Chain End Contribution in Static Secondary Ion Mass Spectrometry of Oligomeric Poly(ethylene glycols)

Poly(ethylene glycols) (PEGs) of a variety of molecular weights ranging from a few hundred Da to 900 kDa, terminated with a range of functional groups, were cast as thin films and examined by static secondary ion mass spectrometry (SSIMS). The intensities of ions in the SSIMS spectra are examined and it is demonstrated that the m/z 45 ion relative intensity (compared to the m/z 43 intensity) for hydroxy-terminated PEGs increases from 68% at very high molecular weight to 84% at 2 kDa and below. Those PEGs terminated with deuteroxy groups demonstrate a strong ion at m/z 46 at low molecular weight, which indicates that the m/z 45 intensity in hydroxy-PEG is strongly influenced by the presence of terminal groups in the PEG film. It is shown that amino- and methoxy-terminated PEGs have a similar m/z 45 relative intensity to high-molecular-weight PEG, suggesting that hydrogen abstraction in these systems does not occur to any great extent and the excess intensity at m/z 45 for hydroxy-PEG derives from fragment ions formed by the direct cleavage of polymer end groups.