wyv NaBH4 in 0.10 mol ly1 of NaOH) and 1.0 ml of reference or sample solution in 0.25 mol ly1 of HNO3 were carried to the gas– liquid separator using the peristaltic pump. The mercury vapor was carried out to the modified graphite tube by argon flow (200 ml miny1), and pre-concentrated for 120 s. The characteristic mass for 1.0 ml of reference solution was 26 pg (R.S.D.s0.12%, ns5). The detection limit obtained was 93 pg (ns20, 3d). The reliability of the entire procedure was confirmed by addition and recovery tests and cold vapor atomic absorption spectrometry. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Graphite furnace; Permanent chemical modifier; Electrodeposition; Mercury determination
*Corresponding author. Tel.: q55-11-38183837; fax: q55-11-38155579. E-mail address: [email protected] (P.V. Oliveira).
0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 2 . 0 0 0 0 9 - 5
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1. Introduction Cold vapor atomic absorption spectrometry (CVAAS) is one of the most widely used techniques for the selective determination of mercury in environmental, biological, clinical and industrial samples w1,2x. When high sensitivity is required, mercury pre-concentration prior to CVAAS can be achieved by amalgamation onto a gold w3,4x or goldyplatinum trap w5x, followed by thermal desorption, getting absolute masses of less than 0.26 ng mercury w4x. Compatible detection limits can also be obtained with the solid phase using a flow-injection system w6–8x, and also by solvent extraction techniques w9x for mercury separation and pre-concentration. On the other hand, electrothermal atomic absorption spectrometry (ETAAS) has rarely been used for mercury determination. The volatility of mercury and its compounds, which cause difficulty in the stabilization during the solvent vaporization and consequent damage to the sensitivity, is the main hindrance. Nevertheless, several attempts have been made to overcome this obstacle. Due to the more pronounced volatility of mercury metal in relation to inorganic mercury compounds, several oxidizing agents (hydrogen peroxide, mixture of hydrogen peroxide and hydrochloric acid, potassium permanganate, potassium dichromate, ammonium sulfide and tellurium), complexes or precipitants reagents were used to prevent the reduction of mercury w10,11x. One of the problems encountered with some of these reagents was that they could not be applied to solutions containing higher concentrations of organic compounds because the reagent was consumed, at least partially, by the organic species w11x. Gold w10,16,17x, iridium w10,13,14,19,20x, palladium w10–12,15,18,19x, rhodium w10,18x, tungsten w14x and zirconium w14x have been investigated as chemical modifiers for mercury determination by ETAAS. These metals were thermally reduced on the inner of graphite surface prior to every heating cycle w12x or as permanent modifier obtained by thermal w10–18x or electrochemical w10,19x processes. In the last case, the graphite tube was wrapped with a Teflon䉸 band to
protect the outside surface from metal deposition w21,22x. Palladium electrochemically deposited onto the graphite surface tube showed better long-term stability (up to 450 firings) than thermally deposited (up to 50 firings) for mercury determination w10x. Characteristic masses of 114 pg (50 ml of solution) w12x and 300 pg (1.5 ml of solution) w13,14x by combining cold vapor generation and trapping in graphite furnace modified with palladium and iridium, respectively, were found. Recent contributions have investigated the distribution and in-depth of palladium, iridium and rhodium electrodeposited onto the inner surface of pyrolytic graphite-coated tube at different stages in the tube history w22x. Noble metals used for permanent modification do not form a compact layer on the surface, but penetrate into the pyrolytic graphite structure during the deposition step. Some advantages related to the electrochemical permanent modification are: noble metals are deposited in their elemental form, reducing the amount of oxygen which is present as metal oxide, and minimizing the corrosion of the graphite walls w22x; low consumption of noble metals reagents; low blank signals due to the contaminants volatilization during the pre-treatment steps; and shortening the time instrument run, and enlarging the graphite tube lifetime. In this work, a new electrochemical flow-cell design for permanent modification of the pyrolytic graphite-coated tube inner wall with palladium is presented. The mercury trace levels pre-concentration and determination in rain, potable, and nonpotable water and lake sediment by using a homemade cold vapor generation and separation system coupled to the ETAAS is proposed to demonstrate the permanent modification procedure efficiency. 2. Experimental 2.1. Instrumentation The spectroscopic measurements were made with an Analytik Jena AAS 6 Vario atomic absorption spectrometer (Analytik Jena, Jena, Germany) equipped with transversally heated graphite tubes
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Fig. 1. Schematic diagram of the tubular electrochemical flowcell: (A) solution inlet; (B) wall silicone rubber; (C) working electrode (graphite tube); (D) reference electrode; (E) auxiliary electrode; (F) outlet solution; and (G) silicone tube.
(Part No 07-8101225), deuterium hollow cathode lamp for background correction and MPE 50 autosampler. Mercury and deuterium hollow cathode lamps (Analytik Jena) used were electronically modulated with a frequency of 150 Hz. A slitwidth of 0.8 nm was selected to isolate the 253.7nm resonance line. All measurements were based on integrated absorbance values computed by OSy 2 WARP 4 Operating System, with at least three replicates. Argon 99.999% (vyv) (Air Liquid Brasil SyA, ˜ Paulo, Brazil) was used as a purge gas and Sao carrier gas of the mercury vapor. A scanning electron microscope Stereoscan 440 model (LEO, England) was used for the topochemical inspection of the modified graphite tube inner surface. A Qwave-3000 microwave digestion system (Questron Corporation, Merceville, NJ, USA) equipped with 100 ml PTFE vessels, a fluoropolymer coated oven cavity, exhaust fan, removable 10-position sample carrousel, a hose to permit venting of fumes into a fume hood and a pressure and temperature sensor were used. To ensure similar amounts of microwave energy for all sediments samples, the carrousel was rotated 1808 forwards and backwards by an internal motor. 2.1.1. Electrochemical flow-cell The tubular electrochemical flow-cell for palladium deposition in the inner of the graphite tube is shown in Fig. 1. It was assembled on the original
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cylindrical geometry of a transversal heated graphite tube (20 mm long and 5.5 mm i.d.) that operates as the working electrode (c). The palladium solution is carried into the flow-cell (a) by using a 0.8-mm PTFE tube connected to a short length (;5 mm) of thick walled silicone tube (b) fitted at inlet flow-cell. A stainless steel tube (10 mm long and 2.5 mm i.d.) fixed at the opposite extremity of the graphite tube with a help of a silicone tube (g) serves as the auxiliary electrode (e) and outlet solution (f). The micro AgyAgCl (saturated with NaCl) reference electrode (d) w23x manufactured on a 10ml polypropylene disposable pipette tip (Cole Parmer catalog 噛 25710-6) is positioned downstream from the working electrode. It is vertically inserted into the flow channel by means of an orifice (diameter, ds1.8 mm) made on the auxiliary electrode surface. To avoid leakage of the flowing solution through the graphite tube sampling hole, it was closed with a PTFE conical pin fixed under pressure with the help of a thin silicone tube. The geometric volume of the cell, defined as the solution volume that completely embraces the three-electrodes, is approximately 340 ml. 2.1.2. Continuous flow system for electrodeposition The palladium electrodeposition in the graphite tube surface was done using an Ismatec Model IPC-8 peristaltic pump (Ismatec, Switzerland) furnished with Tygon䉸 pumping tubes and PTFE tubes (0.8 mm i.d.). The electrochemical flow-cell previously described and the potentiostat were developed in the author’s lab. 2.1.3. Gas–liquid separator The homemade cold vapor generation system (Fig. 2) consisted of an Ismatec Model IPC-8 MSRe peristaltic pump furnished with Tygon䉸 pumping tubes, a injector–commutator w24x made of Perspex䉸, PTFE tubes (0.8 mm i.d.), a Dwyer series RM Rate Master flow meter and a disposable micro polyethylene gas–liquid separator flask (approx. 4.0 ml volume). A volume of 1.0 ml of the reference or sample solutions and 1.0 ml of the reagent solution are selected by the loops LS and LR, respectively, mixed in the confluence T and carried out to the
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Fig. 2. Schematic diagram of cold vapor generation system for pre-concentration of mercury on THGA tube: GT (graphite tube); GLS (gas–liquid separator); MC (anhydrous calcium carbonate mini column); Ar (argon flow rates200 ml miny1 ); T (confluence); C (carrier); S (sample); R (reagent); LS (sample loop); and LR (reagent loop).
gas–liquid separator (GLS). The carrier C was pure air, flowing at 3.0 ml miny1. The mercury vapor was blown on over the modified graphite tube surface with palladium through PTFE tube (20 cm long and 0.8 mm i.d.) by argon carrier stream (200 ml miny1). A mini column (MC), 25 mm long, filled with anhydrous calcium carbonate was placed intercepting the GLS and the graphite tube to absorb water vapor. The gas–liquid separator was fixed in the front of the AA spectrometer, as shown in Fig. 2.
Analytical reference solutions containing 0.5– 5.0 mg ly1 Hg2q in 0.1% (vyv) HNO3 were daily prepared by successive dilution of the 1000-mg ly1 Hg2q Tritisol standard solution (Merck, Darmstadt, Germany). Sodium borohydride and NaOH was used for the reductor preparation. Rain, potable, and non-potable waters and sed˜ lake (Vitoria ´ iments from Gaviao da Conquista, BA, Brazil) were used for mercury determination. 2.3. Procedure
2.2. Reagents, reference solutions and samples All solutions were prepared using high purity deionized water (resistivity 18.2 MV cm) obtained from a Milli-Q䉸 water purification system (Millipore, Bedford, USA). High purity reagents or analytical reagent-grade (Merck, Rio de Janeiro, Brazil) were used in all experiments. Nitric and hydrochloric acids were purified by distillation in quartz sub-boiling still ´ (Marconi Equipamentos de Laboratorio, Piracicaba, Brazil). A solution of 1000 mg ly1 of palladium was prepared by dissolving Pd(NO3)2 in a solution of 100 mmol ly1 of acetate buffer, pH 4.8.
All glassware and high-density polypropylene bottles were cleaned with detergent solution, soaking in 10% (vyv) HNO3 for 24 h, rinsed with Milli-Q䉸 water, filled with clean solution of 10% (vyv) HNO3 and stored until the usage. Reference and sample solutions were prepared in a Class-100 laminar flow hood. The solution of 1000 mg ly1 of palladium in 100 mmol ly1 of acetate buffer flowed through the electrochemical flow-cell at 0.5 ml miny1 for 60 min. All electrochemical modifications in the graphite tube were done without any previous pretreatment or thermal conditioning and were performed at ambient temperature (25"2 8C).
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Table 1 Instrumental operating conditions and THGA heating program for mercury determination by using the cold vapor proposed system Spectrometer Wavelength (nm) Bandpass (nm) HCL current (mA) Read delay (s) Read time (s) Measurements
253.7 0.7 15 1 7 Peak area
Heating programa Step
Temperature (8C)
Ramp (8Cys)
Hold (s)
Ar (ml miny1)
Drying Atomization Cleaning
110 1100 1400
18 FPb 300
10 7 7
300 0 300
a b
Program time: 31 s; Hg trapping temperature: 20 8C. Full power.
The selective palladium deposition on the inner wall of graphite tube was performed at a controlled potential (y0.600 V) measured against the reference electrode. This potential was previously selected through voltammetric experiments. After electrodeposition, the graphite tube was inserted into the graphite furnace and thermal surface conditioning was performed (ramp, hold): 90 8C (30 8Cys, 15 s); 110 8C (30 8Cys, 15 s); 800 8C (4 8Cys, 10 s) and 2000 8C (50 8Cys, 5 s). For optimization of the mercury vapor generation system, the reagent concentration (0.2–6.0% wyv NaBH4 in 0.1 mol ly1 of NaOH), the volume (0.2–1.0 ml) of sample (LS) and reagent (LR) (Fig. 2), the mercury carrier stream (0–700 ml miny1 Ar) and the trapping time (30–240 s) were investigated. The polyethylene gas–liquid separator (GLS) flask was discarded after each sampling to avoid sample inter-contamination. The mercury vapor was conducted to the graphite furnace by using the MPE 5 autosampler. Previously, the pipettor arm was fixed by following the menu serves of MPE 5 in the depth tube position and fixing the best distance between the tip of PTFE tube and the graphite surface for mercury trapping. Before each sampling in the gas–liquid separator, the pipettor arm was positioned in the depth tube position and kept during the pre-concentration time. After that time, the autosampler arm was returned to the wash position
and the heating program was carried out. The optimization of heating program was obtained from dry and atomization temperatures curves of 1.0 mg ly1 Hg2q in 0.1% (vyv) HNO3 pre-concentrated onto the graphite furnace modified with palladium. The operating conditions and the heating program employed for the atomization of the mercury are shown in Table 1. The rain and potable water samples were directly analyzed and the non-potable water sample was diluted five times to 0.1% (vyv) HNO3. Volumes of 1.0 ml of these solution samples were taken for analysis. The sediment river samples were pre-treated in the Questrom-3000 microwave oven. The microwave vessels were decontaminated through a previous clean-up program. Sediment samples were weighed (0.5 g) transferred to the cleaned PTFE vessel, 10.0 ml of sub-boiling nitric acid was added, and the extraction program EPA 3051 was executed. Afterwards, the solution was transferred to a 25.0-ml volumetric flask and the volume was completed with Milli-Q䉸 water. Before the analysis, the solutions were diluted 1:20 with deionized water. Recovery tests were performed by adding 1.0 mg ly1 Hg2q to the rain, potable, and non-potable waters and to the sediment samples before the digestion procedure.
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3. Results and discussion In order to ensure hydrodynamically favorable characteristics for the tubular electrochemical flow-cell, the reference and auxiliary electrodes were positioned downstream with respect to the working electrode. This configuration avoids contamination of the palladium solution flowing through the tubular flow-cell to the reference electrolyte and the auxiliary electrode reaction products. In comparison with the usual thermal procedure, where only a small area of the graphite tube is modified, the electrochemical modification process obtained with the flow-cell described above has the advantages of assuring a uniform metal film over a large internal surface of the graphite tube. This is attractive for pre-concentration of the gas samples, e.g. mercury vapor or hydrides, due to the higher available area for the gas trapping. This fact was confirmed by using a thermally modified graphite tube with 5, 10, 20, 50 and 100 ml of 1000-mg ly1 palladium. It was observed that the modified area was proportional to the volumes of palladium solution injected over the graphite tube wall. The integrated absorbance obtained for 1.0 ml of 1.0-mg ly1 Hg2q in 0.1% (vyv) HNO3, preconcentrated onto these surfaces, increased proportionally with the increase in modified area. The highest modified area and integrated absorbance signal were obtained with the proposed electrochemical modification procedure. The palladium mass electrodeposited onto the graphite tube was approximately 1000 mg, which is twice the amount found by Bulska et al. w20x. Another advantage of all internal surface modification is related to the reduction of the contact area between graphite and oxygen formed during drying and pyrolysis steps, thus improving the graphite tube lifetime. In this work, the potentiostatic palladium electrodeposition for permanent modification of the graphite tube surface was preferred due to the selectivity obtained with an appropriate choice of applied potential to the electrochemical cell. Nevertheless, galvanostatic electrodeposition could be performed with this flow-cell without the reference electrode and using a constant current source. The
electrolytic solution (100 mmol ly1 of acetate buffer, pHs4.8) was selected, considering the best hydrogenionic concentration to avoid hydrolysis of PdII ion and the hydrogen evolution in acid media. The potential value of y0.600 V vs. Agy AgCl(Sat) was selected to ensure the reduction of PdII to Pd0, avoiding the hydrogen evolution which could damage the efficiency of the electrodeposition and the metallic film homogeneity. The electrical current generated during the electrodeposition was approximately 550 mA. Palladium film images, before and after the thermal conditioning of the electrochemical modified pyrolytic graphite tube surface with palladium were made by scanning electron microcopy (SEM), Figs. 3 and 4, respectively. In these figures, white areas represent palladium, and black the pyrolytic graphite surface. Images were taken at different regions of the graphite surface in order to evaluate the palladium distribution. The SEM micrograph of the graphite tube just after electrodeposition showed a homogeneous palladium distribution over the graphite surface (Fig. 3). However, as observed by Bulska et al., palladium does not form a metal compact layer, it is deposited by island formation w22x. Otherwise, when the modified tube was submitted to the thermal conditioning program and heated to 2000 8C for 5 s, a more homogeneous palladium distribution over the pyrolytic surface was observed (Fig. 4). The maximum temperature used in the conditioning program (2000 8C) was between the palladium melting (1552 8C) and boiling points (2927 8C). After this thermal conditioning, palladium distributes over the graphite surface like many small islands in formation, enlarging the contact area and improving the mercury trapping efficiency. It was observed that the palladium film lifetime and the sensitivity were negatively affected without thermal conditioning, and high background signals were observed in the first heating cycles. The efficiency of the film for mercury trapping was reduced for lower than 50 heating cycles. The improvement of the permanent chemical modifier lifetime after the thermal conditioning could be related to the formation of intercalation compounds between palladium and graphite surface w22,25x.
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Fig. 3. Scanning electron micrograph of a graphite tube inner wall modified with palladium by electrodeposition (magnification 1000=).
Fig. 4. Scanning electron micrograph of a graphite tube inner wall modified with palladium by electrodeposition after thermal conditioning (magnification 1000=).
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Considering the clean-out temperature of 1400 8C, below the melting point of the palladium (1552 8C), the lifetime of the modified graphite tube was higher than 500 heating cycles. The combination of a cold vapor generation system with in situ trapping in a whole inner modified graphite tube with palladium and subsequent determination by ETAAS provides a powerful means of separation and pre-concentration of low levels of mercury. For the mercury separation and pre-concentration, the best conditions for cold vapor generation were: 2.0% wyv NaBH4, 120 s of trapping time and 212 ml miny1 of argon gas flow used to carry out the generated mercury vapor from the reaction cell to graphite modified surface. The distance between the tip of PTFE tube and the graphite surface for mercury trapping was 2 mm. It could be supposed that gaseous reactions products formed could pull out the mercury trapped on the palladium film. It was observed that mercury collection efficiency may be impaired by moisture or other gaseous reaction products, which poison the palladium film surface w13,26x. The formation of intermetallic compounds or alloys between mercury and palladium could be the main process of mercury vapor trapping onto the modified graphite surface w13x. Mercury was volatilized at drying temperatures higher than 110 8C. This step was critical to eliminate some water that was carried out with mercury vapor. The temperature of 110 8C (holding time of 10 s) was adopted to evaporate the water accumulated over the palladium surface. The atomization and the clean-up temperatures were 1100 and 1400 8C, respectively. Progressive damage in the sensitivity and repeatability for higher cleaning temperatures ()1500 8C) was observed. These results could be related to the inner film deformation on the graphite furnace when temperatures analogous to and above the melting point of palladium were used (1552 8C). A mini-column of anhydrous calcium carbonate (25 mm) was efficient to adsorb the water vapor. No negative effect in the mercury vapor transport to the modified graphite surface was observed until 30 pre-concentration cycles. After that, the mini-column became moistened and the efficiency
of transport of mercury to the graphite surface was affected. After 30 pre-concentration cycles, a new column was inserted into the flow stream without damaging the results. Without the mini-column of anhydrous calcium carbonate, the water vapor would negatively affect the efficiency of intermetallic formation between mercury and palladium. The performance of the palladium film was satisfactory up to 500 heating cycles (cleaning temperature of 1400 8C). After reaching this number of cycles, other new palladium films were electrodeposited onto the same graphite tube surface and the mercury trapping was carried out. Again, the graphite tube was modified three times (lifetime of almost 1400 heating cycles). After 1400 heating cycles, with the same graphite tube, the random mercury absorbance signal was observed (R.S.D.)20%). At this time, the outside pyrolytic graphite tube surface was very damaged, which could interfere in the heating rate process. It is important to emphasize that palladium electrodeposition can be carried out in a previously used graphite tube. The characteristic mass (mo) and the detection limit based on integrated absorbances were 26 pg (R.S.D.s0.12%, ns5) and 93 pg (ns20, 3d), respectively. In both cases, the volume of the mercury reference solution used was 1.0 ml. The limit of detection was calculated based on the variability of the 20 measurements of the blank solution 0.1% (vyv) HNO3 according to 3dblk ym, where d is the standard deviation (S.D.) of the blank measurements and m is the calibration curve slope. Regarding the pre-concentration step and the heating cycle time, it was possible to analyze 20 samples per hour. The determination of mercury in rain, potable and non-potable waters, and sediment lake samples were performed in order to evaluate the analytical applicability of the electrochemically modified graphite tube. The results are shown in Table 2. Each water sample was spiked with 1.0 mg ly1 of mercury to ensure the quality of the results. The recoveries obtained with the proposed method were approximately 100% for water and between 95 and 103% for sediment. The sediment samples were analyzed by cold vapor atomic absorption spectrometry (CVAAS) and the results were
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Table 2 Mercury contents in rain, potable, non-potable water and sediment lake and recovery values Samples
In this worka
CVAASa
Recovery (%)d
Rain water Potable water Non-potable water Sediment 1c Sediment 2c
Not detected 2.0"0.04b 11.25"0.02b 1380"0.01 1462"0.02
– – – 1387"0.08 1232"0.01
103 103 101 95 103
a
Mean"S.D. (ns5). Concentration unit: mg ly1. c Concentration unit: ng gy1. d Spike of 1.0 mg ly1 of Hg2q. b
according to those obtained by the proposed method (Table 2).
Moreno Moreno thanks Conselho Nacional de ´ ´ Desenvolvimento Cientıfico e Tecnologico (CNPq — Processo 136583y99-7) for fellowship.
4. Conclusions References The tubular electrochemical flow-cell proposed for permanent modification of graphite tube is simple, robust and easy to handle. It presents some attractive features, such as hydrodynamic characteristics to produce a uniform modification of the whole inner surface of the graphite tube, versatility to operate in continuous or stopped flow solution under galvanostatic or potentiostatic conditions, and could be used for the modification of the internal surface of transversal or longitudinal graphite tubes. After palladium electrodeposition, a heating program is recommended to perform the conditioning of the permanent chemical modifier. Considering a clean-up temperature of 1400 8C, the same palladium permanent chemical modifier supported more than 500 heating cycles. The presented cold vapor generation system is simple and cheap. The gas–liquid separator was discarded to avoid inter-contamination between the samples, thus excluding laborious decontamination procedures. The restricted volumes for the reaction allowed us to save the reagent (1.0 ml) and sample (1.0 ml) solutions and reduced the quantity of residues produced during the analysis. Acknowledgments ¸ ˜ de Amparo a` Pesquisa do We thank Fundacao ˜ Paulo (FAPESP-Processo 2000y Estado de Sao ´ Gregorio 06796-1) for financial support. Ruben
w1x B. Welz, M. Sperling, Atomic Absorption Spectrometry, Wiley-VCH, Verlag GmbH, Weinheim, 1999. w2x W.L. Clevenger, B.W. Smith, J.D. Winefordner, Trace determination of mercury: a review, Crit. Rev. Anal. Chem. 27 (1) (1997) 1–26. w3x J.M. Ombaba, Total mercury determination in biological and environmental standard samples by gold amalgamation followed by cold vapor atomic absorption spectrometry, Microchem. J. 53 (2) (1996) 195–200. w4x C.E.C. Magalhaes, ˜ F.J. Krug, A.H. Fostier, H. Berndt, Direct determination of mercury in sediments by atomic absorption spectrometry, J. Anal. At. Spectrom. 12 (1997) 1231–1234. w5x W. Frech, D.C. Baxter, G. Dyvik, B. Dybdahl, On the determination of total mercury in natural gases using the amalgamation technique and cold vapour atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 769–775. w6x P.C. Rudner, A.G. Torres, J.M.C. Pavon, ´ F.S. Rojas, Online preconcentration of mercury by sorption on an anion-exchange resin loaded with 1,5-bisw(2-pyridyl)-3sulphophenyl methylenex thiocarbonohydrazide and determination by cold-vapour inductively coupled plasma atomic emission, Talanta 46 (5) (1998) 1095–1105. w7x J.C. Rocha, J.J. Sene, P. Burba, D.J. Klockow, Interaction of mercury (II) with humic substances from the Rio Negro (Amazonas state, Brazil) by means of ion exchange procedure, J. Braz. Chem. Soc. 9 (1998) 79–84. w8x J.A.G. Neto, L.F. Zara, J.C. Rocha, A. Santos, C.S. ´ Dakuzaku, J.A. Nobrega, Determination of mercury in agroindustrial samples by flow-injection cold vapor atomic absorption spectrometry using ion exchange and reductive elution, Talanta 51 (3) (2000) 587–594.
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w9x A.L. Bihan, J.Y. Cabon, Determination of 1 ngyl levels of mercury in water by electrothermal atomization atomic-absorption spectrometry after solvent extraction, Talanta 37 (12) (1990) 1119–1122. w10x E. Bulska, W. Kandler, A. Hulanicki, Noble metals as permanent modifiers for the determination of mercury by electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 51 (1996) 1263–1270. w11x B. Welz, G. Schlemmer, J.R. Mudakavi, Palladium nitrate–magnesium nitrate modifier for electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 7 (1992) 499–503. w12x X.-P. Yan, Z.-M. Ni, Q.-L. Guo, In situ concentration of mercury vapour in a palladium-coated graphite tube: determination of mercury by atomic absorption spectrometry, Anal. Chim. Acta 272 (1993) 105–114. w13x P.B. Barrera, J.M. Pineiro, ˜ ˜ A.M. Pineiro, A.B. Barrera, Iridium-coated graphite tubes for the direct determination of As, Cd, Hg and Pb in seawater by vapor generation ETAAS, At. Spectrosc. 19 (3) (1998) 100–106. w14x P.B. Barrera, J.M. Pineiro, ˜ ˜ A.M. Pineiro, A.B. Barrera, Use of flow injection cold vapour generation and preconcentration on coated graphite tubes for the determination of mercury in polluted seawaters by electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 12 (1997) 317–321. w15x S.H. Lee, K.H. Jung, D.S. Lee, Determination of mercury in environmental samples by cold vapour generation and atomic-absorption spectrometry with a gold-coated graphite furnace, Talanta 36 (10) (1989) 999–1003. w16x Z. Grobenski, W. Erler, U. Voellkopt, Determination of mercury with Zeeman graphite furnace atomic absorption spectrometry, At. Spectroc. 6 (4) (1985) 91–93. w17x Z. Hladky, J. Risova, ˇ ´ M. Fisera, ˇ Determination of mercury in concentrated mineral acids by electrothermal
w18x
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w20x
w21x
w22x
w23x w24x w25x w26x
atomic absorption spectrometry using gold amalgamation, J. Anal. At. Spectrom. 5 (1990) 691–692. D.L. Tsalev, A. D’Ulivo, L. Lampugnani, M. Di Marco, R. Zamboni, Thermally stabilized iridium on an integrated, carbide-coated platform as a permanent modifier for hydride-forming elements in electrothermal atomic absorption spectrometry, part 1. Optimization studies, J. Anal. At. Spectrom. 10 (1995) 1003–1009. E. Bulska, K.L. Ilkowska, A. Hulanicki, Optimization of electrochemical deposition of noble metals for permanent modification in graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 53 (1998) 1057–1062. H.W. Sinemus, H.H. Standel, B. Radziuk, J. Kleiner, Determination of mercury following sequestration on an iridium coated graphite surface, Spectrochim. Acta Part B 48 (1993) 1719–1722. E. Bulka, W. Jedral, Application of palladium- and rhodium-plating of the graphite furnace in electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 49–53. E. Bulska, B. Thybusch, H.M. Ortner, Surface and subsurface examination of graphite tubes after electrodeposition of noble metals for electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 56 (2001) 363–373. J.J. Pedrotti, L. Agnes, I.G.R. Gutz, Miniaturized reference electrodes with microporous polymer junctions, Electroanalysis 8 (7) (1996) 673–675. F.J. Krug, H. Bergamin Filho, E.A.G. Zagatto, Commutation in flow injection analysis, Anal. Chim. Acta 179 (1986) 103–118. A.B. Volynsky, Mecanism of action of platinum group modifiers in electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 55 (2000) 103–150. D.-Q. Zhang, L.-L. Yang, H.-W. Sun, Determination of mercury by cold vapour atomic absorption spectrometry with derivate signal processing, Anal. Chim. Acta (1999) 173–178.
An electrochemical flow-cell for permanent modification of graphite tube with palladium for mercury determination by electrothermal atomic absorption spectrometry Ruben G.M. Morenoa, Elisabeth de Oliveiraa, Jairo J. Pedrottib, Pedro V. Oliveiraa,* b
a ´ ˜ Paulo, CEP 05508-900, C.P. 26077, Sao ˜ Paulo, SP, Brazil Instituto de Quımica, Universidade de Sao ´ ˆ ´ Departamento de Quımica, Faculdade de Ciencias Biologicas, Exatas e Experimentais, Universidade Presbiteriana Mackenzie, ˜ Paulo, SP, Brazil CEP 01239-900, Sao
Received 2 August 2001; accepted 31 December 2001
Abstract An electrochemical procedure for palladium deposition on the inner surface of pyrolytic graphite-coated tubes for permanent chemical modification and a cold vapor generation system for the pre-concentration and determination of mercury trace levels in rain, potable, and non-potable water and lake sediment by electrothermal atomic absorption spectrometry is proposed. A tubular electrochemical flow-cell was assembled on the original geometry of the graphite tube, which operated as the working electrode. A stainless steel tube, positioned downstream from the working electrode, was used as the auxiliary electrode. The applied potential was measured against a micro AgyAgCl(sat) reference electrode inserted in the auxiliary electrode. Palladium solution in acetate buffer (100 mmol ly1, pHs4.8), flowing at 0.5 ml miny1 for 60 min was used to perform the electrodeposition. A homemade cold vapor generation system composed of a peristaltic pump, an injector–commutator, a flow meter and a disposable polyethylene gas– liquid separator flask (approx. 4.0 ml volume) were used. Volumes of 1.0 ml of reagent (2.0% wyv NaBH4 in 0.10 mol ly1 of NaOH) and 1.0 ml of reference or sample solution in 0.25 mol ly1 of HNO3 were carried to the gas– liquid separator using the peristaltic pump. The mercury vapor was carried out to the modified graphite tube by argon flow (200 ml miny1), and pre-concentrated for 120 s. The characteristic mass for 1.0 ml of reference solution was 26 pg (R.S.D.s0.12%, ns5). The detection limit obtained was 93 pg (ns20, 3d). The reliability of the entire procedure was confirmed by addition and recovery tests and cold vapor atomic absorption spectrometry. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Graphite furnace; Permanent chemical modifier; Electrodeposition; Mercury determination
*Corresponding author. Tel.: q55-11-38183837; fax: q55-11-38155579. E-mail address: [email protected] (P.V. Oliveira).
0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 2 . 0 0 0 0 9 - 5
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1. Introduction Cold vapor atomic absorption spectrometry (CVAAS) is one of the most widely used techniques for the selective determination of mercury in environmental, biological, clinical and industrial samples w1,2x. When high sensitivity is required, mercury pre-concentration prior to CVAAS can be achieved by amalgamation onto a gold w3,4x or goldyplatinum trap w5x, followed by thermal desorption, getting absolute masses of less than 0.26 ng mercury w4x. Compatible detection limits can also be obtained with the solid phase using a flow-injection system w6–8x, and also by solvent extraction techniques w9x for mercury separation and pre-concentration. On the other hand, electrothermal atomic absorption spectrometry (ETAAS) has rarely been used for mercury determination. The volatility of mercury and its compounds, which cause difficulty in the stabilization during the solvent vaporization and consequent damage to the sensitivity, is the main hindrance. Nevertheless, several attempts have been made to overcome this obstacle. Due to the more pronounced volatility of mercury metal in relation to inorganic mercury compounds, several oxidizing agents (hydrogen peroxide, mixture of hydrogen peroxide and hydrochloric acid, potassium permanganate, potassium dichromate, ammonium sulfide and tellurium), complexes or precipitants reagents were used to prevent the reduction of mercury w10,11x. One of the problems encountered with some of these reagents was that they could not be applied to solutions containing higher concentrations of organic compounds because the reagent was consumed, at least partially, by the organic species w11x. Gold w10,16,17x, iridium w10,13,14,19,20x, palladium w10–12,15,18,19x, rhodium w10,18x, tungsten w14x and zirconium w14x have been investigated as chemical modifiers for mercury determination by ETAAS. These metals were thermally reduced on the inner of graphite surface prior to every heating cycle w12x or as permanent modifier obtained by thermal w10–18x or electrochemical w10,19x processes. In the last case, the graphite tube was wrapped with a Teflon䉸 band to
protect the outside surface from metal deposition w21,22x. Palladium electrochemically deposited onto the graphite surface tube showed better long-term stability (up to 450 firings) than thermally deposited (up to 50 firings) for mercury determination w10x. Characteristic masses of 114 pg (50 ml of solution) w12x and 300 pg (1.5 ml of solution) w13,14x by combining cold vapor generation and trapping in graphite furnace modified with palladium and iridium, respectively, were found. Recent contributions have investigated the distribution and in-depth of palladium, iridium and rhodium electrodeposited onto the inner surface of pyrolytic graphite-coated tube at different stages in the tube history w22x. Noble metals used for permanent modification do not form a compact layer on the surface, but penetrate into the pyrolytic graphite structure during the deposition step. Some advantages related to the electrochemical permanent modification are: noble metals are deposited in their elemental form, reducing the amount of oxygen which is present as metal oxide, and minimizing the corrosion of the graphite walls w22x; low consumption of noble metals reagents; low blank signals due to the contaminants volatilization during the pre-treatment steps; and shortening the time instrument run, and enlarging the graphite tube lifetime. In this work, a new electrochemical flow-cell design for permanent modification of the pyrolytic graphite-coated tube inner wall with palladium is presented. The mercury trace levels pre-concentration and determination in rain, potable, and nonpotable water and lake sediment by using a homemade cold vapor generation and separation system coupled to the ETAAS is proposed to demonstrate the permanent modification procedure efficiency. 2. Experimental 2.1. Instrumentation The spectroscopic measurements were made with an Analytik Jena AAS 6 Vario atomic absorption spectrometer (Analytik Jena, Jena, Germany) equipped with transversally heated graphite tubes
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Fig. 1. Schematic diagram of the tubular electrochemical flowcell: (A) solution inlet; (B) wall silicone rubber; (C) working electrode (graphite tube); (D) reference electrode; (E) auxiliary electrode; (F) outlet solution; and (G) silicone tube.
(Part No 07-8101225), deuterium hollow cathode lamp for background correction and MPE 50 autosampler. Mercury and deuterium hollow cathode lamps (Analytik Jena) used were electronically modulated with a frequency of 150 Hz. A slitwidth of 0.8 nm was selected to isolate the 253.7nm resonance line. All measurements were based on integrated absorbance values computed by OSy 2 WARP 4 Operating System, with at least three replicates. Argon 99.999% (vyv) (Air Liquid Brasil SyA, ˜ Paulo, Brazil) was used as a purge gas and Sao carrier gas of the mercury vapor. A scanning electron microscope Stereoscan 440 model (LEO, England) was used for the topochemical inspection of the modified graphite tube inner surface. A Qwave-3000 microwave digestion system (Questron Corporation, Merceville, NJ, USA) equipped with 100 ml PTFE vessels, a fluoropolymer coated oven cavity, exhaust fan, removable 10-position sample carrousel, a hose to permit venting of fumes into a fume hood and a pressure and temperature sensor were used. To ensure similar amounts of microwave energy for all sediments samples, the carrousel was rotated 1808 forwards and backwards by an internal motor. 2.1.1. Electrochemical flow-cell The tubular electrochemical flow-cell for palladium deposition in the inner of the graphite tube is shown in Fig. 1. It was assembled on the original
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cylindrical geometry of a transversal heated graphite tube (20 mm long and 5.5 mm i.d.) that operates as the working electrode (c). The palladium solution is carried into the flow-cell (a) by using a 0.8-mm PTFE tube connected to a short length (;5 mm) of thick walled silicone tube (b) fitted at inlet flow-cell. A stainless steel tube (10 mm long and 2.5 mm i.d.) fixed at the opposite extremity of the graphite tube with a help of a silicone tube (g) serves as the auxiliary electrode (e) and outlet solution (f). The micro AgyAgCl (saturated with NaCl) reference electrode (d) w23x manufactured on a 10ml polypropylene disposable pipette tip (Cole Parmer catalog 噛 25710-6) is positioned downstream from the working electrode. It is vertically inserted into the flow channel by means of an orifice (diameter, ds1.8 mm) made on the auxiliary electrode surface. To avoid leakage of the flowing solution through the graphite tube sampling hole, it was closed with a PTFE conical pin fixed under pressure with the help of a thin silicone tube. The geometric volume of the cell, defined as the solution volume that completely embraces the three-electrodes, is approximately 340 ml. 2.1.2. Continuous flow system for electrodeposition The palladium electrodeposition in the graphite tube surface was done using an Ismatec Model IPC-8 peristaltic pump (Ismatec, Switzerland) furnished with Tygon䉸 pumping tubes and PTFE tubes (0.8 mm i.d.). The electrochemical flow-cell previously described and the potentiostat were developed in the author’s lab. 2.1.3. Gas–liquid separator The homemade cold vapor generation system (Fig. 2) consisted of an Ismatec Model IPC-8 MSRe peristaltic pump furnished with Tygon䉸 pumping tubes, a injector–commutator w24x made of Perspex䉸, PTFE tubes (0.8 mm i.d.), a Dwyer series RM Rate Master flow meter and a disposable micro polyethylene gas–liquid separator flask (approx. 4.0 ml volume). A volume of 1.0 ml of the reference or sample solutions and 1.0 ml of the reagent solution are selected by the loops LS and LR, respectively, mixed in the confluence T and carried out to the
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Fig. 2. Schematic diagram of cold vapor generation system for pre-concentration of mercury on THGA tube: GT (graphite tube); GLS (gas–liquid separator); MC (anhydrous calcium carbonate mini column); Ar (argon flow rates200 ml miny1 ); T (confluence); C (carrier); S (sample); R (reagent); LS (sample loop); and LR (reagent loop).
gas–liquid separator (GLS). The carrier C was pure air, flowing at 3.0 ml miny1. The mercury vapor was blown on over the modified graphite tube surface with palladium through PTFE tube (20 cm long and 0.8 mm i.d.) by argon carrier stream (200 ml miny1). A mini column (MC), 25 mm long, filled with anhydrous calcium carbonate was placed intercepting the GLS and the graphite tube to absorb water vapor. The gas–liquid separator was fixed in the front of the AA spectrometer, as shown in Fig. 2.
Analytical reference solutions containing 0.5– 5.0 mg ly1 Hg2q in 0.1% (vyv) HNO3 were daily prepared by successive dilution of the 1000-mg ly1 Hg2q Tritisol standard solution (Merck, Darmstadt, Germany). Sodium borohydride and NaOH was used for the reductor preparation. Rain, potable, and non-potable waters and sed˜ lake (Vitoria ´ iments from Gaviao da Conquista, BA, Brazil) were used for mercury determination. 2.3. Procedure
2.2. Reagents, reference solutions and samples All solutions were prepared using high purity deionized water (resistivity 18.2 MV cm) obtained from a Milli-Q䉸 water purification system (Millipore, Bedford, USA). High purity reagents or analytical reagent-grade (Merck, Rio de Janeiro, Brazil) were used in all experiments. Nitric and hydrochloric acids were purified by distillation in quartz sub-boiling still ´ (Marconi Equipamentos de Laboratorio, Piracicaba, Brazil). A solution of 1000 mg ly1 of palladium was prepared by dissolving Pd(NO3)2 in a solution of 100 mmol ly1 of acetate buffer, pH 4.8.
All glassware and high-density polypropylene bottles were cleaned with detergent solution, soaking in 10% (vyv) HNO3 for 24 h, rinsed with Milli-Q䉸 water, filled with clean solution of 10% (vyv) HNO3 and stored until the usage. Reference and sample solutions were prepared in a Class-100 laminar flow hood. The solution of 1000 mg ly1 of palladium in 100 mmol ly1 of acetate buffer flowed through the electrochemical flow-cell at 0.5 ml miny1 for 60 min. All electrochemical modifications in the graphite tube were done without any previous pretreatment or thermal conditioning and were performed at ambient temperature (25"2 8C).
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Table 1 Instrumental operating conditions and THGA heating program for mercury determination by using the cold vapor proposed system Spectrometer Wavelength (nm) Bandpass (nm) HCL current (mA) Read delay (s) Read time (s) Measurements
253.7 0.7 15 1 7 Peak area
Heating programa Step
Temperature (8C)
Ramp (8Cys)
Hold (s)
Ar (ml miny1)
Drying Atomization Cleaning
110 1100 1400
18 FPb 300
10 7 7
300 0 300
a b
Program time: 31 s; Hg trapping temperature: 20 8C. Full power.
The selective palladium deposition on the inner wall of graphite tube was performed at a controlled potential (y0.600 V) measured against the reference electrode. This potential was previously selected through voltammetric experiments. After electrodeposition, the graphite tube was inserted into the graphite furnace and thermal surface conditioning was performed (ramp, hold): 90 8C (30 8Cys, 15 s); 110 8C (30 8Cys, 15 s); 800 8C (4 8Cys, 10 s) and 2000 8C (50 8Cys, 5 s). For optimization of the mercury vapor generation system, the reagent concentration (0.2–6.0% wyv NaBH4 in 0.1 mol ly1 of NaOH), the volume (0.2–1.0 ml) of sample (LS) and reagent (LR) (Fig. 2), the mercury carrier stream (0–700 ml miny1 Ar) and the trapping time (30–240 s) were investigated. The polyethylene gas–liquid separator (GLS) flask was discarded after each sampling to avoid sample inter-contamination. The mercury vapor was conducted to the graphite furnace by using the MPE 5 autosampler. Previously, the pipettor arm was fixed by following the menu serves of MPE 5 in the depth tube position and fixing the best distance between the tip of PTFE tube and the graphite surface for mercury trapping. Before each sampling in the gas–liquid separator, the pipettor arm was positioned in the depth tube position and kept during the pre-concentration time. After that time, the autosampler arm was returned to the wash position
and the heating program was carried out. The optimization of heating program was obtained from dry and atomization temperatures curves of 1.0 mg ly1 Hg2q in 0.1% (vyv) HNO3 pre-concentrated onto the graphite furnace modified with palladium. The operating conditions and the heating program employed for the atomization of the mercury are shown in Table 1. The rain and potable water samples were directly analyzed and the non-potable water sample was diluted five times to 0.1% (vyv) HNO3. Volumes of 1.0 ml of these solution samples were taken for analysis. The sediment river samples were pre-treated in the Questrom-3000 microwave oven. The microwave vessels were decontaminated through a previous clean-up program. Sediment samples were weighed (0.5 g) transferred to the cleaned PTFE vessel, 10.0 ml of sub-boiling nitric acid was added, and the extraction program EPA 3051 was executed. Afterwards, the solution was transferred to a 25.0-ml volumetric flask and the volume was completed with Milli-Q䉸 water. Before the analysis, the solutions were diluted 1:20 with deionized water. Recovery tests were performed by adding 1.0 mg ly1 Hg2q to the rain, potable, and non-potable waters and to the sediment samples before the digestion procedure.
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3. Results and discussion In order to ensure hydrodynamically favorable characteristics for the tubular electrochemical flow-cell, the reference and auxiliary electrodes were positioned downstream with respect to the working electrode. This configuration avoids contamination of the palladium solution flowing through the tubular flow-cell to the reference electrolyte and the auxiliary electrode reaction products. In comparison with the usual thermal procedure, where only a small area of the graphite tube is modified, the electrochemical modification process obtained with the flow-cell described above has the advantages of assuring a uniform metal film over a large internal surface of the graphite tube. This is attractive for pre-concentration of the gas samples, e.g. mercury vapor or hydrides, due to the higher available area for the gas trapping. This fact was confirmed by using a thermally modified graphite tube with 5, 10, 20, 50 and 100 ml of 1000-mg ly1 palladium. It was observed that the modified area was proportional to the volumes of palladium solution injected over the graphite tube wall. The integrated absorbance obtained for 1.0 ml of 1.0-mg ly1 Hg2q in 0.1% (vyv) HNO3, preconcentrated onto these surfaces, increased proportionally with the increase in modified area. The highest modified area and integrated absorbance signal were obtained with the proposed electrochemical modification procedure. The palladium mass electrodeposited onto the graphite tube was approximately 1000 mg, which is twice the amount found by Bulska et al. w20x. Another advantage of all internal surface modification is related to the reduction of the contact area between graphite and oxygen formed during drying and pyrolysis steps, thus improving the graphite tube lifetime. In this work, the potentiostatic palladium electrodeposition for permanent modification of the graphite tube surface was preferred due to the selectivity obtained with an appropriate choice of applied potential to the electrochemical cell. Nevertheless, galvanostatic electrodeposition could be performed with this flow-cell without the reference electrode and using a constant current source. The
electrolytic solution (100 mmol ly1 of acetate buffer, pHs4.8) was selected, considering the best hydrogenionic concentration to avoid hydrolysis of PdII ion and the hydrogen evolution in acid media. The potential value of y0.600 V vs. Agy AgCl(Sat) was selected to ensure the reduction of PdII to Pd0, avoiding the hydrogen evolution which could damage the efficiency of the electrodeposition and the metallic film homogeneity. The electrical current generated during the electrodeposition was approximately 550 mA. Palladium film images, before and after the thermal conditioning of the electrochemical modified pyrolytic graphite tube surface with palladium were made by scanning electron microcopy (SEM), Figs. 3 and 4, respectively. In these figures, white areas represent palladium, and black the pyrolytic graphite surface. Images were taken at different regions of the graphite surface in order to evaluate the palladium distribution. The SEM micrograph of the graphite tube just after electrodeposition showed a homogeneous palladium distribution over the graphite surface (Fig. 3). However, as observed by Bulska et al., palladium does not form a metal compact layer, it is deposited by island formation w22x. Otherwise, when the modified tube was submitted to the thermal conditioning program and heated to 2000 8C for 5 s, a more homogeneous palladium distribution over the pyrolytic surface was observed (Fig. 4). The maximum temperature used in the conditioning program (2000 8C) was between the palladium melting (1552 8C) and boiling points (2927 8C). After this thermal conditioning, palladium distributes over the graphite surface like many small islands in formation, enlarging the contact area and improving the mercury trapping efficiency. It was observed that the palladium film lifetime and the sensitivity were negatively affected without thermal conditioning, and high background signals were observed in the first heating cycles. The efficiency of the film for mercury trapping was reduced for lower than 50 heating cycles. The improvement of the permanent chemical modifier lifetime after the thermal conditioning could be related to the formation of intercalation compounds between palladium and graphite surface w22,25x.
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Fig. 3. Scanning electron micrograph of a graphite tube inner wall modified with palladium by electrodeposition (magnification 1000=).
Fig. 4. Scanning electron micrograph of a graphite tube inner wall modified with palladium by electrodeposition after thermal conditioning (magnification 1000=).
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Considering the clean-out temperature of 1400 8C, below the melting point of the palladium (1552 8C), the lifetime of the modified graphite tube was higher than 500 heating cycles. The combination of a cold vapor generation system with in situ trapping in a whole inner modified graphite tube with palladium and subsequent determination by ETAAS provides a powerful means of separation and pre-concentration of low levels of mercury. For the mercury separation and pre-concentration, the best conditions for cold vapor generation were: 2.0% wyv NaBH4, 120 s of trapping time and 212 ml miny1 of argon gas flow used to carry out the generated mercury vapor from the reaction cell to graphite modified surface. The distance between the tip of PTFE tube and the graphite surface for mercury trapping was 2 mm. It could be supposed that gaseous reactions products formed could pull out the mercury trapped on the palladium film. It was observed that mercury collection efficiency may be impaired by moisture or other gaseous reaction products, which poison the palladium film surface w13,26x. The formation of intermetallic compounds or alloys between mercury and palladium could be the main process of mercury vapor trapping onto the modified graphite surface w13x. Mercury was volatilized at drying temperatures higher than 110 8C. This step was critical to eliminate some water that was carried out with mercury vapor. The temperature of 110 8C (holding time of 10 s) was adopted to evaporate the water accumulated over the palladium surface. The atomization and the clean-up temperatures were 1100 and 1400 8C, respectively. Progressive damage in the sensitivity and repeatability for higher cleaning temperatures ()1500 8C) was observed. These results could be related to the inner film deformation on the graphite furnace when temperatures analogous to and above the melting point of palladium were used (1552 8C). A mini-column of anhydrous calcium carbonate (25 mm) was efficient to adsorb the water vapor. No negative effect in the mercury vapor transport to the modified graphite surface was observed until 30 pre-concentration cycles. After that, the mini-column became moistened and the efficiency
of transport of mercury to the graphite surface was affected. After 30 pre-concentration cycles, a new column was inserted into the flow stream without damaging the results. Without the mini-column of anhydrous calcium carbonate, the water vapor would negatively affect the efficiency of intermetallic formation between mercury and palladium. The performance of the palladium film was satisfactory up to 500 heating cycles (cleaning temperature of 1400 8C). After reaching this number of cycles, other new palladium films were electrodeposited onto the same graphite tube surface and the mercury trapping was carried out. Again, the graphite tube was modified three times (lifetime of almost 1400 heating cycles). After 1400 heating cycles, with the same graphite tube, the random mercury absorbance signal was observed (R.S.D.)20%). At this time, the outside pyrolytic graphite tube surface was very damaged, which could interfere in the heating rate process. It is important to emphasize that palladium electrodeposition can be carried out in a previously used graphite tube. The characteristic mass (mo) and the detection limit based on integrated absorbances were 26 pg (R.S.D.s0.12%, ns5) and 93 pg (ns20, 3d), respectively. In both cases, the volume of the mercury reference solution used was 1.0 ml. The limit of detection was calculated based on the variability of the 20 measurements of the blank solution 0.1% (vyv) HNO3 according to 3dblk ym, where d is the standard deviation (S.D.) of the blank measurements and m is the calibration curve slope. Regarding the pre-concentration step and the heating cycle time, it was possible to analyze 20 samples per hour. The determination of mercury in rain, potable and non-potable waters, and sediment lake samples were performed in order to evaluate the analytical applicability of the electrochemically modified graphite tube. The results are shown in Table 2. Each water sample was spiked with 1.0 mg ly1 of mercury to ensure the quality of the results. The recoveries obtained with the proposed method were approximately 100% for water and between 95 and 103% for sediment. The sediment samples were analyzed by cold vapor atomic absorption spectrometry (CVAAS) and the results were
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Table 2 Mercury contents in rain, potable, non-potable water and sediment lake and recovery values Samples
In this worka
CVAASa
Recovery (%)d
Rain water Potable water Non-potable water Sediment 1c Sediment 2c
Not detected 2.0"0.04b 11.25"0.02b 1380"0.01 1462"0.02
– – – 1387"0.08 1232"0.01
103 103 101 95 103
a
Mean"S.D. (ns5). Concentration unit: mg ly1. c Concentration unit: ng gy1. d Spike of 1.0 mg ly1 of Hg2q. b
according to those obtained by the proposed method (Table 2).
Moreno Moreno thanks Conselho Nacional de ´ ´ Desenvolvimento Cientıfico e Tecnologico (CNPq — Processo 136583y99-7) for fellowship.
4. Conclusions References The tubular electrochemical flow-cell proposed for permanent modification of graphite tube is simple, robust and easy to handle. It presents some attractive features, such as hydrodynamic characteristics to produce a uniform modification of the whole inner surface of the graphite tube, versatility to operate in continuous or stopped flow solution under galvanostatic or potentiostatic conditions, and could be used for the modification of the internal surface of transversal or longitudinal graphite tubes. After palladium electrodeposition, a heating program is recommended to perform the conditioning of the permanent chemical modifier. Considering a clean-up temperature of 1400 8C, the same palladium permanent chemical modifier supported more than 500 heating cycles. The presented cold vapor generation system is simple and cheap. The gas–liquid separator was discarded to avoid inter-contamination between the samples, thus excluding laborious decontamination procedures. The restricted volumes for the reaction allowed us to save the reagent (1.0 ml) and sample (1.0 ml) solutions and reduced the quantity of residues produced during the analysis. Acknowledgments ¸ ˜ de Amparo a` Pesquisa do We thank Fundacao ˜ Paulo (FAPESP-Processo 2000y Estado de Sao ´ Gregorio 06796-1) for financial support. Ruben
w1x B. Welz, M. Sperling, Atomic Absorption Spectrometry, Wiley-VCH, Verlag GmbH, Weinheim, 1999. w2x W.L. Clevenger, B.W. Smith, J.D. Winefordner, Trace determination of mercury: a review, Crit. Rev. Anal. Chem. 27 (1) (1997) 1–26. w3x J.M. Ombaba, Total mercury determination in biological and environmental standard samples by gold amalgamation followed by cold vapor atomic absorption spectrometry, Microchem. J. 53 (2) (1996) 195–200. w4x C.E.C. Magalhaes, ˜ F.J. Krug, A.H. Fostier, H. Berndt, Direct determination of mercury in sediments by atomic absorption spectrometry, J. Anal. At. Spectrom. 12 (1997) 1231–1234. w5x W. Frech, D.C. Baxter, G. Dyvik, B. Dybdahl, On the determination of total mercury in natural gases using the amalgamation technique and cold vapour atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 769–775. w6x P.C. Rudner, A.G. Torres, J.M.C. Pavon, ´ F.S. Rojas, Online preconcentration of mercury by sorption on an anion-exchange resin loaded with 1,5-bisw(2-pyridyl)-3sulphophenyl methylenex thiocarbonohydrazide and determination by cold-vapour inductively coupled plasma atomic emission, Talanta 46 (5) (1998) 1095–1105. w7x J.C. Rocha, J.J. Sene, P. Burba, D.J. Klockow, Interaction of mercury (II) with humic substances from the Rio Negro (Amazonas state, Brazil) by means of ion exchange procedure, J. Braz. Chem. Soc. 9 (1998) 79–84. w8x J.A.G. Neto, L.F. Zara, J.C. Rocha, A. Santos, C.S. ´ Dakuzaku, J.A. Nobrega, Determination of mercury in agroindustrial samples by flow-injection cold vapor atomic absorption spectrometry using ion exchange and reductive elution, Talanta 51 (3) (2000) 587–594.
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R.G. Moreno et al. / Spectrochimica Acta Part B 57 (2002) 769–778
w9x A.L. Bihan, J.Y. Cabon, Determination of 1 ngyl levels of mercury in water by electrothermal atomization atomic-absorption spectrometry after solvent extraction, Talanta 37 (12) (1990) 1119–1122. w10x E. Bulska, W. Kandler, A. Hulanicki, Noble metals as permanent modifiers for the determination of mercury by electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 51 (1996) 1263–1270. w11x B. Welz, G. Schlemmer, J.R. Mudakavi, Palladium nitrate–magnesium nitrate modifier for electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 7 (1992) 499–503. w12x X.-P. Yan, Z.-M. Ni, Q.-L. Guo, In situ concentration of mercury vapour in a palladium-coated graphite tube: determination of mercury by atomic absorption spectrometry, Anal. Chim. Acta 272 (1993) 105–114. w13x P.B. Barrera, J.M. Pineiro, ˜ ˜ A.M. Pineiro, A.B. Barrera, Iridium-coated graphite tubes for the direct determination of As, Cd, Hg and Pb in seawater by vapor generation ETAAS, At. Spectrosc. 19 (3) (1998) 100–106. w14x P.B. Barrera, J.M. Pineiro, ˜ ˜ A.M. Pineiro, A.B. Barrera, Use of flow injection cold vapour generation and preconcentration on coated graphite tubes for the determination of mercury in polluted seawaters by electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 12 (1997) 317–321. w15x S.H. Lee, K.H. Jung, D.S. Lee, Determination of mercury in environmental samples by cold vapour generation and atomic-absorption spectrometry with a gold-coated graphite furnace, Talanta 36 (10) (1989) 999–1003. w16x Z. Grobenski, W. Erler, U. Voellkopt, Determination of mercury with Zeeman graphite furnace atomic absorption spectrometry, At. Spectroc. 6 (4) (1985) 91–93. w17x Z. Hladky, J. Risova, ˇ ´ M. Fisera, ˇ Determination of mercury in concentrated mineral acids by electrothermal
w18x
w19x
w20x
w21x
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atomic absorption spectrometry using gold amalgamation, J. Anal. At. Spectrom. 5 (1990) 691–692. D.L. Tsalev, A. D’Ulivo, L. Lampugnani, M. Di Marco, R. Zamboni, Thermally stabilized iridium on an integrated, carbide-coated platform as a permanent modifier for hydride-forming elements in electrothermal atomic absorption spectrometry, part 1. Optimization studies, J. Anal. At. Spectrom. 10 (1995) 1003–1009. E. Bulska, K.L. Ilkowska, A. Hulanicki, Optimization of electrochemical deposition of noble metals for permanent modification in graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 53 (1998) 1057–1062. H.W. Sinemus, H.H. Standel, B. Radziuk, J. Kleiner, Determination of mercury following sequestration on an iridium coated graphite surface, Spectrochim. Acta Part B 48 (1993) 1719–1722. E. Bulka, W. Jedral, Application of palladium- and rhodium-plating of the graphite furnace in electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 10 (1995) 49–53. E. Bulska, B. Thybusch, H.M. Ortner, Surface and subsurface examination of graphite tubes after electrodeposition of noble metals for electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 56 (2001) 363–373. J.J. Pedrotti, L. Agnes, I.G.R. Gutz, Miniaturized reference electrodes with microporous polymer junctions, Electroanalysis 8 (7) (1996) 673–675. F.J. Krug, H. Bergamin Filho, E.A.G. Zagatto, Commutation in flow injection analysis, Anal. Chim. Acta 179 (1986) 103–118. A.B. Volynsky, Mecanism of action of platinum group modifiers in electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 55 (2000) 103–150. D.-Q. Zhang, L.-L. Yang, H.-W. Sun, Determination of mercury by cold vapour atomic absorption spectrometry with derivate signal processing, Anal. Chim. Acta (1999) 173–178.