Spectrochimica Acta Part B 86 (2013) 142–145

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Spectrochimica Acta Part B

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Analytical Note

Determination of molybdenum in plants by vortex-assisted emulsification solidifiedfloating organic drop microextraction and flame atomic absorption spectrometry

Jenny A. Oviedo, Lucimar L. Fialho, Joaquim A. Nóbrega ⁎Departamento de Química, Universidade Federal de São Carlos, P.O. Box 676, São Carlos-SP, 13560-970, Brazil

⁎ Corresponding author. Tel.: +55 16 33518058; fax:E-mail address: djan@terra.com.br (J.A. Nóbrega).

0584-8547/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.sab.2013.02.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 November 2012Accepted 18 February 2013Available online 27 February 2013

Keywords:SFODME8-HydroxyquinolineMicroextractionMolybdenumPlant

A fast and sensitive procedure for extraction and preconcentration of molybdenum in plant samples based onsolidified floating organic drop microextraction combined with flame atomic absorption spectrometry anddiscrete nebulization was developed. 8-Hydroxyquinoline (8-HQ) was used as complexing agent. The experi-mental conditions established were: 0.5% m v−1 of 8-HQ, 60 μL of 1-undecanol as the extractant phase, 2 minvortex extraction time, centrifugation for 2 min at 2000 rpm, 10 min into an ice bath and discrete nebulizationby introducing 200 μL of solution. The calibration curve was linear from 0.02 to 4.0 mg L−1 with a limit of detec-tion of 4.9 μg L−1 and an enhancement factor of 67. The relative standard deviations for ten replicate measure-ments of 0.05 and 1.0 mg L−1 Mo were 6.0 and 14.5%, respectively. The developed procedure was applied fordetermining molybdenum in corn samples and accuracy was proved using certified reference materials.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Molybdenum is an essential element required in low concentrationsfor growth of many organisms. In plants, there is a narrow range of con-centrations that cause deficiency or toxicity [1].

Trace concentrations of Mo can be determined using UV–visiblespectrophotometry [2–4], flame atomic absorption spectrometry (FAAS)[5], graphite furnace atomic absorption spectrometry (GFAAS) [6,7],inductively coupled plasma optical emission spectrometry (ICP OES) [8],inductively coupled plasma mass spectrometry (ICP-MS) [9], andvoltammetry [10]. Flame AAS is commonly used due to its simplicityand relatively low cost. However, the direct determination of Mo inenvironmental samples is difficult due to its low concentration, theoccurrence of matrix interferences and formation of refractory com-pounds [11]. However, all these effects can be minimized by adoptingsteps of separation and preconcentration prior to analytical measure-ments by FAAS.

The most widely used method for separation and preconcentrationof trace amounts of metal ions is liquid–liquid extraction (LLE) [3].However, LLE is time-consuming and requires large volumes of expen-sive and toxic organic solvents. In order to overcome this limitation,new methods of microextraction have been developed. These methodsuse a small volume of organic solvent and the determination of analytesat low concentrations is possible due to the enrichment factor achieved.Solvent microextraction or liquid phase microextraction is commonlyused as sample pre-treatment methods. In both methods, extraction

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occurs in a small amount of a water-immiscible solvent from an aque-ous sample containing the analyte [12,13]. This procedure can be classi-fied into three main categories: single drop microextraction [14],dispersive liquid–liquid microextraction (DLLME) [7], and hollowfiber microextraction [15].

A new method of liquid–liquid microextraction based on DLLMEnamed solidified floating organic drop microextraction (SFODME)was firstly reported in 2007 [16]. This method was applied for the ex-traction and determination of polycyclic aromatic hydrocarbons bygas chromatography/flame ionization detection. In SFODME, a smallamount of an organic solvent with a melting point close to ambienttemperature is added in aqueous solution and floated on the surface.The aqueous phase is stirred, and then the flask containing the sampleis transferred into an ice bath.When the organic solvent phase solidifies,it is transferred to a small conical tube and when it melts at room tem-perature it can be used for the determination of analytes [16]. Thismeth-od has the advantage of simplicity, fast extraction time, low cost, lowconsumption of organic solvent, and high enrichment factor.

Afterwards, SFODME was applied to the determination of inorganicspecies using different complexing agents depending on the analyte.Dadfarnia and co-workers determined Pb in water samples by GFAAS[17]. Cobalt and Ni were also determined in water samples by GFAAS[18]. Cadmium was determined in water samples based on the extrac-tion of an ion-pair ([CdI4]2−) into 1-undecanol by flow injection-FAAS[19]. Solidified FODME has been applied for the determination of Cu[20], Cd and Pb [21], V [22], Cr [23], and Hg [24].

Vortex mixing has been combined with microextraction for speed-ing up the process [25]. Jia and co-workers developed a procedure fordetermination of pesticides in water samples by a gas chromatographymicro electron-capture detector named vortex-assisted liquid–liquid

143J.A. Oviedo et al. / Spectrochimica Acta Part B 86 (2013) 142–145

microextraction. Limits of the detection varied from 0.01 to 0.07 μg L−1

[25].The aim of the study here described was to develop a simple and

accurate procedure for determination of Mo in plant samples basedon SFODME and coupled to FAAS with discrete nebulization.

2. Material and methods

2.1. Apparatus

Measurements were performed with a Varian AA240FS atomic ab-sorption spectrometer (Mulgrave, Australia) equippedwith a backgroundcorrectorwith adeuteriumarc lamp. AMohollowcathode lampwasusedas radiation source. The analytical wavelength (313.3 nm), spectral band-width (0.5 nm) and lamp current (7 mA)were used as recommended bythe manufacturer. A nitrous oxide-acetylene flame was used with flowrates of 11.0/7.6 L min−1, respectively, and burner height was kept at10 mm.

Peak height and peak area were used to monitor the analytical sig-nals for continuous nebulization and discrete nebulization, respectively.A volume of 200 μL of sample was manually introduced with a pipettetip connected to the nebulizer aspiration tube for discrete nebulization.

All pH measurements were carried out using a PHS-3B digital pHmeter (Phtek, China). A centrifuge (Hermle/Labnet Z200A, Germany)was used to accelerate phase separation. A vortex mixer (Thermolynetype 37600 mixer, Dubuque, IA, USA) was used to assist the processesof extraction and separation.

2.2. Reagents and analytical reference solutions

All reagents used were of analytical grade. Solutions were preparedwith ultrapure water obtained from a Milli-Q® purification system(Millipak-40 Filter Unit 0.22 μmNPT, Bedford,MA, USA)with resistivityhigher than 18.2 MΩ cm. Laboratory glassware and polypropyleneflasks were kept in 10% v v−1 nitric acid solution for 24 h and thenwashed with ultrapure water before use. Analytical reference solutionswere prepared by appropriate dilutions of the stock solution of Mocontaining 1000 mg L−1 (Qhemis, High Purity, Hexis, Jundiaí, SP,Brazil) with ultrapure water. The complexing agent, 0.5% m v−1

8-HQ (Vetec, Rio de Janeiro, RJ, Brazil) solution was daily preparedby dissolving the appropriate amount of 8-HQ in 0.15 mol L−1

hydrochloric acid solution and stored in a brown glass flask. Concen-trated nitric acid, hydrochloric acid, and hydrogen peroxide wereobtained fromMerck (Darmstadt, Germany). Nitric acid was purifiedin a sub-boiling apparatus (Milestone, Sorisole, Italy) and diluted asneeded. Acetate buffer (1.5 mol L−1)was prepared to adjust sample pHto 4.75. Extracting solvents, 1-undecanol 99% v v−1 and 1-dodecanol98% v v−1, were obtained from Sigma-Aldrich (Saint Louis, MO, USA).Ethanol 99.5% v v−1 (Tec Lab, São Paulo, SP, Brazil) was also used.Two standard reference materials were employed for checking the ac-curacy of the developed procedure: rice flour NIST 1568a (NationalInstitute of Standards and Technology, Gaithersburg, MD, USA) andwhite cabbage BCR-679 (Institute for ReferenceMaterials andMeasure-ments, Geel, Belgium).

2.3. Sample preparation

Corn roots and leaves were collected from plants cultivated usingmineral nutrient solutions in water. Afterwards, samples were washedby stirring for a few seconds in distilled–deionizedwater containing de-tergent; then, were rinsed with distilled–deionized water in successiveportions to remove all residual detergents and then placed on absorbentpaper. Samples were dried at 65–70 °C and sieved through a 1 mmsieve. Ground sampleswere stored in bottles of highdensity polyethyleneat room temperature. Microwave-assisted sample decomposition wasperformed using a single reaction chamber oven (Ultrawave, Milestone),

and sample masses of 500 mg were microwave-assisted digested using5 mL sub-boiling distilled HNO3 solution 2.0 mol L−1 plus 3 mL of H2O2

of 30% m m−1. After completing the digestion program, vessels werecooled down, and the digests were transferred to a 15 mL conical tubeand dilutedwithwater to 13.0 mL. Themicrowave oven heating programwas performed in three steps: (1) 14 min to reach 120 °C; (2) 11 min toreach 250 °C; and (3) 8 min at 250 °C. In all steps 1.5 kW of microwavepower was applied.

2.4. Microextraction procedure

In a 15 mL conical tube, 120 μL of standard solution containing Mo,1 mL of acetate buffer and 2.5 mL of 8-HQ solution were mixed andthe solution was diluted to 8 mL with ultrapure water and then left atroom temperature for about 10 min to allow complete formation ofthe Mo–8-HQ complex. Then, a volume of 60 μL of 1-undecanol wasadded and the mixture was shaken using a vortex mixer for 2 min. Sep-aration of the two phases occurred upon centrifugation at 2000 rpm for2 min. After this process, the conical tube was transferred into an icebath and the organic solvent was solidified after 10 min. The solidifiedsolvent was then separated from the solution and it melted when leftat ambient temperature. Finally, the extract was diluted to 500 μL withethanol for reaching a volume compatible with duplicate measurementsby discrete nebulization and a volume of 200 μL was manually injectedinto the FAAS.

3. Results and discussion

In this work, SFODME, vortex mixing and discrete nebulizationwere combined for the determination of trace concentrations of Moin plants using FAAS. Aiming to reach a high enrichment factor, effectsof different parameters on extraction conditions, such as type and vol-ume of extraction solvent, and effect of vortex mixing, were optimizedadopting a multivariate approach. The organic solvent selected as anextracting solventmust have amelting point (mp) around ambient tem-perature (i.e. in the range of 10 to 30 °C) [16]. We tested 1-undecanol(mp 15 °C) and 1-dodecanol (mp 24 °C) for extractingMo. The compat-ibility of these solvents with vortex-assisted solidified floating organicdrop microextraction (VA-SFODME) was investigated by adding 60 and90 μL, respectively, of each one to an 8.0 mL aqueous solution containing1.5 mg L−1 of Mo. The complexing agent used was 8-HQ.

3.1. Complexing agent concentration

The separation of metal ions by VA-SFODME involves prior forma-tion of a complex with sufficient hydrophobicity that allows it to beextracted in a small volume of the organic phase. Before evaluatingthe microextraction procedure, the effect of 8-HQ concentrations onanalytical signals obtained by FAAS was studied using concentrationsfrom 0.25 to 1% m v−1. A reference solution of 5 mg L−1 Mo, com-plexation time of 10 min and pH adjusted to 4.75 with acetate bufferwas adopted for this study [9]. In our experiments, the highest incrementin absorbance was reached when working with 8-HQ concentration of0.5% m v−1 and it was selected as the complexing agent concentrationfor further experiments.

3.2. Evaluation of VA-SFODME procedure

The aim of this study was to develop a faster microextraction proce-dure whereby dispersion of the extractant phase into the aqueous phaseis achieved using vortexmixing. The following variables were evaluated:type of solvent, solvent volume, and agitation time using a factorialdesign 23 with a total of 8 experiments (Table 1). The response studiedwas the enrichment factor for Mo absorbance signals measured byFAAS. It may be seen in Table 2 that the interaction between the vari-ables type of solvent and solvent volume had a significant effect at a

Table 1Levels of variables in factorial design 23.

Variable Level (−) Level (+)

Solvent 1-Undecanol 1-DodecanolSolvent volume (μL) 60 90Time (min) 2 4

Table 2Experimental conditions and results of factorial design 23 for VA-SFODME procedure.

Condition Solvent Volume(μL)

Time(min)

RSD (%),n = 3

Enrichmentfactora

1 1-Undecanol 60 2 12.1 16.12 1-Dodecanol 60 2 10.1 10.83 1-Undecanol 90 2 12.3 7.24 1-Dodecanol 90 2 14.6 15.25 1-Undecanol 60 4 1.4 15.26 1-Dodecanol 60 4 8.6 8.27 1-Undecanol 90 4 8.0 6.38 1-Dodecanol 90 4 14.1 15.0

a The enrichment factor is the ratio of the slope of the calibration curve after and beforeextraction.

Table 4Determination of Mo in plant materials (mean ± standard deviation, n = 6).

Sample Found (mg kg−1) Certified (mg kg−1) Recovery (%)

Rice flour NIST-1568a 1.59 ± 0.35 1.46 ± 0.08 108.9White cabbage BCR-679 14.84 ± 1.39 14.8 ± 0.5 100.3Corn roots 5.08 ± 1.23 – –

Corn leaves 1.17 ± 0.66 – –

144 J.A. Oviedo et al. / Spectrochimica Acta Part B 86 (2013) 142–145

confidence level of 95%. Values of RSD for these experimentswere lowerand values of enrichment factors between 6.3 and 16.1 were obtained.Time did not exert a critical effect and a condition with only 2 minof agitation by vortex for simplicity and speediness of the procedurewas chosen. Condition 1 using 60 μL of 1-undecanol with vortex mixingfor 2 min was chosen as the optimal condition for the microextractionprocess and this was used for the determination of Mo in microwave-assisted acid-digested corn samples. Condition 4 also showed a high en-richment factor, however it had the difficulty in the solvent solidificationbefore extraction at the laboratorywork temperature, and this is a disad-vantage for practical purposes and for routine applications.

3.3. Figures of merit

Performance of the VA-SFODME procedure was established byprocessing reference solutions of Mo. The calibration graph was linearfrom 0.02 to 4.0 mg L−1 under the optimized conditions. The equationof the linear calibration graph obtained by the preconcentration proce-durewas A = 0.9388C − 0.0604 (where A is the integrated absorbanceand C is the concentration of Mo (mg L−1) in organic phase) with a lin-ear correlation coefficient of 0.9979. The limits of detection and quanti-fication defined as 3Sb/m and 10Sb/m (where Sb is standard deviation ofthe blank and m is the slope of the calibration graph) were 4.9 and16.5 μg L−1, respectively. The RSDwere 6.0 and 14.5% (n = 10) for solu-tions containing 0.05 and 1.0 mg L−1 of Mo(VI). The enrichment factorcalculated as the ratio of the slope of the calibration curve of the analytesfor the VA-SFODME method to that obtained without preconcentrationwas 6 without discrete nebulization and the enhancement factor was

Table 3Comparison of the limits of detection of procedures for determination of Mo in plants.

Procedure Detection LOD (μg L−1)

Solid phase extraction Spectrophotometry 38Liquid–liquid extraction Spectrophotometry 4.6Liquid–liquid microextraction FO-LADSa 1.43

HR-CS FAASb 140Cloud point extraction ICP-MS 0.8Micelle-mediated extraction Spectrophotometry 1VA-SFODME FAAS 4.9

a Fiber optic-linear array detection spectrophotometry.b High-resolution continuum source flame atomic absorption spectrometry.

67 when using with discrete nebulization, showing that there is a pro-nounced increase of sensitivity for the determination of Mo by FAASwhen combining discrete introduction of the sample solution with themicroextraction process. The enhancement factor contains the solventeffect or other improvements related to the nebulization process [26].Solutions obtained by microextraction presented relatively high viscosi-ty, consequently, continuous aspiration of the sample and subsequentnebulization and atomization were negatively affected. The determina-tion of Mo in plant samples was carried out using discrete nebulization.

A comparison of the limits of detection obtained by the developedprocedure with those reported by several other approaches for thedetermination of Mo in plants is shown in Table 3.

3.4. Determination of Mo in plant samples

The accuracy of the developed procedure was evaluated by deter-mination of Mo in two standard reference materials. According to at-test the determined concentrations agreed with certified values ata 95% confidence level (Table 4). These results indicate that thematricesof the plant samples did not cause any major effect on the VA-SFODMEfor determination of Mo by FAAS. The developed procedure was appliedto the determination ofMo in samples of corn roots and leaves (Table 4).Taking into account the adopted digestion conditions the LOD in digestswas 0.68 mg kg−1.

4. Conclusions

A simple procedure for the determination ofMo in plants combiningVA-SFODME and FAAS with discrete nebulization was developed usingvortex mixing for dispersing the organic phase in the aqueous phase.The developed procedure improved the sensitivity for determiningMo in plants without requiring high sample masses or high volumesof organic solvents leading to minimum generation of residues. Theselatter aspects are critical needs for green chemistry procedures andare compatible with modern trends in analytical chemistry [28].

Acknowledgments

The authors are grateful to the ConselhoNacional deDesenvolvimentoCientífico e Tecnológico (CNPq) for the fellowship provided to J.A.Oand the researchship provided to J.A.N. Financial support providedby Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Enhancement factor Linear range (μg L−1) Reference

– Up to 5000 [2]– 25–150 [3]72.6 – [4]– – [5]70 – [9]– 3–50 [27]67 20–4000 This work

145J.A. Oviedo et al. / Spectrochimica Acta Part B 86 (2013) 142–145

and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior(CAPES) are gratefully acknowledged. The authors also express theirgratitude do Dr. Gilberto Batista de Souza (Embrapa Pecuária Sudeste)for providing corn samples.

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