Rice Slurry Analysis Using Mixed-Gas Plasma and AxiallyViewed ICP OES

Guilherme Luiz Scheffler & Valderi Luiz Dressler &

Dirce Pozebon

Received: 9 September 2013 /Accepted: 14 November 2013# Springer Science+Business Media New York 2013

Abstract This study deals with determination of essential andtoxic elements in slurry of rice using axially viewed induc-tively coupled plasma optical emission spectrometry (ICPOES) andmixed gas plasma (N2-Ar ICP). Nitrogenwas added(flow rate of 20 mL min−1) to the nebulizer gas through thespray chamber. It was observed that the addition of N2 in-creased the ICP robustness and mitigated the slurry matrixeffects. Several parameters associated with the slurry prepa-ration (extractant concentration, dispersant concentration,mass volume ratio of the slurry and sonication) wereevaluated. The slurry was prepared in 10 % (m/v)HNO3+0.2 % (m/v) Triton X-100. Calibration was carriedout with aqueous standards whereas Y(II) (371.029 nm) wasused as internal standard (IS) in order to overcome matrixeffects. The IS and sample solutions were mixed online.Accuracy was evaluated by the analysis of certified referencematerials and by comparison with an independent method ofsample preparation (acid digestion). The concentrations wereconsistent with those certified or found in digested rice sam-ple. Calcium, Mg, K, Cd Cu, Fe, Mn, Mo, Cr, Zn, Co, Pb andNi were investigated whose limits of detections were 0.080,0.033, 0.050, 0.048, 0.050, 0.015, 0.005, 0.107, 0.036, 0.056,0.040, 0.367 and 0.132 mg kg−1, respectively. The RSD(relative standard deviation) was typically lower than 10 %.Samples of parboiled, white, and brown rice were analysedand the concentrations found corroborate with those reported.It was concluded that calibration with aqueous standard is

feasible, making the analysis simpler. The method is robustand can be routinely applied.

Keywords Axially viewed ICPOES . Slurry . Rice . N2-ArICP

Introduction

Rice is largely consumed worldwide and is an importantsource of minerals, carbohydrates, proteins, vitamins andlipids for human nutrition (Heinemann et al. 2005; Falahiet al. 2010). The cereal is the basic dietary product for billionsof people in the world. Rice is mostly grown in irrigated fields,method that favours the absorption of metals and other ele-ments by the plant rice. The irrigated rice plants and soil aresubjected to anaerobic conditions that along with excess ofwater results in greater element mobility in the environment.In this way, rice is subject to contamination by elements fromcontaminated soils (Xiao et al. 2004), water (Williams et al.2006) used for irrigation as well as use of pesticides andfertilizers (Batista et al. 2011). Thus, despite being a sourceof nutrition, rice may be contaminated with toxic elements.For that reason, multivariate analysis has been used in order toidentify rice according to its geographic region (Shen et al.2013) and/or mineral composition (Da Silva et al. 2013).

Inductively coupled plasma optical emission spectrometry(ICP OES) is an appropriate technique for routine determina-tion of toxic and essential elements in rice, since it combinesgood sensitivity, high sample throughput and low chemicalinterference (Oliveira et al. 2012). Matrix effect is the mainshortcoming of ICP OES, requiring different sample pre-treatment and/or calibrations strategies (such as internal stan-dardization, standard addition and matrix matching) to ensureaccurate results in analysis of complex matrices such as slurries(Todolí et al. 2002; Agatemor and Beauchemin 2011a, b).

G. L. Scheffler :D. Pozebon (*)Instituto de Química, Universidade Federal do Rio Grande do Sul,Av. Bento Gonçalves, 9500, 91501-970 Porto Alegre, RS, Brazile-mail: dircepoz@iq.ufrgs.br

V. L. DresslerDepartamento de Química, Universidade Federal de Santa Maria,97105-900 Santa Maria, RS, Brazil

Food Anal. MethodsDOI 10.1007/s12161-013-9764-8

Sample preparation is the most time-consuming and labo-rious step of the analysis, which usually exceeds the timenecessary for the analyte measurement. Microwave-assisteddigestion in closed vessel is considered a standard procedurefor solid samples preparation due to the high efficiency, lowrisk of contamination and analyte loss. Despite these advan-tages and worldwide application, microwave-assisted digestionshows some classical limitations. These include expressive re-agent consumption, dilution of the sample solution,maintenanceand acquisition costs of the microwave oven and respectiveflasks and impossibility to perform microanalysis dependingon the type of flask available for sample decomposition. Theselimitations have stimulated the research of procedures and/ortechniques such as slurry analysis or direct solid samplinganalysis (Ebdon et al. 1997; Ebdon and Wilkinson 1987). Therisks of analyte loss, volume of acid needed and dilution in-volved can be reduced if a given solid sample is analysed asslurry (Goodall et al. 1993; Martins et al. 2002). Additionally,the limit of detection (LOD) can be improved and costs reduced.

However, significant changes in the nebulization process aswell as in the energy available for excitation and ionizationprocesses in the ICP are prone to occur in the case of slurryanalyses. In addition, special nebulizers (Ebdon et al. 1997)are required owing to the high solids content that may easilyclogs the nebulizer tubing and capillaries used.

Mixed-gas plasmas are those produced when argon (Ar) ispartially replaced by foreign gases such as N2, O2, H2, He andCH4 or even when these gases are introduced in the Ar ICP asadditional flows. As a result, thermodynamic properties of theICP are modified and better energy transfer and/or plasmarobustness obtained. Due to these changes, matrix effects may bereduced, particularly in the case of slurry whose matrix is usuallycomplex (Foulkes et al. 1988; Agatemor and Beauchemin2011b; Holliday and Beauchemin 2004; Verbeek and Brenner1989). In such plasmas, accurate and precise results may beobtained by means of calibration with aqueous standards andinternal standardization.

Slurry of refractory materials and coal have been analysedusing Ar–O2 and Ar–N2 plasmas in radially viewed ICP OES(Verbeek and Brenner 1989; Long and Brenner 1990), whichresulted in an improvement of accuracy and precision whencompared with the standard Ar ICP. Addition of H2 and He tothe Ar ICP has improved the vaporization of the slurry ofceramic powder in the ICP (Heitland and Broekaert 1997),being the mixed gas plasma very efficient for determination ofAl and Si by ICP OES with radial configuration. However, ithas been observed (Montaser and Van Hoven 1987) thataddition of a huge amount of N2 to the outer gas flow candegrade the excitation capability of the ICP and increase thebackground due to emission by molecular nitrogen speciesgenerated in the ICP.

In axially viewed ICP OES, the observation path length islonger than that of the radial view and the sensitivity improved

as a consequence. However, matrix effects may be moresevere since the emission of species in the entire centralchannel is detected. The use of mixed-gas plasma and axiallyviewed ICP OES for direct analysis of slurry has not beeninvestigated so far. Hence, the main purpose of the presentstudy is to investigate axially viewed ICP OES and N2-Ar ICPfor direct analysis of rice slurry. A very low flow of N2 isadded to the Ar ICP in order to improve the ICP robustnessand decrease matrix effects. Yttrium (II)-371.029 nm is usedas internal standard (IS) to compensate differences among theslurry sample and aqueous standards in the nebulization andionization/excitation process in the ICP.

Material and Methods

Instrumentation

Measurements weremade using anOptima 2000DV ICPOESspectrometer (Perkin Elmer, MA, USA). Operating conditionsfor the spectrometer, accessories, spectral lines monitored andrespective energies are summarized in Table 1.

Argon (purity of 99.998 %, White Martins/Praxair, Brazil)was used as plasma gas, auxiliary and nebulizer gas. Nitrogen(purity of 99.996 %, White Martins/Praxair, Brazil) was usedas the purge gas in the optical system of the spectrometer. Thisgas was also used to obtain the mixed gas plasma (N2-Ar ICP).All gases used were stored in corresponding cylinders. Theplasmawas ignited in a pure Ar environment as usual and after5 min, N2 (flow rate of 20 mL min−1) was added to thenebulizer gas. The N2 pressure was set at 50 psi to avoid backpressure of the nebulizer gas. Complete transition from the ArICP to the N2-Ar ICP has typically occurred within2 min as observed through the stabilization of the centralchannel of the ICP. The plasma was then run in a mixed-gasenvironment for another 15 min for stabilization (followingthe manufacturer recommendation for the Ar ICP) before anymeasurement.

A baffled cyclonic spray chamber (Jacketed Tracey)associated with a GemCone nebulizer (PerkinElmer)was employed. This nebulizer is appropriate for nebu-lization of slurry or solution having high content ofdissolved solids. The spray chamber employed has an inletfor an additional gas.

A homemade confluence of acrylic was connected to theperistaltic pump of the ICPOES spectrometer and used for on-line mixing of IS and the slurry of the sample or solutions.Two Tygon tubes were used: orange-red (0.19 mm i.d.) for theIS and black-black (0.76 mm i.d.) for the analyte in solution orin slurry. After being mixed in the confluence, they weretransported to the nebulizer through a PTFE tubing (50 cmlong×0.76 mm i.d.). In such conditions, the reference solutionor slurry was only 7 % diluted while that of the IS was 93 %,

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preserving the sensitivity. The concentration of the IS solutionwas initially 2.0 mg L−1 and after on-line dilution it was0.14 mg L−1.

Sample digestion was carried out in PFA vessel in micro-wave oven (Anton Paar, Multiwave 3000). An ultrasonic bath(Unique, Brazil) operated at 40 kHz of frequency and 135 Wof power was employed for slurry sonication. The ricesamples were milled in a cryogenic mill (SPEX, Metuchen,NJ, USA).

Reagents and Solutions

The solutions used were prepared with high purity water(18.2 MΩ cm) obtained in a Milli-Q system (MilliporeCorporation, Billerica, MA, USA). Nitric acid (65 % m/m),

HCl (37 %m/m) and H2O2 fromMerck (Darmstadt, Germany)were used. Calibration solutions were prepared in 10 % (v /v)HNO3, by serial dilution of a multi-element stock solution(SPC33MS, SPC Science, Canada). The concentration of thecalibration solutions ranged from 10 to 500 μg L−1. Yttriumstock solution (1,000 mg L−1, SpecSol, São José dos Campos/SP, Brazil) was diluted in 10 % (v /v) HNO3 in order to obtain a2 mg L−1 Y solution, which was used for on-line internalstandardization. Triton X-100 (Sigma, St. Louis, USA) wasweighted as necessary and diluted in 10 % (v /v) HNO3 toobtain 0.2 % (m/v) Triton X-100, which was used for slurrypreparation. A stock solution ofMg (1,000mg L−1,Merck) wasproperly diluted to obtain a solution containing 100 μg L−1 ofthe element. This solution was used to evaluate plasmarobustness.

Samples

Nine samples of rice (brown, parboiled and white rice) grownin irrigated fields in Rio Grande do Sul/Brazil were analysed.The analysed samples were purchased in local markets andfairs. Aliquots of the samples were milled in cryogenicmill (particles lower than 100 μm) and then stored inpolypropylene vials. One of the samples was randomlyselected for the method development. Two certified referencematerial (wheat flour NIST 1567a and rice flour NIST 1568a)were also analysed.

Sample Decomposition

The following procedure was used for sample decomposition.0.2 g of sample were exactly weighted and transferred to thevessel of the microwave oven to which 4 ml of HNO3 and2 mL of HCl were added. The mixture was allowed tostand for 30 min and then 1 mL of H2O2 was slowlyadded. After 5 min of reaction, the vessel was closed,placed in the microwave oven and irradiated as follows:400 W for 10 min; 900 W for 10 min and 0 W for20 min (cooling). After cooling, the obtained solutionwas transferred to an appropriate volumetric vial and thevolume adjusted to 20 mL by water addition. Thesolutions were analysed directly (without further dilu-tion) with the exception of Mg, Ca and K. For thedetermination of these three elements, the sample solutionswere 2- or 30-fold diluted.

Slurry Preparation

Slurries were prepared in 15-mL vials; 5 mL of 10 % (v /v )HNO3+0.2 % (m/v) Triton X-100 were added to 0.20 g ofmilled rice. Next, the mixture was magnetically stirred for5 min, followed by sonication in ultrasonic bath for 10 min.Sequentially, the mixture was stirred again for 5 min in order

Table 1 Instrumental parameters, spectral lines and respective energiesand accessories employed for ICP OES

Parameter Setting

RF power 1,500 W

Argon gas flowrate (L min−1)

Principal (15), auxiliary (0.2), nebulizer (0.70)

Nebulizer GemCone

Spray chamber Baffled cyclonic (Jacketed-Tracey)

Sample uptake rate 2 mL min−1

Injector tube Alumina (2 mm i.d.)

Plasma view Axial

Replicates 2

Integration time,signal processing

5 s (auto), peak area

Element Spectral line(nm)

Function Energy (eV)a

K(I) 766.490 Analyte 1.61

Cu(I) 327.393 Analyte 3.78

Mg(I) 285.213 Diagnostic/analyte 4.34

Cd(I) 228.802 Analyte 5.41

Ca(II) 393.366 Analyte 9.26

Y(II) 371.029 Internal standard 9.90

Mg(II) 280.270 Diagnostic 12.07

Mn(II) 257.610 Analyte/diagnostic 12.25

Cr(II) 267.716 Analyte 12.95

Fe(II) 238.204 Analyte 13.07

Mo(II) 202.031 Analyte 13.23

Co(II) 228.616 Analyte 13.70

Ni(II) 231.604 Analyte 14.03

Pb(II) 220.353 Analyte 14.79

Zn(II) 206.200 Analyte 15.40

Ar(I) 420.069 Diagnostic –

H(I) 486.133 Diagnostic –

a Excitation energy for atomic lines (I) and total energy sum (excitation+ionization) for ionic lines (II).

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to homogenize the slurry. Then, the volume of the slurryobtained was adjusted to 10 mL by adding a solution contain-ing 10 % (v /v ) HNO3+0.2 % (m/v) Triton X-100, priornebulization and analysis. The slurry was kept under magneticstirring during sampling and subsequent nebulization.

Results and Discussion

Evaluation of Instrumental Parameters and N2 Effect

Initially, the RF power and nebulizer gas flow rate wereevaluated for the sample introduction system employed (abaffled cyclonic spray chamber associated with a GemConenebulizer). The plasma robustness criterion (the ratio Mg(II)/Mg(I)), as proposed by Mermet (Mermet 1991), was used.The ratio Mg(II)/Mg(I) indicates how efficient is the energytransfer in the ICP. If the ratio is ≥8.0, the ICP is consideredrobust, indicating efficient energy transfer. As expected, theICP robustness increased with the RF power increase. Con-sidering the direct analysis of slurry, which demands highenergy, the RF power was fixed at 1,500 W. Besides, mixedplasmas generated from molecular gases usually requirehigher RF power than the standard Ar ICP. Similarlyto the RF power, the nebulizer gas flow rate has greatimpact on plasma robustness and analyte sensitivity. TheICP robustness is higher at low nebulizer gas flow rate,but at the expense of sensitivity since less amount ofsample will reach the ICP. On the other hand, matrixeffects may be exacerbated at higher nebulizer gas flowrate and consequently inaccurate results will be obtain-ed. Therefore, as a compromise condition, the nebulizergas flow was fixed at 0.70 L min−1.

The ratio Mg(II)/Mg(I), the Ar(I) 420.069-nm line emis-sion and the hydrogen Stark broadening line (486.133 nm)emission were monitored with respect to the N2 effect, asshown in Fig. 1. As can be seen, the plasma robustnessincreases from 12 to 26, indicating that a more robust ICPwas achieved with the addition of N2.

In Fig. 1, it is observed that the Mg(II) emission increaseswith the N2 flow rate increase while that of Mg(I) decreases.However, an initial decrease is observed for both Mg(II) andMg(I) emission as well as for Ar(I). The initial decrease ofemission observed is attributed to energy expenditure to dis-sociate the N2 molecules in the ICP and/or is related with theenergy stored by N2 in molecular vibration (Montaser and VanHoven 1987). However, the N2 addition to the Ar ICP mayaffect on several concomitant mechanisms such as charge-transfer reactions, collisions, equilibrium shift (decrease ofelectron density and increase of Ar*), which depend on theenergy of the emission line and atomic configuration of theelements involved. The intensity of Ar(I) increases like theplasma robustness, which indicates that better excitations

conditions occurs in the mixed-gas plasma. By increasingexcited argon species (Ar*) in the ICPmore suitable excitationconditions are created as a consequence. Collision with Ar* isan important mechanism of analyte excitation (known asPenning ionization). However, additional charge-transferreactions among the analyte and nitrogen species (N2

+, N+ andNO+) cannot be excluded in N2–Ar ICP.

The signal of the hydrogen Stark broadening line (H(I)486.133 nm) is commonly used to determine the electronnumber density in plasmas (Acon et al. 2001). According tothe results obtained, it is possible to affirm that the relativeemission intensity of H(I) 486.133 nm increases in virtue of adeformation on the measured peak (with the N2 addition the H486.133 peak becomes broader than that obtained in a pure ArICP). This broadening is a consequence of the electron num-ber density change that can occur when N2 is added to the ArICP, which modifies the analyte excitation path with respect toelectron impact (Sesi et al. 1994). The emission of lines withhigher energy was more affected by N2 as can be seen inFig. 2. The ionization potential of N2 is 15.58 eV and thecloser is the energy of the emission line, the more likely is thecharge transfer reaction in the ICP. The N2

+ and N+ speciesgenerated may react with oxygen in the ICP and give rise toNO+ (ionization potential of 9.26 eV), which could alsoenhance the analyte ionization by charge-transfer.

According to Fig. 2, atomic and ionic lines are affected byN2 in a different extent. Higher enhancement is observed forionic lines with higher energy sum, probably due to excitationby Ar* species (the addition of N2 promotes Ar excitation andanalyte excitation as a consequence). The different enhance-ment observed is not only related with charge-transfer but also

0 10 20

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N2 Flow Rate, mL min-1

Ar(I)

H(I)

Mg(I)

Mg(II)

15

20

25

Mg(

II)/

Mg(

I)Mg(II)/Mg(I)

-

Fig. 1 Effect of N2 on the emission intensity (normalized signal) ofMg(I) , Mg(II) , Ar(I) , H(I) and plasma robustness. RF power, 1,500 W;nebulizer gas flow rate, 0.70 L min−1 and sample uptake rate,2.0 mL min−1. A solution containing 100 μg L−1 of Mg was analysed

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with collision processes. In addition, charge transfer reactionsin the ICP are favourable if the spin configuration of thespecies involved is maintained (Chan and Hieftje 2007).

Evaluation of Slurry

Extractant

To access accurate results, several parameters related withslurry formation must be evaluated (particle size, concentra-tion of dispersant, mass to volume ratio, partial extraction, andslurry stability). In this context, the rice samples were milledin a cryogenic mill that ensured low particles size. The anal-ysis of slurry is more easily accomplished when the analyte ispartially extracted. Nitric acid is the most used extractantbecause it provides good mobility of the analyte in the liquidmedium, production of soluble compounds and low back-ground. Figure 3 shows the effect of the HNO3 concentrationon the emission of Cr, Mn, Cu, Zn and Fe. In this figure, it ispossible to observe that the analyte signal increases with theHNO3 concentration increase. However, the increase is lesspronounced for HNO3 higher than 5 % (v /v ). Accurate resultswere obtained by using 5% (v /v ) HNO3, with the exception ofFe. At this condition, the Fe concentration was 30 % lowerthan that found for the acid digested sample. However, the

concentration of Fe was accurate by using 10 % (v /v )HNO3 as extractant. Taking into consideration the re-sults obtained and to avoid the handling of slurry withhigh acid concentration, 10 % (v /v ) HNO3 was selectedas extractant.

Additional tests were conducted to verify the influence ofthe extractant. By using water instead of 10 % (v /v ) HNO3 forpreparing the slurry, the Cr, Mn, Cu, Zn and Fe concentrationsfound were, respectively, 46, 19, 27, 32 and 54 % lower thanthose found for the digested sample. The same elements werealso measured in the supernatant of the slurry prepared with10 % (v /v ) HNO3 after centrifugation thereof. In this case, theconcentrations of Cr, Mn, Cu, Zn and Fe were, respectively,50, 10, 9, 20 and 50 % lower than those found in the digestedsample. The results obtained in these tests demonstrated thatthe analyte must be at least partially extracted with acid andthe particles of the slurry have to be introduced into the ICP inorder to obtain accurate results.

Dispersant

The addition of a dispersant agent gives rise to stable andhomogeneous slurries, preventing flocculation and/or parti-cles sedimentation in a short period of time (Ebdon et al.1997). Thus, a representative part of the slurry is nebulized

1

2

3

Cu(I) 327.393

Cd(I) 228.802

Zn(II) 206.200

Pb(II) 220.353

Ni(II) 231.604

Co(II) 228.616

Mo(II) 202.031

Fe(II) 238.204

Cr(II) 267.716

Mn(II) 257.610

3.78

5.41

12.25

13.07

12.95

14.03

14.79

15.40

13.70

13.23

Spectral Line, nm

Sig

nal

Rat

io

Cu (II) Mn

Fig. 2 Ratio of the emissionsignal in N2–Ar ICP/Ar ICP forthe spectral lines. (I) and (II)represent atomic and ionicemission lines, respectively. Eachvalue quoted inside the figurecorresponds to the energy of theemission line (in eV)

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and introduced into the ICP, providing more accurate andprecise results for the analysis. The dispersant (Triton X-100) concentration that was evaluated ranged from 0.01 to2 % (m/v). Although the analyte signal has remained invari-able as a function of the Triton X-100 concentration, the slurrystability has varied remarkably. Fast particles sedimentationand insufficient dispersion of particles were observed forTriton-X 100 ≤0.2 % (m/v). On the other hand, slurry floccu-lation was observed for Triton-X 100 in the range of 1–2 %(m/v). In addition, the high viscosity of the slurry affecteddrastically the nebulization efficiency and the ICP becameunstable due to the excessive carbon load (Ebdon et al.1997). Therefore, the surfactant concentration was fixed at0.2 % (m/v), leading to a satisfactory signal stability over 25minutes of permanent slurry analysis..

Slurry Concentration

The sample mass in the slurry must to be evaluated because itimpacts directly on the slurry stability, matrix effects andsensitivity. The LOD is improved by increasing the samplemass in the slurry, but the nebulization process can be im-paired. Moreover, nebulizer and capillaries clogging andmemory effects may occur, demanding longer washing time(Ebdon et al. 1997; Ebdon andWilkinson 1987; Goodall et al.1993). In contrast, much diluted slurries result in lower sensi-tivity and poorer precision owing to the lack of sample repre-sentativeness. The relationship among the slurry concentra-tion and the analyte signal is shown in Fig. 4 for Zn, Cr, Cu

and Mn, where one can note the linear correlation (r =0.9833to 0.9926) for the concentration range evaluated (0.5–3.0 %(m/v)). As a compromise condition among sensitivity, preci-sion and plasma stability, the slurry concentration was fixed at2 % (m/v). With such slurry concentration, solid particlesdeposited on the wall of the spray chamber were easilywashed with the sample blank (10 %v /v HNO3+0.2 % m/vTriton X-100), the ICP remained stable and solid depositioninside the injector tube in the torch was not observed within4 hours of continuous slurry nebulization. For slurry concen-tration higher than 2 % (m/v), clogging of the capillary of theperistaltic pump was observed within this period.

Slurry Sonication

Sonication not only generates free-radicals that can assist theanalyte extraction (Collasiol et al. 2004; Bentlin et al. 2012)but also homogenises the slurry. To this end, in the presentstudy the slurry was sonicated in an ultrasonic bath in differentperiods of time (for 5, 10, 15, 20, 25 and 30 min). It wasobserved that sonication during 10 min has lead to good slurryhomogenization and precision. By increasing the sonicationperiod, the precision obtained was only slightly improved.Therefore, the time of sonication selected for the rice slurrypreparation was 10 min.

Matrix Effect in N2–Ar Plasma

In order to evaluate how effective the N2 addition was forminimizing the rice slurry matrix effects, the slurry ofparboiled rice spiked or not with the analytes was run in ArICP and N2–Ar ICP. The strategy employed eliminates

0 5 10 150.4

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HNO3, % (v/v)

Cr

Mn

Cu

Zn

Fe

Fig. 3 Effect of nitric acid concentration on the relative (normalized)emission signal of the analytes; 200 mg sample and 0.2% (m/v) Triton X-100 were used for the slurry. Error bars are the standard deviation ofthree determinations

1 2 3

1

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0.9926

0.9905

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0.9833

Rel

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Slurry Concentration, % (m/v)

Cr

Mn

Cu

Zn

Fig. 4 Normalized signal of Cr, Mn, Cu and Zn in slurry of rice as afunction of the slurry concentration. The slurry was prepared using 10 %(v /v) HNO3 and 0.2 % (m/v) Triton X-100. Error bars are the standarddeviation of three determinations. Values inside the figure correspondsthe linear correlation coefficient

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differences on sample preparation (or analyte spiking) andnebulization efficiency, being the effect observed only related

with the ICP composition. For that reason, internal standard-ization was not used in the present step. According to Fig. 5,matrix effects are in general less severe in the N2-Ar ICP. Theaddition of N2 promotes better energy transfer in the ICP andchanges of excitation/ionization temperature and/or electronnumber density. Nitrogen has a higher thermal conductivityand heat capacity, which gives raise a more diffuse centralchannel, increasing power density in the ICP (Sesi et al. 1994).

In general, the matrix of the slurry enhanced the analytesignal (20 to 25 %) as well as that of the IS in the N2-Ar ICP,which indicated that the matrix effects could be correctedusing internal standardization.

Calibration, Accuracy, Precision and Limit of Detection

Calibration is one of the main difficulties associated withslurries nebulization and detection using ICP OES. The pres-ence of suspended particles and surfactant (usually added tostabilize the slurry) changes significantly the sample transport,nebulization and excitation processes in the ICP. For thesereasons, simple external calibration using aqueous standardscannot be adequate. In the present work, internal standardiza-tion was employed to compensate matrix effects. Yttrium(371.029 nm) was employed as internal standard (IS) due toits intermediate energy (9.90 eV).

Precision was evaluated by 10 consecutive measurementsof the slurry of parboiled rice spiked with 5.0 μg g−1 of theanalytes, with the exception of Ca, Mg and K. The RSD was

4 12 14 16

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1.2

1.4

1.6

1.8

With N2

Without N2

No

rmal

ized

Em

issi

on

Sig

nal

Line Energy, eV53

Fig. 5 Effect of N2 for matrix effect reduction as a function of theemission line energy. The dotted horizontal line indicates no matrixeffect. The normalized emission signal was obtained by the ratio IslurryS/(I slurry+I standard); I slurryS: rice slurry spiked with 100 μg L−1 of theanalytes, Islurry: rice slurry without spiking, Istandard: 100 μg L

−1 solutionof the analytes. Error bars are the standard deviation of threedeterminations

Table 2 Limit of detection (LOD), compared with that for sampledigestion and reported values. The RSD (relative standard deviation) isfor 10 consecutive runs of slurry of rice spiked with 100 μg L−1 of Cd,Cu, Fe, Mn, Mo, Cr, Zn, Co, Pb and Ni. Yttrium (II) 371.029 nm wasused as internal standard

Element LOD,mg kg−1

RSD, % LOD,mg kg−1

LOD, mg kg−1

Slurry Sampledigestion

Reported

Ca 0.08 1.87 0.15 0.04a; 1.59 b

Mg 0.03 2.23 0.05 0.13 a; 0.64 b

K 0.05 2.28 0.13 48.9 a; 61.1 b

Cd 0.05 0.88 0.08 –

Cu 0.05 1.67 0.06 0.06 a; 1.58 b

Fe 0.01 5.23 0.10 0.03 a; 1.50 b

Mn 0.01 1.27 0.01 0.007 a; 0.87 b; 5.1

Mo 0.11 1.34 0.27 0.14 a

Cr 0.04 2.87 0.16 –

Zn 0.05 1.87 0.12 0.05 a; 1.07b

Co 0.04 1.66 0.06 –

Pb 0.37 5.64 1.00 –

Ni 0.13 1.44 0.27 –

a: Oliveira et al. 2012; b : Cernohorsky et al. 2008.

Table 3 Mean concentration ± standard deviation (n=3) of the elementsfound in parboiled rice submitted to microwave-assisted digestion oranalysed as slurry; all values in milligramme per kilogramme

Element Sample digestion Slurry

Ca 31±3 30±2

Mg 177±11 175±9

K 1,490±71 1,430±57

Cd <0.07 <0.05

Pb <1.0 <0.37

Ni <0.27 <0.13

Cu 1.44±0.12 1.58±0.10

Fe 5.80±0.40 5.20±0.30

Mn 7.40±0.40 7.30±0.25

Mo 0.37±0.06 0.45±0.05

Cr 0.41±0.05 0.35±0.03

Co 0.22±0.02 0.23±0.04

Zn 10.30±0.50 9.50±0.55

– Spiked (5.0 μg g−1)

Cd – 5.04±0.29 (101)a

Pb – 4.60±0.32 (92)a

Ni – 4.91±0.37 (98)a

a Recovery, %

Food Anal. Methods

lower than 6%, as can be observed in Table 2. The LODs (alsoshown in Table 2) were calculated using the 3σ criterion, for10 consecutive runs of the sample blank (10 % (v /v ) HNO3+0.2 % (m/v) Triton X-100). The LOD of the method wascalculated considering 0.20 g of sample mass in 10 mL ofslurry. In the case of sample digestion, 0.20 g of sample in20 mL was considered. Due to the lower dilution involved inthe case of the slurry, the LODs are better than those of thesample digestion method, as can be observed in Table 2. TheLODs found are similar or lower than those reported (seeTable 2) for rice analysed by means of ICP OES; as slurry(Cernohorsky et al. 2008) or extracted with EDTA (Oliveiraet al. 2012). In Table 2, it can be observed that the LOD of K ismuch lower than those reported for the other methods. The

differences may be due to slurry concentration and/or contam-ination of reagents used. With respect to the LODs of Cd, Cr,Co, Ni and Pb, no information was found in literatureconcerning the determination of these elements in rice usingICP OES.

Microwave-assisted digestion was used to evaluate theproposed procedure of sample preparation. The results obtain-ed are summarized in Table 3 where it can be seen goodagreement among the two sample preparation procedures.Therefore, insufficient IS mixing and incomplete analytevaporization/excitation in the ICP can be excluded. SinceCd, Pb and Ni were not detected, recovery tests were carriedout to evaluate potential matrix effects and/or analyte loss (theanalyte was spiked before sample preparation). According toTable 3, good recovery was obtained for the spiked elements.Accuracy was also accessed by the analysis of two certifiedreference materials (wheat flour and rice flour). According toTable 4, the concentrations found agree with the certifiedvalues, excepting for Mn in wheat flour. The lower concen-tration of Mn found in the certified wheat flour may be due toparticles size, since this sample was not subjected to cryogenicgrinding. The difference among the wheat and rice matricesmay have also contributed. However, the concentration valueof Mn found in rice flour (NIST 1568a) is not different of thatcertified.

Analysis of Rice Samples

The concentrations of the investigated elements in the ricesamples are summarized in Table 5. The Cu, Ca, Mg, K, MnZn, Mn and Cu concentrations found in brown rice are ingeneral higher than those in white rice. Other authors (Antoine

Table 4 Analysis of certified reference materials. Mean ± standarddeviation (n=3) concentration, in milligram per kilogram. Results agreewith the certified values according to the Student’s t test at 95 % confi-dence level, with the exception of Mn in wheat flour

Wheat flour (NIST 1567a) Rice flour (NIST 1568a)

Element Certified Slurry Certified Slurry

Ca 191±4 183±12 118±6 110±9

Mg 400±20 385±25 560±20 540±30

K 1,330±30 1,247±65 1,280±8 1,300±20

Cd 0.026±0.002 <0.048 0.022±0.002 <0.048

Cu 2.10±0.20 1.83±0.13 2.40±0.30 2.15±0.15

Fe 14.10±0.50 13.60±0.70 7.40±0.90 6.80±0.50

Mn 9.40±0.90 7.30±0.27 20.0±1.60 19.0±0.80

Mo 0.48±0.03 0.46±0.04 1.46±0.08 1.35±0.20

Zn 11.60±0.40 10.50±0.82 19.40±0.50 18.90±1.10

Table 5 Mean concentration ± standard deviation (n =3) of toxic and nutrient elements found in the analysed rice samples, in slurry form. N2–Ar ICPand internal standardization were used. Concentration in milligram per kilogram

Rice samples

Element Parboiled Polished Polished Polished Brown (organic) Brown (organic) Brown Brown

Ca 30±2 51±3 42±2 49±1 68±5 83±4 63±4 80±6

Mg 180±9 337±33 153±14 130±10 293±21 583±36 252±17 310±20

K 1,443±52 835±64 923±32 390±20 1,235±39 1,493±65 1,214±46 1,490±50

Cd <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048

Cu 1.5±0.1 1.8±0.2 1.5±0.2 2.0±0.0 2.0±0.1 1.9±0.1 1.8±0.2 2.7±0.3

Fe 6.1±0.3 25.0±0.4 20.0±0.1 2.8±0.5 9.1±0.3 14.5±1.3 12.1±1.9 9.5±0.6

Mn 7.5±0.3 7.7±0.3 9.2±1.5 12.9±1.7 19.0±0.9 30.0±1.7 22.6±1.2 29.1±5.8

Mo 0.40±0.05 0.54±0.04 0.65±0.02 0.44±0.00 0.51±0.05 0.47±0.04 0.41±0.02 0.48±0.03

Cr 0.45±0.06 0.31±0.06 0.41±0.04 0.13±0.02 0.22±0.01 0.69±0.07 0.36±0.04 0.16±0.01

Zn 10.1±0.5 17.8±1.4 12.6±1.9 17.5±2.3 18.0±0.3 22.9±1.3 17.4±1.1 27.5±3.5

Co 0.25±0.03 0.34±0.08 0.39±0.09 0.10±0.02 0.24±0.01 0.25±0.01 0.33±0.08 0.13±0.01

Pb <0.37 <0.37 <0.37 <0.37 <0.37 <0.37 <0.37 <0.37

Ni <0.13 <0.13 <0.13 <0.13 0.33±0.08 0.47±0.05 <0.13 <0.13

Food Anal. Methods

et al. 2012) also found higher concentration of Mg, Mn, Cuand Zn in brown rice than in white rice from worldwide.Teixeira et al. (2012) determinedMn and Zn in different typesof Brazilian rice using energy dispersive x-ray fluorescencespectrometry (EDXRF). The authors also found higher Zn andMn concentrations in brown rice than in polished rice. On theother hand, the Fe concentration is higher in white rice where-as those of Mo and Co are similar in polished and brown rice,which also corroborates with the previous cited work (Antoineet al. 2012). According to this reference, similar concentra-tions of Co and Mo were found in brown rice and white rice,and higher concentration of Fe in white rice. Lead and Cdwere not detected in any analysed sample and this is inaccordance with reported values (Antoine et al. 2012;Fangmin et al. 2006). Average concentration of Cd<0.08 mg kg−1 (Antoine et al. 2012) and Pb ranging from0.002 to 0.200 mg kg−1 (Fangmin et al. 2006) were reportedfor rice from other countries. Nickel was detected only inorganic brown rice, while the higher Cr concentration wasalso found in that rice. According to reference (Antoine et al.2012), the average concentration of Cr in brown rice fromworldwide was also higher than that in polished rice. Theconcentration of Ni found in the present study is lower thanthat reported for rice from Bangladesh (Williams et al. 2009)and India (Fazeli et al. 1998) where the concentration of Niwas 5.0 mg kg−1. On the other hand, Ni was not detected inrice cultivated in Nigeria (Orisakwe et al. 2012) whereas theestimated LOD of Ni was 0.05 mg kg−1.

Conclusions

The determination of essential and toxic elements in rice slurrywas possible by using pneumatic nebulization, internal stan-dardization and N2-Ar ICP. Nitrogen addition enhanced theplasma robustness because it promotes excitation of Ar speciesand improves energy transfer in the plasma, reducing matrixeffects. In this manner, accurate results were obtained by directanalysis of rice slurry using axially viewed ICP OES. It wasobserved that the ionic emission lines were more affected by N2

than the atomic ones. On-line internal standardization withY(II) (371.029 nm) overcome matrix effects, allowing calibra-tion with aqueous standards which simplify the analytical work.The proposed method can be routinely applied for accuratedetermination of minor and trace elements in rice.

Acknowledgements The authors would like to thank CNPQ andCAPES for scholarships and FAPERGS (Fundação de Amparo àPesquisa do Estado do Rio Grande do Sul) for financial support.

Conflict of Interest G.L. Scheffler declares that he has no conflict ofinterest. V.L. Dressler declares that he has no conflict of interest. D. Pozebondeclares that she has no conflict of interest. This article does not contain anystudies with human or animal subjects.

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Food Anal. Methods