научная статья по теме DETERMINATION OF ZN(II) IN ALCOHOL FUEL BY FLAME ATOMIC ABSORPTION SPECTROMETRY AFTER ON-LINE PRECONCENTRATION USING A SOLID PHASE EXTRACTION SYSTEM Химия

Текст научной статьи на тему «DETERMINATION OF ZN(II) IN ALCOHOL FUEL BY FLAME ATOMIC ABSORPTION SPECTROMETRY AFTER ON-LINE PRECONCENTRATION USING A SOLID PHASE EXTRACTION SYSTEM»

ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2012, том 67, № 5, с. 504-510

ОРИГИНАЛЬНЫЕ СТАТЬИ

УДК 543

DETERMINATION OF Zn(II) IN ALCOHOL FUEL BY FLAME ATOMIC ABSORPTION SPECTROMETRY AFTER ON-LINE PRECONCENTRATION USING A SOLID PHASE EXTRACTION SYSTEM © 2012 г. Vanessa N. Alves1, Rafael Mosquetta1, Eduardo Carasek2, Nívia M. M. Coelho1

1Instituto de Química, Universidade Federal de Uberlandia Uberlandia 38400-902, MG, Brazil 2Departamento de Química, Universidade Federal de Santa Catarina Florianópolis 88040900, SC, Brazil Received 29.05.2010; in final form 30.08.2011

The method for the determination of zinc in alcohol fuel by flame atomic absorption spectrometry using a solid phase extraction system containing Moringa oleifera seeds as biosorbent material is described. The multivariate optimization of hydrodynamic variables was performed using a full factorial design (24) including the following factors: sorbent mass, preconcentration time, volume of eluent, sample flow rate, sample pH and eluent concentration. It was verified that the aforementioned factors as well as their conditions were statistically significant at the 95% confidence level. With the optimized conditions, the preconcentration factor and the limit of detection were estimated as 23 and 0.9 p.g/L, respectively. The analytical curve was linear from 0 to up to 100 p.g/L, with a correlation coefficient of 0.999. The developed method was successfully applied to alcohol fuel, and accuracy was assessed through recovery tests with results ranging from 96 to 100%.

Keywords: zinc, preconcentration, alcohol fuel, Moringa oleifera.

Brazil has positioned itself on the international stage as the only country to introduce alcohol as fuel alternative to petroleum as a fuel on a large scale [1]. Although less polluting than fuels derived from petroleum, knowledge of the types of organic and inorganic contaminants present in alcohol fuel is essential, since some of these species are harmful to health and the environment when emitted as pollutants in the combustion process.

Metal ions are the main inorganic contaminants present in alcohol fuel, and since they are present in extremely low concentrations they represent the greatest challenge in terms of the development of analytical methodologies. Technical standards adopted for the quantification of heavy metals in alcohol fuel involve procedures that require long analysis times and specialist labor.

The most widely used technique for the determination of metals in alcohol fuel is atomic absorption spectrometry, and many different analytical strategies can be found in the literature for such determinations [2]. These include direct analysis of the sample [3, 4], or analysis in conjunction with preconcentration techniques, such as solid phase extraction, where the sample passes through a column packed with a complexing agent [5, 6] and is subsequently eluted for analysis.

Preconcentration as an initial stage in the analytical procedure is necessary when the concentration of the analyte in the sample is below or near the detection

limit of the analytical tool [6, 7]. Several preconcen-tration techniques have been used to determine trace metals by chemical sorption on modified solid supports, particularly in natural water and ethanol fuel samples [8—16].

Studies involving preconcentration of metals are carried out mostly with the use of commercially available adsorbents. However, biosorbent materials are very attractive when employed in metal adsorption processes, since they present low toxicity and are simple to use, economical, and easy to obtain [19—21].

M. oleifera is the best known species of the Morin-gaceae family. It is a plant native to northwest India but it has spread all over the world, mainly in tropical countries.

Studies involving the use of M. oleifera for the treatment of industrial effluents in terms of the removal of heavy metals have been published [22]. Our research group has been exploring the use of M. oleifera seeds as biosorbent in a flow system for Ag(I) determination in waters [23]. However, the use of this material in preconcentration of zinc in alcoholic matrices has not been reported in the literature.

Therefore, this study aims to develop an on-line preconcentration methodology coupled to flame atomic absorption spectrometry for zinc determination in alcohol fuel samples.

(а)

(b)

Fig. 1. Schematic diagram of the on-line preconcentration system for zinc determination by FAAS. (a) preconcentration position and (b) elution position. C — mini-column containing M. oleifera seeds and L — eluent loop.

EXPERIMENTAL

Instrumentation. A Varían Model SpectrAA 220 (Victoria, Australia) flame atomic absorption spectrometer, with air-acetylene flame, was used for zinc determination. A zinc hollow cathode lamp was run under the conditions recommended by the manufacture and conventional values were used for the wavelength, slit width and burner height.

A Gehaka PG1800 pH meter was used to set the pH of the samples and working solutions. A MinipulsTM 3 (Gilson, Villiers-Le-Bel, France) peristaltic pump equipped with eight channels and Tygon® and polyethylene tubes were used to pump the solutions through the mini-column (60 x 3 mm) in the elution and preconcentration steps.

Reagents and solutions. All working solutions were prepared with ultra pure water obtained from a Milli-Q (Bedford, MA, USA) water purification system. All reagents were analytical grade. All laboratory glassware was previously washed with neutral detergent and then kept overnight in 10% (v/v) nitric acid solution and washed with deionized water.

The working solutions used in this study were prepared through dilution of a 100 mg/L stock solution of zinc (Carlo Erba, Italy) in 95% ethanol (Cromoline, Sao Paulo, Brazil). Solutions of 0.1 M HNO3 and NaOH were used to adjust the pH. The nitric acid solutions used as the eluent was prepared through dilution in water of concentrated nitric acid obtained from Merck (Darmstadt, Germany).

Preparation of mini-column. The M. oleifera seeds used to construct the mini-column were obtained from trees cultivated in the city of Uberlândia (Minas Gerais, Brazil) and collected during September—November 2007. The seeds were separated from the pods, washed in deionized water and dried at 25 ± 2 °C. After drying, the seeds were crushed in a blender for home

use (Black & Decker, Sao Paulo, Brazil) and passed through 850 ^m sieves.

A mini-column with length and internal diameter of 60.0 and 2.0 mm, respectively, was filled with adsorbent material. The ends of this mini-column were sealed with small glass wool beds to prevent material losses.

Preconcentration system. The schematic diagram of the on-line preconcentration system for Zn(II) determination by FAAS is shown in Fig. 1. During the preconcentration step (Fig. 1a), the injector is kept in the loading position, the samples or working solutions are pumped through the mini-column and the effluent is discharged. In the elution step (Fig. 1b), the injector is switched to the injection position, and 100 ^L of eluent percolates at a flow rate of 4.5 mL/min through the mini-column in the opposite direction to that of the preconcentration step. The eluate is carried directly to the nebulization system of the FAAS.

The mini-column was always regenerated in the elution step using an elution time of 30 s. Signals were measured as peak height by using instrument software. The operating conditions were established and the determination was carried out.

It is important to note that the elution was performed in the opposite direction to the sample loading in order to avoid progressive tightening of the packed column.

Optimization of the system. The on-line preconcentration system was optimized using the multivari-ate method in order to determine the best conditions for zinc determination, considering maximum sensitivity and best reproducibility. A two-level full factorial 24 design with a central point and 19 runs in total was carried out, in duplicate, to determine the influence of the selected factors and their interactions in the preconcentration system. The factors selected were: pre-

Table 1. Conditions for Zn(II) preconcentration and analytical response for the study of multivariate optimization using SPE with M. oleifera seeds column and determination by FAAS

Run Preconcentration time, min Volume of eluent, p.L Biosorbent mass, mg Sample flow rate mL/min Absorbance

1 4.0 100 30.0 3.5 0.1833

2 4.0 100 30.0 6.0 0.1459

3 4.0 100 60.0 3.5 0.2319

4 4.0 100 60.0 6.0 0.2014

5 4.0 300 30.0 3.5 0.1721

6 4.0 300 30.0 6.0 0.2508

7 4.0 300 60.0 3.5 0.2091

8 4.0 300 60.0 6.0 0.2412

9 10.0 100 30.0 3.5 0.2040

10 10.0 100 30.0 6.0 0.3188

11 10.0 100 60.0 3.5 0.3890

12 10.0 100 60.0 6.0 0.3960

13 10.0 300 30.0 3.5 0.2335

14 10.0 300 30.0 6.0 0.4641

15 10.0 300 60.0 3.5 0.4033

16 10.0 300 60.0 6.0 0.3736

17 7.0 200 45.0 4.75 0.2288

18 7.0 200 45.0 4.75 0.2041

19 7.0 200 45.0 4.75 0.2215

concentration time, volume of eluent, sample flow rate and mass of adsorbent.

From the optimum hydrodynamic conditions, an assessment was made to verify the influence of the sample pH and eluent concentration on the precon-centration efficiency. The experiments were carried out in duplicate, using a 10.0 ^g/L solution of Zn(II).

Samples. The matrices tested consisted of commercial samples of alcohol fuel obtained from three different gas stations in the city of Uberlandia. These samples showed concentrations of analyte below the detection limit of the method, and thus to assess the recovery of analyte they were fortified at levels of 0— 50.0 |ig/L.

RESULTS

Evaluation of hydrodynamic parameters. The multi-variate optimization strategy was employed in order to achieve optimum levels of hydrodynamic parameters associated with the preconcentration system. The factorial 24 design was useful in establishing the physical parameters, whose results are shown in

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