Determination of lead and cadmium in food samples by the coprecipitation method

Abstract

In this study, the coprecipitation method developed using a combination of 2-mercaptobenzothiazole (MBT) as a chelating reagent and copper as coprecipitate carrier was used for the determination of trace lead and cadmium in various food samples by graphite furnace atomic absorption spectrometry (GFAAS). The method was applied for the determination of Pb(II) and Cd(II) in salami, sausage, chicken, anchovy, spinach, cabbage, onion, dill, parsley, lettuce, tea and rice samples. The matrix modifiers were added as 50 μg NH₄H₂PO₄ + 3 μg Mg(NO₃)₂ for both Pb(II) and Cd(II). The signals were measured as peak area. The concentrations of Pb(II) and Cd(II) in the food samples were found to be in the range of 6.63 ng g⁻¹ (anchovy) −3.30 μg g⁻¹ (spinach) and 2.67 ng g⁻¹ (salami) −0.51 μg g⁻¹ (lettuce), respectively.

Introduction

Lead and cadmium are toxic elements. Both metals cause adverse health effects in humans and their widespread presence in the human environment comes from anthropogenic activities (Viňas, Pardo-Martínez, & Hernández-Córdoba, 2000). The most important sources of lead exposure are industrial emissions, soils, car exhaust gases and contaminated foods. Vegetables with a relatively large leaf area, such as spinach and cabbage can contain high levels when grown near lead sources (Gama, da Silva, & Lemos, 2006). The entrance of lead at levels >0.5–0.8 μg/mL into blood causes various abnormalities. Lead accumulates in the skeleton, especially in bone marrow. It is a neurotoxin and causes behavioural abnormalities, retarding intelligence and mental development. It interferes in the metabolism of calcium and vitamin D and affects haemoglobin formation and causes anaemia (Memon, Hasany, Bhanger, & Khuhawar, 2005). Cadmium ions are easily absorbed by vegetables and, in animal-based food, are principally distributed in the liver and kidneys. The highest cadmium concentrations are found in rice, wheat, oyster, mussels and the kidney cortex of animals (Gama et al., 2006). The determination of trace Pb(II) and Cd(II) in food samples is difficult due to various factors, particularly low concentration and matrix effects. Although graphite furnace atomic absorption spectrometric (GFAAS) method is a powerful analytical tool for determining trace elements in environmental samples, a preliminary separation and preconcentration step are necessary to eliminate any matrix components, and to improve the detection limit. The widely used techniques for the separation and preconcentration of trace metals include liquid–liquid extraction (Tôei, Motomizu, & Yokosu, 1979), coprecipitation (Korn et al., 2006, Tokalıoğlu et al., 2007), flotation (Magda, Dalia, & Ahmed, 2006), ion exchange (Elson & Macdonald, 1979) and solid phase extraction (Tokalıoğlu, Oymak, & Kartal, 2004). In general, a separation/preconcentration method for the Pb and Cd analyses of various food samples by GFAAS in literature is not used. So, the use of a coprecipitation method for food samples can be an advantage.

The coprecipitation method is useful for the concentration of trace metal ions, and is often combined with graphite furnace atomic absorption spectrometry for the determination of the trace metal ions (Minami et al., 2003, Nakajima et al., 2003). The main requirement for this technique is that the collector should be easily separated from the matrix solution. This can be done by filtering or centrifuging and then washing of the precipitate. In addition, it is desirable that the collector should be a pure and readily available substance. The advantages of this technique are its simplicity and the fact that various analyte ions can be preconcentrated and separated simultaneously from the matrix. Various coprecipitation procedures including use of organic and inorganic coprecipitants have been developed. Inorganics which are magnesium (Wu & Boyle, 1998), indium (Şahin, Tokalıoğlu, Kartal, & Ülgen, 2005), aluminium (Döner & Ege, 2005), cerium (Divrikli & Elçi, 2002), terbium (Minami, Sohrin, & Ueda, 2005) and iron (Nakajima et al., 2003) hydroxides and organic coprecipitants, generally dithiocarbamates of bismuth (Sato & Ueda, 2001), copper (Chen et al., 1997, Mao et al., 1998), nickel (Sato & Ueda, 2000) and cobalt (Elçi, Şahin, & Öztaş, 1997) have been widely used as efficient collectors for trace elements.

In the present study, Cu(II)-mercaptobenzothiazole (Cu(II)-MBT) was used as a new coprecipitant in order to determine the trace Pb(II) and Cd(II) in food samples (vegetable, tea, rice, salami, sausage, chicken, liver and fish) by GFAAS. The experimental conditions for coprecipitation of the Pb(II) and Cd(II) onto Cu(II)-MBT, including pH, amount of carrier element, reagent amount, sample volume, standing time, and matrix ions have been optimised in our previous study (Tokalıoğlu et al., 2007). This method has several advantages in comparison with the other coprecipitation methods. It has lower detection limit (DL) and relative standard deviation (RSD) values. The method is reliable, simple, economic, fairly rapid and precise. The time required for the coprecipitation method was about 30 min. The recoveries of elements in the presence of the most common matrix elements containing the alkaline and alkaline earth metals and transition metals were fairly good.