Studies of the ablation rate, particle size distribution and signal intensities from NIST 610 and zircon 91500 were carried out using UV-ns LA-ICP-MS to investigate the limitations for non-matrix matched quantification of the Pb and U system. All major elements (Zr, Si, Hf) as well as Pb and U were measured in both matrices using different crater diameters. The results indicate two different laser ablation dominated sources of fractionation which prohibit non-matrix matched calibration for accurate age determination. Selective loss of Pb (crater diameter independent) and crater diameter dependent retention of U through phase change in the crater during the ablation of zircon are the two sources of fractionation which occur. However, at certain operating conditions the loss of Pb and the retention of U within the crater are equal. This leads to ages, which randomly agree with the reference age.