Evaluation of detector dead time calculation models for ICP-MS

Four methods of pulse counting detector dead time calculation were evaluated on two quadrupole ICP-MS instruments, using the criteria of ease of application and accuracy (i.e., reliable measurement uncertainty). Three of the methods were based on measurements of the Pb-204/Pb-208 isotope ratio in the NIST Standard Reference Material NBS-982, while the fourth method was based on the comparison of pulse counting signals versus the corresponding analog signals across a range of signal intensities. The uncertainty in the dead time result derived from each method was evaluated and found to vary considerably, with some approaches underestimating this parameter, while others yielded large uncertainties. Overall, a method based on studies of the Pb-204/Pb-208 isotope ratio measured for different Pb concentration solutions, recalculated using different dead time settings (herein referred to as Method 2), was found to be the most suitable in terms of the specified criteria, particularly with respect to its uncertainty evaluation.