A calculation method based on isotope ratios for the determination of deadtime and its uncertainty in ICP-MS and application of the method to investigating some features of a continuous dynode multiplier

A method for the determination of dead time (and its uncertainty) of the io n-counting detection system of ICP-MS instruments is presented. It is based on isotope ratio measurements, which are more precise than ion current mea surements and will result in values for the dead time with a measurable unc ertainty. The suggested method allows us to estimate the uncertainty associ ated with the dead time. Typical relative uncertainties of the dead time va lues were found to be in the range of 10-20%. This method has been applied to monitoring the dead time for a detection sy stem over the lifetime of a particular electron multiplier. The dead time w as found to be an important criterion for replacing an old electron multipl ier when optimum isotope ratio measurements are required. The dependence of the dead time on the mass of the measured isotopes has been investigated. No mass dependence of the dead time could be concluded within the measureme nt uncertainty on this particular instrumental set-up.