Spectrochimica Acta Part B 71-72 (2012) 102–106

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Spectrochimica Acta Part B

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Analytical Note

Development of an analytical method for the determination of arsenic in gasolinesamples by hydride generation–graphite furnace atomic absorption spectrometry

Emilene M. Becker a,b, Morgana B. Dessuy c, Wiliam Boschetti c, Maria Goreti R. Vale c,d,⁎,Sérgio L.C. Ferreira d,e, Bernhard Welz d,f

a Universidade Federal do Pampa, Bagé, RS, Brazilb Universidade Federal de Pelotas, Pelotas, RS, Brazilc Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91501‐970 Porto Alegre, RS, Brazild Instituto Nacional de Ciência e Tecnologia do CNPq, INCT de Energia e Ambiente, Universidade Federal da Bahia, Salvador, BA, Brazile Instituto de Química, Universidade Federal da Bahia, Salvador, BA, Brazilf Departamento de Química, Universidade Federal de Santa Catarina, 88040‐900 Florianópolis, SC, Brazil

⁎ Corresponding author. Fax: +55 51 3308 7304.E-mail address: mgrvale@ufrgs.br (M.G.R. Vale).URL's: http://www.inct.cienam.ufba.br (M.G.R. Vale), h

(S.L.C. Ferreira), http://www.inct.cienam.ufba.br (B. Welz

0584-8547/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.sab.2012.04.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 November 2011Accepted 16 April 2012Available online 23 April 2012

Keywords:GasolineSample preparationArsenic determinationIn‐situ trappingHG–GF AAS

The purpose of the present work was to optimize the conditions for the determination of arsenic in gasoline withhydride generation–graphite furnace atomic absorption spectrometry after acid digestion using a full two-levelfactorial designwith center point. The arsinewas generated in a batch systemand collected in a graphite tube coat-edwith 150 μg Ir as a permanentmodifier. The sample volume, the pre-reduction conditions, the temperature pro-gram and modifier mass were kept fixed for all experiments. The estimated main effects were: reducing agentconcentration (negative effect), acid concentration (negative effect) and trapping temperature (positive effect).It was observed that there were interactions between the variables. Moreover, the curvature was significant, indi-cating that the best conditions were at the center point. The optimized parameters for arsine generation were2.7 mol L−1 hydrochloric acid and 1.6% (w/v) sodium tetrahydroborate. The optimized conditions to collect arsinein the graphite furnace were a trapping temperature of 250 °C and a collection time of 30 s. The limit of detectionwas 6.4 ng L−1 and the characteristicmasswas 24 pg. Twodifferent systems for acid digestionwere used: a digesterblock with cold finger and a microwave oven. The concentration of arsenic found with the proposed method wascompared with that obtained using a detergentless microemulsion and direct graphite furnace determination. Theresults showed that the factorial design is a simple tool that allowedestablishing the appropriate conditions for sam-ple preparation and also helped in evaluating the interaction between the factors investigated.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Owing to its toxic, cumulative and harmful effects, arsenic is an el-ement of great importance in different fields, such as environmental,nutritional and clinical; however it is an analyte that is difficult to be de-termined at low levels. In petrochemical plants and various cracking pro-cesses arsenic is important even at trace concentration, as it can causesevere and irreversible catalyst poisoning [1–3]. Although this elementtends to concentrate in heavier fractions, it can also be found in lighterpetroleum products [2]. Thus, its accurate quantification in petroleumderivates is important for the refining process and environmental issues.

Considering the fact that, usually, arsenic is present at low concen-tration in petroleum products, and that this kind of samples presentscomplexmatrix, it is necessary to apply sensitive and selective analytical

ttp://www.inct.cienam.ufba.br).

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techniques for its determination. Graphite furnace atomic absorptionspectrometry (GF AAS) and hydride generation atomic absorption spec-trometry (HG AAS) are frequently used for this purpose [3]. The mainadvantage of using GF AAS is that this technique offers the possibilityof direct analysis with aminimum of sample pretreatment. However,there are only a few examples in the literature for fuel analysis with-out previous sample treatment [2,4,5]. The use of emulsions or micro-emulsions appears to be the most promising approach because of theshort sample preparation time and the low risk of analyte losses by vola-tilization or sorption. Thesemethodswere described in several papers forthe direct determination of trace elements in petroleum derivatives[6–9]; procedures to obtain microemulsions or conventional emulsionswere also proposed for the determination of As [1,10,11]. In addition toproblems related to the complexity of the matrix, organic standards,which are indispensable in the case of direct sample introduction, are un-stable, and there are essentially no certified reference materials availablefor petroleum derivatives. For this reason the accuracy of the developedmethod has to be accomplished by comparison with results obtainedwith independent techniques, particularly with respect to sample prepa-ration [12].

Table 1Graphite furnace temperature program for the deposition of the iridium permanentmodifier; argon flow rate 1 L min−1 in all stages.

Step Temperature/°C Ramp/°C s−1 Time/s

1 90 5 402 110 1 403 130 1 404 1100 300 255 2100 500 15

103EM.. Becker et al. / Spectrochimica Acta Part B 71-72 (2012) 102–106

HG AAS is one of the most acknowledged sample introductiontechniques that is useful for both enhancing sensitivity and providingchemical speciation information for inorganic and some organo-arsenicspecies [13,14]. Since the first report about the determination of arsenicby AAS after generation of its volatile hydride [15], the unquestionableadvantages of the technique led to its application to virtually all elementscapable of forming volatile hydrides, such as arsenic, antimony, bismuth,germanium, lead, selenium, tellurium and tin [16]. One of the inherentadvantages of HG AAS is that the analyte can be easily preconcentratedeither in a special collection device [15] or directly in the atomizer [16].In-atomizer trapping in a graphite tube of GF AAS permits significantenhancement in relative detection power over conventional batch orcontinuous generation approaches for the ultra-trace determination ofhydride-forming elements [17]. After this technique was proposed byDrasch et al. [18] in 1980, it has achieved considerable improvement,and in-situ trapping in a graphite tube is nowadays one of themost pop-ular pre-concentration methods for all analytically important hydrides.

One of the major improvements for trapping of volatile hydrideswas the proposal to use graphite tubes coatedwith permanentmodifiers,such as elements of the platinum group. Shuttler et al. [19] proposed theuse of a mixture of palladium and iridium as permanent modifier forhydride trapping in a graphite furnace. This treatment was applied suc-cessfully to pre-concentrate and atomize arsenic, selenium and bismuth,claiming a trapping efficiency of virtually 100% [19,20]. Iridium wasfound to be one of the most economic modifiers; tubes treated withthis modifier could be used for several hundred measurements withoutany re-coating. The use of iridium coating also significantly improvedthe sensitivity of the method and the efficiency of hydride depositionin comparison with other modifiers.

To be compatible with the HG technique, usually a complete min-eralization of organic samples, such as fuels and biofuels, is mandato-ry. Systems for microwave-assisted mineralization of organic samplesin closed vessels became available only toward the end of the last cen-tury [21]. Such closed, pressurized systems for mineralization preventlosses of analytes, provide results with better reproducibility andhigher accuracy than open-beakermethods, canminimize contamina-tion, allow several samples to be digested simultaneously, and oftenrequire less time than other digestion methods. In spite of these ad-vantages, microwave-assisted mineralization of petroleum and petro-leum products was reported only by a few authors [21–23].

The goal of this work was to optimize the conditions for the determi-nation of arsenic in gasoline samples after an acid digestion using hydridegeneration–graphite furnace atomic absorption spectrometry (HG–GFAAS), applying a factorial design to establish the best experimental condi-tions for hydride generation and trapping. Twomethods of acid digestionas sample preparation for petroleum derivatives were investigated: asemi-open procedure using a cold finger system [24] and a microwave-assisted digestion with a mixture of nitric acid and hydrogen peroxidein a closed system [23]. To validate the results obtained with the two di-gestion methods for HG–GF AAS, they were compared with a direct GFAAS determination using a detergentless micro-emulsion.

2. Experimental

2.1. Instrumentation

Allmeasurementswere carried out using an AAS 5 EA atomic absorp-tion spectrometer (Analytik Jena AG, Jena, Germany), equipped withdeuterium background correction and a transversely heated graphitetube atomizer. The HS 5 hydride generation system (Analytik Jena),which can be used in a batch or continuous-flow mode, was operatedin the batch mode. The spectrometer was interfaced to an IBM PC/ATcompatible computer. An arsenic hollow cathode lamp (Analytik Jena),operated at a current of 6 mA, was used as the line source. Themeasure-ments were performed at 193.7 nm with a spectral slit width of 0.8 nm.Pyrolytically coated graphite tubes without platform (Analytik Jena, Part

No. 407-A81.011)were used for themeasurementswithHG–GFAAS andPIN platform tubes (Analytik Jena, Part No. 407-A81.025) were used forthe measurements with GF AAS. Iridium (Ir) was employed as perma-nent modifier for trapping of arsine and the subsequent determinationof arsenic by GF AAS. The Ir coatingwas thermally deposited by injectingten times 15 μL of a 1000 mg L−1 Ir stock solution and subjecting thetube after every injection to the temperature program shown in Table 1,in order to reduce the salt to the metal and produce a reactive surfacelayer of 150 μg Ir.

An MPE 5 furnace autosampler (Analytik Jena) was used for sampleintroduction with both techniques, HG–GF AAS and GF AAS. To connectthe hydride generator to the graphite furnace, the standard PTFE injectioncapillary was replaced by a titanium capillary (1.0 mm i.d.), which madepossible the introduction of the gaseous hydrides into the pre-heatedgraphite tube. The distance of the capillary tip from the tubewall was ad-justed to 0.5 mm. A PTFE capillary (1.5 mm i.d., length ~1.0 m) was usedto transport the volatile compounds from the hydride generator to the in-jection capillary.

Argon with a purity of 99.996% (White Martins, São Paulo, Brazil)was used as the transport, purge and protective gas. Integrated absor-bance (peak area) was used exclusively for signal evaluation and quan-tification. The standard calibration technique with aqueous calibrationsolutions was used for both, the HG–GF AAS and GF AAS methods. Thegraphite furnace temperature programs for both methods are given inTables 2 and 3, respectively.

The sample digestionswere carried out using two different systems: alaboratory microwave oven (Multiwave, Anton Paar, Graz, Austria), withPTFE vessels, and a digester block (Quimis, São Paulo) with glass tubesand a cold finger system.

2.2. Reagents, solutions and samples

Analytical grade reagents were used exclusively. Distilled, deionizedwater with a specific resistivity of 18 MΩ cm from aMilli-Q water puri-fication system (Millipore, Bedford,MA, USA)was used for the prepara-tion of standards,modifier solutions andmicroemulsions. All containersand glassware were soaked in 1.4 mol L−1 nitric acid for at least 24 hand rinsed three times with water before use. Nitric acid (65% w/v,Merck, Darmstadt, Germany) was further purified by sub-boiling distil-lation in a quartz sub-boiling still (Kürner Analysentechnik, Rosenheim,Germany). The working standards were prepared by serial dilution of a1000 mg L−1 As stock solution (SpecSol, QuimLab, Jacareí, SP, Brazil)with0.014 mol L−1 nitric acid.

A solution of 2.7 mol L−1 hydrochloric acid was prepared fromconcentrated HCl (37% w/v, Merck). Aqueous solutions of 1.6% (w/v)sodium tetrahydroborate, NaBH4, were freshly prepared by dissolvingthe solid reagent in 0.3% (w/v) NaOH (Merck). Pre-reducing solutionscontaining 5.0% (w/v) potassium iodide and 5.0% (w/v) ascorbic acidin water were prepared fresh every day. A solution of 0.1% (v/v) anti-foaming agent (Silicon Antifoaming Agent, Merck) was prepared in2.7 mol L−1 hydrochloric acid.

The detergentless micro-emulsion used to validate the resultswas prepared according to previously established data [1] bymixing 3 mL of the sample with 5 mL of propan-1-ol (Merck) and

Table 2Temperature program for the determination of As by HG–GF AAS.

Step Temperature/°C Ramp/°C s−1 Time/s Ar flow-rate /L min−1

Preheating, capillaryinsertion

250 300 10 0.1

Hydride trapping 250 0 30 0Pyrolysis 1000 500 7 0Atomization 2200 1000 5 0Cleaning 2250 1000 5 2.0

Table 4Temperature program for the microwave-assisted digestion of gasoline samples afterover-night pre-digestion with nitric acid.

Step Power/W Time/min

1 100–1000 202 1000 103 0 30

104 EM.. Becker et al. / Spectrochimica Acta Part B 71-72 (2012) 102–106

600 μL of 0.014 mol L−1 nitric acid solution, and filling up to a final vol-ume of 10 mL with propan-1-ol. The mixture was vigorously shaken inorder tomix the components and to obtain a one-phase transparent so-lution. Volumes of 60 μL were used for the measurements. A mixture of1000 mg L−1 Pd+300 mg L−1 Mg+0.05% (v/v) Triton X-100 (Pd andMg as the nitrates from Merck) was used as chemical modifier for thedirect determination of As by GF AAS.

The certified reference material (CRM) NIST SRM 1634c, Trace El-ements in Fuel Oil (National Institute of Standards and Technology,Gaithersburg, MD, USA) was used for method validation. Gasoline sam-ples were obtained from Brazilian companies and from gas stations inPorto Alegre, Brazil. All sampleswere collected in clean bottles and storedin a refrigerator at 4 °C until their analysis.

2.3. Sample digestion

The gasoline samples were digested by two methods. The first onewas using a digester block with a cold finger system; 2.5 mL of con-centrated sulfuric acid (Merck) was added to 1.5 mL of sample inglass tubes, and a cold finger, filled with room-temperature water wasinserted. The tubes were slowly heated to 150 °C (under reflux) andheld at this temperature for 4 h. The cooling water was replaced fromtime to time when its temperature became too high. The mixture wasthen cooled to room temperature and 4.5 mL of concentrated nitric acidwas added and heated again to 150 °C until a colorless or slightly yellowsolution was obtained. To conclude the digestion process, 1.0 mL of 30%(v/v) hydrogen peroxide was added dropwise. The digested sampleswere transferred to 20-mL volumetric flasks and the volume com-pleted with ultrapurewater. This procedurewas performed in triplicatefor each sample.

The digestionwas also performed in a closed system using a laborato-rymicrowave oven. Before each digestion program, the PTFE vesselsweresoaked overnight in 1.4 mol L−1 HNO3 at room temperature, followed bya cleaning program recommended by the manufacturer, using concen-trated nitric acid. For the microwave-assisted digestion 2.0 mL of nitricacid was added to 0.2 mL of sample in the PTFE vessels and the mixturewas left at room temperature overnight. Then, 1 mL of 30% (v/v) hydro-genperoxidewas added, the vesselswere closed and inserted into themi-crowave oven and the digestion program given in Table 4 was executed.After cooling the samples were carefully transferred to 10-mL volumetric

Table 3Temperature program for the determination of As by GF AAS. Purge gas (argon) flowrate: 2 L min−1 in all stages, except during atomization, when the gas flow wasinterrupted.

Step Temperature/°C Ramp/°C s−1 Time/s

Drying 1 90 5 20Drying 2 120 5 20Drying 3 150 5 30Pyrolysis 1500 50 20Atomization 2400 3000 6Cleaning 2500 1000 6

flasks and the volume completedwithultrapurewater. This digestionwasalso done in triplicate for each sample.

2.4. Pre-reduction

After the digestion, a pre-reductionwas necessary in order to reduceany pentavalent arsenic to its trivalent stage before hydride generation.2 mL of 6.0 mol L−1 hydrochloric acid and 0.5 mL of pre-reducing agent(5% (w/v) potassium iodide+5% (w/v) ascorbic acid) were added to0.5 mL or 1.0 mL of the digested sample, depending on its As content.Also, 1.0 mL of 0.1% (v/v) anti-foaming solution was added and the vol-ume was completed to 5 mL with water. After 30 min at room temper-ature, arsenic was determined by HG–GF AAS. The calibration solutionswere prepared in the same way employing volumes of 0.5 mL.

2.5. Hydride generation

After the pre-reduction, samples and standard solutions (5 mL) weretransferred to the reaction vesselwhichwas connected to theHG system.The graphite furnacewas heated to 250 °C and the capillary inserted intothe graphite tube. After this the tetrahydroborate (12 mL min−1) andargon (6 L h−1) flows to the reaction vessel were initiated in order tostart the generation of arsine and its transport to the graphite tube. Thetetrahydroborate flow was stopped after 15 s, while the argon flowwas continued for the total 30 s of trapping time, to ensure that all arse-nic compounds were transferred to the graphite atomizer.

3. Results and discussion

3.1. Factorial design experiments

The conditions for generation and trapping of arsine were opti-mized using a full two-level factorial design (24) with center point.The influence of four factors on the response (integrated absorbance,Aint) has been evaluated: hydrochloric acid concentration (variedfrom 0.6 to 4.8 mol L−1); sodium tetrahydroborate concentration(varied from 0.2 to 3.0%, m/v); trapping temperature of the atomizer(varied from 100 to 400 °C); and trapping time of arsine in the atom-izer (varied from 10 to 50 s). The experimental domains for each fac-tor were defined based on literature data [16–19,23]. All the otherfactors were kept constant, as described in the Experimental section.All experiments of the factorial design were performed using a blankdigestion solution, spiked with 200 ng L−1 arsenic, using the opensystem. Table 5 shows the matrix with real and coded values for six-teen experiments from factorial design and five more experiments forcenter point.

Evaluation of the factorial design as a Pareto chart (Fig. 1) demon-strates that, for the experimental dominions established, HCl and NaBH4

concentrations showed negative effects, i.e., Aint decreased with the in-crease of HCl and NaBH4 concentrations. The trapping temperature hada positive effect on the response, which means that the Aint increasedwhen higher trapping temperatures were used. The collection timewas not significant within the range investigated (pb0.05) and didnot influence the signal. The factorial design also allowed evaluatingthe interactions between the investigated factors. There was significantinteraction between HCl and NaBH4 concentrations and betweenNaBH4

concentration and trapping temperature, as well as betweenHCl, NaBH4

Table 5Matrix of the full two-level factorial design 24 (n=1)a.

Experiment HClconcentration/mol L−1

NaBH4

concentration/% m/v

Trappingtemperature/°C

Trappingtime/s

Aint/s

1 1 (4.8) 1 (3.0) 1 (400) 1 (50) 0.04962 1 (4.8) 1 (3.0) 1 (400) −1 (10) 0.02743 1 (4.8) 1 (3.0) −1 (100) 1 (50) 0.07604 1 (4.8) 1 (3.0) −1 (100) −1 (10) 0.08615 1 (4.8) −1 (0.2) 1 (400) 1 (50) 0.15616 1 (4.8) −1 (0.2) 1 (400) −1 (10) 0.14807 1 (4.8) −1 (0.2) −1 (100) 1 (50) 0.05968 1 (4.8) −1 (0.2) −1 (100) −1 (10) 0.07669 −1 (0.6) 1 (3.0) 1 (400) 1 (50) 0.147110 −1 (0.6) 1 (3.0) 1 (400) −1 (10) 0.144911 −1 (0.6) 1 (3.0) −1 (100) 1 (50) 0.104412 −1 (0.6) 1 (3.0) −1 (100) −1 (10) 0.109413 −1 (0.6) −1 (0.2) 1 (400) 1 (50) 0.130914 −1 (0.6) −1 (0.2) 1 (400) −1 (10) 0.123815 −1 (0.6) −1 (0.2) −1 (100) 1 (50) 0.098216 −1 (0.6) −1 (0.2) −1 (100) −1 (10) 0.084317 0 (2.7) 0 (1.5) 0 (250) 0 (30) 0.154718 0 (2.7) 0 (1.5) 0 (250) 0 (30) 0.159019 0 (2.7) 0 (1.5) 0 (250) 0 (30) 0.150320 0 (2.7) 0 (1.5) 0 (250) 0 (30) 0.140521 0 (2.7) 0 (1.5) 0 (250) 0 (30) 0.1551

a Real values are in parentheses.

Table 6Analytical figures of merit for HG–GF AAS.

Parameter Result

Linear regression aqueous standard Aint=0.1682 mAs+0.0114Correlation coefficient R 0.9982Limit of detection (n=10) 6.4 ng L−1

Limit of quantification (n=10) 21 ng L−1

Characteristic mass 24 pgLinearity 0.2–2.0 ng (40–400 ng L−1)Precision 200 ng L−1 (n=10) 5.3%

Aint=integrated absorbance; mAs=m of arsenic (ng).

105EM.. Becker et al. / Spectrochimica Acta Part B 71-72 (2012) 102–106

and trapping temperature. All interaction effects were negative, in otherwords, the combination of two ormore significant factors can affect neg-atively the response. However, the trapping temperature, by itself, had apositive effect. Thismeans that the trapping temperature can increase ordecrease the response, depending on the other significant factors in-volved. Similarly, this can be observed in the other significant interac-tions, i.e., the three significant factors (NaBH4 and HCl concentrationsand trapping temperature) depend on each other to obtain the best re-sult. It is worth emphasizing that using univariate optimization these in-teractionswould not be observed and different experimental conditionsmight have been found as optimum, such as a higher trapping temper-ature [25]. The curvature effect value is high and positive, whichmeansthat there is a maximum analytical signal around the center point.Hence, the conditions of the center point have been used for furtherexperiments.

Considering these results, the following experimental conditionswere chosen: hydrochloric acid concentration: 2.7 mol L−1, sodiumtetrahydroborate concentration: 1.5%, w/v, trapping temperature ofthe atomizer: 250 °C and trapping time of arsine in the atomizer: 30 s.The data are in agreement with the results found in the literature forAs(III) and total inorganic As, using slurry sampling [13]. The optimizedconditions for total inorganic arsenic determination were 300 °C for

Fig. 1. Pareto chart of effects for the models generated on the evaluation of As det

trapping temperature and 30 s for trapping time, using a graphite tubetreated with 150 μg Ir as permanent modifier. However, the optimizedconcentrations of 3.0% (w/v) NaBH4 and 0.24 mol L−1 HCl reportedin this work were significantly different from the values found in ourexperiments.

3.2. Analytical characteristics and figures of merit

The figures of merit determined under the optimized experimentalconditions are shown in Table 6. The calibration curve was establishedusing a blank and five calibration solutions, prepared in the same wayas the samples in the concentration range from 40 to 400 ng L−1. Thecharacteristic mass was found to be 24 pg, which is very close to thevalue found for the direct GF AAS method, and also in agreement withliterature values [14], indicating an essentially quantitative trapping ofarsine in the graphite tube. The correlation coefficient was higher than0.99, indicating good linearity within the calibration range investi-gated. The limits of detection (LOD) and quantification (LOQ), whichare shown in Table 6, represent the instrumental values. Consideringthe sample volume of 75 μL, which corresponds to the 1 mL of digestedsample solution used for pre-reduction and hydride generation, theLOD and LOQ values were 0.43 and 1.4 μg L−1, respectively. Comparingthe LODwith previously obtained data [1], the value of 1.9 μg L−1, calcu-lated for 18 μL of sample is almost a factor of five higher than the oneobtained in this work. This difference is due to the lower volume of sam-ple introduced into the atomizer. The precision, expressed as the relativestandard deviation of 10 consecutive measurements of a standard solu-tion of 200 ng L−1 As was 5.3%.

Since there is no CRM available that is similar to the investigated sam-ples, the method was validated employing a residual fuel oil (NIST SRM1634c Trace Elements in Fuel Oil). The CRM was submitted to the diges-tion using a digester blockwith coldfinger and the same conditions as de-scribed for the samples. The result of 0.140±0.03 μg g−1 obtained for theCRM is in agreementwith the certified value of 0.1426±0.0064 μg g−1 ata 95% of confidence using an F test and the STATISTICA software.

ermination (Aint) with 95% of confidence; values with p>0.05 are significant.

Table 7Arsenic concentration in gasoline samples after acid digestion using HG–GF AAS and byGF AAS using detergentless micro-emulsions.

Sample As/μg L−1a

HG–GF AAS GF AAS

Cold finger Microwave oven

Gasoline A 32.5±2.5 38.3±6.9 39.0±1.1Gasoline B 13.1±0.4 11.2±1.9 11.2±0.3Gasoline C 13.6±1.4 9.2±1.6 11.6±0.3

a All values are average and the standard deviation of n=3 measurements of threeindependently digested samples.

106 EM.. Becker et al. / Spectrochimica Acta Part B 71-72 (2012) 102–106

3.3. Analytical application

The results for the determination of As in three gasoline samples usingthe two digestion procedures are shown in Table 7. The results of HG–GFAAS were compared with the reference method using GF AAS with thesamples prepared as micro-emulsions. The results for gasoline samplesA, B and C, using the proposed method with acid digestion are in agree-ment with the reference method with 95% of confidence. This was con-firmed using an F test and STATISTICA software. Considering the Asdetermination by HG–GF AAS, the results for digester block with coldfinger showed better precision than those obtained after microwave-assisted digestion. This might be due to the much smaller sample vol-ume that could be digested with the latter system, resulting in loweranalyte concentration in the solution for measurement compared tothe digester block system.

4. Conclusions

Similar to the results of otherswhoused a factorial design formethoddevelopment and optimization, it was found also in this work that it is asimple tool that allows establishing the appropriate conditions for thedetermination of arsenic using the proposed method; it also helped toevaluate the interaction between the investigated factors. Univariate op-timization does not offer this advantage. The results for the three inves-tigated methods were in agreement among themselves. The mainadvantage of the HG–GF AAS method, although it is slower and morelabor-intensive than any direct method, is the improved sensitivity andLOD, which can be decisive in the analysis of the lighter fractions of thepetroleum distillation. In addition, although not of importance for thecurrent application, where only three samples have been investigated,the heating block technique offers clear advantages over microwave-assisted digestion for the routine analysis of a large number of samples.Although the actual heating program in themicrowave oven is relativelyshort, the over-night pre-digestion with nitric acid and the additionalover-night cleaning of the PTFE vessels after the digestion, both rec-ommended by the manufacturer, make the procedure very slow unlessa large number of PTFE vessels or several microwave ovens are used. Incontrast to this, several relatively inexpensive heating blocks can be ar-ranged next to each other in order to accomplish simultaneous digestionof a large number of samples.

Acknowledgment

The authors are grateful to Conselho Nacional de DesenvolvimentoCientífico e Tecnológico (CNPq) and to Coordenação deAperfeiçoamentode Pessoal de Nível Superior (CAPES) for scholarships and financialsupport. M.G.R.V., S.L.C.F. and B.W. have research scholarships fromCNPq; W.B. has a scholarship from CAPES. The authors are also gratefulto Instituto Nacional de Ciência e Tecnologia do CNPq, INCT de Energiae Ambiente for financial support.

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