Analytical note
Surfactant/oil/water system for the determination of selenium in eggs by graphite furnace atomic absorption spectrometry

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Abstract

An oil-in-water formulation has been optimized to determine trace levels of selenium in whole hen eggs by graphite furnace atomic absorption spectrometry. This method is simpler and requires fewer reagents when compared with other sample pre-treatment procedures. Graphite furnace atomic absorption spectrometric (GF AAS) measurement was carried out using standard addition calibration and Pd as a modifier. The precision, expressed as relative standard deviation, was better than 5% and the limit of detection was 1 µg L 1. The validation of the method was performed against a standard reference material Whole Egg Powder (RM 8415), and the measured Se corresponded to 95.2% of the certified value. The method was used for the determination of the Se level in eggs from hens treated with Se dietary supplements. Inorganic and organic Se sources were added to hen feed. The Se content of eggs was higher when hens were fed with organic Se compared to the other treatments. The proposed method, including sample emulsification for subsequent Se determination by GF AAS has proved to be sensitive, reproducible, simple and economical.

Introduction

Selenium (Se) is an essential trace element for animals and humans. It has been implicated with the protection of body tissues against oxidative stress, maintenance of defense against infection, and modulation of growth and development. Chronic Se deficiency may enhance susceptibility to viral infection, cancer, cardiovascular disease, thyroid dysfunction and various inflammatory conditions. However, in spite of its important action, excessive intake can cause harmful effects in the organism [1].

Food is the main source of selenium, and the content of this element is related with the food origin and processing. There is a widespread use of selenium supplementation by enriching commercial foodstuffs for farmed animals with sodium selenite or selenium-enriched yeast [2]. Eggs are of great importance as they provide a significant amount of biologically available forms of Se due to their high nutritional value and capacity to provide 50% of the daily required Se intake in humans [3]. In addition, Se-enriched eggs have been designed and introduced in various markets and are becoming popular [4].

Owing to the essentiality/toxicity dichotomy of selenium, there are numerous procedures for its determination in biological samples and, especially, in foods [5]. It is often determined by graphite furnace atomic absorption spectrometry (GF AAS) or hydride generation absorption spectrometry (HG AAS) [6]. The choice depends on the matrix composition and selenium species present in the sample [7]. While the HG AAS response is strongly dependent on the selenium form, GF AAS is adequate for the measurement of both organic and inorganic species.

Methods for determining trace elements should involve minimal sample handling; however, literature for selenium determination in eggs reports methods of sample treatment involving digestion with acids and peroxides [8], [9], and some have attempted to improve sample digestion by combining the use of these reagents with ultraviolet [10] or microwave radiation [2], [11]. This results in additional steps, which may lead to inconveniences such as sampling errors, contamination and losses during handling [3]. In addition, acids might interfere in HG AAS and GF AAS measurements and, therefore, inaccurate results for the determination of selenium can be attributed to the resistance of selenium compounds to oxidation and the volatility of selenium species present or formed [6].

The egg is a system emulsified naturally by components such as phospholipids, lipoproteins and proteins [12], [13]; however due to high viscosity and high content of organic matter its direct introduction in the furnace for GF AAS measurement is particularly difficult. Direct emulsification with surfactants provides a rapid procedure for sample preparation since this approach does not require any destruction of the organic matrix [14]. It simply reduces the viscosity and the organic content of the sample, maintaining its homogeneity and stability, making the properties of the egg emulsion appropriate to being analyzed by GF AAS.

The purpose of this study was to develop, for the first time, a method for Se determination by GF AAS in whole chicken eggs preparing an emulsified system containing non-ionic surfactant, egg sample, oil and water.

Section snippets

Instrumentation/procedure

A Model ZEEnit 600 atomic absorption spectrometer (Analytik Jena, Jena, Germany) equipped with a, transversely heated graphite tube atomizer, a MPE 60z auto-sampler and transversal Zeeman-effect background correction system operating in two or three-field mode was used for all measurements. A SpectrAA (Varian, Australia) selenium hollow cathode lamp was used as radiation source. Argon 99.996% (White Martins, São Paulo, Brazil) was employed as a purge and protective gas. The optimum operating

Formulation studies

The emulsion type (oil-in-water (O/W) or water-in-oil (W/O)), stability and rheological behavior are known to depend upon three types of variables, i.e. physicochemical formulation, composition and fluid mechanical conditions, as well as the way these variables are handled or programmed during the formulation protocol [15]. Thus, the amount of oil, surfactant, water and form of agitation were optimized in order to produce stable and low viscosity emulsions.

Four criteria were used to choose the

Conclusion

The main advantages of the proposed method over traditional digestion techniques are that it does not require a time-consuming sample treatment, large amounts of organic solvents or inorganic acids; it is simple and shows good accuracy and reproducibility. The major challenges and main requirement for emulsion application in atomic spectrometry were to obtain stable emulsions with low viscosity. The proposed method including sample emulsification for subsequent Se determination for GF AAS has

Acknowledgement

The authors thank the CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for scholarships.

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    As the stability of the system depends on the conditions of preparation, the procedure was carried out carefully and exactly the same way for all samples. Ieggli et al. [65–69] stipulated 2 min of manual mechanical stirring to reach emulsification, followed by magnetic stirring at 3 000 rpm for 15 min at room temperature for complete stabilization of the system. The choice of the most suitable emulsifier for a given emulsion requires a certain level of experience due to the variety of available agents.

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This paper was presented at the 10th Rio Symposium on Atomic Spectrometry, held in Salvador-Bahia, Brazil, 7–12 September 2008, and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference.

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