Mercury determination with ICP-MS: signal suppression by acids

When mercury is quantified by ICP-MS under routine conditions (external calibration) in reference materials, which require mineralization with nitric acid, the experimental concentrations are almost always unacceptably 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 solutions, with which the external calibration curves were established, 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 concentration in the digest, different from the concentration in the external calibration solutions, is corrected mathematically based on a pre-established function [Hg2+] = f [HNO3]. The concentrations found by this mathematically based correction 2.04+/-0.01 mg Hg/kg (n = 5) is in good agreement with the values obtained by acid matched calibration or by the standard addition method. For practical work with large numbers of samples the mathematical correction appears to be the method of choice. For occasional mercury determinations, the standard addition method seems to be the most practicable.