VACUUM UV PHOTOIONIZATION MASS-SPECTROMETRY OF SMALL POLYMERS USING JET COOLING

Many polymers with short chains and molecular weights up to several th ousand atomic mass units can be transferred to the vapor phase whole b y pulsed laser desorption, in contrast with the well-known, destructiv e ablation process encountered with long-chain, high-molecular-weight polymers. However, the vaporized whole polymers are hot and thus fragm ent extensively when photoionized. For their detection as molecular io ns, it is necessary first to cool the vaporized polymers, which is acc omplished by entraining them in an Ar jet expansion. Near-threshold, s ingle-photon ionization at 125 nm (9.9 eV) was used in all cases and c ompared with 193 nm, two-photon ionization for polystyrene. Mass spect ra showing oligomeric distributions for samples of poly(dimethylsiloxa ne), poly(ethylene oxide), poly(isoprene), poly(perfluorotrimethylene oxide) and polystyrene are reported. Application of the technique to s amples of increasing average molecular weight shows that, as the chain length increases, thermal decomposition eventually predominates over vaporization. (C) 1997 Elsevier Science S.A.