Direct and rapid determination of ultratrace heavy metals in solid plant materials by ET-AAS ultrasonic-assisted slurry sampling.

Introduction: Plants can be used as bioindicators in the study of contamination processes by heavy metals. Most of the analytical methodologies used for determination of metals in plants are based on atomic techniques with previous wet digestion of the solid samples. Methodologies that allow direct metal measurements in solid samples are very attractive alternatives.

Objective: To develop a new procedure for direct analysis of copper, nickel, cadmium and lead at very low concentration levels in leaves based on electrothermal atomic absorption spectroscopy (ET-AAS) with introduction of samples as a slurry.

Methodology: In order to obtain accurate and precise results even at very low concentrations, the different parameters that influence the sample slurry preparation such as acid percentage, presence of stabilising agents and ultrasonic probe operation were studied. Instrumental parameters such as chemical modifier and temperature and times for drying, pyrolysis and atomisation steps that influence ET-AAS measurement were optimised.

Results: Optimal slurry conditions for copper and nickel determination were 0.5% Tween 85 with 5% nitric acid. For lead and cadmium analysis the best results were obtained in 5% nitric acid without stabilising agents. The achieved detection limits were 0.023 mg/kg for copper, 0.018 mg/kg for nickel, 0.0002 mg/kg for cadmium and 0.009 mg/kg for lead. For validation purposes, the method was applied to metal analysis in a pine needles reference material.

Conclusion: According to our knowledge, the detection limits obtained are the best reported in the literature. The methodology was successfully used in metal determinations in actual leaf samples.