Random/internal uncertainties are those relating to the measurement of isotopic ratios of the sample, standards and blank and are used in the derivation of errors of the radiogenic ratios. These uncertainties are intrinsic to each analysis and represent the minimum uncertainty that must be considered. Most of these sources of random uncertainty relate to the mass spectrometry measurements and our ability to measure and reproduce isotopic ratios. Factors such as the electronic noise of detectors place a theoretical limit on the precision which can be achieved by detecting a certain number of ions over a finite period of time (counting statistics). However for almost all geochronologic applications other factors such as correction for mass dependent fractionation that occurs during sample ionisation, and correction for common and/or initial parent and daughter nuclide dominate the analytical uncertainty budget. It is possible to reduce the uncertainty in the mass dependent fractionation via ‘double-spiking’, where two tracer isotopes of the same element (202Pb-205Pb, or 233U-235U for example) are used for real time mass fractionation correction.