1, University of Duisburg, Duisburg, , Germany
3, Center for Nanointegration Duisburg-Essen, Duisburg, , Germany
Inline characterization of nanoparticle formation with particle mass spectrometry (PMS) is well established to investigate the evolution of particle size distributions during the nanoparticle synthesis in low-pressure gas-phase reactors. It is commonly used to investigate the synthesis of metal and metal oxide nanoparticles to determine the influence of process conditions (temperature, precursor concentration, pressure, fuel/oxygen equivalence ratio) on the particle growth. The method is based on molecular-beam sampling by vacuum expansion of the particle-laden gas at variable locations in the reaction chamber through a nozzle/skimmer system. The vacuum conditions immediately suppress further reactions and particle growth. The system is equipped with an electrostatic deflection unit and a Faraday cup detector to measure potential-energy-filtered currents of charged particles. Due to limitations in the achievable pumping rates and (too high) particle loading, conventional PMS systems with a two-step skimmer/nozzle arrangement are limited to sampling from low-pressure (< 100 mbar) environments at low particle number concentrations. There are only few reports on molecular-beam mass spectrometry (MBMS) of particles from atmospheric-pressure systems, mostly used for the investigation of sooting flames.
In this study, we present a newly designed PMS that can be operated at atmospheric pressure. It is based on a three-stage system that consists of the conventional nozzle/skimmer system combined with an additional expansion chamber in front of the PMS nozzle between reactor and PMS. The PMS is connected to the reactor via a thin pipe (1.7 mm inner diameter). Fluid-dynamic calculations as well as Schlieren measurements show that a supersonic expansion develops in the first chamber. Dependent on the length of the pipe, the supersonic expansion moves upstream. This enables the determination of the best skimmer position that ensures the charge preservation of the sampled particles by extracting them before interacting with the shock wave. Thus, the sampled aerosol is directly transferred into a particle-laden molecular beam, which than passes the nozzle and the skimmer of the PMS. The additional expansion chamber enables sampling with the PMS within a wide pressure range between 5 mbar and atmospheric pressure. Results concerning Schlieren measurements and spatially resolved investigation of nanoparticles from sooting flames as well as from a spray-flame nanoparticle reactor, both operated at ambient pressure, will be discussed.