DETERMINATION OF SOME HEAVY METALS BY WET-DRY ASHING INEDIBLE OILS USING FLAME ATOMIC ABSORPTION SPECTROSCOPY

Transcription

1 DETERMINATION OF SOME HEAVY METALS BY WET-DRY ASHING INEDIBLE OILS USING FLAME ATOMIC ABSORPTION SPECTROSCOPY K.A.B.Najm 1,M.Al-Mahabis 2 and R.M.Atia 3 1 Faculty of Education, Al-Asmaryia Islamic university, Zliten-Libya 2 Faculty of Science, University of Tripoli, Tripoli-Libya 3 Faculty of Science, University of Misurata, Misurata-Libya ABSTRACT The determination of Cu, Cr, Fe, Mn, Ni, Sn and Zn in some Edible oils was performedby flame atomic absorption spectroscopy. The study assinged that wet-dry ashing method using sulfuric acid gave better results than dry and wet methods. Analysis by standard calibration curve and standard addition methods was more precise than solvent extraction method. Keywords: Heavy Metals, Wet-Dry Ashing, Edible Oils, Atomic Absorption Spectroscopy. 1. INTRODUCTION Vegetable oils are widely used in the cooking and food processing, cosmetics, pharmaceutical and chemical industries[1]. Plant and animals depend on some metals as micronutrients. Metal elements such as Na, K, Ca Mg, Fe, Cu, Zn and Mn, are essential nutrients for human growth. However, certain forms of some metals can also be toxic, even in relatively small amounts, and therefore pose a risk to the health of animals and people. Metal elements such as Cd, Pb, Co, and Cu, could also have detrimental effects on health. While the effects of chronic exposure to health. While the effects of chronic exposure to trace amounts of some metals are not well understood, many incidents tells us about the seriousness of high levels of

2 exposure to some toxic metals, especially cadmium, chromium, cobalt, metals, cadmium,,nickel and lead[2-4]. The presence of metals in vegetable oilsdue to the association of trace metal with origin of oils(soils and fertilizers),metal processing equipment,catalysts used for hydrogenation, toxicity of edible oils and the effect of metals on characteristics of finished products,such as color and teste[5]. Exact determination of traces of metals is a major step in evaluation and testing oils and fats, especially edible grades. It is very important to determine cumulative poisons such as Pb, contamination promoting rancidity,like Cu, trace metals affecting taste(cu and Fe) and metals building up in kidenys due to Sn(II).Sample preparation is acritical step in the analytical procedure for thedetermination of heavy metals in vegetable oils.classical methods usually employed are wet-digestion, dry-ashing, acid extraction, closdvessel and focused open-vessel microwavedissolution and dilution[6,7].thedetermination of these metals inthe vegetable oils requires specific analyticalprocedures such as emission and atomicabsorption spectrophotometric techniques as wellas electroanalytical techniques[8-12]. The determination of heavy metals in edible oils using dry-ashing,wetashing and solvent extraction methods suffer disadvantages such as the possible loss of volatile metal species duringashing, contamination in the course of the digestion or chelation process, or nonquantitative recoveries especially when numerous extraction steps are involved [13,14]. In this paper we report an investigation on the feasibility of wet-dry ashing for determination of some heavy metals in edible oils using mineral acids and

3 their mixtures. The comparsion with dry-ashing,wet- ashing and solvent extractionwas also discussed. 2. MATERIALS AND METHODS All metals solutions under investigation were prepared from high-purity metals or its salts from Riedel-De-Hean,CosmicChemicas using double distilled water. The concentrated acids employing were high purity specific for trace analysis. Flame atomic absorption spectrophotometer(model Hitachi ) was used fordetermination of metal ions.samples of edible oils (Olive,Sunflower and Corn) commercially available were purchased from the market usedthought out study. Procedures of dry ashing,wet ashing and solvent extraction methodes were utilized by Oome and Van Pee[14],while wet-dry ashing used by Al-Ammar [15].Only mineral acids or mixtures of acids in different volume ratio wereused such as H2SO4, HNO3, HClO4, H2SO4: HNO3(1:1), HNO3:H2O2(1:1), HNO3:HClO4(1:1) and HNO3:HClO4(1:2).The procedure used by Al-Ammar as followed: 50.00gm of oil was weighted exactly into 400ml beacker containing 25ml con. H2SO4. The beacker was heated on hot plate for destruction of oil with gradual increase temperature, inordor to prevent suppetering of the solution. The solution evaporated to dryness until the white vapor ellapsed. The remaing precipitate transferred quantitatively into porcelain crucible and ignited in muffle furnace at 525 C, starting from 50 C until 525 C at the rate of 50 C for each half hour to get red of carbonate material for 15 hours. The crucible cooled in desiccator, then the remain ash dissolved in

4 10 ml (1:1)HNO3volume ratioand heate d to boiling for 2 mints. The solution cooled and filtrated with whatman NO.41, then trnsferred to volumetric flaskand diluted to mark with dist. water. Blank solution was employed using (1:1) HNO3. The procedure was repeated using the above acids and their mixture.the results of analysis are listed intable(1).in this study H2SO4 was considered as best choice to wet-dry ashing method. 3. RESULTS AND DISCUSSION Table(1) shows the results of heavy metals analysis using minerals acids and its mixtures for samples of olive,sunflower and corn oils respectivelly. H 2 SO 4 andh 2 SO 4 :HClO 4 (1:2) were considered to be the best acids for Cu metal by wet-dry ashingfor all oils samples. HNO 3 or HNO 3 :HClO 4 (1:1) was excluded due to partial loss during digestion,which happened largely in HNO 3 relative to other acids and its mixtures.for Zn metal, H 2 SO 4 and H 2 SO 4 :HNO 3 (1:1) mixture gave better results than other acids(lower precssion). HNO 3 :HClO 4 (1:1) and HNO 3 :HClO 4 (2:1) mixtures gave lower concentration compared with H 2 SO 4,althougth the coefficient of variation nearly same. This might be due to partial voltalization of Zn as aresult of precence HNO 3 during digestion.this confirmed the right conclusion to partial lose for Zn happened highly in the case of HNO 3, HClO 4 and their mixtures. Also, H 2 SO 4, H 2 SO 4 :HNO 3 (1:1) and, HClO 4 were considered best acids for digestion to Cr metal. For digestion of Mn metal H 2 SO 4, HClO 4 and HNO 3 :HClO 4 (2:1) mixture were the best choice.h 2 SO 4 and HClO 4 gave best results(good precssion) for Fe metal. Also, we noticed that the usage ofhno 3 or mixtures excepth 2 SO 4 :HNO 3 (1:1) mixture gave lower

5 concentration level relative to H 2 SO 4 orhclo 4. This attributed to partial lose of Fe due to oxidizing property of HNO 3 during digestion processe.h 2 SO 4, HClO 4 and H 2 SO 4 :HNO 3 (1:1) mixture was considered best choice to ashing processe for Ni metal. The results of analysis for sunflower using various ashing methods and comparsion with standard calibration, standard addition and solvent extraction methods are precentedin Table(2). It was seen from Table(2) that solvent extraction gave lower concentrations for all metals with higher coefficient of variation especially for Cu, Cr, Mn and Sn compared with other metals. This was due to low efficiency of extraction to heavy metals as a result strong organo-metallic bonding between metal ion and oil which is difficult to break it by solvent extraction.theconcentrations of all metals obtained by dry-ashing were low compared with wet-dry ashing,in addition the coefficient of variation for most metals under study was high. This variation might be due to partial voltalization for few metals and adsorption on crucible surface. The results of analysisby wet-ashing were better precssion than the dry-ashing except Mn metal which might be due to low efficiency of oxidizing agent resulting incomplete ashing. The results of analysis obtained by wet-dry ashing were more precise than both dry-ashing and wet-ashing. The results of heavy metals obtained using standard addition method was better precssion than standard calibration curve due to low effect of interferences. Finally standard calibration curve was axcceptable relative to standard addition method using wet-dry ashing by H 2 SO

6 The concentrations of Cu and Cr metals were within standard permissible limit( 0.1 and 0.3ppm respectively), while for Fe metal was above the standard limit (5 ppm) regardless of oil type. The same was also applicable to Mn metal in which the concentration above the standard limit of (0.1 ppm). For Zn metal the concentration was within standard limit of (0.6 ppm), while for Sn metal above standard limit of (0.1 ppm)

7 - 80 -

8 4. CONCLUSION The results of analysis of heavy metals in edible oils reaveled that wet-dry ashing using H 2 SO 4 gave better results compared with other acids and their mixtures. Also the results appeared that metals presented in this study were within international standard limit except Fe,Mn and Sn metals. REFERENCES [1] Dugo. G, la Pera L, La Torre GL, Giuffrida D Determination of Cd (II), Cu (II), Pb (II), and Zn (II)content in commercialvegetableoilsusing derivative potentiometric stripping analysis. Food Chem. 87,

9 [2] Buldini PL, Ferri D, Sharma JL Determination of some inorganic species in edible vegetable oils and fats by ion chromatography. J. Chromatogr. A, 789, [3] Demirbas.A Concentrations of 21 metals in 18 species of mushrooms growing in the East Black Sea region. Food Chem. 75, [4] Garrido DM, Frias I, Diaz C, Hardisson A Concentrations of metals in vegetable edible oil.food.chem. 50, [5] La Pera L, Lo Coco F, Mavrogeni E, Giuffrida D, Dugo G.2002a.Determination of copper (II), lead (II), cadmium(ii) and zinc (II) in virgin olive oils producedin Sicily and apulia by derivative potentiometricstripping analysis. Italian J. Food Sci. 14, [6] Allen LB, Siitonen PH, Thompson HC Jr Determination of copper, lead, and nickel in edible oilsby plasma and furnace atomic absorption spectroscopies. J. Am Oil Chem. Soc. 75, [7] Garrido DM, Frias I, Diaz C, Hardisson A Concentrations of metals in vegetable edible oil.food Chem. 50, [8] Hendrikse PW, Slikkerveer FJ, FolkersmaA,Dieffenbacher A Determination of copper, ironand nickel in oils and fats by direct graphite furnaceatomic absorption spectrometry. Pure Appl. Chem. 63, [9] Coco FL, Ceccon L, Ciraolo L, Novelli V Determination of cadmium (II) and zinc (II) in Olive oilsby derivative potentiometric stripping analysis. Food Control 14, [10] Buldini PL, Ferri D, Sharma JL Determination ofsome inorganic species in edible vegetable oils andfats by ion chromatography. J. Chromatogr. A, 789, [11] Zeiner, M., Steffan, I., Cindric, I., J. (2005). Determination of trace elements in Olive oil by ICP-AES and ETA-AAS: A pilot study on the geographical characterization.microchem. J. 81, [12] Cindric I J, Michaela Z, Steffan I Trace elemental characterization of edible oils by ICP AES andgfaas. Microchem. J. 85, [13] Karadjova I, Zachariadis G, Boskou G, Stratis. J Electrothermal atomic absorption spectrometricdetermination of aluminium,cadmium,chromium,copper, iron, manganas, nickel and lead in olive oil. J.Anal. Atom. Spect.13, [14] Ooms.R and Van Pee.W.1983.Determination of trace metal content in Corn oil by atomic absorption spectroscopy. J. Am Oil Chem. Soc.60, [15] Al-Ammar.A, Najm K. A. B and Hussan.A.M.1997.Application of Generalized Internal Reference Method for Accuracy and Precision Improvement In Trace Elements Analysis In Biological Samples, M.Sc.Thesis,Iraq