On-line sampling and intact mass analysis of nanometer-size aerosols via time-of-flight high-mass spectrometry

A device capable of continuously sampling and mass analyzing aerosols in the 1-10 nm diameter size range (masses up to 1 MDa) at part per billion (ppb, 10¹⁰ cm^-3) concentrations is described. Small aliquots of an aerosol flowing at atmospheric pressure are sampled into a time-of-flight mass spectrometer (TOFMS) via a pulsed molecular beam valve at 10-50 Hz. The aerosol molecular beam is singly and nondestructively ionized with light from an ultraviolet excimer laser and accelerated across a 20 kV electrostatic field. Ionized particle packets are separated in mass during a free flight and re-accelerated across an additional 30 kV into a stainless steel conversion dynode plate. Signals from ejected electrons and negative ion fragments resulting from the particle-dynode collisions are amplified in a dual microchannel plate detector, digitized, and stored in a fast transient recorder. Sampling of He flow streams bearing benzene, fullerenes, as well as Na, Mg, and CsI particles (nanocrystals) is demonstrated. In addition, the gas-phase reaction between a pre-formed Na nanocrystal and alkane thiols is monitored in real-time.