Stability of Mercury Dilute Aqueous Solutions: An Open Issue

In the framework of the activities of the European Commission Reference Laboratory (CRL) for residues at the Istituto Superiore di Sanita a number of analytical quality control trials on determination of trace analytes are being carried out. As regards trace elements it soon became apparent that the stability of Hg dilute aqueous solutions is rather unsatisfactory. An investigation was thus undertaken to ascertain the conditions under which the concentration of this metal does not appreciably change. Mercury solutions were prepared in high purity deionized water in the concentration range 1-10 ng g-1 and the effect of various amounts of HNO3 and K2Cr2O7 (alone or in combination) was elucidated by measuring the variations with time in the actual concentration of Hg. Determinations were performed by both inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry. Results show that solutions added with 5% (w/w) HNO3 and 0.01% (w/w) K2Cr2O7 do not undergo any significant alteration in the nominal content of the analyte for at least 1 month. Similar conclusions are reached in the case of solutions enriched in CaCO3, NaCl, NaNO3, KH2PO4, KNO3, and Mg(NO3)2.6H2O to mimic acid digests of meat samples. The improvement ensuing from the adoption of this approach on the performance of the Hg analytical quality control trials is discussed in detail.