Abstract When mercury is quantified by ICP-MS under routine conditions (external calibration) in reference ma- terials, which require mineralization with nitric acid, the experimental concentrations are almost always unaccept- ably low in comparison to certified values. Sorption of mercury on the Teflon surfaces of the digestion vessels, changes in the viscosity of the aspirated solutions, in the efficiency of the nebulization, in the aerosol transport, and memory effects cannot be responsible for the low results. The intensity of a mercury signal is strongly dependent on the concentration of nitric acid (and other mineral acids) in the measured solutions. Correct results for mercury in the SRM GBW-90101 (Chinese human hair; 2.16 ± 0.21 mg Hg/kg certified) can only be obtained, when the solu- tions, with which the external calibration curves were es- tablished, have exactly the same nitric acid concentration as the aspirated digests (2.03 ± 0.01 mg Hg/kg; n = 5), when mercury is determined by the standard addition method (2.10 ± 0.01 mg Hg/kg; n = 5), or when the experimental mercury concentration obtained at a nitric acid concentra- tion in the digest, different from the concentration in the external calibration solutions, is corrected mathematically based on a pre-established function [Hg 2+ ] = f [HNO 3 ]. The concentrations found by this mathematically based correction 2.04 ± 0.01 mg Hg/kg (n = 5) is in good agree- ment with the values obtained by acid matched calibration or by the standard addition method. For practical work with large numbers of samples the mathematical correc- tion appears to be the method of choice. For occasional mercury determinations, the standard addition method seems to be the most practicable. Introduction Generally, the toxic element mercury is present at very low concentrations in the atmosphere, hydrosphere, lithos- phere, and biosphere. Mercury emissions from natural sources exceed anthropogenic emissions, for instance, by coal- or oil-fired electric power plants, by alkali chloride and polyvinyl chloride plants, and through agricultural use of organic mercury compounds [1]. To monitor mer- cury in the environment, several reliable analytical meth- ods such as cold-vapor atomic absorption spectrometry (CVAAS) [2], cold-vapor atomic fluorescence spectrome- try (CVAFS) [3], and gas chromatography with electron capture detection (GC-ECD) [4] are available. These methods have low detection limits for mercury, but lack the capability for the simultaneous determination of other trace elements. Inductively-coupled argon-plasma mass spectrometry (ICP-MS) valued for its multielement capability and its excellent detection limits (< 0.01 μg/L for many elements including mercury) could be the method of choice for the determination of mercury and many other elements. Whereas many other elements were and are routinely quantified by ICP-MS in biological and non-biological samples, mercury was determined only occasionally. The study of the relevant literature raises the suspicion that mercury is difficult to determine accurately by ICP-MS under routine operating conditions. Reports claim that the thermal ionization of mercury, an element with the high first ionization potential of 10.44 eV, is suppressed by el- ements with low ionization potentials when present at high concentrations [5–8]. Mercury determinations are in- fluenced by high concentrations of dissolved solids and very likely by the ratio of inorganic to organic mercury compounds in the sample solutions. Mineralization, a necessary prerequisite for the deter- mination of mercury by liquid sample introduction, is of- ten carried out in closed vessels with microwave-assisted heating to prevent losses of mercury or its compounds. However, when cod muscle was digested with HNO 3 /H 2 O 2 Liu Jian · Walter Goessler · Kurt J. Irgolic † Mercury determination with ICP-MS: signal suppression by acids Fresenius J Anal Chem (2000) 366 : 48–53 © Springer-Verlag 2000 Received: 9 June 1999 / Revised: 25 August 1999 / Accepted: 28 August 1999 ORIGINAL PAPER Dedicated to the late Professor Kurt J. Irgolic L. Jian · W. Goessler (Y) · K. J. Irgolic † Institute for Analytical Chemistry, Karl-Franzens-University Graz, Universitätsplatz 1, A-8010 Graz, Austria