Determination of Heavy Metals in Soil and Plants by Atomic Absorption Without Prior Dissolution of the Sample

Determination of total heavy metals in soil and plant material normally requires the dissolution of samples, often achieved by dissolution in dilute acid following dry ashing or by wet oxidation using oxidising acids. This gives poor recovery unless hydrofluoric acid is used to destroy the silicate matrix. Such techniques may also lead to contamination and are time consuming unless large batch digestion systems are used to reduce the actual time required per sample. Non-dissolution methods such as Emission Spectroscopy in a DC carbon arc often suffer from poor precision and requires complex standards to account for severe matrix interferences. The use of solid samples when analysing by atomic absorption has been less popular until recently when applications have been reported in which samples are analysed, without prior dissolution using either electrothermal atomisation or atomisation in a flame using the Delves microsampling cup technique.
Methods are described in which soil and plants are analysed by atomic absorption using both the above approaches to solid sample introduction. The introduction of a slurried finely ground soil sample to an electrothermal atomiser to determine cadmium and lead and the direct introduction of unground solid samples of vegetation into a flame using the Delves cup technique.
The slurry ETA-AAS method showed sufficient accuracy and precision for the determination of lead and cadmium in soil for the purpose of environmental surveys. It certainly would merit further work to ascertain its usefulness for less volatile elements which may be of interest in broader geochemical studies. Other work using thixotrophic thickening agents has been successful and there is no reason to believe that the simpler approach used by ourselves should not be equally successful.
The direct Microsampling cup solid sampling technique offers much by way of increased sensitivity for the determination of lead in small sections of plant material dissected from an individual plant. It offers great promise for applications involving determining the distribution of lead in vegetation that has been affected by lead absorbed from the soil or from the air. The technique has certain operational disadvantages since it is time consuming to weigh individual milligram samples into cups. Whilst contamination errors from reagents and grinding are overcome, weighing errors and contamination from handling can be a problem if great care is not taken.
In this paper we wished to point out some of the advantages and disadvantages of solid sampling-AAS and to point to potential environmental applications which, we contend, are great.