Direct determination of chromium in infant formulas employing high-resolution continuum source electrothermal atomic absorption spectrometry and solid sample analysis

Author’s Accepted Manuscript Direct determination of chromium in infant formulas employing High-resolution continuum source electrothermal atomic absorption spectrometry and solid sample analysis Arlene S. Silva, Geovani C. Brandao, Geraldo D. Matos, Sergio L.C. Ferreira www.elsevier.com/locate/talanta

PII: DOI: Reference:

S0039-9140(15)30001-1 http://dx.doi.org/10.1016/j.talanta.2015.05.046 TAL15638

To appear in: Talanta Received date: 11 February 2015 Revised date: 19 May 2015 Accepted date: 20 May 2015 Cite this article as: Arlene S. Silva, Geovani C. Brandao, Geraldo D. Matos and Sergio L.C. Ferreira, Direct determination of chromium in infant formulas employing High-resolution continuum source electrothermal atomic absorption spectrometry and solid sample analysis, Talanta, http://dx.doi.org/10.1016/j.talanta.2015.05.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Direct determination of chromium in infant formulas employing high-resolution continuum source electrothermal atomic absorption spectrometry and solid sample analysis

Arlene S. Silva, Geovani C. Brandao, Geraldo D. Matos, Sergio L. C. Ferreira*

Universidade Federal da Bahia, Instituto de Química, Grupo de Pesquisa em Química e Quimiometria, Campus Ondina, 40170-115, Salvador, Bahia, Brazil Instituto Nacional de Ciência e Tecnologia, INCT, de Energia e Ambiente, UFBA, Salvador, BA 40170-115, Brazil

*Corresponding author: Sergio L. C. Ferreira, E-mail: [email protected], FAX: + 55 71 32374117 Abstract The present work proposed an analytical method for the direct determination of chromium in infant formulas employing the high-resolution continuum source electrothermal atomic absorption spectrometry combined with the solid sample analysis (SS-HR-CS ET AAS). Sample masses up to 2.0 mg were directly weighted on a solid sampling platform and introduced into the graphite tube. In order to minimize the formation of carbonaceous residues and to improve the contact of the modifier solution with the solid sample, a volume of 10 µL of a solution containing 6% (v/v) H2O2, 20% (v/v) ethanol and 1% (v/v) HNO3 was added. The pyrolysis and atomization temperatures established were 1600 and 2400 ºC, respectively, using magnesium as chemical modifier. The calibration technique was evaluated by comparing the slopes of calibration curves established using aqueous and solid standards. This test revealed that chromium can be determined employing the external calibration technique using

aqueous standards. Under these conditions, the method developed allows the direct determination of chromium with limit of quantification of 11.5 ng g-1, precision expressed as relative standard deviation (RSD) in the range of 4.0 to 17.9% (n = 3) and a characteristic mass of 1.2 pg of chromium. The accuracy was confirmed by analysis of a certified reference material of tomato leaves furnished by National Institute of Standards and Technology. The method proposed was applied for the determination of chromium in five different infant formula samples. The chromium content found varied of 33.9 to 58.1 ng g-1 (n = 3). These samples were also analyzed employing ICP-MS. A statistical test demonstrated that there is no significant difference between the results found by two methods. The chromium concentrations achieved are lower than the maximum limit permissible for chromium in foods by Brazilian Legislation. Keywords: Chromium determination; Direct solid sampling; Infant formulas; High-resolution

continuum

source

electrothermal

atomic

absorption

spectrometry 1. Introduction Chromium is an element that can be toxic or essential for humans, depending on their chemical form and daily intakes [1,2]. In Brazil and in the world, legislation and medical community have set the daily recommended intakes (DRI) of chromium for infants, children and adults, according to the life stage [3,4]. In addition, Brazilian legislation has also set the permissible maximum level of chromium in foods at 0.10 ppm [5]. This way, the determination of chromium in foods is relevant for nutritional and toxicological reasons. Infant formulas have been often used as a substitute for breast milk during the infant’s first six months of life in several countries. These products must have high nutritional value to supply the infant’s energy and nutrients requirements. However, infant formulas can also present toxic elements due to raw materials used or from manufacturing process [6]. Therefore, the trace

analysis of these foods is of great relevance to obtaining data on infant exposure to trace elements. The electrothermal atomic absorption spectrometry (ET AAS) is an analytical technique often employed for the determination of trace elements due to its high sensibility and selectivity. In addition, alternate sample introduction techniques can be easily combined with ET AAS as the direct analysis of solid samples (DSS) [7,8]. This feature is very attractive for the analysis of complex matrices, including foods. The employment of DSS in trace analysis has several advantages, such as: less sample manipulation and operational work; minimum or no reagents consumption; less risk of sample contamination; high sensitivity and analysis frequency; besides reduce or avoid the toxic waste generation [7,9,10]. Since the advent of high-resolution continuum source electrothermal atomic absorption spectrometry (HR-CS ET AAS), the DSS has been extensively applied for the elemental analysis of several complex matrices, such as: crude oil [11], airborne particulate matter [12,13,14,15], medicinal plants [2,16], biomass [17], biological samples [18 27], coal [28 31], activated charcoal [32], carbon nanotubes [33], geological samples [34 36], cosmetic [37], plastic materials [38] and glass [39]. This technique has singular features in comparison to conventional line source atomic absorption spectrometry in the context of DSS, as more efficient background correction, with several strategies to eliminate spectral interferences; better ratio signal-noise, providing higher sensitivity; and possibilities to calibrate with aqueous standards solutions [40,41]. However, the HR-CS ET AAS combined with DSS has been little employed for the foods analysis: dos Santos et al developed two routine screening methods for the simultaneous determination of Cd and Fe in beans [42] and grain products [43]. This work proposes the development of a rapid, reliable and simple method for the determination of chromium in infant formulas based on cereals employing high-resolution continuum source electrothermal atomic absorption spectrometry combined with direct analysis of solid samples.

2. Experimental 2.1. Instrumentation A model ContrAA 700 high-resolution continuum source electrothermal atomic absorption spectrometer (Analytik Jena AG, Jena, Germany) was used for all absorbance measurements of chromium. The equipment has a XBO 301 xenon short-arc lamp (GLE, Berlin, Germany) with nominal power of 300 W, operating in the “hot spot” mode as continuum radiation source; a double monochromator with a prism pre-monochromator and high-resolution echelle monochromator with spectral resolution of about 2 pm per pixel; a chargecouple device (CCD) array detector; and a transversely heated graphite tube atomizer. Pyrolytically coated graphite tubes and platforms were used for the determinations and obtained from same manufacturer. As purge and protective gas, it was used argon with a purity of 99.996% (White Martins, São Paulo, Brazil) and flow rate of 2.0 L min−1 during all stages, except during atomization, when the gas flow was stopped. An SSA 600 solid sampling accessory (Analytik Jena AG) was used to introduce the solid sampling platforms into the graphite tube, after weighting in a Sartorius WZPW micro-balance (Göttingen, Germany) with precision of 1 µg. A stainless steel micro-spatula was used to transfer the samples to the solid sampling platforms. Micropipettes were used to inject the working solutions on the solid sampling platform. A model XSeries II quadrupole inductively coupled plasma mass spectrometer (Thermo Fisher Scientific, Bremen, Germany) was used for the Cr determinations in the infant formula samples after acid digestion procedure, as comparative method. The spectrometer has a hexapole collision cell, nickel sampler and skimmer cones. A model ASX-520 autosampler (Teledyne CETAC Technologies, Omaha, NE, USA) was used for the automatic sample introduction. 45Sc and 72Ge were used as internal standards to compensate acid effects and instrument drifts. The operating conditions were as follows: radiofrequency applied power of 1.3 kW, nebulizer gas flow-rate of 0.89 L min−1, plasma gas flow-rate of 13 L min−1, dwell time of 10 ms and three readings per replicate.

A model TE 040/25 heating block (TECNAL, Piracicaba, SP, Brazil) was utilized for the acid digestion procedure of the infant formula samples. 2.2. Reagents, solutions and samples All solutions were prepared by using ultrapure water obtained from a MilliQ water purification system (Millipore, Bedford, MA, USA) with a resistivity of 18.2 Mȍ cm. All reagents utilized were of analytical grade: 70% (m/m) nitric acid (J.T.Baker, Hexis Cientifica, Brazil), Triton X-100 (J.T.Baker), 30% (v/v) hydrogen peroxide (Merck, Darmsdat, Germany), 96% (v/v) ethanol (Merck), chromium and magnesium stock solutions (Qhemis, Joinville, SC, Brazil) containing 1000 mg L-1 of element and palladium stock solution (Qhemis) containing 10000 mg L-1 of element. A working solution containing 20 µg L-1 of Cr was prepared by appropriate diluting of the Cr stock solution in 0.5% (v/v) nitric acid. Two solutions containing oxidant and surfactant agents were prepared: a solution containing 0.05% (m/v) Triton X-100, 1% (v/v) H2O2 and 0.5% (v/v) HNO3; and other containing 20% (v/v) ethanol, 6% (v/v) H2O2 and 1% (v/v) HNO3. Chemical modifiers solutions were prepared from Mg and Pd stock solutions in 0.5% (v/v) HNO3: a solution containing 0.1% (m/v) Mg; and other containing a mixture of 0.1% (m/v) Pd plus 0.05% (m/v) Mg. The tomato leaves certified reference material (CRM), NIST 1573a, furnished by National Institute of Standards and Technology (Gaithersburg, MD, USA) was used in this work. The samples analyzed work were of infant formulas based on cereals: one based on rice and corn, one based on oat and rice and one each of corn, rice and oat. All these were acquired from supermarkets localized in Salvador City, Brazil. Before instrumental analysis, the samples were homogenized using porcelain mortar and pestle, except the infant formula sample based on rice and corn.

2.3. Evaluation of the thermal conditions The study of the thermal behavior of Cr was performed using an infant formula sample based on rice and corn. The salts Mg(NO3)2 and mixture Pd(NO3)2/Mg(NO3)2 were preliminarily investigated as chemical modifiers, according to data reported in literature [1,44 46]. The studies were performed by using sample masses up to 2.0 mg, 5 µg of Mg, 5 µg Pd + 2.5 µg Mg and 10 µL of solutions containing oxidant and surfactant agents. Pyrolysis and atomization temperatures were evaluated using univariate methodology with and without chemical modifier. 2.4. Calibration of the method The calibration of the method was evaluated by comparing the slopes of calibration curves obtained with aqueous standards and solid standards. The aqueous calibration curve was obtained by pipetting different aliquots in range from 4.0 to 20 µL of an aqueous standard solution containing 20 µg L-1 of chromium in 0.5% (v/v) nitric acid. The solid calibration curve was obtained by introducing different masses from tomato leaves CRM (NIST 1573a) in the range from 30 to 167 µg. In this case, the analytical blank measurements were performed according to the “zero mass” concept [9] by introducing a SS platform containing the mixture of 5 µL of the chemical modifier solution and 10 µL of the solution containing oxidant and surfactant agents. 2.6. Procedure for the digestion of infant formula samples A mass of about 0.5 g of each infant formula sample was directly weighted in a digestion tube, and 5.0 mL of 65% (m/m) nitric acid and 1.0 ml of 30% (v/v) hydrogen peroxide were added. “Cold finger” was used as reflux system. The samples were digested for 4 h being the temperature of the digester block at 130 °C. After cooling at room temperature, the residual solution was transferred to a volumetric flask and the volume completed to 14 mL with ultrapure water. Before analysis by ICP-MS, the solutions of the digested samples were diluted five times with ultrapure water.

2.5. Procedure for the direct determination of chromium in infant formula samples Masses up to 2.0 mg of infant formula samples were directly weighted on the solid sampling platform. Afterwards, a volume of 5 µL of 0.1% (m/v) Mg solution was added. In order to improve the contact of the modifier solution with the solid sample and to avoid the formation of carbonaceous residues, a volume of 10 µL of a solution containing 6% (v/v) H2O2, 20% (v/v) ethanol and 1% (v/v) HNO3 was also added. Then, the solid sampling platform was introduced into the graphite tube and the sample subjected to graphite furnace temperature program shown in Table 1. The absorbance measurements were performed using the main line of chromium at 357.8687 nm. The signals were obtained by peak area with integration time of 8 s using five pixels. All determinations were performed in triplicate and the absorbance signal was normalized for 1 mg of sample. Table 1 3. Results and discussion 3.1. Preliminary studies All preliminary studies were performed using an infant formula sample based on rice and corn. The pyrolysis and atomization temperatures were fixed at 1300 and 2300 ºC, respectively. During the preliminary study of chemical modifiers, it was observed a surface tension between the modifier solution and solid sample. Furthermore, it was also observed the formation of carbonaceous residues on the SS platform after each analytical measurement. This way, solutions containing surfactant and oxidant agents were evaluated in order to improve the contact between modifier solution and solid sample, and also prevent the formation of carbonaceous residues [8]. Initially, a solution containing 0.05% (m/v) Triton X-100, 1% (v/v) H2O2 and 0.5% (v/v) HNO3 was tested and the results achieved were not satisfactory, since this solution not avoided the formation of carbonaceous residues. Then, a

second solution containing 20% (v/v) ethanol, 6% (v/v) H2O2 and 1% (v/v) HNO3 was subsequently

tested. This second solution prevented the formation of

carbonaceous residues and also improved the contact between modifier solution and sample. A volume of 10 µL of this solution was found be sufficient for sample masses up to 2.0 mg. For the preliminary study of chemical modifiers two solutions were tested: the first containing 0.1% (m/v) Mg and another that was the mixture 0.1% (m/v) Pd + 0.05% (m/v) Mg. The integrated absorbance signals obtained for chromium without modifier and using the two chemical modifiers are shown in Table 2. The signals obtained no present significant differences, at 95% confidence level. However, it was observed that the mixture Pd + Mg promoted the corrosion of the SS platform surface in the presence of the solution containing oxidant and surfactant agents. Additionally, the formation of carbonaceous residues was not avoided under this condition. Probably, these facts can be explained of following way: transition metals of the platinum group, as palladium, cause the catalyst decomposition of hydrogen peroxide [47,48]. Under this condition, the sample matrix cannot be completely removed. Furthermore, oxygen formed from H2O2 decomposition can interact with the carbon of the SS platform surface, causing its corrosion. This way, 0.1% (m/v) Mg was chosen as chemical modifier for the further studies. Table 2 3.2. Study of the thermal conditions The thermal behavior of chromium in infant formula sample was investigated with and without Mg as chemical modifier. In Fig. 1 are shown the pyrolysis and atomization curves obtained for this study. In absence of modifier the pyrolysis temperature was varied in the range from 1200 to 1700 ºC, being the atomization temperature fixed at 2300 ºC. Under these conditions, the pyrolysis temperature established was of 1300 ºC. Above of this temperature, Cr presented low thermal stability, causing decrease of the analytical signal. When Mg was used as chemical modifier, the pyrolysis temperature was investigated in the range from 1200 to 1800 ºC, while the atomization

temperature was fixed also at 2300 ºC. In this case, the results demonstrated that chromium is stable thermally in all range of temperature studied (Fig. 1). This way, the pyrolysis temperature established was 1600 ºC, being that Mg was maintained as chemical modifier. Afterwards, the atomization temperature was evaluated in the range from 2100 to 2600 ºC. The temperature of 2400 ºC was chosen for the chromium atomization, considering both signal intensity and peak profile (Fig. 2). Figure 1 Figure 2 3.3. Validation and application The calibration of the method was evaluated by comparing the slopes of calibration curves obtained with aqueous standards and solid standards. The aqueous calibration curve was obtained in the mass range from 80 to 320 pg of chromium by pipetting different aliquots of an aqueous standard solution containing 20 µg L-1 of chromium in 0.5% (v/v) nitric acid. The solid calibration curve was obtained in the mass range from 59.7 to 332.3 pg of chromium using the tomato leaves CRM (NIST 1573a) furnished by National Institute of Standards and Technology. No significant difference was observed between the slopes of the calibration curves, since the slope of the aqueous curve was of (0.0013 ± 0.0001) s ng-1, while the slope of the solid curve was of (0.0012 ± 0.0001) s ng-1, both with correlation coefficients better than 0.9987, at 95% confidence level. These results demonstrated that the external calibration technique using aqueous standards can be used for the direct quantification of Cr in infant formula samples. Under this condition, the proposed method presented linearity up to 960 pg of Cr. Limits of detection (LOD) and quantification (LOQ) were obtained as being: (3s)/S and (10s)/S, respectively, in which “s” is the standard deviation of ten measurements of the analytical blank and “S” is the slope of the calibration curve using aqueous standards. This way, LOD and LOQ were of 3.4 and 11.5 ng g-1, respectively. Characteristic mass obtained for the proposed method was of 1.2 pg of Cr. The accuracy of the proposed method was confirmed by

analysis of the tomato leaves CRM (NIST 1563a). The chromium concentration found by this method using external calibration was (1.90 ± 0.09 mg kg-1) being that the certified value for chromium is (1.99 ± 0.06 mg kg-1). The proposed method was applied for the analysis of five different infant formula samples based on cereals. The chromium concentrations found varied from 33.9 to 58.1 ng g-1, with RSD values between 4.0 and 17.9%, as shown in Table 3. The RSD values obtained by the proposed method were satisfactory, considering the direct analysis of solid samples. All these five samples were also analyzed by ICP-MS after acid decomposition procedure. The results obtained by direct method were compared with those found by ICP-MS method using paired t-test. This statistical test revealed that there is no significant difference between the results obtained by two methods. The chromium concentrations found in the five samples analyzed were lower than the maximum limit for Cr in foods stipulated by the Brazilian Legislation, which is of 0.10 ppm [5]. Table 3 4. Conclusions The employment of HR-CS ET AAS combined with the solid sampling technique allowed the development of a simple and rapid method for the quantification of chromium in infant formulas based on cereals. The possibility of the direct quantification of chromium in the solid samples employing the external calibration technique using aqueous standards provides higher simplicity for the method. Furthermore, the proposed method presents analytical parameters satisfactory, such as: limit of quantification, precision and accuracy. The chromium contents found in the samples analyzed are lower than the maximum limit permissible by Brazilian Legislation.

Acknowledgements The

authors

gratefully

acknowledge

to

Conselho

Nacional

de

Desenvolvimento Científico e Tecnológico (CNPq), PRONEX/Fundação de Amparo à Pesquisa do Estado da Bahia (FAPESB), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for grants and fellowships. Figure captions Fig. 1. Pyrolysis and atomization curves of chromium in an infant formula sample based on rice and corn with 0.1% (m/v) Mg and without chemical modifier. Normalized absorbance signals for a sample mass of 1 mg. Fig. 2. Profile of the analytical signal of chromium in an infant formula sample based on rice and corn at the pyrolysis and atomization temperatures of 1600 ºC and 2400 ºC, respectively. TABLES Table 1 Graphite furnace temperature program used for the direct determination of chromium in infant formula samples by SS-HR-CS ET AAS. Step Drying1 Drying2 Pyrolysis Atomization Clean

Temperature (°C) 120 150 1600 2400 2550

Heating rate (°C s-1) 3 5 20 2500 500

Hold time (s) 25 15 20 8 4

Ar flow rate (L min-1) 2.0 2.0 2.0 0 2.0

Table 2 Results of the preliminary study of chemical modifiers for the direct determination of chromium in infant formula (n = 3, at 95% confidence level). Chemical modifier

Integrated absorbance* (s)

RSD (%)

Without modifier

0.07066 ± 0.04260

24.3

5 µg Mg

0.06660 ± 0.01928

11.7

5 µg Pd + 2.5 µg Mg

0.07060 ± 0.01838

10.5

*Normalized values for a sample mass of 1 mg.




Table 3 Results of the direct determination of chromium in infant formula samples by SS-HR-CS ET AAS and ICP-MS (n = 3, at 95% confidence level). Infant formula sample

SS-HR-CS ET AAS -1

ICP-MS

Found (ng g )

RSD%

Found (ng g-1)

Corn

33.9 ± 13.0

15.4

33.8 ± 3.7

Oat

47.4 ± 19.5

16.6

45.2 ± 9.5

Oat and rice

58.1 ± 20.1

14.0

49.0 ± 0.8

Rice and corn

55.1 ± 5.4

4.0

51.7 ± 4.5

Rice

52.9 ± 22.9

17.4

56.0 ± 4.4

Highlights:

A rapid and simple method was developed for the Cr determination in infant formulas. The method is based on the direct solid sampling combined with HR-CS ET AAS. No sample decomposition procedure is required. The method was successfully applied to analysis of infant formulas based on cereals.


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*Graphical Abstract (for review)

Figure

Figure

Highlights:

A rapid and simple method was developed for the Cr determination in infant formulas. The method is based on the direct solid sampling combined with HR-CS ET AAS. No sample decomposition procedure is required. The method was successfully applied to analysis of infant formulas based on cereals.