Talanta 49 (1999) 135–142

Direct determination of bismuth, indium and lead in seawater by Zeeman ETAAS using molybdenum containing

chemical modifiers

Orhan Acar a, A. Rehber Turker b,*, Ziya Kılıc c

a TAEA, Ankara Nukleer Arastırma 6e Egitim Merkezi, 06983, Ankara, Turkeyb Gazi U8 ni6ersitesi, Fen Edebiyat Fakultesi, 06500, Ankara, Turkeyc Gazi U8 ni6ersitesi, Gazi Egitim Fakultesi, 06500, Ankara, Turkey

Received 9 July 1998; received in revised form 20 October 1998; accepted 12 November 1998

Abstract

Direct determination of Bi, In and Pb in sea water samples has been carried out by ETAAS with Zeemanbackground correction using molybdenum containing chemical modifiers and tartaric acid as a reducing agent.Maximum pyrolysis temperatures and the effect of mass ratios of the mixed modifier components on analytes havebeen investigated. Mo+Pd+TA or Mo+Pt+TA mixture was found to be powerful for the determination of 50 mgl−1 of Bi, In and Pb spiked into synthetic and real sea waters. The accuracy and precision of the determination werethereby enhanced. The recoveries of analytes spiked were 94–103% with Mo+Pd+TA or Mo+Pt+TA and theyare only 49–61% without modifier. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: ETAAS–Zeeman background correction; Bismuth; Indium; Lead; Sea water; Chemical modifiers

1. Introduction

In recent years, determination of volatile ele-ments such as bismuth, indium and lead in seawater samples has become increasingly importantin order to monitor the pollution of this environ-ment. Owing to its high sensitivity and specificity,electrothermal atomic absorption spectrometry(ETAAS) with Zeeman background correction is

one of the most promising methods for the directdetermination of trace elements in water samples[1–3]. However, the high salt contents of seawater matrix and very low concentrations ofvolatile elements cause considerable difficulty inthe direct analysis. During the determination ofvolatile elements by ETAAS, chemical interfer-ences and significant background absorbance of-ten occur in graphite furnace due tocovolatilization of the analyte with the matrix[2,3].

To overcome most of the matrix interferencesand to increase the accuracy of determination in

* Corresponding author. Tel.: +90-312-212-2900; fax: +90-312-212-2279.

E-mail address: rturker@quark.fef.gazi.edu.tr (A.R. Turker)

0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.

PII: S0039 -9140 (98 )00358 -0

O. Acar et al. / Talanta 49 (1999) 135–142136

ETAAS, the application of chemical modificationfor the stabilization of volatile elements duringpyrolysis stage is one of the methods that has metwith reasonable success for analyte elements [1].For this purpose many metal salts, their mixturesand some organic substances have been recom-mended and used as chemical modifiers [2–11].The aim of matrix modification is to permit thepyrolysis temperature to be high enough to re-move the balk of the interfering substances duringthe pyrolysis stage without loss of analyte beforeatomization stage. In our previous works [10,11],the effects of tungsten, palladium and molybde-num containing chemical modifiers as well astartaric acid (TA) for the thermal stabilization ofBi, In and Pb have been investigated systemati-cally and applied to the determination of Bi andPb in geological samples. Several matrix modifiersand organic substances have been recommendedand used to determine lead in water samples [7,8].

The aim of presented work is to determine Bi,In and Pb in synthetic and real sea water samplesby using molybdenum containing chemicalmodifiers. Tartaric acid (TA) was added as chem-ical reductant to the sample solutions togetherwith the mixed modifiers to provide higher ther-mal pre-treatment temperatures of analytes, tomodify the form of modifier and help to reducechemical interferences. Tartaric acid produces re-ductants such as C, CO and H2 by thermal de-composition, similar to ascorbic acid or oxalicacid used by Byrne et al. [12]. Addition of tartaricacid should assure an early reduction of modifiersand analytes to highly dispersed metallic forms inthe temperature programme and elimination ofchlorides as HCl (g). Efficient reductants pro-duced from TA at the surface of the tube mightreact with the chlorides in the aqueous matrices toform HCl (g) which is removed at low tempera-tures [13], thus preventing the loss of analyte aschloride. Therefore, Mo+Pd+TA and Mo+Pt+TA mixed modifiers that provided the higherpre-treatment temperatures for the analytes [11]were applied to the direct determination of spikedBi, In and Pb in synthetic and real sea watersamples by ETAAS with Zeeman backgroundcorrection system.

2. Experimental

2.1. Instrumentation

A Hitachi Model 180/80 flame and graphitefurnace atomic absorption spectrometer equippedwith a Zeeman effect background corrector, au-tosampler (P/N-170/126) and an automatic dataprocessor was used for all measurements. Detailsabout the AAS equipment, thermal stabilizingstudies and optimized graphite furnace tempera-ture programme for wall atomization is given inprevious works [10,11]. The operating parameterswere set as recommended by the manufacturerexcept that a spectral band pass of 1.3 nm andalternate wavelength was used for Bi (306.8 nm).The light sources are single-element hollowcathode lamps. A Varian model 9176 recorderwas connected to AAS to obtain atomizationprofiles of analytes in a 20 mV/FS spans. Hitachipyrolytically coated graphite tubes (P/N-180/7444) were used for atomization throughout. Allabsorbance values were obtained by using inte-grated mode. Argon served as a carrier gas in 300ml min−1 flow rate.

2.2. Reagents

All solutions were prepared by dissolving ana-lytical grade reagents in deionized water distilledin an Elgastat type C114 distillation unit. Stockstandard solutions (2000 mg ml−1) of Pd and Ptwere prepared according to [9]. 0.4% (m/v) of Mo(VI) was prepared from H2MoO4 (Merck) dis-solved in 1% (v/v) ammonia solution. All modifiersolutions were diluted as required. 4% (m/v) tar-taric acid (TA) solution was prepared daily. 1000mg ml−1 of Bi, In and Pb standard solutions(BDH chemicals) were used. Working standardsolutions were freshly prepared by successive dilu-tion to the desired concentrations using 0.2%HNO3.

2.3. Procedure

Synthetic sea water was prepared according toCantle [14]. 2.67 g of NaCl, 0.54 g of MgCl2, 0.11g of CaCl2 and 0.08 g of KCl salts were dissolved

O. Acar et al. / Talanta 49 (1999) 135–142 137

in a teflon beaker with deionized water. 1 ml ofeach Bi, In and Pb standard solutions (20 mgml−1) were spiked into it and this solution wastransferred into a 100 ml glass calibrated flask.The Teflon beaker was washed two times with0.2% (v/v) nitric acid. 600 ml of conc. nitric acidwas also added to final solution to avoid theabsorption of analytes onto glass walls and di-luted to the mark with deionized water. Sea wa-ter sample was collected from the surface waterof the Marmara sea coast in Turkiye in a 2 Iployethylene bottle. A sample portion was trans-ferred into 100 ml volumetric flasks and imme-diately acidified with 1 ml of conc. nitric acid. 1ml of each Bi, In and Pb standard solutions (20mg ml−1) were spiked into these flasks becauseof natural contents of the analytes are below thedetection limits. All of the flasks were filled withsample (sea water) to the mark.

Synthetic or Marmara sea water sample con-taining 200 mg l−1 analytes was diluted by afactor 4 (0.5 ml of sample +1.5 ml of deion-ized water) in order to decrease interferences inthe absence of modifier. In the presence ofmodifier, 0.5 ml of sample solution and 0.5 mlof water were added to 1 ml of modifier solu-tion (20 000 mg ml−1 of TA, 2000 mg ml−1 ofMo, 400 mg ml−1 of Pd or Pt, 2000 mg ml−1 ofMo+20 000 mg ml−1 of TA, 400 mg ml−1

of Pd or Pt). 0.5 ml of sample solution and0.5 ml of TA (4% (m/v)) stock solution insteadof water were added to 1 ml of modifiersolution (2000 mg ml−1 of Mo+400 mg ml−1

of Pd or Pt) for the triple modifier mixturessuch as Mo+Pd+TA. 20 ml of the sample so-lution (analyte concentration 50 mg l−1) pre-pared in the presence or absence of single ormixed modifiers were injected into the graphitetube.

The optimized graphite furnace temperatureprogramme for wall atomization is given in Ref.[11]. The optimum modifier mass and massratio of the mixed modifier components werefound to be 20 mg of Mo, 4 mg of Pd and Pt,and 20/4 (mg/mg) of Mo/Pd and Mo/Pt. 200 mgof TA was used together with the mixedmodifiers [11].

3. Results and discussion

3.1. Stabilizing effects of modifiers on analytes

In order to remove the high contents of salts insea water, higher pyrolysis temperatures foranalytes may be preferable. A chemical modifierhas been recommended for a long time to increasemaximum permissible temperature and to mini-mize interferences in the determination of volatileelements in ETAAS [2]. Maximum pyrolysistemperatures (Tmax) of the analytes studied andthe optimum mass ratios of the mixed modifiercomponents were found in our previous work [11].Obtained pyrolysis temperatures are approxi-mately in agreement with Tsalev’s results [13,15].When Mo+Pd+TA was used, higher pyrolysistemperatures were obtained than the othermodifier mixtures studied by Havezov et. al. [16].

In this study, thermal pre-treatment curves foranalytes with TA, Pd+TA and Pt+TA modifierswere also studied and lower pyrolysis tempera-tures were obtained. The addition of TA to Pdand Pt did not increase the pyrolysis temperature,but it increased the formation of analyte atoms atlower temperatures [12]. The effect of tartaric acidon atomic absorption signal of analyte can beexplained primarily by reaction that occurbetween gaseous analyte-containing moleculessuch as PbO (g) and the reducing gases producedby pyrolysis of tartaric acid during the atomi-zation temperature ramp [12].

When the Mo+Pd+TA or Mo+Pt+TA wasadded to the solution, the pyrolysis temperaturescould be increased up to 1350–1400°C for Bi andIn and 1250–1300°C for Pb to remove much ofsample matrix without the risk of analyte loss [11].The mechanism of increased thermal stability canbe explained from the reaction between analyteand modifier elements. Shan and Wang fromX-ray photoelectron spectra, suggested theformation of Pb–Pd and Bi–Pd intermetallicbonds on graphite furnace [17].

3.2. Effect of modifiers on the atomizationprofiles of analytes

One advantage of chemical modification is that

O. Acar et al. / Talanta 49 (1999) 135–142138

the atomization signals become fairly symmetricaland shifted to higher pre-treatment temperaturesand appearance times [18]. Welz et al. [4] demon-strated this behaviour by comparing the shapes ofatomization signals of various elements in boththe absence and presence of a Pd–Mg modifier.In order to demonstrate how the Mo, Mo+Pd+TA and other Mo-based modifiers affect the at-omization and background profiles of analytes, acomparative study was conducted. Fig. 1 showsthat the analyte and background atomization

profiles of sea water spiked with 50 mg l−1 of leadas an example. The atomization and backgroundabsorption profiles of spiked Pb for sea water andfor aqueous Pb standard solution obtained withand without modifiers were compared. As can beseen in Fig. 1, although similar symmetrical atom-ization signal shapes were obtained for Pb in seawater, both in the absence and presence of chemi-cal modifiers, the atomic signals appeared at anearlier time in the absence of a modifier mixturethan in its presence. The appearance times of thepeaks were identical for sea water and foraqueous standard. However, the maximum peaktimes of Pb in sample were slightly later thanthose in the aqueous Pb standard. When Mo+Pd+TA mixture has been used, the appearancetime of atomization signal for Pb in sample and inaqueous standard was shifted to a later time whileincreasing pyrolysis temperature and no reductionin atomic absorption signal was observed up to amaximum pyrolysis temperature above 1200°C. Itmay be due to early reduction of analyte andmodifiers to reactive metallic forms. Thermal sta-bility of these compounds is higher and the corre-sponding inter-metallic compounds (or alloysbetween them) [17] are formed when Mo+Pd+TA is used. With no modifier was used, smallanalyte signal and higher background absorbancewere obtained for Pb in sample even when thesolution was diluted by a factor of 1+3. WhenMo+Pd+TA was used, it was observed thatanalyte signal increased while the backgroundabsorption decreased. As can be seen in Fig. 1,higher signal/noise ratios of Pb was obtained inthe presence of Mo+Pd+TA than this obtainedin the absence of modifier. The reason for this isattributed to the behavior of mixed chemicalmodifier. Tartaric acid may reduce the modifiersand analytes to their free reactive metals at pyrol-ysis temperatures less than 800°C [18] and there-fore the stabilizing effect of modifier is increased.It can be seen in Fig. 1 that under these condi-tions the background absorbance from the matrixis perfectly corrected using Zeeman correctorwhich allows direct determination of sea waterinto furnace for the determination of analytes.

Fig. 1. Atomization profiles of Pb in aqueous standard and insea water sample; where S1, standard (50 mg l−1); S2, sample(50 mg l−1); B1 and B2, background absorption profiles of Pbin standard and sample respectively.

O. Acar et al. / Talanta 49 (1999) 135–142 139

Table 1Detection limits (LOD, 3s-criterion) and characteristic masses (m0) for Bi, In and Pb determination using some modifiers

AnalytesaMatrix modifiers

PbInBi

m0, pgLOD, mg l−1m0, pgLOD, mg l−1LOD, mg l−1 m0, pg

111.4 6.26No modifier 22.69 175.0 15.79 43.05.4494.4 37.2TA 13.2118.43 168.4

10.15 81.1 5.07 31.1Mo 17.03 161.978.2 4.83Mo+TA 15.57 131.6 8.74 30.2

4.7776.4 28.9Pd+TA 7.9914.64 120.65.44 65.9 2.25Mo+Pd 27.110.26 105.33.68 33.2 0.97Mo+Pd+TA 7.03 23.551.6

a Analytical range: 20–400 mg l−1 for Bi and In; 5–80 mg l−1 for Pb with or without of single or mixed modifier mixtures. Samplevolume: 20 ml.

3.3. Analytical conditions and calibration

The determination of Bi, In and Pb in samplesolutions were performed with and without Mo-based modifiers by using the calibration on thebase of single element solutions in nitric acid forcomparison. Calibration against standard solu-tions in the presence or absence of modifiers wasperformed for analytes by using optimumparameters such as furnace temperature pro-gramme, modifier mass and mass ratios of themixed modifiers as described in [11]. To obtaincalibration graph, working standard solutions ofanalytes in analytical ranges given in Table 1 wereadded to the modifier solutions as described inSection 2.3. All calibration graphs were linear andcorrelation coefficients were about \0.999.

Detection limits (LOD, 3s-criterion) and char-acteristic mass (m0) of Bi, In and Pb spiked to seawater (20 mg l−1 of each analyte) were determinedin the presence or absence of some modifiers andgiven in Table 1. As can be seen in Table 1, thelowest detection limits and characteristic massesof analytes were obtained by using Mo+Pd+TA mixed modifier.

3.4. Application

For new modifiers, they are particularly impor-tant that their stabilizing powers for analyte ele-ments persist also in the presence of complex

matrix and of high salt concentrations. Therefore,Mo+Pd+TA and Mo+Pt+TA mixedmodifiers providing higher pyrolysis temperaturesfor the analytes and their components providinglower pyrolysis temperatures were applied to thedetermination of Bi, In and Pb in synthetic andreal sea water samples. Results were given inTables 2 and 3. The determination of analytes insample solutions were performed with and with-out modifier by using calibration graph methodand seven parallel sample solutions. As can beseen in Tables 2 and 3, a good accuracy wasobtained by using mixed modifiers containing TA.When higher chloride concentrations are to beexpected in samples such as sea water, the chlo-ride interferences are to be important to deter-mine volatile elements. As can be seen in Tables 2and 3, Mo+Pd+TA or Mo+Pt+TA mixedmodifiers could be used for the determination ofBi, In and Pb in sea water with about 5% relativeerror. Tartaric acid acts on the chloride interfer-ence in the matrix. It can be plausibly concludedthat tartaric acid is a valid modifier for reducingthe chloride ions in samples [13]. The products ofTA can be convert chloride ions to HCl (g) andhigh contents of chloride ions vaporize at lowtemperatures in the pyrolysis stage. EspeciallyMo+Pd+TA and Mo+Pt+TA mixtures wereefficient to allow the use of pyrolysis temperaturesin the range of 1350–1400°C for Bi and In and1250–1300°C for Pb. These temperatures could be

O. Acar et al. / Talanta 49 (1999) 135–142140

Table 2Recovery tests performed for bismuth, indium and lead determination in synthetic sea water

Pyrolysis temperature (°C) ConcentrationsElement determined Modifier Recovery (%)

FoundaSpiked (mg l−1)

800 50.0 24.494.6 49Bi No modifier5829.193.750.0750TA

50.0 36.192.4Mo 1000 7250.0 41.292.5Pd 1200 82

42.592.150.0 851150Pd+TA1100 50.0 39.792.2 79Pt

8441.992.050.01100Pt+TA50.0 40.791.9Mo+Pd 1300 8150.0 47.291.8Mo+Pd+TA 1400 94

8843.991.950.01300Mo+Pt1350 50.0 48.491.5 97Mo+Pt+TA

28.992.550.0 581000In No modifier50.0 31.592.3TA 6385050.0 36.492.1Mo 1100 73

7939.692.150.01250Pd41.991.9 84Pd+TA 1150 50.0

7738.392.450.01200Pt50.0 39.392.0Pt+TA 1150 7950.0 42.591.8Mo+Pd 1400 85

9748.391.550.01350Mo+Pd+TA1300 50.0 43.491.9 87Mo+Pt

9648.291.51350 50.0Mo+Pt+TA

50.0 24.392.6Pb No modifier 4980050.0 31.992.5TA 750 64

35.692.250.0 711000Mo7738.492.1Pd 1100 50.08040.191.850.01150Pd+TA

50.0 37.991.9Pt 1050 7650.0 39.292.0Pt+TA 1100 78

9145.392.150.01250Mo+Pd9547.591.6Mo+Pd+TA 1300 50.0

50.0 42.991.9Mo+Pt 1200 8610351.491.51250 50.0Mo+Pt+TA

a Mean of seven replicate measurements with 95% confidence level.

sufficient to volatilize many matrix constituentswithout loss of analyte elements. When nomodifier is used, the error is very high because ofmatrix interferences. Such effects are negligiblewhen Mo+Pd+TA or Mo+Pt+TA is presentand accuracy and precision are satisfactory. It canbe expected, therefore, that the recommendedchemical modifiers are applicable to the determi-nation of Bi, In and Pb in sea water samples.

4. Conclusion

Direct determination of volatile elements suchas Bi, In and Pb in sea water by ETAAS usingMo-based modifiers were examined. In generalterms, the analytical problems arising from thesample matrix can be controlled only with dilu-tion and using Mo+Pd+TA or Mo+Pt+TAmixed modifier and Zeeman background correc-

O. Acar et al. / Talanta 49 (1999) 135–142 141

Table 3Recovery tests performed for bismuth, indium and lead determination in Marmara sea water

Pyrolysis temperature (°C) ConcentrationsElement determined Recovery (%)Modifier

FoundaSpiked (mg l−1)

30.694.450 61800Bi No modifier50 32.793.4TA 6575050 36.492.6Mo 1000 73

37.692.550 751050Mo+TA50 40.592.3Pd+TA 1150 81

7839.292.4501100Pt+TA50 43.892.1Mo+Pd 1300 8850 48.891.6Mo+Pd+TA 1400 98

8441.992.3501300Mo+Pt1350 50 47.191.8 94Mo+Pt+TA

30.794.250 611000In No modifier50 32.593.7TA 6585050 35.492.9Mo 1100 71

7437.292.2501150Mo+TA50 41.292.6Pd+TA 1150 82

39.892.3 80Pt+TA 1150 5050 42.892.0Mo+Pd 1350 8650 49.091.4 981400Mo+Pd+TA50 45.392.3Mo+Pt 1300 91

9547.591.61350 50Mo+Pt+TA

50 28.394.1Pb No modifier 5780050 32.893.7TA 750 66

35.293.250 701000Mo37.492.6 75Mo+TA 1050 50

7839.192.4501150Pd+TA50 38.392.5Pt+TA 1100 7750 41.592.3Mo+Pd 1250 83

10150.391.4501300Mo+Pd+TA50 41.192.4Mo+Pt 1200 8250 47.591.7Mo+Pt+TA 951250

a The mean of seven replicate measurements with 95% confidence level.

tion. Recoveries of analytes spiked to water sam-ples were 94–103% with Mo+Pd+TA or Mo+Pt+TA. The recoveries of analytes are only49–61% without modifier.

Acknowledgements

The supports of Turkish Atomic Energy Au-thority and Gazi University Research Found aregratefully acknowledged.

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