Generation of doubly charged species from small synthetic polymers in a high pressure MALDI source

Doubly charged adducts were unexpectedly observed when performing MALDI of small synthetic polymers (Mn < 5 kDa) in a quite high pressure ionization source (∼60 mTorr). This effect was best observed for alkali adducts of polyethylene glycol (PEG, Mn = 2 kDa) using a solvent-free sample preparation protocol, but doubly protonated molecules were also observed for poly(2-vinylpyridine) (Mn = 4.8 kDa). In contrast, only singly charged adducts were detected for poly(methylmethacrylate) (Mn = 2 kDa), poly(caprolactone) (Mn = 3 kDa) or polystyrene (Mn = 2 kDa). To get insight in this gas phase cationization process, formation of doubly charged PEG adducts was investigated with different cationizing agents and MALDI matrices. Consistent with the high affinity of these small PEG chains towards small alkali, higher yields of doubly charged adducts were obtained with lithium salts, but the clear influence of the counter-anion could not be rationalized. PEG double charging was also observed to be a matrix-dependent event. Although no strict correlation could be established between the extent of PEG double charging and relative lithium affinity of tested matrices, cation transfer would be a major process for synthetic polymer ionization in MALDI.

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