Direct determination of nickel in heroin and cocaine by electrothermal atomic absorption spectrometry using deuterium arc background correction combined with chemical modification

Abstract

A comparative study of the use of nitric acid and magnesium nitrate as chemical modifiers, using a deuterium arc background corrector, in the direct determination of nickel in cocaine and heroin was developed. Cocaine samples were dissolved in 2 ml of 35.0% nitric acid whereas heroin was dissolved in water. Pyrolysis temperatures of 1500 and 1600 °C were obtained for nitric acid and magnesium nitrate, respectively, for cocaine and heroin solutions, a deuterium lamp being adequate to correct the background signals obtained. The sensitivity was better with the use of magnesium nitrate, the limit of detection being 32.1 µg kg^–1 compared with 34.3 µg kg^–1 with nitric acid. The within-batch precision (n= 11) and the analytical recovery were adequate with the use of both chemical modifiers. The introduction of a cooling step was studied through the within-run precision, resulting in an optimum temperature of 200 °C. However, the sensitivity obtained by using cooling was poorer than that given by programmes without a cooling step. Similarly, the omission of the pyrolysis step did not give satisfactory results. Studies on interferences and on the variation of the analytical performance with the amount of sample were also carried out with the use of magnesium nitrate as the most suitable chemical modifier. Finally, the method was applied to determine nickel in heroin and cocaine samples confiscated by the Spanish Police, the concentrations found in heroin being between 0.50 and 1.35 mg kg^–1 and in cocaine between 0.07 and 0.34 mg kg^–1.