HYPERTHERMAL SURFACE-IONIZATION - A NOVEL ION-SOURCE WITH ANALYTICAL APPLICATIONS

We have found that a wide range of organic molecules with hyperthermal kinetic energy (1-20 eV) can undergo an efficient molecular ionization or dissociative ionization upon scattering from a surface. The molecular kinetic energy is obtained in a simple supersonic expansion of the organic heavy molecule seeded in hydrogen (or helium) carrier gas through a pinhole nozzle. Hyperthermal surface ionization (HSI) is a sensitive, selective and informative ionization method. It is characterized by several properties that make it attractive for analytical applications. The HSI mass spectra (negative and positive ions) exhibit unique fragmentation patterns. These HSI mass spectra are different from electron impact (EI) ionization mass spectra, and reveal structural and isomeric information. The ionization yield is determined by the molecular mass, electron affinity or ionization potential and thus extreme selectivity against common light gases is observed. The absence of the vacuum residual gases in the mass spectra combined with the high molecular ionization efficiency contributes to an improved signal-to-noise ratio and detection sensitivity. The supersonic jet chamber serves as an efficient high-load jet separator in the tail-free coupling of a gas chromatograph (GC) to a mass spectrometer (MS) and allows the use of beam modulation and lock-in amplification. The HSI ion source itself can serve as a selective and sensitive GC detector for many functional groups of molecules such as I, Br, Cl, F, PO2, NO2, CN, NH2 polycyclic aromatic hydrocarbons and organometallics. Most notable among these are the organofluorine molecules whose mass spectra are demonstrated and discussed. A simple GC detector based on HSI is described. Intra-nozzle chemical reactions can be used for the selective and sensitive determination of olefins among paraffins. © 1990.