MS spectra contained peaks with a positive mass offset of 16 Da relative to the original C-PEG molecular masses, indicating the presence of PEG reaction products in which a hydrogen was replace by a hydroxyl group. Furthermore, the intensities of peaks with a mass difference of -2 Da increased with increasing reaction time, consistent with end-group aldehyde formation on some products. These chemical changes were confirmed by collecting C-NMR spectra on aliquots of the initial and the three irradiated PEG solutions ( 13 C-NMR spectra are provided in section S2; Supplementary Information). The