A broad continuum of anions with m/z ~ 1000 to >3000 Da were detected with evidence of n-octyl substitution (Figure ) while the corresponding positive ion spectrum showed the [Me2Al(OMTS)] + and [(n-C8H17)MeAl(OMTS)] + cations in amounts consistent with the composition of MMAO 12 ( 1 H NMR spectroscopy).31 Finally, h-MAO (W. R. Grace 10 or 30 wt%) shows the same [Me2Al(OMTS)] + cation in positive mode and a very simple spectrum in negative mode consisting primarily of a single anion with m/z = 1375 Da (Figure , top spectrum).32 This latter result is remarkable given the complex nature of h-MAO, though higher m/z anions were present with sufficient intensity at higher levels of OMTS to be characterized by MS/MS experiments.32 It is worth noting that the [26,8] -anion, which could be formed from 26,9 and OMTS by [Me2Al] + abstraction, is a minor component of this mixture (Figure (top)) and not present in MMAO 12. Storage of 10 wt% MAO at room temperature leads to formation of higher m/z anions32 with fully aged samples having a negative ion mass spectrum similar to that of h-MAO earlier supplied by Sigma Aldrich. Ironically, the average m/z ratio of material from Sigma Aldrich (ca. 1800 Da) was in good agreement with published work about MAO which appeared contemporaneously.20 In contrast, the major anion in unaged h-MAO with composition [(MeAlO)16(Me3Al)6Me] -, hereinafter abbreviated [16,6] -is significantly lower in MW, and is hardly prominent, though present, in the MMAO 12 sample. More recently, the anion distribution of synthetic h-MAO was shown to vary with the conditions of synthesis; higher m/z anions such as [18,6] -and [23,7] -were more prominent in material prepared at elevated temperature. Any differences in efficacy between h-MAO (or MMAO 12) formulations which have very different anion distributions, remains unexplored though h-MAO and MMAO 12 have been used interchangeably in polymerization studies. Finally, activation of metallocene catalysts by MAO has been studied by ESI-MS36 , and of all the donors studied, metallocene complexes behave in a unique manner. Regardless of precursor, higher m/z anions are detected with increased intensities as the Al:Zr ratio is increased at constant [Al].36 , 37 It is known from other spectroscopic work that the nature of the ion-pairing and ion-pair speciation is sensitive to this ratio, with both inner-and outer-sphere ion-pairs being partnered with more weakly coordinating anions at higher Al:Zr ratios. The interpretation given in these papers relied on the argument that higher MW anions would be more weakly coordinating. Having said that, the major anion detected by ESI-MS at all ratios investigated using unaged h-MAO from W. R. Grace was [16,6] -.37 Recent DFT studies of propene insertion featuring a sheet model for this anion, analogous to the structure of neutral 26,9, show that the contact ion-pair derived from rac-Me2Si(η 5 -Ind)2ZrMe2 would not be very reactive towards insertion, at least in comparison to complexes featuring [B(C6F5)4] -or [MeB(C6F5)3] -anions. The theoretical results involving [16,6] -were in disagreement with experimental studies of propene insertion. While ESI-MS is a selective and sensitive technique for detection and characterization of ions it has limitations; principally that (solvated) ions must be present in solution in order to generate gas phase ions. ESI-MS experiments are generally not feasible in low dielectric media because the necessary electrochemistry that generates the excess charge on the droplets formed during the ESI process is not supported by the presence of solvated ions. Further, ESI-MS is not a quantitative method as ion intensity is a function of both surface activity (the propensity for an ion to be found at the surface of a droplet rather than ionpaired in the interior) and m/z ratio in addition to solution concentration. 49 Absent authentic samples of the analyte in question, it is disingenuous to make conclusions about solution concentration. Therefore, some of the effects discussed above cannot be easily interpreted, at least in a quantitative manner, while ESI-MS by its very nature is not sensitive to the neutral components of mixtures unless they can easily acquire a charge. Evidently, ion-pair speciation during metallocene catalyst activation using MAO is still an open question despite the battery of experimental and theoretical studies brought to bear on this matter over the years. In this paper we show through ESI-MS and PSGE NMR studies that the anion distribution and average dimensions of MAO are sensitive to the formulation in use, demonstrate there is a correspondence between the two techniques when it comes to aging of MAO and that activator content as measured by NMR spectroscopy is significantly higher in a formulation featuring higher m/z anions, in agreement with Brintzinger's hypothesis.22