Analytical methods and quality assurance

Abstract

Three procedures for the determination of mercury (Hg) in plant material and biological tissue are compared. All use closed vessel microwave heating followed by flow injection cold vapour atomic absorption spectrometry (FI‐CV‐AAS). Mercury results for reference materials digested with a mixture containing nitric acid (HNO₃), hydrochloric acid (HCl) and hydrogen peroxide (H₂O₂) were elevated by up to 3 times at low concentrations (e.g. <0.05 mg Hg kg^‐1). The magnitude of the apparent increase in Hg concentration decreased as the time interval between digestion and analysis increased. The addition of sulphamic acid to an aliquot of the digestate, prior to FI‐CV‐AAS gave Hg values in agreement with certified values. The apparent elevation of Hg concentrations was probably due to interference from volatile nitrous oxides, which have absorption bands close to the Hg wavelength. No evidence of this interference was apparent when only HNO₃ and H₂O₂ were used in the digestion procedure. However, the presence of HCl in the digestion mixture was necessary to facilitate the determination of chromium (Cr) in the digestate.