The incorporation of metal cations into polymer backbones: an important consideration in the interpretation of ESI-MS spectra

Poly(acrylic acid) (polyAA), poly(N-isopropylacrylamide) (polyNIPAAm) and poly(diisopropyl vinyl phosphonate) (polyDISP) samples were subjected to electrospray ionisation-mass spectrometry (ESI-MS) analysis employing quadrupole-ion trap as well as high resolution quadrupole time-of-flight (Q-TOF) detectors, and the resulting spectra screened for the detection of polymers in which metal cations have become incorporated into the chain backbone following deprotonation reactions during the ESI process. Adducts formed from such polymer-metal salts were detected in spectra produced from each of the samples under investigation, indicating that the formation, ionisation and detection of polymer-metal salts can be readily achieved under typical ESI-MS experimental conditions. Specifically (under conditions of MeOH containing 0.1 mM sodium acetate) polyAA is most susceptible to the incorporation of metal ions (up to 3 replacements), followed by polyNIPAAm with two replacements and polyDISP with one replacement. The results obtained from the present investigation clearly demonstrate that for synthetic polymers capable of undergoing deprotonation under ESI conditions, the potential detection of polymer-metal salt adducts must be considered during spectral data interpretation to avoid erroneous species assignments.