Formation and decomposition of water clusters as observed in a triple quadrupole mass spectrometer

Water clustering up to mass 4000 has been observed using the Finnigan TSQ-700 electrospray mass spectrometer operating in either the normal or discharge modes with the capillary just above room temperature. The mechanism of cluster formation and structure has been studied by changing instrumental parameters including capillary temperature, auxiliary gas flow, and tube lens (skimmer) voltage. As expected, and in agreement with earlier work, enhanced abundances were observed at clusters of n = 21 and 28 molecules of water. Abundances of these same clusters are enhanced after fragmentation of higher mass clusters by MS/MS. Existing models including the clathrate structure are examined and it is suggested that with the exception of the structures at n = 21 and 28, the clusters are based on ice I _c . Also in agreement with previous work, water clusters were found to contain both protons and ammonium ions and the presence of the latter was proved by accurate mass measurement. In one case, these ions also attached to decomposition products formed by the discharge from traces of residual polyethylene glycol.