Detection and analysis of cadmium in foods by chromatographic methods or universal luminescence detection
School of Science and Technology, CSU-Riverina
Cadmium was identified as a distinct element in 1817. Its acute toxicity was soon recognised. Early recorded cases of cadmium poisoning were generally due to industrial exposure involving inhalation of cadmium dusts. Nowadays acute intoxication is relatively rare whereas long-term chronic exposure is of considerable concern. The dangers of chronic exposure were noted in the 1950s following several suggestive incidents in the previous decade. However, the greatest concern over cadmium pollution was generated by the so-called Itai Itai incident in Japan. The source of the cadmium was rice which had been grown in irrigation water contaminated with the effluents of a mining operation. The relationship between cadmium intake and toxicity is complex with factors such as sex, age and calcium nutrition affecting susceptiblity. The Itai Itai incident focused attention on cadmium by demonstrating that chronic cadmium poisoning constituted a health hazard to the general population and was not restricted to industrial workers.
Cadmium finds wide application in plastics as a stabiliser, as a phosphor in television screens, in Ni-Cd batteries and in ceramic glazes. As a result of this use increasing quantities of cadmium find their way into the environment. The main anthropogenic sources can be identified as mining operations, waste incineration, coal/oil combustion and fertilisers. Cadmium is distinguished from other pollutants such as organochlorine pesticides by the fact that it is not biodegradable and, once in the environment, its potential toxicity is controlled largely by its physico-chemical form. Cadmium is therefore characterised by a long environmental persistence which accounts for its bioaccumulation in individuals. However, unlike the organochlorine pesticides there is little evidence for the biomagnification of cadmium in the food chain.
Reported concentrations of cadmium in various samples (Table 1) vary widely reflecting the variation in environmental distribution and also analytical methods. In collaborative surveys (Department of the Environment Analytical Quality Control Harmonized Monitoring Committee and the Guildford Trace Element Quality Assurance Scheme) many participating laboratories did not achieve the required accuracy for determination of dissolved cadmium. This failure was attributed to both random and systematic errors. The situation with more complex samples, as represented by foodstuffs, is likely to be less favourable. Numerous analytical procedures have been applied to the determination of cadmium (Table 2). Of these, spectrometric methods feature in approximately 90% of reported methods.
Work is currently proceeding on the determination of cadmium levels in pasture and sheep kidneys using atomic absorption and anodic stripping voltametry for measurement. One aspect of this project is the development of an inexpensive procedure for cadmium determination based on luminescence detection. The system developed involves flow injection and detection of the cadmium via the hydrogen peroxide-luminol reaction.
The reaction of luminol with hydrogen peroxide to produce light is catalysed by a number of metal ions, notably copper and cobalt. However, this direct approach is limited in its application to metal ions exhibiting a catalytic effect on the luminol reaction. One solution to this problem is the use of ion displacement for the detection. In this approach a cation exchange column in the copper or cobalt form is incorporated in the flow injection manifold (Figure 1). The sample is injected into the carrier stream (C) and displaces the copper or cobalt ions from the cation column. The latter ions react with the luminol-hydrogen peroxide to produce light which is detected. This procedure has been examined and provides a universal detection method for mono-, di- and tri-valent metal ions. It has the advantage of lower detection limits relative to fluorescence systems because of the elimination of the radiation source. Specificity can be provided by incorporating ion chromatography in the manifold. Work is proceeding on this application with the intention of applying it to measurement of cadmium levels in pasture and animals.
The contribution of Professor P.J. Worsfold and the late Bolei Yan is acknowledged.