Direct analysis of Antarctic krill by slurry sampling: determination of copper, iron, manganese and zinc by flame atomic absorption spectrometry

Journal of the Brazilian Chemical Society

Alves, Flávia L.¹  Universidade Estadual de Campinas, Campinas, Brazil¹  Centro Atómico Constituyentes, Buenos Aires, Argentina¹  Centro Atómico Ezeiza, Buenos Aires, Argentina¹  Arruda, Marco A. Z.¹  Smichowski, Patricia¹  Farías, Silvia¹  Marrero, Julieta¹ 

关键词: Antarctic krill;    slurry;    FAAS;    ICP-AES;   

DOI  :  10.1590/S0103-50532000000400007

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Most analyses by flame and plasma atomization techniques are carried out on solutions using continuous nebulization to introduce the sample. In this context, the mineralization of solid samples which is necessary prior to the analysis, which increases both the time of analysis and the risk of sample contamination inherent in the dissolution of samples. For these reasons, there is considerable interest in the development of analytical methods which allow the direct analysis of solid samples. In recent years, numerous methods have been developed in order to introduce solid samples into a variety of sources for subsequent elemental analysis1,2. Electrothermal atomization, laser ablation and glow discharge have been employed to quantify minor and trace elements in solid materials. However, a number of factors must be considered to obtain an analytical performance comparable to that achieved with liquid sampling. The introduction of the sample as a slurry is an alternative that has been used to analyze a diversity of elements in different matrices3-8. Comprehensive reviews on this methodology have been reported in the literature9,10. Slurry sampling is now an accepted technique and a very convenient way to analyze solid samples.The slurry sample technique arose in 1974, developed by Brady et al.11,12 and combines the advantages of solid and liquid sampling13. The main advantages of the slurry sampling technique, through direct solid sample atomization, include simplification in the prior sample treatment, time reduction in sample decomposition, minimization of contamination and analyte losses, reduction of costs and reagent consumption, as well as avoiding the use of hazardous chemicals.Particle size and the lack of homogeneity in the sample can especially affect the precision and accuracy of the analysis, accomplished through the slurry technique. Particle size is a critical factor in slurry preparation and must be optimized to a particle diameter that will also contribute to good sample agitation14. The limited range of slurry concentration is a drawback that must also be considered. This technique permits the use of aqueous standards and/or methods of standard addition10,15 to be applied for quantification.The slurry technique associated with flame atomic absorption spectrometry (FAAS) was first applied by Willis,16 who determined several metals in geological samples. Some problems can occur in the flame; the principal difficulty is related to the slurry particle transport through the nebulizer.16-21 In order to solve these problems related to the transport efficiency of the suspended particles in the slurry, some authors proposed the use of special nebulizers.22-25Krill (Euphasia superba) is a small shrimp extremely abundant in the Southern Ocean that can vary in length from less than 1 cm to ca. 15 cm, being mostly transparent, with large black eyes, a brught red shell, and often a vividly green stomach, but it becomes curled up and opaque when dead. The importance of krill sample analysis is based on the fact that this crustacean is a prime food source for seals, penguins and whales and these higher animals probably would not survive in the absence of the krill. The krill can accumulate trace elements and other chemicals26 and this shows promise for use as an indicator of ongoing contamination in the most pristine continent of the planet. Finally, krill is also consumed by humans, more specifically, in countries such as Chile, Russia, Japan, Poland, South Korea and Ukraine26.The Cu, Zn, Mn and Fe determinations in krill samples are based on the importance of these elements in animal and human nutrition. Also, these elements constitute some of those that are included in the certification program for Antarctic samples. In this way, the aim of this study is to explore the development of a rapid and simple slurry method to determination of Cu, Fe, Mn and Zn in Antarctic krill and to compare the results obtained with those performed in digested krill samples by flame atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry (ICP-AES). The analysis was carried out on a certified reference material (MURST-ISS-A2). This material was prepared in the framework of the Programma Nazionale per la Richerca in Antarctica (PNRA, Italian National Program for Antarctic Research) and was coordinated by the Istituto Superiore di Sanità (ISS, Itálian National Institute of Health). ExperimentalInstrumentationA Perkin-Elmer model AAnalyst 300 flame atomic absorption spectrometer, equipped with a deuterium lamp background correction system (Norwalk, CT, USA) was used throughout this work for the analysis of slurries and digested krill samples. Hollow cathode lamps (Perkin-Elmer, Darmstadt, Germany) of Cu, Mn, Fe and Zn were used as the radiation source and the analytical measurements were based on time averaged absorbance. The principal FAAS operating conditions are summarized in Table 1.    A Perkin-Elmer (Norwalk, CT, USA) ICP 400 sequential inductively coupled Ar plasma atomic emission spectrometer was used for the analysis of mineralized krill samples. Instrumental operating conditions are summarized in Table 2.    Solutions of Antarctic Krill were prepared by acid-assisted microwave (MW) digestion using a MLS-2000 (Milestone-FKV, Sorisole, Bergamo, Italy) equipped with temperature and pressure sensors, TFM (tetrafluoromethaxil) digestion bombs and with a magnetron of 2450 MHz.A Cole-Parmer model 8890 ultrasonic bath (Vernon Hills, Illinois 60061, USA) was used for homogenization of the slurry prior to its introduction into the nebulizer system.The material to be analyzed was observed under a Philips 515 (Netherlands) scanning electron microscope (SEM) that was operated at 30 kV for secondary electron (SE) imaging.Standards and reagentsWelding Ar from AGA (Buenos Aires, Argentina) was found to be sufficiently pure for Cu, Mn, Fe and Zn ICP-AES determination. Acetylene from Air Liquid (Campinas, SP, Brasil) was employed for FAAS measurements.All solutions were prepared with analytical quality chemicals (Merck, Darmstadt, Germany) and distilled/deionized water was used throughout this work. The HNO3 used for sample digestion and slurry preparation was distilled in a quartz sub-boiling still (Marconi, Piracicaba, Brasil). The H2O2 used for sample mineralization was of Suprapure grade (Merck, Darmstadt, Germany). For FAAS measurements a 1000 mg L-1 of Zn(II) [from Zn(NO3)2] and Fe(III) [from Fe(NO3)3] solutions were prepared as stock solutions. Cu and Mn stock solutions were prepared from their metallic forms (99.99% of purity). The standard solutions for Cu, Fe and Zn were prepared daily by making serial dilutions with 2 mol L-1 HNO3 and with 4.0 mol L-1 for Mn. For ICP-AES, the standards were prepared from Titrisol (Merck) flask.Sample preparation proceduresTwo different sample introduction approaches (slurry sampling and nebulization of solutions) were set up and compared to evaluate the possibility of using slurry sampling as a valid alternative for the determination of trace metals in Antarctic krill.Slurry preparation of krill samplesThe slurry was prepared by weighing 250 mg of lyophilized krill in a volumetric flask and completing the volume to 25 mL with 2.0 mol L-1 HNO3 for Cu, Fe and Zn. In the

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