Evaluation of tungsten–rhodium coating on an integrated platformas a permanent chemical modifier for cadmium, lead, and seleniumdetermination by electrothermal atomic absorption spectrometry

Eder C. Limaa, Francisco J. Kruga,* , Kenneth W. Jacksonb

aCentro de Energia Nuclear na Agricultura, Universidade de Sa˜o Paulo, Postal Box 96, 13400-970, Piracicaba-SP, BrazilbWadsworth Center, New York State Department of Health, and School of Public Health, State University of New York, Albany NY 12201-0509,

USA

Received 6 July 1998; accepted 7 September 1998

Abstract

A tungsten–rhodium coating on the integrated platform of a transversely heated graphite atomizer is proposed as a permanentchemical modifier for the determination of Cd, Pb, and Se by electrothermal atomic absorption spectrometry. It was demon-strated that coating with 250mg W 1 200mg Rh is as efficient as the conventional Mg(NO3)2 1 NH4H2PO4 or Pd1 Mg(NO3)2

modifiers for avoiding most serious interferences. The permanent W–Rh modifier remains stable for 300–350 firings of thefurnace, and increases tube lifetime by 50%–100% when compared to pyrolytic carbon integrated platforms. Also, there is lessdegradation of sensitivity during the atomizer lifetime when compared with the conventional modifiers, resulting in a decreasedneed of re-calibration during routine analysis. The characteristic masses and detection limits achieved using the permanentmodifier were respectively: Cd 1.10.4 pg and 0.020mg L21; Pb 30 3 pg and 0.58mg L21 and Se 42 5 pg and 0.64mg L21.Results from the determination of these elements in water reference materials were in agreement with the certified values, sinceno statistical differences were found by the pairedt-test at the 95% level.q 1998 Elsevier Science B.V. All rights reserved.

Keywords:Permanent chemical modifier; Rhodium; Tungsten; Cadmium; Lead; Selenium; Electrothermal atomic absorptionspectrometry

1. Introduction

The use of modifiers became an essential part ofelectrothermal atomic absorption spectrometry(ETAAS) with the introduction of the stabilizedtemperature platform furnace concept (STPF) [1].Since first proposed by Ediger [2], the main purposeof chemical modifiers is to either thermally stabilizethe analyte or increase the volatility of the matrix.Hence, the bulk of the matrix may be removed during

the pyrolysis stage, by volatilization or decomposi-tion, prior to atomization of the analyte. A mixtureof Pd 1 Mg(NO3)2 has been the most employedchemical modifier for stabilizing the majority ofelements determined by ETAAS [3–5]. However,other noble metals, including Pt, Rh, Ru, and Irhave been employed as chemical modifiers for awide range of elements in ETAAS, for direct sampleintroduction [6–10] as well as for hydride [11–14]and Hg trapping [15] in the graphite atomizer. Analytethermal stabilization is considered to occur by itsentrapment in droplets of the metallic reduced form

Spectrochimica Acta Part B 53 (1998) 1791–1804

0584-8547/98/$ - see front matterq 1998 Elsevier Science B.V. All rights reserved.PII: S0584-8547(98)00227-4

* Corresponding author. Fax: 0055-19-429-4610.

of the modifier, forming a solid-phase solution, and/orby forming intermetallic compounds between theanalyte and modifier [6,9,16]. Tungsten has alsobeen used as a chemical modifier for several elementsin ETAAS [17–19]. Analyte thermal stabilizationmay occur through interactions with several Wspecies formed in the graphite atomizer, e.g., isomor-phous substitution of one W atom from the WOx

lattice by an analyte atom; the formation of Wcompounds with the analyte (such as PbxWO3,AsW12O40

32, etc); or the simple effect of volatileelements embedding in refractory W compounds, assuggested by Slaveykova and Tsalev [18].

Chemical modifiers are generally used by introdu-cing an aliquot of the modifier solution into the atomi-zer together with sample solution, or the modifier canbe introduced first onto the graphite surface, followedby drying and pyrolysis, and then the sample solutioncan be delivered into the atomizer. Both proceduresare repeated at each firing and have their advantagesand limitations. The first method requires high puritychemical modifier reagents in order to avoid elevatedblank values. In the second procedure, the modifier isreduced over the graphite surface, often allowingbetter analyte stabilization [6]. Additionally, thisprocedure eliminates volatile impurities in the modi-fier solution during its pretreatment cycle, allowing adecrease in the detection limits. Contrarily, the totalheating program time is greatly increased, becomingunsuitable when analysis of a large number of samplesis required. These disadvantages of the use of a chemi-cal modifier might be solved if it were present in thetube, as apermanent chemical modifier. The use ofpre-reduced noble metal permanent modifiers such asPd, Rh, Ru, Pt, and Ir [11–14,20–22], carbide formingelements (W, Zr, Nb, Ta) [23,24] and mixed carbide-forming elements (W, Zr) with Ir [25–27] have beenapplied successfully for trapping of hydride-formingelements and mercury [15] in ETAAS. Also, some ofthese modifiers have been used with the direct intro-duction of sample solution into the graphite furnacefor determination of Cd, Pb, and Se [28,29].

Permanent modifiers provide attractive advantagessuch as simpler and faster heating programs forETAAS determination; elimination of volatile impu-rities of the modifier during its coating process ongraphite surface, which allows a decrease in detectionlimits; and improvement in hydride trapping [25-27,30].

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Table 1Sequence for coating the integrated platform of a THGA withtungsten followed by Rh

Step Actions and parameters

Treatment with W1 Pipet 50ml of 1.0 g l21 W onto

the platform2 Run the following heating

programa (ramp, hold time) fordrying and pyrolysis: 1208C (5,25 s), 1508C (10, 60 s), 6008C(20, 15 s), 10008C (10, 15 s)

3 Repeat steps 1 and 2 three moretimes

4 Repeat step 1 and run thefollowing heating program(ramp, hold time): 1208C (5,25 s), 1508C (10, 60 s), 6008C(20, 15 s), 10008C (10, 15 s),14008C (10, 5 s), 20008C (3, 2 s)

5 Run four times the temperatureprogram (ramp, hold time) fortungsten carbide conditioning atmild temperature: 1508C (1,10 s), 6008C (10, 15 s), 11008C(10, 5 s), 14008C (10, 10 s)

6 Run four times the temperatureprogram (ramp, hold time) fortungsten carbide conditioning athigh temperature: 1508C (1,10 s), 6008C (10, 15 s), 11008C(10, 5 s), 14008C (10, 10 s),15008C (3, 5 s), 16008C (1, 1 s),17008C (1, 1 s), 18008C (1, 1 s),19008C (1, 1 s), 20008C (1, 1 s)

Coating with Rh7 Pipet 20–50ml of 1.0–1.0 g l21

Rh onto the platform8 Run the following heating

program (ramp, hold time) fordrying and pyrolysis: 1208C (1,10.25 s), 1508C (5, 30–60 s),10008C (10, 10 s), 14008C (1,5 s)

9 Repeat steps 7 and 8 more threetimes

10 Repeat step 7 and run thefollowing heating program(ramp, hold time): 1208C (1,25 s), 1508C (5, 60 s), 10008C(10, 10 s), 14008C (1, 5 s),20008C (1, 3 s)

a All steps of the heating programs were performed under 250 mlmin21 of Ar.

However some potential drawbacks for the permanentmodifiers, such as, multiple peaks; over-stabilizationof some analytes; limitation of the maximum appliedtemperature in the heating program in order to noteliminate the permanent modifier from the graphitesurface were also reported [25–27,30].

The aim of this work was to propose an easilyprepared W–Rh permanent chemical modifier, coatedon the integrated platform of a transversely heatedgraphite tube atomizer, for the determination of Cd,Pb, and Se. Water reference materials were employedto test the proposed modifier.

2. Experimental

2.1. Apparatus

A Perkin-Elmer 4100ZL atomic absorption spectro-meter with a longitudinal Zeeman effect backgroundcorrection system, furnished with a transverselyheated graphite atomizer (THGA) and an AS-71 auto-sampler were used. The standard THGA (Part no.B050-4033) with integrated platforms was used,either without any previous treatment (referred to asthe pyrolytic carbon platform), or after pretreatment

first with W and then with Rh (referred to as the W–Rh treated platform).

Measurements were made at the analytical wave-lengths recommended by the manufacturer (Cd:228.8 nm, slit 0.7 nm; Pb: 283.3 nm, slit 0.7 nm; andSe: 196.0 nm, slit 0.7 nm). A Perkin-Elmer electrode-less discharge lamp EDLII system was used.

2.2. Tungsten–rhodium permanent chemical modifier

A detailed procedure for platform coating withtungsten followed by Rh is shown in Table 1. Theprogram (total time is about 45 min) was convenientlyperformed by the autosampler and automatically setby employing the Furnace and Sequence pages of theElement Parameter window of the GEM version soft-ware. In Table 2 are shown the heating programsemployed for Cd, Pb, and Se determination usingpyrolytic carbon platforms with the conventionalmodifier and the W–Rh permanent modifier. Unlessotherwise stated, Ar (AGA) as purge gas was usedthroughout. All measurements of integrated absor-bance were made with at least three replicates.

The surface of a treated platform was analyzed byscanning electron microscopy (SEM), employing a

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Table 2Heating program for the determination of Cd, Pb and Se in water reference materials

Step Temp (8C) Ramp (s) Hold (s) Ar flow rate (ml min21)

1 150 25 20a, 25b 2502 200 5 10a, 15b 2503 Var-a Var-b Var-c 2504 Var-d 0 5 05 2200 1 4 250Injection temperature 1008CPyrolytic carbon platform with conventional modifierMetal Modifier a b c dCd 3mg Mg(NO3)2 1 50mg

NH4H2PO4

6508C 5 s 40 s 13008C

Pb 5mg Pd1 3mg Mg(NO3)2 10008C 10 s 25 s 18008CSe 5mg Pd1 3mg Mg(NO3)2 14008C 10 s 25 s 20008CPyrolytic carbon platform with permanent modifierMetal Modifier a b c dCd 250mg W 1 200mg Rh 5008C 5 s 40 s 12008CPb 250mg W 1 200mg Rh 8008C 10 s 25 s 18008CSe 250mg W 1 200mg Rh 12008C 10 s 25 s 20008C

a Permanent modifier.b Conventional modifier.

Jeol JSM T300 Scanning Microscope operating at anaccelerating voltage of 20 kV.

2.3. Reagents, reference solutions, and samples

High purity deionized water obtained using a Milli-Q water purification system (Millipore) was usedthroughout. Analytical reagent grade HNO3 and HClwere distilled in quartz sub-boiling stills (Ku¨merGermany). The carbide-forming chemical modifier(1.0 g l21 W) was prepared by dissolving 0.1794 gof Na2WO4·2H2O (Merck) in 100 ml of water.Rhodium solutions of 0.1 and 1.0 g l21 were obtaineddissolving suitable amounts of (NH4)3RhCl6·11/2H2O(Johnson & Matthey) in 10 ml of 10% v/v HCl.Conventional chemical modifiers (added at eachfiring), used for analyte stabilization with pyrolyticcarbon platform, were 0.05% m/v Pd1 0.03% m/vMg(NO3)2 (for Pb and Se) and 0.03% m/vMg(NO3)2 1 0.50% m/v NH4H2PO4 (for Cd). Thesewere prepared from NH4H2PO4 salt (SuprapurMerck), 10.0 g l21 Pd solution (Pd(NO3)2, Merck)and 10.0 g l21 Mg(NO3)2 solution (Merck).

Stock solutions of Cd, Pb, and Se (1.00 g l21) wereobtained from spectrographically pure CdO,Pb(NO3)2, and SeO2 (Johnson & Matthey). Analyticalcalibration standards were prepared over the range of0.25–2.5mg l21 Cd and 2.5–20.0mg l21 Pb and Se bysuitable serial dilution of stock solutions in 0.2% v/vHNO3.

The effects of concomitants on analyte atomizationwere investigated with solutions containing up to10 000 mg l21 Na (NaCl, Johnson & Matthey),10 000 mg l21 K (KCl, Johnson & Matthey),10 000 mg l21 Ca (CaCO3 1 HNO3, Johnson &Matthey), 10 000 mg l21 Mg (MgO 1 HNO3, John-son & Matthey), 10 000 mg l21 Cl (HCl Merck),10 000 mg l21 sulfate (H2SO4, Suprapur Merck),10 000 mg l21 phosphate (NH4H2PO4, SuprapurMerck), 1000 mg l21 Zn (ZnSO4·7H2O, Johnson &Matthey), 1000 l21 Fe (Fe2O3 1 HCl, Johnson &Matthey), 500 mg l21 Mn (MnO2 1 HNO3, Johnson& Matthey), and 500 mg l21 Al (AlCl 3, Merck-Triti-sol), in 0.2% v/v HNO3.

Water reference materials TM24, TM26, andTM28, from the National Water Research Instituteof Canada, were used for validation of the permanentchemical modifier for Cd, Pb, and Se determinations.

2.4. Materials

All solutions were stored in polypropylene bottles(Nalgene). Plastic bottles, autosampler cups andglassware materials were cleaned by soaking in20% v/v HNO3 for 24 h, rinsing five times withMilli-Q water and dried and stored in a class 100laminar flow hood.

2.5. Evaluation of tungsten–rhodium as a permanentchemical modifier

Pyrolysis and atomization curves, concomitanteffect studies, and determination in water referencematerials for Cd, Pb, and Se, were carried out usingpyrolytic carbon platforms (10–20ml analyte1 10mlconventional modifier), and with W–Rh treated plat-forms (10–20ml analyte), using the heating programsdescribed in Table 2.

3. Results and discussion

3.1. Preparation of the tungsten–rhodium permanentchemical modifier

Initial coating experiments were carried out byintroducing multiple 20ml aliquots of W solutiononto the integrated platform (total mass 250mg W),followed by stepwise drying and pyrolysis. The inte-grated platform was not completely covered when thisprocedure was adopted, due to the small volume ofmodifier, which did not spread throughout theplatform during the graphite surface treatment.When the same W mass was introduced in aliquotsof 50ml into the atomizer, the platform was thor-oughly covered. Higher W masses were tried, butdouble and broader peaks were observed for Pb andSe, requiring high atomization temperature to avoidmemory effects. However, when the atomizationtemperature exceeded 21008C–22008C, the coating-lifetime was drastically diminished to less than 200firings per treatment. For these reasons, the coatingprocedure was carried out by introducing into theatomizer 250mg of W in five 50ml aliquots of1.0 g l21 W. Under these conditions, the coating-lifetime per treatment was about 300–350 firings,without significant changes in sensitivity andreproducibility.

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Fig. 1. Pyrolysis curves for: (A) 50 pg Cd; (B) 1 ng Pb; (C) 1 ng Se.B – Permanent modifier; 250mg W.X – Permanent modifier, 250mg W 1

6mg Rh.O – Permanent modifier, 250mg W 1 200mg Rh. × – Pyrolytic carbon platform with conventional modifier (for Cd: 3mg Mg(NO3)2 1 50mg NH4H2PO4; for Pb and Se: 5mg Pd1 3mg Mg(NO3)2.

The adopted procedure for platform treatment withW was based on previous reports of hydride trappingon W–Ir and Zr–Ir permanent modifiers [25–27], andis described in Table 1. After depositing 250mg of Wonto the platform, two conditioning steps were carriedout four times each, in order to transform the tungstenoxides and/or oxycarbides to tungsten carbides[18,25]. After this treatment, Rh metal was depositedon the pretreated integrated platform, followed bystepwise drying and pyrolysis. This heating programreduced the noble metal on the coated tungstencarbide platform [6,7]. The resulting W–Rh perma-nent modifier could be employed for typical 20mldirect sample solution introduction, and each platformtreatment lasted for about 300–350 firings. The usedtreated platform could be reconditioned by perform-ing 4–5 firings at 24008C for 5 s for clean out, andsubsequently carrying out the treatment proceduredescribed in Table 1. The new re-coated platformagain lasted for 300–350 firings, with sensitivity notsignificantly different (, 3%) from the first coating.The total time for treatment of the graphite surfacewith W–Rh is about 45 min, and this procedure couldbe carried out during the required warm-up of theEDLs. A coating would typically last for a workingday of 8–10 h of instrument operation (i.e. 300–350firings). The clean-out temperature was not allowedto exceed 22008C (1 s ramp plus 4 s holding time),in order to avoid removal of the Rh coating fromthe platform. The W–Rh permanent modifier life-time would be limited to 250 firings if a clean outtemperature of 24008C was used for the same timeperiod.

3.2. Behavior of cadmium, lead, and selenium on thetungsten–rhodium permanent modifier

Pyrolysis curves for Cd, Pb, and Se on pyrolyticcarbon and several treated platforms are presented inFig. 1. Several coatings with Rh masses ranging from6–500mg (W mass was kept at 250mg) were testedfor optimizing the amount of the noble metal thatwould promote higher sensitivity and thermal stabili-zation. For all analytes, the comparative sensitivitieswere in the order: 250mg W 1 200mg Rh . 250mgW 1 6 mg Rh . 250mg W. The lower sensitivitiesobtained with the 250mg W 1 6 mg Rh and 250mg Wpermanent modifiers were probably due to analyte

losses during the atomization stage of the heatingprogramme, since there is no dependence on pyrolysistemperature. For Rh masses. 200mg, a decrease ofabout 15% in sensitivity was observed. This depres-sive effect with increasing amount of noble metalchemical modifier was also observed previously[11, 25].

The maximum pyrolysis temperature that could beused for Pb and Se with the 250mg W 1 200mg Rhpermanent modifier was 1008C–2008C higher thanthat possible with the direct introduction of the recom-mended chemical modifier for these analytes in aTHGA (3mg Mg(NO3)2 1 5 mg Pd) [32]. Tsalevand Slaveykova observed similar behavior for theseelements when Rh and Pd were employed as conven-tional modifier [6]. Although the maximum usablepyrolysis temperature for Se was 15008C for all testedpermanent modifiers, a remarkable increase in itssensitivity was obtained when 250mg W 1 200mgRh permanent modifier was employed indicating thatRh plays a role in this analyte’s thermal stabilization.For Cd and Pb the effect of Rh mass was not soremarkable as for Se atomization.

Significant variations in Cd and Se peak profileswere not observed during the several coatings thatwere applied during the total tube lifetime. Lead hada shoulder on its peak profiles when 200mg of W wasemployed as permanent modifier, whereas welldefined peaks were always obtained in the presenceof Rh. Shifts in appearance time were not perceptiblefor the three analytes with the several tested perma-nent modifiers.

3.3. Morphological study of the tungsten–rhodiumcoating

Transversely heated graphite atomizers were cutopen to permit removal of the platforms. The surfacesof pyrolytic carbon and W–Rh treated integrated plat-forms were then analyzed by SEM. In Fig. 2 arepresented micrographs at 500 fold magnification,showing the morphology of the platform during analy-tical firings. Although there is a non-uniform distribu-tion of modifier over the platform surface, therequired analyte thermal stability is attained underthese conditions.

As can be seen from Fig. 2(A), granuli of graphitewere observed throughout the pyrolytic carbon

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platforms. After 300 firings the graphite surfacebecame worn-out, and the graphite granuli started todisappear (Fig. 2B). After 600 firings, holes in thegraphite surface were noticeable [Fig. 2(C)]. when

the graphite platform was treated with W–Rh, alayer of chemical modifier covered the graphitesurface [Fig. 2(D)]. It can be seen that, after 350analytical firings, there is a need for re-coating the

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Fig. 2. Electron micrographs of graphite platforms at 500 fold magnification [from left to right: scalebar (mm),accelerating voltage (kV), date(day-month), photonumber]. (A) Pyrolytic carbon platform without firings; (B) Pyrolytic carbon platform after 300 firings with 0.05% Pd1

0.03% Mg(NO3)2 modifier; (C) Pyrolytic carbon platform after 600 firings with 0.05% Pd1 0.03% Mg(NO3)2 modifier; (D) Platform treatedwith 500mg W 1 200mg Rh permanent modifier without analytical firings; (E) Platform treated with 250mg W 1 200mg Rh permanentmodifier after 350 analytical firings; (F) Platform treated with 250mg W 1 200mg Rh permanent modifier after 700 analytical firings (platformwas recoated after 350 analytical firings).

surface with W–Rh, since the coating is sparselydistributed over the graphite surface, and further a12% decrease in the analytical signal was observed[Fig. 2(E)]. An advantage of W–Rh treatment is anextension of the platform lifetime when compared topyrolytic carbon surfaces. A twice treated platform

can be successfully re-used after 700 analytical firingsif treated again with W–Rh, since few cavities wereobserved in its surface [Fig. 2(F)]. Contrarily, thepyrolytic carbon platform presented large cavitiesafter 600 firings, preventing its use with goodreproducibility. On average, W–Rh treatment increase

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Fig. 3. Interferences of NaCl, KCl, Ca(NO3)2, FeCl3 and AlCl3 on atomization of 50 pg Cd. Amounts of interferents are based on massed of Na,K, Ca, Fe, Al. (A) Conventional modifier (3mg Mg(NO3)2 1 50mg NH4H2PO4); (B) Permanent modifier (250mg W 1 200mg Rh).B – Na(NaCl). × – K (KCl). f – Ca (CaNO3)2. O – Fe (FeCl3). W – Al (AlCl 3).

the total tube lifetime by at least 50% (without consider-ing the firings for coating), when compared to pyrolyticcarbon platforms.

It is important to point out that no significant differ-ences on the maximum analyte pyrolysis temperatureswere observed during the lifetime of W–Rh treatedplatform as well as carbon platforms.

3.4. Performance of the tungsten–rhodium permanentmodifier for cadmium, lead, and selenium atomizationin the presence of concomitants

Some authors [28,31] did not recommend the use ofdirect injection as an efficient process for coating thegraphite surface, due to a non-uniform distribution of

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Fig. 4. Interferences of NaCl, KCl, FeCl3 and HCl on atomization of 1 ng Pb. Amounts of interferents are based on masses of Na, K, Fe, Al, Cl.(A) Conventional modifier (3mg Mg(NO3)2 1 5 mg Pd); Mg (NO3)2. (B) Permanent modifier (250mg W 1 200mg Rh).B – Na (NaCl).× – K(KCl). O – Fe (FeCl3). W – Al (AlCl 3). f – Cl (HCl).

the coating throughout the platform, when comparedwith those obtained by physical deposition [28,29],which provides dense and uniform coatings.However, when the permanent modifiers producedby sputtering onto a graphite surface were used inreal samples, they were not effective in overcominginterferences [29]. It is known that carbon has a role in

the action of noble metal modifiers for reduction ofoxides [31]. If the graphite surface were completelycovered with the noble metal modifier, as occurs whenthe coating is produced by sputtering [28,29], therewould not be available carbon active sites on thegraphite surface to promote the reduction of analyteoxide to free atoms. This may inhibit the formation of

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Fig. 5. Interference of NaCl, KCl, Ca(NO3)2, Mg(NO3)2 and H2SO4 on atomization of 1 ng Se. Amounts of interferents are based on masses ofNa, K, Ca, Mg, SO4

22.(A) Conventional modifier (3mg Mg(NO3)2 1 5 mg Pd); (B) Permanent modifier (250mg W 1 200mg Rh).B – Na(NaCl). × – K (KCl). f – Ca (Ca(NO3)2). W – Mg (Mg(NO3)2). O – SO4

22 (H2SO4).

intermetallic compounds and/or a solid solutionbetween the analyte and noble metal-modifier,increasing analyte losses by volatilization. As a conse-quence the modifier action for analyte stabilizationwould be impaired. Therefore, it is expected thatgraphite surfaces incompletely coated with the perma-nent modifier will be more effective for overcominginterferences compared with sputtered permanentmodifiers [29].

An extensive interference study, with concomitantconcentrations usually found in environmental andfood samples [33], was carried out in order to comparethe performance of 250mg W 1 200mg Rh perma-nent modifier with the recommended conventionalchemical modifiers [32], for overcoming matrixeffects on Cd, Pb, and Se atomization (Figs. 3–5,respectively). It can be seen in Fig. 3 that interferencesby high amounts of NaCl, Ca(NO3)2, and AlCl3 on Cdatomization were significantly lower for the proposedpermanent modifier when compared with 3mgMg(NO3)2 1 50mg NH4H2PO4. For both modifiers,FeCl3 had practically the same effect on Cd atomiza-tion. Hence the permanent modifier was quite similarto the recommended conventional modifier for avoid-ing interferences by the main components of environ-mental and food samples. Mineral acids, as well asother species (up to 100mg of Mg as Mg(NO3)2,10mg of Zn as ZnSO4, 5mg of Mn as Mn (NO3)2,and 100mg of PO4

32 as NH4H2PO4) did not signifi-cantly affect the atomization of 50 pg of Cd.

The stabilization of Pb with the permanent modifierin the presence of the concomitants is shown in Fig. 4.High chloride amount as HCl (up to 10% v/v HCl),which had a depressive effect on Pb atomization forthe conventional Pd Mg(NO3)2 modifier, did not inter-fere significantly when using the permanent modifier.Contrarily, chloride as NaCl and KCl presentedgreater interference on Pb atomization for theproposed permanent modifier. For AlCl3 and FeCl3,the interference effects were higher for the conven-tional chemical modifier. Other species (up to 100mgof PO4

3 as NH4H2PO4, 100mg of SO42 as H2SO4,

100mg of Ca as Ca(NO3)2, 100mg of Mg asMg(NO3)2, 5mg of Zn as ZnSO4, and 5mg of Mn asMn(NO3)2) did not have significant effects on theatomization of 1 ng of Pb when either type of chemi-cal modifier was used.

Efficient control of interference on Se atomization

for a wide range of concomitant amounts was notobtained for either modifier (Fig. 5). Sulfate, in theH2So4 form, interfered strongly when conventional aswell as permanent modifiers were employed. A differ-ent strategy must be utilized to overcome sulfate inter-ference on Se atomization if the sulfate concentrationis . 50 mg l21, as already suggested by Ni et al. [33].Better interference control on Se atomization forlarger amounts of Na as NaCl and K as KCl wasachieved with the proposed permanent modifier, butfor Ca as Ca(NO3)2, more severe interference wasobserved with this modifier. Up to 100mg of Cl (asHCl), 5mg of Fe (as FeCl3), 5mg of Al (as AlCl3), and5mg of Mn (as Mn(NO3)2) did not interfere signifi-cantly with either modifier. Generally, if the samplematrix is not too complex, it might be possible todetermine Se. It is beyond the scope of this work tofind better analytical conditions for Se determinationin more complex real samples.

3.5. Analytical characteristics

All experiments were performed in 0.2% v/vNHO3, although concentrations as high as 5.0% v/vcould be tolerated by the permanent modifier withoutsignificant variations in the Cd, Pb, and Se analyticalsignals. Linear ranges extending up to 2.5mg l21 forPb and Se were obtained by using the heating programdescribed in Table 2 for recommended chemicalmodifiers [31] and 250mg Rh permanent modifier.With the permanent modifier, characteristic massesfor Cd, Pb, and Se, based on integrated absorbance,were respectively, 1.1 0.4 pg; 30^ 3 pg, and42 ^ 5 pg (uncertainly based on ten average resultsobtained on different days). These values are close tothose quoted by the manufacturer with the use ofconventional modifiers [32].

Tube lifetimes were increased by 50%–100% whenW–Rh coatings were employed compared to pyrolyticcarbon platforms for the three analytes. Presented inFig. 6 are long-term stability curves for pyrolyticcarbon platform with conventional modifier and W–Rh treated platforms when Cd was determined. Withthe permanent modifier, the tube lifetime was doubledcompared with the use of 3mg Mg(NO3)2 1 50mgNH4H2PO4 on pyrolytic carbon platforms, whenreconditioning treatments were performed duringfive working-days. It can also be seen that variations

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of sensitivity are smaller for the permanent modifiercompared with pyrolytic carbon platforms withconventional modifiers. Hence, there will be lessneed for re-calibration during the routine analysis oflarge number of samples, compared with the situationwhen conventional modifiers are employed. It isstressed that the platform treatment did not increaseanalysis time, because the procedure was performed

during the required heating time of EDLs (40–45 min).

Detection limits, calculated from 20 consecutivemeasurements of the blank solution (0.2% v/v nitricacid) according to IUPAC [34], were: Cd, 0.023mgl 21 and 0.020 ng ml21 for 3mg of Mg(NO3)2 1 50mgof NH4H2PO4 and 250mg of W � 200mg of Rhpermanent modifier, respectively; Pb 0.5mg l21 and

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Fig. 6. Long-term stability curves for pyrolytic carbon platform and W–Rh treated platform. Each point represents an average of tenmeasurements after injection of 10mg l21 Cd in 0.2% v/v NHO3. Arrows indicate a new coating with W followed by Rh (details in Table1). W – Pyrolytic carbon platform; 3mg Mg(NO3)2 1 50mg NH4H2PO4. B – 250mg W 1 200mg Rh permanent modifier.

Table 3Determination of Cd, Pb and Se in Water Reference Materials from the National Water Research Institute of Canada by using 250mg W 1

200mg Rh permanent modifier and conventional modifiers. Results are an average of three measurements^ standard deviation

Sample Certified valuea (mg l21) Permanent modifier (mg l21) Conventional modifier (mg l21)

TM24 Cd: 12.5 ^ 3.4 Cd: 12.6^ 0.05 Cdb: 13.5 ^ 0.08TM24 Pb: 7.2 ^ 2.9 Pb: 8.7^ 0.14 Pbc: 9.1 ^ 0.23TM24 Se: 4.0^ 1.8 Se: 86^ 0.11 Sec: 3.75 ^ 0.22TM26 Cd: 17.0 ^ 3.8 Cd: 15.3^ 0.28 Cdb: 15.9 ^ 0.12TM26 Pb: 13.6^ 4.8 Pb: 14.5^ 0.12 Pbc: 15.9 ^ 0.19TM26 Se: 7.0^ 2.5 Se: 6.52^ 0.09 Sec: 5.20 ^ 0.25TM28 Cd: 1.4 ^ 1.2 Cd: 1.31^ 0.03 Cdb: 1.37 ^ 0.08TM28 Pb: 2.9 ^ 1.3 Pb: 3.8^ 0.06 Pbc: 3.8 ^ 0.18TM28 Se: 5.0^ 1.7 Se: 4.48^ 0.27 Sec: 4.05 ^ 0.24

a 3mg Mg(NO3)2 1 50mg NH4H2PO4.b 5mg Pd1 3mg Mg(NO3)2.c Uncertainty limits of reference materials.

0.31mg l21 for 5mg of Pd1 3mg of Mg(NO3)2 and250mg of W 1 200mg of Rh permanent modifier,respectively; and Se 1.53 ng ml21and 0.64 ng ml21

for 5mg Pd 1 3 mg Mg(NO3)2 and 250mg W 1200mg Rh permanent modifier, respectively. Thehigher detection limits obtained with the conventionalmodifiers may be attributed to high palladium blankvalues.

A comparison of permanent modifier and conven-tional modifier for Cd, Pb, and Se determination inwater reference materials was performed (Table 3).The concentrations of these analytes determined inthe samples were in agreement with their certifiedvalues for both modifiers. Applications of the perma-nent modifier for the determination of theses analytesin environmental and food samples are under investi-gation in the authors’ laboratory with very promisingresults.

4. Conclusions

In general, the graphite treatment with W followedby Rh presented better analytical performance to theconventional chemical modifiers for Cd, Pb, and Sedetermination in waters. The tube lifetime wasincreased by 50%–100% in relation to an pyrolyticcarbon platforms, leading to a remarkable decrease inthe variable analytical costs [36]. The longer termstability achieved with 250mg W 1 200mg Rhpermanent modifier (rsd 9.3%,n � 1700) whencompared with pyrolytic carbon platforms with 3mgof Mg(NO3)2 1 50mg of NH4H2PO4 (rsd 11.4%,n�800) allows a decrease of the re-calibration during theroutine analysis, hence, a higher number of samplescan be analyzed in a working-day. Furthermore, atleast the same accuracy and analyte thermal stabiliza-tion provided by the conventional modifiers wereattained by using the 250mg W 1 200mg Rh perma-nent modifier.

Acknowledgements

The authors are grateful to Prof. Dr. Marco A.Z.Arruda (University of Camoinas) for scanning elec-tron microscopy experiments. We are also thankful toAlessandra Tomazzini Ferreira and Uelinton Guaita(CENA-USP) for technical support. We are grateful to

FundaCao de amparo a` Pesquisa do Estado de Sa˜oPaulo (FAPESP), Financiadora de Estudos e Projetos(PRONEX) and Conselho Nacional de Desenvolvi-mento Cientifico e Technolo´gico (CNPq) for financialsupport and fellowships.

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