Optimized microwave digestion procedure for cadmium analysis of mussel samples

Abstract

The determ i n ation of cadmium and other trace metals in foods such as mussels provides information on the extent of a q u atic ecosystems contaminat i o n , because these biva l ve s have been widely used as bioindicators of coastal contamination [1], and lead to an evaluation of their suitability for human consumption. Analysis of cadmium in biological and food samples tra d i t i o n a l ly has been perfo rmed by at o m i c absorption spectrometry. This requires a sample preparation b e fo re photometric identifi c ation. Decomposition of the organic matrices of these samples is a neccesary preliminary task for accurate determination of trace elements, because a complete or partial destruction or re m oval of the orga n i c m at rix is re q u i red to minimize chemical and/or phy s i c a l i n t e r fe rences. Exceptions to this are certain beve rage s , t o which it is possible to analyse directly without sample pretreatment. Therefore, digestion of the organic matter is a requisite for cadmium analysis in mussel samples. A great number of wet and dry ashing methods are know n , but it is generally considered that dry ashing causes a loss of cadmium at temperatures over 500 °C [2][3], and wet digestions are tedious and time consuming. Microwave energy is curre n t ly used, since their implementation results in the improvement of existing processes such as digestion. The activation of processes by exposure to microwaves gives rise to faster and cleaner reactions when compared to conventional heating. This type of radiation is characterised by a high penetration effect into the materials, easy handling and rapid energy transfer to the whole sample without surface overheating, which reduces the digestion time to a few minutes [4][5]. The vessels used for microwave acid digestion can be low [6][7] and high pressure bombs [8]. With high pressure bombs, the pressures and temperatures needed for the total digestion of the sample are reached very quickly, and volatile elements as cadmium are not loss from these sealed vessels.The purpose of this paper is to study the digestion of mussel samples using microwave energy in high pre s s u re bombs with nitric acid for cadmium determination by a continuous flow injection system, including a preconcentration ion-exchange process, coupled to a flame atomic absorption s p e c t rometer similar to that rep o rted by Cespón-Romero et al. [9]. Experimental ApparatusTo homo genize the samples an Ultra-Turrax T25 (Janke and Ku n ke l , GmbH and Co. K) and an Omni-Mixer and H o m oge n i zer (Omni Intern at i o n a l , USA) we re used. Th e homogenized samples were freeze-dried with the Bench Top Shell Fre e ze r, model 77570, 7757-01 (Lab c o n d o Corporation, USA). The digestion was carried out in a Parr 4782 Microwave Digestion Bomb (Pa rr Instru m e n t C o rp o rat i o n , U S A ) , wh i ch is designed specifi c a l ly fo r microwave heating. The bomb is made with a new, highstrength microwave-transparent material to enclose a chemically inert teflon sample cup of 45 mL. A...