AnalyticalMethods

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Universidade Federal do Rio Grande do Sul

Alegre, RS, Brazil. E-mail: dircepoz@iq.ufr

33087215

† Electronic supplementary informa10.1039/c3ay40723c

Cite this: Anal. Methods, 2013, 5, 4371

Received 1st May 2013Accepted 22nd June 2013

DOI: 10.1039/c3ay40723c

www.rsc.org/methods

This journal is ª The Royal Society of

Internal standardization in axially viewed inductivelycoupled plasma optical emission spectrometry (ICP OES)combined with pneumatic nebulization and aerosoldesolvation†

Guilherme Luiz Scheffler and Dirce Pozebon*

Internal standardization using axially viewed inductively coupled plasma optical emission spectrometry (ICP

OES) combined with pneumatic nebulization/aerosol desolvation was evaluated in the present study. The

sample and internal standard (IS) solutions were mixed on-line. Spectral lines of In, Y, Ga and Ar were

evaluated for internal standardization under robust plasma conditions to overcome matrix effects on Ba,

Cd, Co, Cr, Zn, Pb, Sr, Mn, Cu, Ni and V quantification in digests of plant materials. In(II)230.606 nm was

only effective for Cd and Pb whereas Y(II)371.029 nm or Ga(I)417.206 nm or Ga(I)294.364 nm were

effective for the other investigated elements. Argon was not effective, suggesting that matrix effects

were related to aerosol generation and its transport to the ICP. By using internal standardization

accurate results were obtained for certified reference samples of plant materials and the relative

standard deviation (RSD) was lower than 1% even for solutions with high acid concentration. The limits

of quantification (LOQ) of Cr(I)357.869, Cr(II)267.716, Cu(I)327.393, Cu(II)224.700, Cd(I)228.802, Cd(II)

214.440, Pb(I)217.000, Pb(II)220.353, Zn(I)213.857, Zn(II)206.200, Sr(II)407.771, Ba(II)233.527, V(II)292.464,

Mn(II)257.610, Ni(II)232.003 and Co(II)228.616 nm were 0.070, 0.041, 0.065, 0.094, 0.033, 0.056, 1.33,

0.45, 0.065, 0.10, 0.005, 0.20, 0.12, 0.015, 0.10 and 0.047 mg g�1, respectively. It was concluded that the

method is feasible for routine applications.

1 Introduction

Inductively coupled plasma optical emission spectrometry (ICPOES) is a well-established technique for determination of a greatnumber of elements.1–4 The main advantages of ICP OES are theargon excitation source that brings minimum chemical inter-ferences, high analytical throughput, high linear working range,rich emission spectra, low sample consumption and goodprecision and accuracy. However, spectral and non-spectralinterferences are prone to occur and they are the main short-comings associated with the technique.5 Non-spectral interfer-ences are commonly caused by easy ionizable elements such asNa, K and Ca, organic compounds, and inorganic acids. Spectralinterferences may be overcome by proper analyte line selection,while non-spectral interferences may be reduced or avoided bymeans of sample dilution, matrix separation, matrix matching,standard addition calibration or internal standardization.6,7

Non-spectral interferences are more severe when high-effi-ciency nebulizers are used to introduce the sample solution into

, Instituto de Quımica, 91501-970, Porto

gs.br; Fax: +55 33087304; Tel: +55 51

tion (ESI) available. See DOI:

Chemistry 2013

the ICP because the amount of matrix transported to the ICPincreases. Sample dilution is a common way to overcomematrixeffects, but more dilution increases the limit of detection (LOD)and hinders quantication of trace elements. Standard additioncalibration and matrix separation are considered laborious forroutine applications. However, several matrix interferences maybe just corrected by internal standardization.8,9,21 By calibrationusing the ratio analyte signal/internal standard (IS) signal,changes of plasma temperature and energy are equalized, aswell as viscosity and surface tension of the sample solution,which affect the nebulization and aerosol transport to the ICP.It is important to remark that the selected element used as ISmust be undetected in the sample and be free of spectralinterferences. For efficient correction, the analyte and the ISmust fulll several requirements. In this sense, the selection ofIS has received attention in atomic emission spectrometry.10–20

Multivariate analysis has been employed for selecting proper ISelements, following specic characteristics of emission linesand matrix components.14,15,17 It has been suggested10 that asingle element can be used to compensate changes in thenebulization (aerosol generation and transport). Spectral lineswith similar excitation and ionization energies are preferablefor compensating shis in the plasma equilibrium (vapor-ization, atomization, excitation and ionization processes). Due

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to their intermediate energies, spectral lines of Y and Sc havebeen recognized as universal IS.

Yttrium was successfully employed as the IS element in theanalysis of a plant material slurry using axially viewed ICP OESand conventional pneumatic nebulization.16 Effects of acidpresent in the sample solution were determined, whereas the ISwas selected using a multivariate approach.17 It was veried thatthe analytical errors due to HCl, HNO3, H2SO4, and theirmixtures were reduced from 10–40% to less than 5%, using Scor Co as IS for As, Cu, Mn, Ni, P, Pb and Zn.17 The use of Sc424.682 nm as IS provided accurate results for the sameelements in marine sediments digested with aqua regia.14

Yttrium and Be were investigated for correction of interferencescaused by C in ICP OES.18 It was observed that Be was efficientfor atomic lines while Y was not. Simultaneous measurement ofspectral lines of the analyte and the respective signals ratio hasbeen proposed19,20 for compensation of acid and Na interfer-ences. This method was nominated as a common analyteinternal standardization (CAIS) technique. Despite thesuccessful results achieved in correcting Na interference, twospectral lines of the analyte (with different excitation energies)needed to be monitored, that was a clear limitation of themethod.

While several researchers11 have observed that the efficiencyof internal standardization is only met under robust plasmaconditions, others have stated that its full effectiveness dependson specic spectral line properties and plasma view.13 Theapproaches for the selection of IS elements are still empirical inview of the difficulties to create quantitative models forexplaining matrix effects in ICP. Thus, the correct selection of ISmust be evaluated for each type of matrix and analyte.

In the present study, emission lines of In, Y, Ga and Ar areinvestigated and compared as IS for determination of traceelements in digests of plant materials using axially viewed-ICPOES. The IS is on-line added and a high-efficiency pneumaticnebulizer is used for introducing the sample solution into theICP. This nebulizer is employed for improving the sensitivity inthe determination of trace elements. The main objective is toestablish a robust method for routine determination of traceelements in plant materials using ICP OES.

Table 1 Operating conditions and accessories employed for ICP OES

Parameters and accessories Setting

Plasma power (W) 1500Plasma gas ow rate (L min�1) 15Auxiliary gas ow rate (L min�1) 0.2Nebulizer gas ow rate (L min�1) 0.65Purge gas (mL min�1) 2.5Nebulizer Concentric/APEX-QInjector tube Alumina (2.0 mm i.d.)Resolution HighSignal processing mode Peak area

(7 points per peak)Integration time Auto (1–5 s)Sample uptake rate (mL min�1) 0.65Plasma view Axial

2 Experimental2.1 Reagents, standards and solutions

The solutions used were prepared with high purity water(18.2 MU cm) obtained in a Milli-Q system (Millipore Corpora-tion, Billerica, MA, USA). Nitric acid (65% m/m) and HCl(37% m/m) from Merck (Darmstadt, Germany) were used.Calibration solutions were prepared in HNO3 5% (v/v), by serialdilution of a multi-element stock solution (SpecSol, QuimlabQuımica e Metrologia, S~ao Jose dos Campos/SP, Brazil) con-taining 5.0 mg L�1 of Ba, Cd, Co, Cr, Zn, Pb, Sr, Cu, Mn and Ni,and a 1000 mg L�1 stock solution of V (Merck). Stock solutionsof Y (1000 mg L�1, Spex), Ga (1000 mg L�1, Spex) and In(1000 mg L�1, Merck) were properly diluted in 5% (v/v) HNO3

and used for internal standardization. A stock solution of Mg(1000mg L�1, Merck) was sequentially diluted in water to obtain

4372 | Anal. Methods, 2013, 5, 4371–4377

the nal concentration of 50 mg L�1 Mg. This solution was usedfor plasma robustness evaluation.

2.2 Instrumentation

All measurements were carried out using an OPTIMA 2000 DVICP OES spectrometer (PerkinElmer Corporation, Massachu-setts, USA). Argon (99.998%, White Martins/Praxair, Brazil) wasused as the plasma, auxiliary and nebulizer gas. Nitrogen(99.996%, White Martins/Praxair, Sapucaia do Sul/RS, Brazil)was used as the purge gas of the optical system. A high-effi-ciency nebulizer APEX-Q (ESI, Omaha, NE, USA) with aerosoldesolvation was employed for introducing the solutions into theICP. The system consists basically of a PFA concentric micro-nebulizer, a heated (140 �C) cyclonic spray chamber and aPeltier-cooled (2 �C) multipass condenser. The instrumentaloperating conditions and accessories employed are summa-rized in Table 1 and the spectral lines monitored are in Table 2.

A microwave oven equipped with quartz vessels (Anton PaarMultiwave 3000, Graz, Austria) was used to assist the sampledigestion.

2.3 Experimental setup

A homemade conuence of acrylic was connected to the peri-staltic pump of the ICP OES spectrometer and used for on-linemixing of IS and sample solutions. Two Tygon tubes were used:orange-red (0.19 mm i.d.) for the IS and black-black (0.76 mmi.d.) for the analyte solution. The solutions mixed in theconuence were transported to the nebulizer by PTFE tubing(50 cm long � 0.76 mm i.d.). The sample uptake rate was keptconstant at 0.65 L min�1. In this manner, the sample solutionwas only 7% diluted while that of the IS was 93%, preserving thesensitivity. The concentration of the IS solution was initially0.75 mg L�1 and aer on-line dilution it was 50 mg L�1. Theexperimental setup is schematized in Fig. 1.

2.4 Samples and sample preparation

The following certied reference materials were analyzed:orchard leaves (SRM 1571) and tomato leaves (SRM 1573) fromthe National Institute of Standards and Technology (NIST),

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Table 2 Emission lines measured

Element Atom/ion Wavelength (nm) Energya (eV)

Cr I 357.869 3.46Cu I 327.393 3.79Cd I 228.802 5.42Pb I 217.000 5.71Zn I 213.857 5.80Sr II 407.771 8.73Ba II 233.527 11.21V II 292.464 11.37V II 290.880 11.40Mn II 257.610 12.25Cr II 267.716 12.92Ni II 232.003 12.98Co II 228.616 13.70Ni II 231.604 14.03Cd II 214.440 14.77Pb II 220.353 14.79Zn II 206.200 15.40Cu II 224.700 15.96Gab I 471.206 3.07Inb

I 325.609 4.08Gab I 294.364 4.31Yb II 371.029 9.90Inb

II 230.606 11.16Arb I 404.443 —Arb I 363.268 —

a Excitation potential for atomic lines or energy sum (excitation +ionization potential) for ionic lines. b Used as internal standards (IS).

Fig. 1 Experimental setup for on-line IS addition.

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oriental tobacco leaves (CTA-OTL-1) from the Institute ofNuclear Chemistry and Technology of Poland, and bushbranches and leaves (NIM-GBW07602) from the NationalResearch Centre for Certied Reference Materials of China. Theanalyzed samples were digested as follows: 250 mg of samplewere exactly weighed, transferred to a quartz vessel of themicrowave oven and 6 mL of HNO3 + 2 mL of HCl were added tothe sample in the ask. The ask was closed and the mixturewas subsequently irradiated for 3 minutes at 300 W, 1 minute at0 W, 6 minutes at 500 W and 5 minutes at 650 W, followed bycooling for 15 minutes. Aer cooling, the sample solution wastransferred to a graduated polypropylene vial and the volumewas adjusted to 25 mL by addition of water.

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3 Results and discussion3.1 Evaluation of RF power, nebulizer gas ow rate andsample uptake rate

The inuence of the RF power, nebulizer gas ow rate and sampleuptake rate on the intensity of spectral lines of IS candidateelements (Ga, Y and In) was determined. The emission signalsremainednearly constant between1100and1500W,probably duetoaerosoldesolvation.Thisbehaviordiffers fromthat observed forconventional (without aerosol desolvation) pneumatic nebuliza-tion where the analyte signal generally increases with plasmapower increasing in the studied range. In the case of aerosol des-olvation, the cooling of plasma is lower andmore energy remainsavailable for analyte excitation in the ICP. However, plasmarobustness is degraded because the water is partially removed;water is themain source of hydrogen in the ICPwhere it improvesenergy transfer.22–25 In the present study, the plasmapowerwas setat 1500 W in order to have a robust plasma since internal stan-dardization for the analyte in the presence of a sample matrix iswell known to be more effective at this condition.11

The effects of the nebulizer gas ow rate and sample uptakerate are shown in Fig. 2(a)–(d). According to Fig. 2(a), theemission intensity of ionic lines (In-230.606 nm andY-371.029 nm) reaches a maximum around 0.65 L min�1 whilethe maximum for atomic lines (In-325.609, Ga-417.206, Ga-294.364 nm) does about 0.75 L min�1. Similar behavior isobserved for emission lines of Cr and Cu (Cr-357 and Cu-327) asshown in Fig. 2(c). It can be concluded that atomization ispredominant instead of ionization at higher nebulizer gas owrates. The main reasons would be the lower plasma robustnessand lower residence time of the analyte in the ICP. The resultsobtained demonstrate that in selecting an internal standard forICP OES not only the characteristics of the IS element must beconsidered, but also its spectral lines. In Fig. 2(a) it is observedthat the behavior of the signals of the Ar emission lines(404.443 nm and 363.268 nm, superimposed on Fig. 2(a)) differsmarkedly from that of the other emission lines. This demon-strates that Ar is not a suitable IS, as further discussed in thetext. In order to have good sensitivity and plasma robustness,the nebulizer gas ow rate was xed at 0.65 L min�1.

With respect to the sample uptake rate, as shown in Fig. 2(b)and (d), it is observed that the sample ow rate has less inuenceand the behavior of the signals of atomic and ionic lines isindistinguishable. The signals of the analytes increase with theincreasing sample uptake rate, as expected. The emission signalincreases because a larger amount of analyte is introduced intothe ICP. However, the signal remains almost constant from 0.95to 1.2 mLmin�1 due to plasma loading, leading to less energeticplasma. Thus, the sample uptake ratewasxed at 0.65mLmin�1.

Under the selected conditions the ratio Mg(II)280.271 nm/Mg(I)285.213 was 12.3, which indicated a robust plasma.22 Theplasma is considered robust if the Mg(II)/Mg(I) ratio is $8.

3.2 Evaluation of the system for on-line addition of IS

The addition of IS to the sample solution is commonly carriedout off-line, but it can be done on-line, reducing the work

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Fig. 2 Influence of the nebulizer gas flow rate (a) and (c) and sample uptake rate (b) and (d) on the signals of spectral lines of IS candidate elements and analytes.

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demanded and the time spent for analysis. An additionaladvantage of on-line addition is lower contamination risks.However, complete mixing of the IS and sample solutions mustbe ensured. In the present study, PTFE tubing of 0.76 mm i.d. �50 cm long was inserted aer the conuence (see Fig. 1). Thesystem was projected for ensuring adequate mixing of thesolutions and to dilute the sample solution as little as possible.The efficiency of mixing was checked by comparison to off-lineaddition of IS. Additional tubing (1.65 mm i.d. � 20 cm long)was inserted aer the conuence. Under this condition (90 cmof sample path) the relative standard deviation (RSD) remainedthe same (see Table 1S in the ESI†), but a longer period ofwashing was necessary. Therefore, this additional tubing wasnot necessary.

A multielement solution containing 100 mg L�1 of the ana-lytes was run for evaluating the time necessary for completewashing of the sample introduction system. By using a 2.5%(v/v) HNO3 solution it was observed that 30 s was sufficient forwashing the system, as shown in Fig. 3. Solutions of HNO3 with

4374 | Anal. Methods, 2013, 5, 4371–4377

different concentrations of the acid (2.5, 5.0 and 10% v/v) wereinvestigated and it was observed that the time necessary forwashing did not decrease by increasing the HNO3 concentra-tion. This was not only due to the sample path, but also due tothe nebulization/desolvation system (heated cyclonic spraychamber and cooled condenser) employed that requires alonger period of washing in comparison to a conventionalpneumatic nebulizer.

3.3 Precision, calibration curves and gures of merit

The stability of the sample introduction system has utmostimportance so that matrix effects can be overcome using IS.Short-term precision (for 50 measurements within 10 min) isshown in Fig. 4 for IS and analytes where we can see that theRSD is lower than 3%. The RSDs for 10 measurements of allanalyte elements and IS are shown in Fig. 1S in the ESI.†

The effect of the sample matrix on the emission signal of IScandidates was investigated. In general, the signals of ionic

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Fig. 3 Time necessary for washing the sample introduction system; analyteconcentration: 100 mg L�1. The washing solution was 2.5% (v/v) HNO3.

Fig. 4 Short-term precision (n¼ 50, 10 min) using the system shown in Fig. 1 forintroducing the solution into the ICP; values in parentheses are the respectiveRSDs; a solution containing 100 mg L�1 analyte and 50 mgL�1 IS was run.

Fig. 5 Influence of a plant material matrix (bush branch and leaves) on thesignal of emission lines of IS (upper figure) and analyte elements (lower figure); (a)blank solution (2.5% v/v HNO3) and (b) sample solution, obtained as described inSection 2.4. Al measurements were carried out sequentially.

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lines (In-230.606, Y-371.029) were more suppressed by thematrix than those of atomic lines (Ga-294.364 and In-325.609),as shown in Fig. 5. The exception is Ga-417.206 whose signalwas enhanced by the sample matrix. It is worth citing that evenunder robust plasma conditions the matrix effect was still verypronounced. As expected, the matrix effect on atomic and ioniclines of analyte elements is also different (shown in Fig. 5 forCu, Pb and Cr), which had also been previously observed for thenebulizer gas ow rate (Fig. 2(a) and (c)).

Calibration curves were obtained using the evaluated IS andthe setup shown in Fig. 1. Calibration curves with a linearcorrelation coefficient of 0.999 were typically obtained for allinvestigated IS (see Table 2S in the ESI†) by plotting the ratio(analyte signal/IS signal) versus analyte concentration. The limitof detection (LOD) and quantication (LOQ) are given in

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Table 3. They were calculated using 3s and 10s criterion,respectively (s is the standard deviation of ten consecutive runsof sample blank). The LOQ was calculated taking into account0.25 g of sample mass in 25 mL. According to Table 3, low LODsare obtained by internal standardization, combined withnebulization/aerosol desolvation and axially viewed-ICP OES.Aerosol desolvation improves sensitivity because the sampletransport efficiency to the ICP is enhanced and the backgroundof water is reduced, whereas the use of IS improves precision.

3.4 Certied reference material analysis

Certied reference materials were analyzed using internalstandardization with In, Y and Ga. The results are presented inTables 4, 3S, 4S and 5S (see the ESI†). The concentrations foundwere generally in agreement with those certied, especiallyusing Y and Ga as IS. On the other hand, with few exceptions theresults were inaccurate (30 to 50% lower) by using externalcalibration without IS. Since the same spectral line of Y or Gawas suitable as IS for spectral lines covering a wide range ofenergy (from 3.46 to 15.96 eV) it is assumed that matrix effects

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Table 3 Limit of detection (LOD) and quantification (LOQ)

AnalyteWavelength(nm)

InstrumentalLOD (mg L�1)

Method LOQ(mg g�1)

Cr(I) 357.869 0.21 0.070Cu(I) 327.393 0.20 0.065Cd(I) 228.802 0.10 0.033Pb(I) 217.000 4.0 1.33Zn(I) 213.857 0.19 0.065Sr(II) 407.771 0.015 0.005Ba(II) 233.527 0.62 0.20V(II) 292.464 0.54 0.18V(II) 290.880 0.35 0.12Mn(II) 257.610 0.05 0.015Cr(II) 267.716 0.12 0.041Ni(II) 232.003 0.81 0.27Co(II) 228.616 0.14 0.047Ni(II) 231.604 0.31 0.10Cd(II) 214.440 0.17 0.056Pb(II) 220.353 1.4 0.45Zn(II) 206.200 0.29 0.10Cu(II) 224.700 0.28 0.094

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were not only plasma-related. They were mostly related to thesample introduction system and different physical properties ofthe solution. It is important to remember the concentration ofions and salts present in the sample solution (250 mg of samplewere decomposed by adding 6 mL of HNO3 + 2 mL of HCl for anal volume of 25 mL of solution), which changes the viscosityand surface tension of the solution. The nebulization systememployed contains a PFA micronebulizer that produces anaerosol with low droplet size, allowing aerosol nucleation.5 Theprimary aerosol generated in the nebulizer is heated in the wallof a hot chamber (at 140 �C), cooled at room temperature (25 �C)and then at 2 �C by the Peltier effect, before being introduced in

Table 4 Element concentration (in mg g�1) found in certified tobacco leaves (n ¼confidence level

Calibration

Analyte line Certied value Without IS In(II)230.606

Ba 233.527 (II) 88.2 � 11.5 41.0 � 1.1 75.6 � 4.1Cd 228.802 (I) 1.12 � 0.12 0.521 � 0.001 1.08 � 0.02Cd 214.440 (II) 1.12 � 0.12 0.568 � 0.002 1.18 � 0.09Co 228.616 (II) 0.879 � 0.039 0.274 � 0.026 0.542 � 0.045Cr 267.716 (II) 2.59 � 0.32 0.875 � 0.017 1.827 � 0.031Cr 357.869 (I) 2.59 � 0.32 0.731 � 0.027 1.55 � 0.046V 292.464 (II) 3.08 � 0.42 1.016 � 0.039 2.10 � 0.06V 290.880 (II) 3.08 � 0.42 1.237 � 0.063 2.91 � 0.10Zn 206.200 (II) 49.9 � 2.4 22.14 � 0.57 41.04 � 0.65Zn 213.857 (I) 49.9 � 2.4 20.62 � 0.47 38.47 � 0.48Pb 220.353 (II) 4.91 � 0.80 2.10 � 0.013 4.79 � 0.18Pb 217.000 (I) 4.91 � 0.80 1.7 � 0.30 3.97 � 0.44Sr 407.771 (II) 201 � 20 92.6 � 3.0 169.1 � 4.0Cu 327.393 (I) 14.1 � 0.5 6.54 � 0.18 12.01 � 0.26Cu 224.700 (II) 14.1 � 0.5 6.54 � 0.21 12.75 � 0.16Mn 257.610 (II) 412 � 14 230 � 1 294 � 9Ni 231.604 (II) 6.32 � 0.65 2.29 � 0.07 4.32 � 0.02Ni 232.003 (II) 6.32 � 0.65 2.32 � 0.07 4.44 � 0.09

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the ICP torch. Matrix effects may take place as follows:(i) change on aerosol formation (related to solution viscosity),(ii) changes in the aerosol transport (related to the sample path,heating and cooling stages), (iii) aerosol nucleation and(iv) shis in the ICP equilibrium. Nevertheless, the last isexpected to be low in robust plasma conditions.

The results obtained in the present study demonstrate thatY(II)371.029 nm (9.90 eV) and Ga(I)417.206 nm (3.07 eV) areefficient IS for compensatingmatrix effects caused by the matrixof the digestate of plants; Y(II)371.029 more than Ga(I)417.206.The unsuccessful role of In as IS can be explained by specicproperties of the element (ionization potential, excitation/ioni-zation zone in the ICP, statistical probability of transitions, etc.).

3.5 Internal standardization with an Ar emission line

Internal standardization using Ar emission lines is repor-ted.23–25 They have been effective in compensating shis in theICP equilibrium. Once Ar emission lines are always availablefor measurement in the Ar-ICP source any addition is notnecessary, reducing the work demanded, possible contamina-tion and selection of a useful element absent in the sample. Inthe present study, Ar(I)404.443 nm and Ar(I)363.268 nm wereevaluated. Although the RSD of the argon emission lines wasabout 4%, the analyte concentrations found in the certiedreference material samples were inaccurate. This again high-lighted that matrix effects were mainly related to the samplesolution nebulization and respective transport to the ICP, sinceinternal standardization using Ar may compensate only ICP-related effects. Hence, internal standardization using Aremission lines is a limited approach. However, in any analysisusing ICP OES the Ar signal might be useful to check if thematrix effect occurs in the ICP or in the sample introductionsystem.

3, mean � standard deviation). Values in bold agree with those certified at 95%

In(I)325.609 Y(II)371.029 Ga(I)417.206 Ga(I)294.364

32.4 � 1.8 89.32 � 2.88 91.15 � 16 72.7 � 40.26 � 0.01 1.26 � 0.02 1.53 � 0.16 1.12 � 0.070.27 � 0.01 1.24 � 0.03 1.71 � 0.18 1.24 � 0.080.12 � 0.02 0.592 � 0.06 0.856 � 0.07 0.612 � 0.0300.50 � 0.02 2.37 � 0.05 2.42 � 0.20 1.80 � 0.130.40 � 0.02 1.83 � 0.05 2.11 � 0.20 1.54 � 0.120.60 � 0.04 2.47 � 0.12 2.73 � 0.32 2.04 � 0.110.60 � 0.04 3.35 � 0.20 3.85 � 0.40 2.79 � 0.17

17.01 � 0.31 49.7 � 1.4 48.27 � 4.90 38.23 � 2.6315.85 � 0.24 47.6 � 1.4 45.36 � 5.0 35.86 � 2.41.16 � 0.08 5.30 � 0.09 6.12 � 0.88 4.53 � 0.4301.80 � 0.38 4.55 � 0.40 4.55 � 1.45 3.52 � 1.072.3 � 1.9 204 � 6 198 � 19 157 � 125.01 � 0.12 14.4 � 0.4 14.3 � 1.4 11.3 � 0.84.72 � 0.08 15.0 � 0.5 15.1 � 1.0 12 � 0.8187 � 4 405 � 5 347 � 45 268 � 231.61 � 0.01 5.70 � 0.20 5.44 � 0.51 4.2 � 0.231.62 � 0.05 5.64 � 0.15 5.59 � 0.57 4.3 � 0.32

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4 Conclusions

Internal standardization associated with pneumatic nebuliza-tion/aerosol desolvation and axially viewed ICP OES was provedto be efficient for determination of trace elements in plantmaterials. Precise and accurate results were obtained byinternal standardization using Y(II)371.029 nm or Ga(I)417.206nm as IS. On the other hand, inaccurate results were obtainedby means of external calibration without IS due to matrix-induced interferences mostly generated on sample nebulizationand aerosol transport to the ICP. Sample solution with high acidcontent need not be diluted since the acid interference could becorrected by internal standardization, by measuring the abovementioned spectral lines. Therefore, it can be stated that theproposed method is useful for routine application.

Although similar energy is always the principal criterion forselecting an IS for ICP OES, a simple relationship between theanalyte and the IS was not found in this study, which indicatedthat matrix effects were not plasma-related in the present case.The same conclusion was drawn by the invariability of the argonemission signal observed when the sample solution was intro-duced into the ICP.

Acknowledgements

The authors thank CAPES, CNPq and FAPERGS for nancialsupport and scholarships provided.

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