Vacuum UV photoionization mass spectrometry of small polymers using jet cooling

Many polymers with short chains and molecular weights up to several thousand atomic mass units can be transferred to the vapor phase whole by pulsed laser desorption, in contrast with the well-known, destructive ablation process encountered with long-chain, high-molecular-weight polymers. However, the vaporized whole polymers are hot and thus fragment extensively when photoionized. For their detection as molecular ions, it is necessary first to cool the vaporized polymers, which is accomplished by entraining them in an Ar jet expansion. Near-threshold, single-photon ionization at 125 nm (9.9 eV) was used in all cases and compared with 193 nm, two-photon ionization for polystyrene. Mass spectra showing oligomeric distributions for samples of poly(dimethylsiloxane), poly(ethylene oxide), poly(isoprene), poly(perfluorotrimethylene oxide) and polystyrene are reported. Application of the technique to samples of increasing average molecular weight shows that, as the chain length increases, thermal decomposition eventually predominates over vaporization. (C) 1997 Elsevier Science S.A.