Comparative analysis of dry ashing and wet digestion methods for the determination of trace and heavy metals in food samples

doi:10.1016/j.foodchem.2014.10.097

Food Chemistry

Volume 173, 15 April 2015, Pages 682-684

https://doi.org/10.1016/j.foodchem.2014.10.097 Get rights and content

Highlights

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Two processing methods were compared in heavy metal analysis of five food groups.
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Dry ashing method is recommended based on four major advantages.
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The modified processing methods proposed achieved excellent recoveries.

Abstract

This study compared the dry ashing and wet digestion methods of processing food samples for elemental analysis. The concentrations of trace elements (manganese, iron, copper and zinc) and heavy metals (chromium, cadmium, lead and nickel) were determined in varieties of samples classified as fruits, leafy and fruity vegetables, tubers, legumes and cereals, obtained from Abeokuta, South-West, Nigeria. The metal concentrations were determined using atomic absorption spectrophotometer according to standard methods. The accuracy of the procedures was confirmed by spiking some samples and evaluating their recoveries. The metal levels evaluated were relatively higher in the dry-ashed samples than the wet-digested samples. However, the results showed non-significant differences in most data obtained after processing with the two methods. The dry ashing method is recommended for digestion of food items in these categories because: it involves lesser amount of chemical reagents and related hazards; it requires simple equipment and achieved better recovery.

Introduction

Heavy metals are among the major contaminants of food supply and may be considered the most important problem to the environment (Zaidi, Asrar, Mansoor, & Farooqui, 2005). Such a problem is becoming more serious all over the world especially in developing countries. Considering the level of technological advancement in the developing world, it becomes very necessary to obtain accurate and reliable data on the concentrations of elements in the commonly consumed foods. Furthermore, the elemental composition of food items differs from one group to another due to their different particulate nature and structural matrix. This makes it quintessential to analyse each food group with the method that will provide such accurate and reliable data.

Flame/graphite furnace atomic absorption spectrometry (F/GFAAS) is the most utilised instrument for the determination of trace and heavy metal contents of food samples in most analytical laboratories. Many analytical methods including AAS for trace element determination in food materials require decomposition of the sample. The dry ashing, wet and microwave digestions are used for sample decomposition prior to the determination of trace elements by AAS (Saracoglu, Saygi, Uluozlu, Tuzen, & Soylak, 2007). The dry ashing and wet digestion methods have been applied in several analyses of food samples (Akinyele and Osibanjo, 1982, Bahemuka and Mubofu, 1999, Demirel et al., 2008, Onianwa et al., 1999, Onianwa et al., 2001, Onianwa et al., 2000, Saracoglu et al., 2007, Tuzen, 2003). However, in most cases, these methods were applied separately on the samples processed and fewer metals were evaluated. At other times when they were applied together with more metals involved, fewer food groups were considered. Thus, the aim of this study was to contribute to the establishment of more reliable, routine method for the determination of manganese, iron, copper, zinc, chromium, cadmium, lead and nickel in five food groups (fruits, leafy and fruity vegetables, tubers, legumes and cereals). Dry ashing and wet digestion methods were applied in processing the samples in an effort to recommend the more appropriate digestion method in determining the eight metals in these food groups. The reliability of the methods for estimation of trace elements and heavy metals in the samples has been checked by spiking some samples with metal standards and determining their recovery rates.

Section snippets

Apparatus

The dry ashing was done using the Uniscope muffle furnace (model SM 9080, Surgifriend Medicals, England), while the wet digestion was done using the Digestion System 40, 1000 heating unit of JC Tecator. A Buck Scientific Atomic Absorption Spectrophotometer (210 VGP model, East Norwalk, Connecticut, USA) was used for determination of the metals. The determination was carried out in an air/acetylene flame. The operating parameters for the elements determined were set as recommended by the

Results and discussion

The average recovery rates (%) of the minerals analysed after the dry ashing and wet digestion are presented in Table 2. The recovery values from both dry-ashed and wet-digested samples were nearly quantitative (>90%), except in the chromium analysis, where the recoveries were about 80%. This did not vary significantly with the food type spiked. In most of the samples spiked, dry-ashed samples had slightly elevated recovery rates. The recovery values of iron and copper from samples processed

Conclusion

In this study, we have been able to analyse food samples covering wider scope (five food groups) with the two methods. The dry ashing procedure will be recommended because of four reasons: it requires the use of smaller amount of chemicals hence cost effective; it involves lesser risks associated with chemical usage; it requires simple equipment (muffle furnace) that is easily handled; and it achieved better recovery in the samples. Moreover, the obtained analytical values make this modified

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Short communicationComparative analysis of dry ashing and wet digestion methods for the determination of trace and heavy metals in food samples

Highlights

Abstract

Introduction

Section snippets

Apparatus

Results and discussion

Conclusion

Food Chemistry

Food Chemistry

Journal of Hazardous Materials

Food Chemistry

Food Chemistry

Short communication
Comparative analysis of dry ashing and wet digestion methods for the determination of trace and heavy metals in food samples