Cold vapour atomic fluorescence determination of mercury in milk by slurry sampling using multicommutation

A highly sensitive mechanized method has been developed for the determination of mercury in milk by atomic fluorescence spectrometry (AFS). Samples were sonicated for 10 min in an ultrasound water bath in the presence of 8% (v/v) aqua regia, 2% (v/v) antifoam A and 1% (m/v) hydroxilamine hydrochloride, and after that, they were treated with 8 mmol l^-1 KBr and 1.6 mmol l^-1 KBrO3 in an hydrochloric medium. Atomic fluorescence measurements were made by multicommutation, which provides a fast alternative in quality control analysis, due to the easy treatment of a large number of samples (approximately 70 h^-1), and is an environmentally friendly procedure, which involves a waste generation of only 94.5 ml h^-1 as compared with the 605 ml h^-1 obtained by using continuous AFS measurements. The limit of detection found was 0.011 ng g^-1 Hg in the original sample. The method provided a relative standard deviation of 3.4% for five independent analysis of a sample containing 0.30 ng g^-1 Hg. To validate the accuracy of the method, a certified reference material NIST-1459 (non-fat milk powder) containing 0.3(plus-minus)0.2 ng g^-1 Hg was analysed and a value of 0.27(plus-minus)0.06 ng g^-1 Hg was found. A comparison made between data found by the developed procedure and those obtained by microwave-assisted digestion and continuous AFS measurements evidenced a good comparability between these two strategies. Results obtained for commercially available milk samples varied between 0.09 and 0.61 ng g^-1 Hg depending on the type of sample and its origin. The confluence of the analytical waste with a 6 mol l^-1 NaOH allowed us to reduce the waste generation in a working session from 1 l to 5 g solid residue with a matrix of Fe(OH) 3 which contributes to the deactivation of traces of heavy metals presents in the samples that does not form volatile hydrides.