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Method development for the determination of cadmium in fertilizer samples usinghigh-resolution continuum source graphite furnace atomic absorption spectrometryand slurry sampling

Aline R. Borges a,b,1, Emilene M. Becker a, Céline Lequeux a,c, Maria Goreti R. Vale a,b,⁎,1,Sergio L.C. Ferreira b,d,1, Bernhard Welz b,e,1

a Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazilb Instituto Nacional de Ciência e Tecnologia do CNPq-INCT de Energia e Ambiente, Universidade Federal de Bahia, Salvador, BA, Brazilc Université de Rennes 1, Rennes, Franced Instituto de Química, Universidade Federal da Bahia, Salvador, Bahia 40170-290, Brazile Departamento de Química, Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil

a b s t r a c ta r t i c l e i n f o

Available online 7 April 2011

Keywords:High-resolution continuum source graphitefurnace atomic absorption spectrometryCadmium determinationSlurry samplingFertilizer analysisModifiers

The determination of cadmium (Cd) in fertilizers is of major interest, as this element can cause growthproblems in plants, and also affect animals and humans. High-resolution continuum source graphite fur-nace atomic absorption spectrometry (HR-CS GF AAS) with charge-coupled device (CCD) array detectionovercomes several of the limitations encounteredwith conventional line source AAS, especially the problemof accurate background measurement and correction. In this work an analytical method has been devel-oped to determine Cd in fertilizer samples by HR-CS GF AAS using slurry sampling. Both a mixture of 10 μgPd+6 μg Mg in solution and 400 μg of iridium as permanent modifier have been investigated and aqueousstandards were used for calibration. Pyrolysis and atomization temperatures were 600 °C and 1600 °C forthe Pd-Mg modifier, and 500 °C and 1600 °C for Ir, respectively. The results obtained for Cd in thecertified reference material NIST SRM 695 (Trace Elements in Multi-Nutrient Fertilizer) of 16.7±1.3 μg g−1

and 16.4±0.75 μg g−1 for the Pd–Mg and Ir modifier, respectively, were statistically not different from thecertified value of 16.9±0.2 μg g−1 on a 95% confidence level; however, the results obtained with the Irmodifier were significantly lower than those for the Pd–Mg modifier for most of the samples. Thecharacteristic mass was 1.0 pg for the Pd–Mg modifier and 1.1 pg Cd for the Ir modifier, and the correlationcoefficients (R2) of the calibration wereN0.99. The instrumental limits of detection were 7.5 and 7.9 ng g−1,and the limits of quantification were 25 and 27 ng g−1 for Pd–Mg and Ir, respectively, based on a samplemass of 5 mg. The cadmium concentration in the investigated samples was between 0.07 and 5.5 μg g−1 Cd,and hence below the maximum value of 20 μg g−1 Cd permitted by Brazilian legislation.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Fertilizers are natural or industrially produced materials, whichcontain essential nutrients for normal plant growth [1]. Mineralfertilizers contain elements that occur naturally in the raw materialsand that are essential for plant life; they are used to: (i) supplementthe natural availability of minerals in soil in order to satisfy thedemand for cultures that have a high productivity potential and resultin economically viable production; (ii) compensate for the loss of

nutrients arising from the removal of crops or leaching from the soil;(iii) improve the conditions in case they are not favorable for cropproduction.

Primary nutrients are nitrogen (N), phosphorus (P) and potassium(K), which need to be applied in large quantities, as they are con-stituents of many plant components. Fertilizers may also containsecondary and micro-nutrients in smaller quantities. These elementshave a wide variety of essential functions in the metabolism ofplants, are constituents of enzymes that control different processes inplants, and the deficiency of any of these nutrients can affect plantgrowth [1–4].

However, fertilizers may also contain toxic trace elements in theircomposition that might have entered the raw materials by weath-ering, erosion and volcanic activity, or from anthropogenic sources [2].Heavy metals are dangerous because they tend to bio-accumulate,cause toxicity to plants and contaminate the food chain. Heavy metals

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⁎ Corresponding author at: Instituto de Química, Universidade Federal do Rio Grandedo Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazil. Tel.: +55 51 33086278; fax: +55 51 3308 7304.

E-mail address: mgrvale@ufrgs.br (M.G.R. Vale).1 www.inct.cienam.ufba.br.

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are associated with a great number of adverse effects on humanhealth [2].

When the final product is of relatively high value, the potentiallyharmful trace elements were removed; however, there is noinexpensive procedure to do that in the production of fertilizer.Among these elements, most attention has been given to Cd, as it isthe element that has the highest transfer factor for plants and,therefore, is the most toxic component in fertilizers and in the soil[2,3].

The procedures found in the literature for the determination oftrace elements in fertilizers usually involve a wet digestion, followedby a determination using flame atomic absorption spectrometry(FAAS) [5], graphite furnace atomic absorption spectrometry (GFAAS) [6], inductively coupled plasma mass spectrometry (ICP-MS)[7,8] or inductively coupled plasma optical emission spectrometry(ICP OES) [9,10]. ICP-MS is the most sensitive technique, but, unlesselectrothermal vaporization (ETV) is used for sample introduction, itrequires prior sample digestion, like most of the other techniques.This procedure might be tedious, time-consuming and introducesystematic errors due to incomplete analyte extraction or solubility[11]. One of the potential disadvantages of ETV sample introduction isanalyte loss during transport from the ETV unit to the plasma [12].Moreover, ICP-MS is the most expensive spectrometric technique,both regarding purchase price and running cost.

For all these reasons the direct analysis using solid or slurrysampling with GF AAS appears to be an excellent alternative and isbeing widely used for determination of trace elements [13–15]. Thesetechniques reduce sample processing to a minimum, reducing at thesame time possible errors inherent in all sample preparation steps[13,16]. With the introduction of equipment for high-resolutioncontinuum source atomic absorption spectrometry (HR-CS AAS) withCCD array detection [17], several limitations of classical line sourceAAS could be eliminated, particularly those associated with spectralinterferences, a problem that was quite common in the analysis ofsolid samples [15].

It is kind of mandatory in GF AAS, to use chemical modifiers for thedetermination of volatile trace elements, such as Cd, in complexsamples in order to facilitate the removal of the matrix without losingthe analyte in the pyrolysis stage. The mixture of Pd+Mg(NO3)2 wasproposed as kind of a universally applicable chemical modifier some25 years ago [18], and it is still widely used nowadays. Permanentmodifiers, on the other hand, simplify the procedure, increase the lifetime of the graphite tube and reduce the blank value, as they arepurified in the deposition process [19,20]. Iridium as permanentmodifier was studied for the determination of Cd in coal, water andbiological samples [21–23] or in combination with other modifiers forthe analysis of grain products, soil, sediment and biological samples[23–25].

In this paper slurry sampling HR-CS GF AAS has been investigatedfor the determination of Cd in N–P–K fertilizers and limestone,comparing the palladium andmagnesium (Pd–Mg)modifier, added insolution, and iridium (Ir) as permanent modifier. The two quitedifferent materials have been chosen on order to investigate therobustness of the developed procedure.

2. Experimental

2.1. Instrumentation

AModel contrAA 700 high-resolution continuum source flame andgraphite furnace atomic absorption spectrometer (Analytik Jena, Jena,Germany) equipped with a xenon short-arc lamp with a nominalpower of 300 W, operating in a hot-spot mode, was used throughout.The high-resolution double monochromator with a linear CCD arraydetector with 588 pixels has a spectral resolution of 1.2 pm per pixelat the 228.802-nm Cd resonance line. Atomic absorption was

measured using the center pixel ±1, corresponding to a spectralinterval of 3.6 pm; however, the entire spectral range ±0.12 nmaround the analytical line was displayed by the 200 pixels that areused for analytical purposes.

The transversely heated graphite tube atomizer, which is part ofthe contrAA 700, was used for all measurements with pyrolyticallycoated PIN platform tubes (Analytik Jena, Part No. 407-A81.025). AnMPE 60 furnace autosampler (Analytik Jena) was used for introduc-tion of slurries and solutions. The optimized graphite furnace tem-perature program (refer to Sections 3.2.2 and 3.2.3) used for alldeterminations is shown in Table 1. Argon with a purity of 99.996%(White Martins, São Paulo, Brazil), with a flow-rate of 2 Lmin−1, wasused as purge and protective gas during all stages, except duringatomization where the flow was stopped. Peak volume selectedabsorbance (PVSA) [26], i.e., the integrated absorbance (Aint)summated over three pixels around the line core (AΣ3, int) was usedfor signal evaluation and quantification. An M2P micro-balance(Sartorius, Göttingen, Germany) was used for weighing the samples.

2.2. Reagents

The nitric acid (Merck, Germany) used to prepare the slurries andaqueous calibration standards, was further purified by sub-boilingdistillation in a quartz apparatus (Kürner Analysentechnik, Rosenheim,Germany). Distilled (in a quartz apparatus) and then deionized water(DDW) with a specific resistivity of 18 MΩ cm, from a Milli-Q waterpurification system (Millipore, Bedford, MA) was used throughout. Allcontainers and glassware were soaked in 1.4 mol L−1 HNO3 for at least24 h and rinsed three timeswithDDWbefore use. The Cd stock standardsolution (1000 mg L−1 Cd in 0.014 mol L−1 nitric acid) was preparedfrom Titrisol concentrate (Merck). The working standards wereprepared by serial dilution of the stock solution with 0.014 mol L−1

nitric acid.The following chemical modifier solutions were used (all solu-

tions and reagents from Merck): palladium modifier stock solu-tion, 10.0±0.2 gL−1 Pd in 15% (v/v) HNO3, magnesium modifierstock solution, 10.0±0.2 gL−1 Mg(NO3)2 in 15% (v/v) HNO3, and1000 mg L−1 Ir.

In order to apply Ir as a permanent modifier, ten aliquots of 40 μLeach of 1000 mg L−1 Ir solution were injected onto the platform andthe graphite tube submitted to the first four stages of the temperatureprogram shown in Table 2 after each injection. After the last treatmentthe full temperature program was executed completing a deposit of400 μg Ir. An argon flow rate of 2 Lmin−1 was maintained during allprogram stages. This temperature programwas optimized in previouswork of our group [21].

2.3. Samples and slurry preparation

The fertilizer samples used in this study were acquired at localagricultural stores, Rio Grande do Sul, Brazil. The samples used in thiswork were: N:P:K fertilizer in percentage by weight 4:14:8 and10:10:10, limestone, fertilizer with composition 23.1% K2O, 11.3% Mg,

Table 1Graphite Furnace Temperature Program for the determination of cadmium by HR-CS GFAAS using the Pg–Mg modifier and Ir as permanent modifier, respectively.

Stage Temp/°C Ramp/°C s−1 Hold time/s

Drying 90 10 20Drying 110 10 20Drying 150 10 10Pyrolysis 600a,500b 50 20Atomization 1600 2000 5Cleaning 2200 500 4

a Pd–Mg modifier.b Ir modifier.

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22.5% S and the certified reference material NIST SRM 695, TraceElements in Multi-Nutrient Fertilizer (National Institute of Standardsand Technology, Gaithersburg, MD, USA).

Sample pre-treatment included only milling in three cycles of20 min each in a vibrating ball mill (Fritsch, Idar-Oberstein, Germany)with stainless steel mortar and balls. After that the samples werepassed through a 45-μm sieve and placed into an oven at 50 °C for 1 hto reach constant weight. The dried samples were kept in a desiccatoruntil they were analyzed. The slurries were prepared in triplicate byweighing the sample directly into special glass cups that could beintroduced into the autosampler and the ultrasound bath. Quantitiesbetween 0.39 and 5.50 mg of fertilizer were weighted to an accuracyof 0.001 mg. A volume of 1.0 mL of diluent was added to the cups,resulting in slurry concentrations of 0.039–0.55% (m/v). The diluentsolution consisted of 5% (v/v) HNO3, 0.05% (v/v) Triton X-100 and10% (v/v) ethanol. The slurries were homogenized for 30 minutes inan ultrasonic bath and placed into the autosampler. Prior to sampleinjection, the slurry was homogenized manually one more time for5 s. In order to avoid possible sedimentation errors, the autosamplercapillary was immersed 6 mm below the surface of the liquid.Volumes between 5 and 30 μL of slurry were taken up automaticallyand delivered into the atomizer; six aliquots of each sample slurryhave been analyzed.

2.4. Digestion of fertilizer samples

For comparison the samples were also digested using the referencemethod EPA Method 3050B [27], recommended by the United StatesEnvironmental Protection Agency. A mass of 1.0 g of fertilizer wasweighed directly into a 50-mL beaker, 10 mL of sub-boiled HNO3,diluted 1+1 with DDW were added, and the beaker covered with awatch glass. The mixture was heated to 95±5 °C under reflux for15 min without boiling. Then 5 mL of concentrated sub-boiled HNO3

was added and heated under reflux for another 30 min. Thisprocedure was repeated until no brown fumes were given off by thesample, indicating the complete reaction with HNO3. After the samplehad cooled, 2 mL of DDW and 3 mL of 30% H2O2 were added dropwiseto avoid excessively vigorous effervescence. The resultant solutionwas warmed, and 1-mL aliquots of 30% H2O2 were added until nomore effervescence was observed. Then, the mixture was heated andevaporated until the volume was reduced to approximately 5 mL. Theresidual solution was transferred to a 100-mL volumetric flask anddiluted to volume with distilled water. The procedure was done intriplicate.

3. Results and discussion

All method development and optimization experiments werecarried out using a commercial fertilizer N:P:K=4:14:8, a limestone,and NIST SRM 695 in order to find out if a different composition hadan influence on the parameters for the determination of Cd usingslurry sampling.

3.1. Influence of diluents and homogenization

The precision and accuracy of determinations using the slurrytechnique depend critically on the particle size of the solid material,the sample homogenization, the slurry stability, the liquid mediumemployed to prepare the slurry, and the slurry concentration [28].Based on previous studies carried out by our research group [21,29],Triton X-100, HNO3 and ethanol were chosen as diluentmedia, since itwas found to be optimumwith respect to stabilizing effect, sensitivityand precision.

It is well documented that extraction of the analyte from the solidto the liquid phase has a significant influence on the precision andaccuracy of the results in slurry sampling [28]. It has been confirmedin this work that homogenization of the fertilizer samples in anultrasonic bath was mandatory for a better extraction efficiency of theanalyte from samples, and hence better precision, comparedwith onlymanual mixing.

The particle size of the solid material can influence the stabiliza-tion and atomization efficiency of slurries [28], which, in turn, can alsoinfluence accuracy and precision of the measurement. To control thisparameter, all samples were passed through a 45-μm sieve, whichresulted in improved precision and accuracy. The particle fractionN45 μmhas been investigated as well, but did not show any significantdifference in the Cd content, so that it was considered safe todisregard this fraction. The NIST SRM 695 showed no significantdifference with and without sieving, since this reference material isalready a fine powder. In addition, it has been found necessary to drythe samples to facilitate their passage of through sieve, a procedurewhich is not recommended in the certificate of the NIST SRM 695.However, the drying temperature of 50 °C used in this work wassignificantly lower than the temperature of 85 °C investigated by NIST[30].

3.2. Investigation of chemical modifiers

3.2.1. Mass of Pd–Mg modifierThe mass of Pd–Mg modifier that is necessary to stabilize Cd in

the various fertilizer samples was investigated using a solution of1.0 gL−1 Pd+0.6 gL−1 Mg and increasing the volume of thissolution co-injected with the samples from 0 to 20 μL. As shown inFig. 1, the atomic absorption signal increased significantly withthe addition of 5 μL of modifier solution, corresponding to 5 μgPd+3 μg Mg, and increased slightly further for 10 μL of modifier

Table 2Temperature program for treatment of the platform with Ir as a permanent modifier(adopted from Ref. [21]).

Stage Temperature/°C Ramp/°C s−1 Hold time/s

1 130 20 402 160 20 503 1000 100 254 1400 200 55 2000 1000 5

Fig. 1. Study ofmass of Pd–Mgmodifier for ( ) 50 pg Cd in aqueous solution; ( ) 27.5 μgfertilizer N:P:K=4:14:8; (■) 3.9 μg NIST SRM 695; ( ) 159 μg limestone; Tpyr=800 °C;Tat=1600 °C. Error bars are standard deviation of six repetitive measurements of threeindependently prepared slurries (n=18).

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solution (10 μg Pd+6 μg Mg). The only exception was the limestonesample, which apparently did not need a modifier, as the matrixitself was stabilizing the analyte to a sufficiently high pyrolysistemperature. This effect is being investigated later in this work. Avolume of 10 μL of the Pd–Mg modifier solution has been chosen forall future measurements.

3.2.2. Pyrolysis curvesThe pyrolysis curves for cadmium obtained with the Pd–Mg

modifier are shown in Fig. 2a using the atomization temperatureof 1600 °C recommended by the instrument manufacturer. Thecurves for the aqueous standard solution, the NIST SRM 695 andthe limestone are very similar; the integrated absorbance signalwas stable up to a pyrolysis temperature of 800 °C. The curve for thefertilizer N:P:K=4:14:8, however, exhibited a significantly differentbehavior with a slight decrease in sensitivity already between 300 °Cand 600 °C, and a significant loss of sensitivity above this temperature.For this reason, the pyrolysis temperature for all futuremeasurementswith the Pd–Mg modifier was fixed at a compromise temperature of600 °C.

The pyrolysis curves obtained with the Ir permanent modifier areshown in Fig. 2b using the same atomization temperature of 1600 °C.An aqueous standard solution and the same fertilizer samples aswiththe Pd–Mg modifier have been investigated. Only the limestonesample showed a thermal behavior very similar to that found withthe Pd–Mg modifier, as a pyrolysis temperature of 800 °C could beused without any analyte loss. Cd in the aqueous standard solutioncould be stabilized by the Ir modifier up to a pyrolysis temperature of600 °C; above this temperature the sensitivity started to dropslightly, and severe losses occurred above 800 °C. For NIST SRM695 and the fertilizer N:P:K=4:14:8, however, the sensitivity wasdecreasing already for pyrolysis temperatures above 300 °C, andsevere analyte losses were observed above 700 °C. A pyrolysis tem-perature of 500 °C was finally adopted as a compromise for all futureexperiments with Ir as a permanent modifier. This relatively poorstabilizing power of the Ir modifier was already observed previouslyfor the determination of Cd in coal slurries [21]; it was brought intoconnection with the relatively high acid concentration of the solu-tion and/or some concomitant that is co-extracted and deactivatingthe modifier in part, as the same permanent modifier workedperfectly well for the same samples when direct solid sampling wasused [31].

Although the purpose of this work has been to develop a commontemperature program for all kind of fertilizers, it was at least ofacademic interest to investigate a little bit more the significantlydifferent behavior of the limestone sample, particularly in the case ofIr as a permanent modifier. The question was if this was due to ahigher stabilizing power of themodifier in the presence of this matrix,or if the matrix of this sample itself has a significant stabilizing effectfor cadmium, as has already been discussed in Section 3.2.1. For thisreason a pyrolysis curve has also been established for this samplewithout the addition of any modifier; the result is shown in Fig. 2ctogether with the pyrolysis curves obtained with the two modifiers.There is no significant difference between the three curves, and it isobvious that it is the matrix of this sample that stabilizes Cd up to800 °C, and neither the Pd-Mg modifier nor Ir has any additionalstabilizing effect.

3.2.3. Atomization temperature and absorbance signalsThe atomization curves did not show any irregularities for both

modifiers, and the atomization temperature of 1600 °C recommendedby the instrument manufacturer, which was also optimized inprevious work [16,21] was chosen for all future determinations. Thetime- and wavelength-resolved absorbance spectra obtained for thefertilizer N:P:K=4:14:8 without and with automatic backgroundcorrection using both modifiers are shown in Fig. 3a–d. The

uncorrected spectra in Fig. 3a and c essentially only show the lampflicker noise and some continuous background absorption, which isobviously higher with the Ir modifier due to the lower pyrolysistemperature used with this modifier. Nevertheless, Fig. 3b and dshows that both the lamp flicker noise and the continuousbackground are corrected perfectly with both modifiers. There is no

Fig. 2. Pyrolysis curves for Cd; (a) and (b) for (■) 50 pg of aqueous solution of Cd;( ) 27.5 μg fertilizer of compositionN:P:K=4:14:8; ( ) 3.9 μg SRMNIST 695; ( ) 159 μgof limestone; (a) Pd-Mgmodifier; (b) Ir permanent modifier; (c) for 159 μg of limestone(■) without modifier; ( ) Pd–Mgmodifier ( ) Ir permanentmodifier. Tat=1600 °C.Error bars are standard deviation of six repetitive measurements of threeindependently prepared slurries (n=18).

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structured background visible in Fig. 3b and d, which is according toexpectation for an atomization temperature as low as 1600 °C.

3.3. Influence of the slurry concentration

In order to investigate if the sample mass introduced into thegraphite tube as slurry has some influence on the determination, twosamples with different Cd concentration were chosen: SRM NIST 695and fertilizer N:P:K=4:14:8. For both samples, slurries were preparedwith different mass of sample per unit volume (% m/v) and analyzedusing both modifiers. Fig. 4 shows a linear correlation between theintegrated absorbance signal and the samplemass in the range from 2.5to 30 μg for fertilizerN:P:K=4:14:8 andbetween1.0 and8.0 μg forNISTSRM 695, and a deviation from linearity above these values. In bothcases thepointwheredeviation from linearitywasobservedwasaroundAint=0.5–0.6 s, which is close to the value where deviation fromlinearity was observed also for aqueous standard solutions. This meansthat these deviations from linearity are due to stray light, and theminordifferences in the values found forAint in thedifferent samples are due tothe fact that the onset of curvature depends on peak height absorbance,and not on Aint. Consequently, samplemass does not have any influence

on the Cd determination in fertilizers, at least not within themass rangeinvestigated in this work.

3.4. Figures of merit

Calibration curves have been established with a blank and eightcalibration solutions in the concentration range 0.50–6.0 μg L−1 Cd(10–200 pg Cd) using the conditions described in the Experimentalpart and the two modifiers. The results are shown in Table 3. Thelinear range extended from 10 to 100 pg Cd for the Pd–Mg modifierand from 10 to 125 pg for the Ir modifier. The sensitivity was verysimilar for both modifiers. The instrumental limits of detection(LOD) in both situations were calculated as three times the standarddeviation of 10 measurements of a blank divided by the slope ofthe calibration curve. The limits of quantification (LOQ) are based onthe same measurements using 10 times the standard deviation of theblank readings. The characteristic mass (m0) is defined as the mass ofanalyte corresponding to an integrated absorbance of 0.0044 s. It isinteresting to note that the blank value with the Pd–Mg modifier isabout ten times higher than that with the Ir modifier. This is verytypical, as any contamination present in the modifier solution isremoved during the application of the permanent modifier.

Fig. 3. Time- andwavelength-resolved absorbance spectra for Cd in 27.5 μg of fertilizer N:P:K=4:14:8 introduces as slurry; (a) and (b) Pd–Mgmodifier, Tpyr=600 °C; Tat=1600 °C;(c) and (d) Ir permanent modifier, Tpyr=500 °C; Tat=1600 °C; (a) and (c) without background correction; (b) and (d) after automatic correction for continuous backgroundabsorption.

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3.5. Validation of the method

The result obtained for Cd in the certified fertilizer referencematerial using calibrationwith aqueous standards is shown in Table 4.There is no significant difference between the values obtained withthe two modifiers and the certified value at a 95% confidence level,based on a Student t-test. Another certified reference material, NISTSRM 1635 Coal has been investigated as well due its lower cadmiumconcentration, although the matrix is for sure significantly different.There is no statistical difference between the results obtainedwith thePd–Mgmodifier in solution; however, the results obtained with the Irmodifier were significantly lower.

This is in agreement with the results obtained previously for thedetermination of Cd in coal using slurry sampling [21]. The precision(RSD) of the methods was evaluated by analyzing 10 replicates of thesame sample; the RSD obtained was 4.5% and 1.7% for Pd–Mg and Ir

modifiers, respectively; this difference is most likely due to the lowerblank value obtained with the latter modifier.

3.6. Analytical results

The proposed method was applied for the determination of Cd infive different fertilizer samples; the results expressed as confidenceinterval at the 95% level, (paired t test) are summarized in Table 5.The Cd concentration found in the investigated samples was be-tween 0.071 and 5.54 μg g−1 for the Pd-Mg modifier and between0.13 and 3.35 μg g−1 for the Ir permanent modifier. Except for thelimestone sample, the results obtained with the Pd–Mg modifierwere significantly higher than those obtained with the Ir permanentmodifier. Based on the shape of the pyrolysis curves with the Irmodifier, which show losses of Cd already at pyrolysis temperatureslower than 500 °C, it might be assumed that the values obtained withthis modifier are biased low, and those obtained with the Pd–Mgmodifier are accurate. This assumption is further supported by themeasurements shown in Fig. 4. The integrated absorbance signalsfor both samples are lower with the Ir modifier compared to thePd–Mg modifier, whereas the sensitivity for aqueous standardsolutions is essentially identical for the two modifiers.

The concentrations found in this study are higher thanthose reported by Zaccone et al. [6] who found 0.12 μg g−1 Cd infertilizer samples. Nevertheless, in most other publications values of2–40 μg g−1 Cd in fertilizers samples are reported [2,10,32]. In thepresent study, it can be seen that the highest concentrations of Cdwere found in samples with higher concentrations of phosphate,which was also observed in other investigations [10,32]. In all cases,however, the Cd values found in this study were well below themaximum level of 20 μg g−1 established by the Brazilian Ministry ofAgriculture.

In order to obtain more information on the accuracy of the resultsfor fertilizers samples using the two modifiers, the digestion methodadopted by the Ministry of Agriculture [33], based on EPA Method3050B [27] was used for comparison. The results obtained aresummarized in Table 6. Again the values obtained with the Irpermanent modifier were significantly lower than those with thePd-Mg Modifier, supporting the assumption that the acid content ofthe solution, similar to that of the slurry, and/or some dissolvedmatrix component deactivates in part the stabilizing power of thepermanent modifier. In case of the limestone sample all results were

Fig. 4. Correlation between the sample mass introduced into the graphite furnace asslurry and the integrated absorbance for a) fertilizer N:P:K=4:14:8; and b) NIST SRM695. (■) Pd-Mg modifier, Tpyr=600 °C, Tat=1600 °C; ( ) Ir permanent modifier,Tpyr=500 °C, Tat=1600 °C. Error bars are standard deviation of six repetitivemeasurements of three independently prepared slurries (n=18).

Table 3Figures of merit for the determination of cadmium by slurry sampling HR-CS GF AAS.

Modifier Linear regression equationa R2 LODb/ng g−1

LOQb/ng g−1

m0/pg

Pd–Mg Aint=0.0267+0.0041 mCd 0.9978 7.4 25 1.0Ir Aint=0.0023+0,0040 mCd 0.9991 7.9 27 1.1

a Aint=integrated absorbance; mCd=mass of Cd.b Based on the injection of 30 μL of slurry, corresponding to 0.16 mg of sample

introduced into the furnace.

Table 4Cadmium determination in certified reference materials using slurry sampling HR-CSGF AAS.

Certified reference material Certified/μg g−1 Found/μg g−1±sda

Pd–Mg Ir

Fertilizer NIST SRM 695 16.9±0.2 16.7±1.3 16.4±0.75Coal NIST SEM 1635 0.03±0.01 0.027±0.004 0.019±0.001

a All values are average and standard deviation of 6 measurements of threeindependently prepared slurries (n=18).

Table 5Determination of cadmium in fertilizer samples with slurry sampling HR-CS GF AASusing the Pd–Mg modifier in solution and the Ir permanent modifier, respectively.

Sample Cd/μg g−1±sda

Pd–Mg Ir

Fertilizer N:P:K 4:14:8 5.54±0.13 3.35±0.32Fertilizer N:P:K 10:10:10 3.48±0.22 2.55±0.14Fertilizer 23.1% K2O+11.3% Mg+22.5% S 1.10±0.14 0.72±0.071Limestone 0.071±0.005 0.13±0.005TSP 46% P2O5+15% Ca 3.23±0.30 2.29±0.16

a All values are average and standard deviation of 6 measurements of threeindependently prepared slurries (n=18).

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significantly lower than those obtained with slurry sampling.Recoveries between 87% and 84% were obtained when a known con-centration of Cd (100 μL of 1000 mg L−1) was added to the limestonesample prior to the digestion. This suggests some loss of Cd in thedigestion process; however, it cannot completely explain the sig-nificantly lower results found for digested samples compared to theslurry analysis.

4. Conclusion

The results obtained in this work for fertilizer samples support ourprevious experience with coal samples that the Ir permanent modifiercannot stabilize Cd sufficiently well when slurry sampling is used.Similar results have also been obtained for digested samples, whichsuggest that the acid content of the slurry/solution and/or a dissolvedmatrix component impairs the stabilizing power of this modifier.The Pd–Mg modifier, added in solution, in contrast, could stabilizeCd to a sufficiently high pyrolysis temperature allowing trouble-freedeterminations and the use of aqueous standard solutions forcalibration. The relatively small amount of sample preparationinvolved in slurry sampling makes the technique suitable for routineapplication.

Acknowledgements

The authors are grateful to Conselho Nacional de DesenvolvimentoCientífico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento dePessoal de Nível Superior (CAPES) and Fundação de Amparo àPesquisa do Estado do Rio Grande do Sul (FAPERGS) for financialsupport. M.G.R.V., B.W. and S.L.C.F. have research scholarships fromCNPq; A.R.B. has a scholarship from CAPES and E.M.B. from CNPq.

References

[1] K.F. Isherwood, Mineral Fertilizer Use and the Environment by InternationalFertilizer Industry Association, Revised Edition, Paris, 2000.

[2] S. Javied, T. Mehmood, M.M. Chaudhry, M. Tufail, N. Irfan, Heavy metal pollutionfrom phosphate rock used for the production of fertilizer in Pakistan, Microchem.J. 91 (2009) 94–99.

[3] M.J. McLaughlin, K.G. Tiller, R. Naidu, D.P. Stevens, Review: the behaviour andenvironment impact of contaminants in fertilizers, Aust. J. Soil Res. 34 (1996) 1–54.

[4] A.S. Lopes, L.R.G. Guilherme, Uso eficiente de fertilizantes e corretivos agrícolas:aspectos agronômicos, third ed. ANDA, São Paulo, 2000.

[5] J. Milinović, V. Lukić, S. Nikolić-Mandić, D. Stojanović, Concentrations of Heavymetals in NPK fertilizers imported in Serbia, Pestic. Fitomed. (Beograd) 23 (2008)195–200.

[6] C. Zaccone, R. Di Caterina, T. Rotuno, M. Quinto, Soil–farming system–food–health: effect of conventional and organic fertilizers on heavymetal (Cd, Cr, Cu, Ni,Pb, Zn) content in semolina samples, Soil Till. Res. 107 (2010) 97–105.

[7] Y.K. Rui, J.B. Shen, Application of ICP-MS to detecting ten kinds of heavy metals inKCl fertilizer, Spectrosc. Spect. Anal. 28 (2008) 2428–2430.

[8] Y.K. Rui, S.L. Jiang, F.S. Zhang, Effects of nitrogen fertilizer input on thecomposition of mineral elements in corn grain, Agrociência 43 (2009) 21–27.

[9] P.F. Kane, W.L. Hall, Determination of arsenic, cadmium, cobalt, chromium, lead,molybdenum, nickel and selenium in fertilizer by microwave digestion andinductively coupled-optical emission spectrometry detection: collaborativestudy, J. AOAC Int. 89 (2006) 1447–1466.

[10] V.G. Bizarro, E.J. Meurer, F.R.P. Tasch, Cadmium contents of phosphate fertilizermarketed in Brazil, Cienc. Rural 38 (2008) 247–250.

[11] A.T. Duarte, M.B. Dessuy, M.M. Silva, M.G.R. Vale, B. Welz, Determination ofcadmium and lead in plastic material fromwaste electronic equipment using solidsampling graphite furnace atomic absorption spectrometry, Microchem. J. 96(2010) 102–107.

[12] M. Resano, F. Vanhaecke, M.T.C. de Loos-Vollebregt, Electrothermal vaporizationfor sample introduction in atomic absorption, atomic emission and plasmamassspectrometry-a critical review with focus on solid sampling and slurry analysis,J. Anal. At. Spectrom. 23 (2008) 1450–1475.

[13] M.G.R. Vale, N. Oleszczuk, W.L. dos Santos, Current status of direct solid samplingfor electrothermal atomic absorption spectrometry—a critical review of thedevelopment between 1995 and 2005, Appl. Spectrosc. Rev. 41 (2006) 377–400.

[14] S.L.C. Ferreira, M. Miró, E.G.P. da Silva, G.D. Matos, P.S. dos Reis, G.C. Brandão,W.N.L.dos Santos, A.T. Duarte, M.G.R. Vale, R.G.O. Araujo, Slurry sampling an analyticalstrategy for the determination of metals and metalloids by spectroanalyticaltechniques, Appl. Spectrosc. Rev. 45 (2010) 44–62.

[15] B. Welz, M.G.R. Vale, D.L.G. Borges, U. Heitmann, Progress in direct solid samplinganalysis using line source and high-resolution continuum source electrothermalatomic absorption spectrometry, Anal. Bioanal. Chem. 389 (2007) 2085–2095.

[16] R.G.O. Araujo, N. Oleszczuk, R.T. Rampazzo, P.A. Costa, M.M. Silva, M.G.R. Vale, B.Welz, S.L.C. Ferreira, Comparison of direct solid sampling and slurry sampling forthe determination of cadmium in wheat flour by electrothermal atomicabsorption spectrometry, Talanta 77 (2008) 400–406.

[17] B. Welz, H. Becker-Ross, S. Florek, U. Heitmann, High-Resolution ContinuumSource Atomic Absorption Spectrometry, Wiley-VCH, Weinheim, New York, 2005.

[18] G. Schlemmer, B. Welz, Palladium and magnesium nitrates, a more universalmodifier for graphite furnace atomic absorption spectrometry, Spectrochim. ActaPart B 41 (1986) 1157–1165.

[19] B. Welz, M. Sperling, Atomic Absorption Spectrometry, third ed. Wiley–VCH,Weinheim, 1999.

[20] H.M. Ortner, E. Bulska, U. Rohr, G. Schlemmer, S. Weinbruch, B. Welz, Modifiersand coatings in graphite furnace atomic absorption spectrometry—mechanisms ofaction (A tutorial review), Spectrochim. Acta Part B 57 (2002) 1835–1853.

[21] L. Bianchin, D. Nadvorny, A.F. Silva, M.G.R. Vale, M.M. Silva, W.N.L. Santos, S.L.C.Ferreria, B. Welz, U. Heitmann, Feasibility of employing permanent chemicalmodifiers for the determination of cadmium in coal using slurry samplingelectrothermal atomic absorption spectrometry, Microchem. J. 82 (2006) 174–182.

[22] Z. Fan, W. Zhou, Dithizone-chloroform single drop microextraction systemcombined with electrothermal atomic absorption spectrometry using Ir aspermanent modifier for the determination of Cd in water and biological samples,Spectrochim. Acta Part B 61 (2006) 870–874.

[23] O. Acar, Evaluation of V, Ir, Ru, V–Ir, V–Ru, and W–V as permanent chemicalmodifiers for the determination of cadmium, lead, and zinc in botanic andbiological slurries by electrothermal atomic absorption spectrometry, Anal. Chim.Acta 545 (2005) 244–251.

[24] L.M.G. dos Santos, R.G.O. Araujo, B. Welz, S.do.C. Jacob, M.G.R. Vale, H. Becker-Ross,Simultaneous determination of Cd and Fe in grain products using direct solidsampling and high-resolution continuum source electrothermal atomic absorp-tion spectrometry, Talanta 78 (2009) 577–583.

[25] R. Dobrowolski, A. Adamczyk, M. Otto, Comparison of action of mixed permanentchemical modifiers for cadmium and lead determination in sediments and soils byslurry sampling graphite furnace atomic absorption spectrometry, Talanta 82(2010) 1325–1331.

[26] U. Heitmann, B. Welz, D.L.G. Borges, F.G. Lepri, Feasibility of peak volume, sidepixel and multiple peak registration in high-resolution continuum source atomicabsorption spectrometry, Spectrochim. Acta Part B 62 (2007) 1222–1230.

[27] http://www.epa.gov/wastes/hazard/testmethods/sw846/pdfs/3050b.pdf.[28] C. Bendicho, M.T.C. de Loos-Vollebregt, Solid sampling in electrothermal atomic

absorption spectrometry using commercial atomizers. A review, J. Anal. Atom.Spectrom. 6 (1991) 353–374.

[29] M.M. Silva, M.G.R. Vale, E.B. Caramão, Slurry sampling graphite furnace atomicabsorption spectrometry: determination of trace metals in mineral coal, Talanta50 (1999) 1035–1043.

[30] E.A. Mackey, M.P. Cronise, C.N. Fales, R.R. Greenberg, S.D. Leigh, S.E. Long, A.F.Marlow, K.E. Murphy, R. Oflaz, J.R. Sieber, M.S. Rearick, L.J. Wood, L.L. Yu, S.A.Wilson, P.H. Briggs, Z.A. Brown, J. Budahn, P.F. Kane, W.L. Hall Jr., Developmentand certification of the new SRM 695 trace elements in multi-nutrient fertilizer,Anal. Bioanal. Chem. 387 (2007) 2401–2409.

[31] A.F. da Silva, D.L.G. Borges, F.G. Lepri, B. Welz, A.J. Curtius, U. Heitmann,Determination of cadmium in coal using solid sampling graphite furnace high-resolution continuum source atomic absorption spectrometry, Anal. Bioanal.Chem. 382 (2005) 1835–1841.

[32] G. Nziguheba, E. Smolders, Inputs of trace elements in agricultural soils viaphosphate fertilizers in European countries, Sci. Total Environ. 390 (2008) 53–57.

[33] Normative Instruction No 24 SDA June 20, 2007, Brazilian Ministry of Agriculture,2007, p. 15, Published in the Official Gazette 06/09/2006, Section 1.

Table 6Cadmium determination in fertilizer samples after digestion using the Pd–Mg modifierin solution and the Ir permanent modifier, respectively.

Sample Digestion Cd/μg g−1±sda

Pd–Mg Ir

NIST SRM 695b 16.9±0.52 12.1±0.87Limestone 0.052±0.002 0.068±0.04TSP 46% P2O5+15% Ca 3.03±0.07 2.17±0.075

a All values are average and standard deviation of three repetitive measurements ofthree independent digestions (n=9).

b Certified value: 16.9±0.2 μg g−1.

535A.R. Borges et al. / Spectrochimica Acta Part B 66 (2011) 529–535