Estimation of uncertainties in ICP-MS analysis: a practical methodology

The measurement uncertainty associated with the determination of Ni-60 in aqueous samples by ICP-MS has been calculated using a cause-and-effect approach. A cause-and-effect diagram was constructed to aid in the identification of the sources of uncertainty associated with the method. The uncertainty estimate was calculated from a combination of existing quality control data and specially planned experiments. The uncertainty budget was based initially on precision data, followed by separate evaluation of the method bias and the effect of parameters not sufficiently covered by these estimates. The construction of the cause-and-effect diagram, its reconciliation with existing data and the estimation of the individual components of the uncertainty budget are described in detail. The expanded uncertainties for three different nickel concentrations (3,10,35 ng g(-1)) were calculated as 1.1, 1.5 and 5.3 ng g(-1), respectively. These were calculated using a coverage factor of two approximating to a 95% level of confidence. The dominant contributions to the uncertainty budget were method precision, instrument drift and bias measured as method recovery. The uncertainties associated with the concentration of the working standard and sample dilution were found to be insignificant. (C) 1999 LGC (Teddington Ltd). Published by Elsevier Science B.V. All rights reserved.