A comparison of electrospray versus nanoelectrospray ionization Fourier transform mass spectrometry for the analysis of synthetic poly(dimethylsiloxane)/poly(ethylene glycol) oligomer blends

The transfer efficiency of synthetic oligomer ions in monocomponent systems of polymer mixtures from solution phase to gas phase is examined here for electrospray ionization (ESI) and nanoelectrospray (nanoES) ionization. For poly(dimethylsiloxane) (PDMS), one sees a significant bias in favor of met hyl-terminated (ME) PDMS versus 2-methylpropyl-terminated (MP) PDMS in ESI. This ionization dependence (even for two very similar polymers differing o nly in end-group functionality) would be problematic for quantification eff orts of polymers within blends by ESI. It is known that the surface tension of droplets containing polymer blends and solutions is strongly dependent on molecular weight of the polymer and on potential surface activity of the end groups, which occurs primarily via differential adsorption and depleti on at the surface. This theory can be used to describe the solution to gas phase bias for PDMS ions with different end groups. For oligomers of simila r mass but very different polarity, such as PDMS and poly(ethylene glycol) (PEG), this preferential bias is even more notable, as the PDMS ion signal is nearly completely attenuated relative to the PEG ion signal in ESI mass spectra when drying gas is applied. This bias against PDMS may be due to th ermal denaturation and loss of the lone charge or conformational effects. H owever, both the drying gas flow rate and the ESI skimmer potential influen ces the degree of this bias. In the absence of drying gas, the PDMS ion sig nal in the PEG/PDMS mixtures is readily observable. Furthermore, the PEG si gnal scales with drying gas flow rates, which is consistent with the notion that higher surface tension hydrophilic PEG becomes more efficiently desol vated and ionized with increasing collisions with drying gas molecules. The observation that nanoES (which uses no drying gas) has far less solution-t o-gas-phase transfer bias is indicative of the critical role that heat of v aporization of the solvent plays in ion formation for polymers of very diff erent polarities and surface tensions. (Int J Mass Spectrom 202 (2000) 241- 250) (C) 2000 Elsevier Science B.V.