application of factorial designs and doehlert matrix in ... · 10 yqq y100 y108 11 yqy q90 y97.8 12...

Spectrochimica Acta Part B 57(2002) 1939–1950

0584-8547/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S 0 5 8 4 - 8 5 4 7Ž0 2.0 0 1 6 0 - X

Application of factorial designs and Doehlert matrix inoptimization of experimental variables associated with the

preconcentration and determination of vanadium and copper inseawater by inductively coupled plasma optical emission

spectrometry�

Sergio L.C. Ferreira*, Adriana S. Queiroz, Marcelo S. Fernandes, Hilda C. dos Santos´

Universidade Federal da Bahia, Instituto de Quımica, Grupo de Pesquisa em Quımica Analıtica,´ ´ ´Campus Universitario de Ondina, Salvador, Bahia, Brazil 40170-290´

Received 12 March 2002; accepted 30 July 2002

Abstract

In the present paper a procedure for preconcentration and determination of vanadium and copper in seawater usinginductively coupled plasma optical emission spectrometry(ICP OES) is proposed, which is based on solid-phaseextraction of vanadium(IV), vanadium(V) and copper(II) ions as 1-(2-pyridylazo)-2-naphthol(PAN) complexesby active carbon. The optimization process was carried out using two-level full factorials and Doehlert matrix designs.Four variables(PAN mass, pH, active carbon mass and shaking time) were regarded as factors in the optimization.Results of the two-level full factorial design 2 with 16 runs for vanadium extraction, based on the variance analysis4

(ANOVA), demonstrated that the factors pH and active carbon mass, besides the interaction(pH=active carbonmass), are statistically significant. For copper, the ANOVA revealed that the factors PAN mass, pH and active carbonmass and the interactions(PAN mass=pH) and (pH=active carbon mass) are statistically significant. Doehlertdesigns were applied in order to determine the optimum conditions for extraction. The procedure proposed allowedthe determination of vanadium and copper with detection limits(3syS) of 73 and 94 ng l , respectively. They1

precision, calculated as relative standard deviation(R.S.D.), was 1.22 and 1.37% for 12.50mg l of vanadium andy1

copper, respectively. The preconcentration factor was 80. The recovery achieved for determination of vanadium andcopper in the presence of several cations demonstrated that this procedure improved the selectivity required forseawater analysis. The procedure was applied to the determination of vanadium and copper in seawater samplescollected in Salvador City, Brazil. Results showed good agreement with other data reported in the literature.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Vanadium; Copper; Seawater; Doehlert matrix; Inductively coupled plasma optical emission spectrometry(ICP OES)

� This paper was presented at the 7th Rio Symposium on Atomic Spectrometry, held in Florianopolis, Brazil, April 2002 and is´published in the Special Issue ofSpectrochimica Acta Part B, dedicated to that conference.

*Corresponding author. Fax:q55-71-2355166.E-mail address: [email protected](S.L. Ferreira).

1940 S.L. Ferreira et al. / Spectrochimica Acta Part B 57 (2002) 1939–1950

1. Introduction

Procedures for optimization of factors by mul-tivariate techniquesw1,2x have been encouraged,as they are faster, more economical and effective,and allow more than one variable to be optimizedsimultaneously. This optimization can be accom-plished using experimental designsw3x, which canbe of first or second order. The second-orderdesigns have advantages, because they not onlydetermine the influence of the variables to beoptimized on the response, but also enable theresponse function to be obtained and optimized.

The Doehlert matrix w4x is an optimizationsystem, defined as a second-order design. For aprocess involving two variables(A and B) and anexperimental response(Y), the model is describedas:

2 2YsaqbAqcBqdA qeB qfAB (1)

where Y is the experimental response,A and Brepresent the variables to be optimized,a is anindependent term,b and c are coefficients of thelinear terms, d and e are coefficients of thequadratic terms andf is the coefficient of theinteraction term. The identification of the criticalpoints (maximum, minimum or saddle point) iscarried out with application of the Lagrange cri-terion in the equation obtained during the optimi-zation process using experimental data. Inanalytical chemistry, the Doehlert matrixw5–16xhas been widely used in several situations, suchas: development of an on-line procedure for pre-concentration and determination of zinc by induc-tively coupled plasma optical emissionspectrometry (ICP OES) w5x; optimization ofexperimental variables in solid-phase spectropho-tometry w6,7x; optimization process for simultane-ous solvent extraction of several metalsw8,9x;methodology for spectrophotometric determinationw10x; optimization for spectrofluorimetric determi-nation w11x; methodology for separation processusing micellar electrokinetic capillary chromatog-raphy w12x; extraction process using microwavesw13x; optimization for voltammetry determinationw14x; and investigation of matrix effects in ICPOESw15x. In our laboratory, Doehlert designs wereused for the optimization of variables for the

preconcentration and determination of molybde-num in seawater by ICP OESw16x.

Vanadium is an essential trace element for plantsand animals, which stimulates the synthesis ofchlorophyll and promotes the growth of younganimals. Copper is also an essential trace elementfor humans, higher mammals and numerous plants.The blood of marine mollusks and crabs containsthe Cu complex hemocyanin rather than the Fecomplex hemoglobin, which is taken up from theseawater and acts as a respiratory catalystw17x.Thus, these metals are frequently determined inseawater. However, their determination by ICPOES w18x is difficult because of its relatively lowsensitivity and the high saline concentration ofseawater. The concentration ranges for vanadiumand copper in seawater are 2.0–3.0 and 0.2–4.0mg l , respectivelyw19x.y1

The reagent 1-(2-pyridylazo)-2-naphthol(PAN)forms complexes with several metal ions, includingvanadium(IV), vanadium(V) and copper(II). Inpreconcentration procedures, PAN was repeatedlyused for different analytical separation strategies,such as: solid phase extraction using an activecarbon columnw20x; silica gelw21,22x; naphthalenew23x; Amberlite XAD-2000 w24x; alumina w25x;Amberlite XAD-2 w26,27x; Amberlite XAD-4 w28x;and chloromethylated polystyrenew29x, as well asby cloud-point extractionw30,31x.

In this work, a procedure for the preconcentra-tion and determination of vanadium and copper inseawater using ICP OES is proposed. Factorialdesigns and a Doehlert matrix were used foroptimization of the experimental variables. It isbased on the solid-phase extraction of vanadiumand copper ions as PAN complexes on activecarbon.

2. Experimental

2.1. Instrumentation

A Research Laboratories model 3410 minitorchsequential inductively coupled plasma opticalemission spectrometer(Dearborn, MI, USA) cou-pled to an IBM PC-AT computer was used. Emis-sion intensities were measured under the conditionsshown in Table 1. The calibration curves(0–2.0

1941S.L. Ferreira et al. / Spectrochimica Acta Part B 57 (2002) 1939–1950

Table 1Operating parameters for the inductively coupled plasma opti-cal emission spectrometer

Incident output power 650 WReflected power -5 WNebulizer Glass, MeinhardPlasma gas flow rate 7.5 l miny1

Auxiliary gas flow rate 0.81 l miny1

Aerosol carrier gas flow 0.81 ml miny1

Solution uptake rate 2.5 l miny1

Signal integration time 5 sIntegration for determination 3Emission line(nm)

V(II) 309.311Cu(II) 324.754

mg ml ) for vanadium and copper were plottedy1

with solutions prepared from a 100.0mg mly1

stock solution. A Digimed pH meter(Santo Ama-ro, Brazil) was used to measure pH values. AnEtica mechanical shaker(Sao Paulo, Brazil) at˜100 counts min was also used.y1

2.2. Reagents

All reagents were of analytical grade unlessotherwise stated. Ultrapure water was obtainedfrom an EASYpure RF set-up(Barnstedt,Dubuque, IA, USA). Nitric and hydrochloric acidwere of Suprapur quality(Merck). Laboratoryglassware was kept overnight in 10% nitric acidsolution, rinsed with deionized water before use,and dried in a dust-free environment.

Vanadium solution(10.0 mg ml ) was pre-y1

pared by diluting a 1000mg ml vanadiumy1

solution(Merck) with 1% (vyv) hydrochloric acid.Copper solution(10.0 mg ml ) was preparedy1

by diluting a 1000 mg ml copper solutiony1

(Merck) with 1% (vyv) hydrochloric acid.PAN solution 0.25%(wyv) was prepared by

dissolving 1.25 g of 1-(2-pyridylazo)-2-naphthol(Aldrich) in 500 ml of ethanol(Merck).

Acetate buffer(pH 3.75) was prepared by mix-ing 14.76 g of sodium acetate with 104.1 ml ofconcentrated acetic acid and diluting it to 1 l withultrapure water.

Acetate buffer(pH 5.75) was prepared by mix-ing 149.24 g of sodium acetate with 10.3 ml of

concentrated acetic acid and diluting it to 1 l withultrapure water.

Synthetic seawater was prepared with a com-position w5x of: 27.9 mg ml NaCl; 1.4 g ly1 y1

KCl; 2.8 g l MgCl ; 0.5 g l NaBr; and 2.0 gy1 y12

l MgSO .y14

2.3. Surface seawater samples

Seawater samples were collected in polypropyl-ene bottles, previously cleaned by soaking in 2mol l nitric acid. Samples were filtered throughy1

a membrane of 0.45-mm pore size, acidified to 1% (vyv) with concentrated nitric acid, and storedfrozen until they were analyzed. Sampling stationswere beaches on the Atlantic Ocean in SalvadorCity, Brazil.

2.4. General procedure

A sample volume of 800 ml, containing vana-dium and copper ions, was transferred into astoppered flask; 10 ml of acetate buffer solutionand a volume of PAN solution(0.25%) wereadded. After fast shaking, a mass of active carbonwas added and the mixture was shaken again fora certain time. The system was then filtered undervacuum through a 2.5-cm-diameter cellulosemembrane. The residue of active carbon was trans-ferred to an Erlenmeyer flask and digested at 1208C with 4.00 ml of concentrated nitric acid solutionuntil dryness. The residue was treated with 10.0ml of 3 mol l nitric acid, and filtered through ay1

paper filter (Whatman no 40). The filtrate wascollected and used for determination of vanadiumand copper by ICP OES using the emission linesV(II) 309.311 and Cu(II) 324.754 nm.

2.5. Procedure used in the factorial design

The general procedure was applied using thevariable experimental conditions for PAN mass,pH, active carbon mass and shaking time shownin Table 2. Maximum and minimum levels of eachfactor were chosen according to data from previousexperiments.


Table 2Factors and levels used in the factorial design for extraction ofvanadium and copper

Variable Low (y) High (q)

PAN mass(A) (mg) 1250 12500pH (B) 3.75 5.75Active carbon mass(C) (mg) 50 200Shaking time(D) (min) 10 50

Table 3Design matrix and the results of vanadium extraction

No PAN pH Active Shaking Vanadiummass carbon time extraction

mass (%)

1 q q q q 100y1102 q q q y 98y1023 q q y q 90y984 q q y y 94y88.45 q y q q 98y956 q y q y 97y94.47 q y y q 62y70.88 q y y y 53y55.69 y q q q 104y104

10 y q q y 100y10811 y q y q 90y97.812 y q y y 91y9513 y y q q 90y9214 y y q y 88y9415 y y y q 72y76.616 y y y y 55y65

2.6. Procedures used in the Doehlert matrix

The general procedure was applied and theexperimental conditions for pH, PAN mass, activecarbon mass and shaking time were established inagreement with requirement by the optimizationprocess.

2.7. Optimization strategy

The optimization process was carried out usingtwo-level full factorial and Doehlert matrixdesigns. All experiments were carried out in dupli-cate, using 800 ml of synthetic seawater containing10.0 mg of vanadium and copper. Four variables(PAN mass, pH, active carbon mass and shakingtime) were regarded as factors, and the experimen-tal data were processed using theSTATISTICAL

program.

2.8. Lagrange criterion

The Lagrange criterionw5,16x was used fordetermination of the critical point of the second-order equation and is based on the calculation ofthe Hessian determination ofY:

2 2 2 2H(A,B)s(d YydA )(d YydB )2 2y(d YydAdB) (2)

The critical point (a ,b ) is maximum ifo o

H(a ,b ))0 and d YydA (a ,b )-0, and it is2 2o o o o

minimum if H(a ,b ))0 and d YydA (a ,b ))2 2o o o o

0. A saddle point exists ifH(a ,b )-0. If theo o

response surface has a maximum, this point iscalculated by solving the equation systemsd Yy2

dA s0 andd YydB s0.2 2 2

3. Results and discussion

3.1. Factorial design

The procedure proposed is based on the solidphase extraction of vanadium(IV), vanadium(V)and copper(II) ions as PAN complexes usingactive carbon. The following factors were evalu-ated: PAN mass, pH, active carbon mass andshaking time. A two-level full factorial of 2 with4

16 runs was carried out in order to determine themain factors of the extraction process. Table 2 listthe maximum and minimum values given to eachfactor and Tables 3 and 4 show the experimentaldesign matrix and the results derived from eachrun in duplicate for vanadium and copper, respec-tively. The significance of the effects was checkedby analysis of the variance(ANOVA) and usingP-value significance levels.

The ANOVA results for vanadium produced thePareto chartw32,33x of main effects shown in Fig.1. Bar lengths are proportional to the absolutevalue of the estimated effects, which helps incomparing the relative importance of effects. Theinterpretation of this chart demonstrates that thefactors pH and active carbon mass are highlysignificant. An increase in pH and in active carbon


Table 4Design matrix and the results of copper extraction

No PAN pH Active Shaking Coppermass carbon time extraction

mass (%)

1 q q q q 96.7y1022 q q q y 107y1073 q q y q 97.7y1014 q q y y 92y945 q y q q 97.8y1026 q y q y 99.2y1047 q y y q 96.3y998 q y y y 97y1019 y q q q 97y95.4

10 y q q y 92.1y90.211 y q y q 80y8312 y q y y 80y7913 y y q q 75.6y7314 y y q y 70y69.715 y y y q 71y72.516 y y y y 71.5y68.9

Fig. 1. Pareto chart of standardized effects for variables in the vanadium extraction.

mass increases the extraction efficiency. The shak-ing time is a less significant factor. The interaction(pH=active carbon mass) is also statistically sig-nificant. The factor PAN mass has an insignificanteffect.

The Pareto chart in Fig. 2 demonstrates thatPAN mass provides a more significant effect forcopper extraction. An increase in this complexingagent leads to higher extraction efficiency. Thefactors pH and active carbon mass also producesignificant effects. The interactions(PANmass=pH) and(pH=active carbon mass) are alsostatistically significant. The shaking time in therange 10–50 min has no significant effect on theextraction.

3.2. Final optimization by Doehlert design

The factorial design demonstrated that the vari-ables at the levels studied need final optimization,for which the Doehlert designs were used. Firstly,designs were developed for the optimization of pH


Table 5Doehlert matrix for design 1

Experiment pH PAN mass Vanadium recovery(%)(mg)

Experimental Expected

1 3.75 1250 90 902 3.75 12500 90 853 4.75 7500 105 1024 8.00 7500 85 825 2.50 7500 70 666 5.75 12500 100 947 5.75 1250 110 109

R s0.9781.2

Fig. 2. Pareto chart of standardized effects for variables in the copper extraction.

and PAN mass and then designs for the optimiza-tion of active carbon mass and shaking time.

3.2.1. Design 1—conditions of pH and PAN massfor vanadium extraction

In this design the optimized variables were pHand PAN mass, setting the active carbon mass andshaking time at 200 mg and 50 min, respectively.The seven experiments required by the Doehlertdesign are described in Table 5. The pH and PANmass varied from 2.5 to 8.0 and from 1250 to12 500mg, respectively.

The data obtained were used in the Doehlertmatrix and Eq. (3) illustrates the relationshipbetween pH, PAN mass and vanadium extraction(%):

% V extractionsy36.202q48.202 pH2q0.003m y3.998 pHPAN

y4y4.52=10 m pHPANy7 2y1.277=10 m (3)PAN

The corresponding surface response is shown inFig. 3.

Application of the Lagrange criterion in thisequation demonstrates that:

y6H(a ,b )s1.838=10o o

2 2d YydpH sy7.996

These results indicated that there was a maximumon the surface response, which was calculated bythe following equations:

dV extractionydpHs0s48.202y7.996 pH

y4y4.52=10 m (3a)PAN


Fig. 3. Surface response for vanadium extraction(%). Vana-dium concentration, 12.50mg l ; synthetic seawater volume,y1

800 ml; active carbon mass, 200 mg; shaking time, 50 min;pH 2.5–8.0; PAN mass, 1250–12 500mg.


Experiment pH PAN mass Copper recovery(%)(mg)

Experimental Expected

1 3.75 1250 52.6 53.32 3.75 12500 104 107.13 4.75 7500 110 111.84 8.00 7500 108 109.85 2.50 7500 84 85.16 5.75 12500 109 111.17 5.75 1250 79 78.6

R s0.9986.2

Fig. 4. Surface response for copper extraction(%). Copperconcentration, 12.50mg l ; synthetic seawater volume, 800y1

ml; active carbon mass, 200 mg; shaking time, 50 min; pH2.5–8.0; PAN mass, 1250–12 500mg.

dV extractionydm s0PANy4s0.003y4.52=10 pHy2.554

y7=10 m (3b)PAN

The maximum values are pH 5.96 andm sPAN

1197mg.

3.2.2. Design 2—conditions of pH and PAN massfor copper extraction

In this design, the experimental conditions(pH,PAN mass, active carbon mass and shaking time)are in agreement with design 1. The seven exper-iments required by the Doehlert design aredescribed in Table 6.

The data obtained were used in the Doehlertmatrix and Eq. (4) illustrates the relationshipbetween pH, PAN mass and copper extraction(%):

% Cu extractionsy63.968q35.595 pH2q0.017m y2.276 pHPAN

y9.605y4=10 m pH–6.264PANy7 2=10 m (4)PAN



y6H(a ,b )s4.781=10o o

2 2d YydpH sy4.552


dCu extractionydpHs0s35.595y4.552 pH

y9.605y4=10 m (4a)PAN



Experiment Active Time Vanadium recovery(%)carbon (min)

Experimental Expectedmass(mg)

1 50 10 91 922 200 10 99.5 1003 125 30 101 1014 125 60 99 1005 125 5 99.5 98.56 200 50 100 1007 50 50 96 96CP 157 33 – 102.17RC 150 30 – 102.10

CP, critical point; RC, recommended conditions for this pro-cedure.R s0.9624.2

Fig. 5. Surface response for vanadium extraction(%). Vana-dium concentration, 12.50mg l ; synthetic seawater volume,y1

800 ml; pH 5.75; PAN mass, 12 500mg; active carbon mass,50–200 mg; shaking time, 5–60 min.

dCu extractionydm s0PANy4s0.017y9.605=10 pHy1.253

y6=10 m (4b)PAN

The maximum values are pH 5.91 andm sPAN

9035mg.

3.2.3. Design 3—conditions of active carbon massand shaking time for vanadium extraction

In this design, pH and PAN mass were fixed,and active carbon mass and shaking time werevaried. Considering the results obtained in designs1 and 2, pH was fixed at 5.75(maximum pHallowed for the acetate buffer) and PAN mass at12 500 mg. The seven experiments required forthe new Doehlert design are described in Table 7.Time and active carbon mass varied from 5 to 60min and from 50 to 200 mg, respectively.

The data obtained were used in the Doehlertmatrix and Eq. (5) illustrates the relationshipbetween active carbon mass, shaking time andvanadium extraction(%):

% V extractions79.741q0.219mAC

q0.316ty6.178y4 2=10 (m ) y7.5ACy4 2=10 m ty0.003t (5)AC



y6H(a ,b )s6.851=10o o

2 2 y3d Yydm sy1.236=10AC


dV extractionydm s0AC

s0.219y1.236y3=10 m y7.5ACy4=10 t (5a)

dV extractionydts0s0.316y7.5y4=10 mAC

y0.006t (5b)

The maximum values arem s157 mg andtsAC

33 min.

3.2.4. Design 4—conditions of active carbon massand shaking time for copper extraction

In this design, the experimental conditions(pH,PAN mass, active carbon mass and shaking time)are in agreement with design 3. The seven exper-



Experiment Active Time Copper recovery(%)carbon (min)

Experimental Expectedmass(mg)

1 50 10 94 942 200 10 99 993 125 30 102 1024 125 60 100 995 125 5 99 996 200 50 99 987 50 50 98 97CP 143 31 – 101.84RC 150 30 – 101.81

CP, critical point; RC, recommended conditions for this pro-cedure.R s0.9522.2

Fig. 6. Surface response for copper extraction(%). Copperconcentration, 12.50mg l ; synthetic seawater volume, 800y1

ml; pH 5.75; PAN mass, 12 500mg; active carbon mass, 50–200 mg; shaking time, 5–60 min.

iments required by the Doehlert design aredescribed in Table 8.

The data obtained were used in the Doehlertmatrix and Eq. (6) illustrates the relationshipbetween active carbon mass, shaking time andcopper extraction(%):

% Cu extractions83.826q0.18m q0.34tACy4 2y5.595=10 (m )ACy4y6.667=10 m ty4AC

y3 2=10 t (6)



y6H(a ,b )s8.508=10o o

2 2 y3d Yydm sy1.119=10AC


dCu extractionydm s0ACy3s0.18y1.119=10 m y6.667AC

y4=10 t (6a)

dCu extractionydts0s0.34y6.667

y4=10 m y8ACy3=10 t (6b)

The maximum values arem s143 mg andtsAC

31 min.

3.3. Procedure for determination of vanadium andcopper in seawater

Considering the results obtained in the designs,the procedure for the determination of vanadiumand copper in seawater recommends the use of thegeneral procedure, described in the experimentalpart, using a PAN mass of 12 500mg, pH 5.75,active carbon mass of 150 mg and shaking timeof 30 min.

3.4. Analytical features

The precision calculated as the relative standarddeviation(RSD) for a series of 11 replicates was1.22 and 1.37% for 12.50mg l of vanadium andy1

copper, respectively, in synthetic seawater solution.The preconcentration factor was 80, considering

the sample volume of seawater(800 ml) and asolution volume for analysis of 10.0 ml.

The sensitivity w34x was studied by means ofthe limits of detection(LOD) and quantification(LOQ), defined as LODs(3s)yS and LOQs


Table 9Determination of vanadium in unspiked and spiked seawatersamples(ns3)

Seawater sample Vanadium(mg l )y1 Recovery

Added Founda(%)

Synthetic 0 -LOD –12.50 12.26"0.10 98.112.50b 11.55"0.19 92.4

Jardim Alah 0 1.73"0.06 –12.50 14.16"0.17 99.4

Stella Mares 0 3.45"0.08 –12.50 15.38"0.14 95.4

Porto da Barra 0 3.13"0.09 –12.50 14.68"0.17 92

Corsario´ 0 3.05"0.08 –12.50 15.33"0.16 98.2

Ondina 0 1.77"0.04 –12.50 14.90"0.16 105

Sample volume, 800 ml.At 95% confidence level.a

Vanadium in the presence of several metal ions.b

Table 10Determination of copper in unspiked and spiked seawater sam-ples(ns3)

Seawater sample Copper(mg l )y1 Recovery

Added Founda(%)

Synthetic 0 -LOD –12.50 12.38"0.11 99.012.50b 11.78"0.18 94.2

Jardim Alah 0 -LOD –12.50 12.65"0.18 101.2

Stella Mares 0 0.94"0.02 –12.50 13.21"0.14 98

Porto da Barra 0 0.38"0.11 –12.50 13.51"0.17 105

Corsario´ 0 0.38"0.04 –12.50 12.56"0.06 97.4

Ondina 0 0.48"0.10 –12.50 12.25"0.16 94

Sample volume, 800 ml.At 95% confidence level.a

Copper in the presence of several metal ions.b

(10s)yS, where S is the slope of the analyticalcurve and s is the standard deviation of 10consecutive measurements of the blank. For vana-dium, LOD and LOQ were 73 and 243 ng l ,y1

and for copper, 94 and 313 ng l , respectively.y1

3.5. Effect of other metal ions on the procedureproposed

In order to check the effect of other metal ionson the method proposed, vanadium and copper(10.00 mg) and other metal ions(all 10.00 mg)were added to 800 ml of synthetic seawater andthe procedure was applied. The values measuredwere 9.24"0.16 mg (ns3) for vanadium and9.45"0.15 mg (ns3) for copper, with recoveryof 92.4 and 94.5%, respectively. This experimentwas carried out using a multi-elemental ICP OESsolution Quality Control Standards(QCS-19),which had arsenic, antimony, beryllium, cadmium,calcium, chromium, cobalt, iron, molybdenum,nickel, thallium, titanium, zinc, lead, magnesium,manganese and selenium at a concentration of 100mg l each.y1

3.6. Accuracy

In order to evaluate the accuracy of the proce-dure developed, vanadium and copper were deter-

mined in the CASS-4 Nearshore SeawaterReference Material for Trace Metals(NationalResearch Council Canada). For vanadium, theresult achieved was 1.16"0.18 mg l comparedy1

to the certified value of 1.18"0.16 mg l . Fory1

copper, the result achieved was 0.602"0.064 mgl compared to the certified value ofy1

0.592"0.055 mg l . This test was carried outy1

using 150 ml of solution.

3.7. Analytical application

The optimized methodology was applied to theanalysis of seawater samples collected during thewinter of 2001 from several beaches in SalvadorCity, Brazil. The results are shown in Tables 9 and10, together with recovery data for added vanadi-um and copper. The data found in this study wereconsistent with those reported in literaturew19x,including former data for the same city using adifferent procedure for copper determinationw35x.The recovery of vanadium and copper added tothe samples before application of the methodproposed demonstrates its efficiency.

4. Conclusions

Application of factorial designs and a Doehlertmatrix allowed the optimization of a procedure for


the determination of vanadium and copper by ICPOES, based on solid phase extraction, to be moreefficient using a smaller number of experiments.The data obtained for vanadium and copper fromseawater samples collected in beaches in SalvadorCity, Brazil were consistent with those reported inthe literature.

Acknowledgments

The authors acknowledge grants from ConselhoNacional de Desenvolvimento Cientıfico e Tecnol-´ogico (CNPq, CTPETRO) and Coordenacao de´ ˜¸Aperfeicoamento de Pessoal de Nıvel Superior´¸(CAPES).

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