The number average m/z ratio (and any higher averages as well as Ɖ) calculated from the raw MS data is dependent on all anions being detected with the same sensitivity. For higher m/z anions this cannot be true in a general sense given the intrinsic bias of MS towards lower m/z ions. This leads to a pronounced narrowing of the MWD and a bias towards lower m/z averages as measured by low resolution MS vs. other methods such as GPC.63 Even in the case of high-resolution techniques such as ESI FT MS, polydispersity of narrow MWD synthetic polymers is still underestimated by mass spectrometry. If there are also differences in surface activity between homologous ions even more pronounced intensity differences can be observed. 49 For an example obtained under the same conditions as we use for the analysis of MAO, see the Supporting Information Figure S-3 where the sensitivity difference for [n-Bu4N] + (m/z 242) vs. [(n-C18H37)4N] + (m/z 1026) is ca. 150-fold at equivalent concentrations. If differences of this magnitude also pertain to the aluminoxane anions, particularly those with higher m/z ratios, it is easy to understand why 𝑀 ̅ 𝑛 , 𝑀 ̅ 𝑤 etc. are underestimated using the ESI-MS data.