Nano Research Facility School of Engineering & Applied Science

AMS-TOF

 

 Aerosol Mass Spectrometer-Time of Flight

 

   

    Aerodyne Research, Inc.

 

 

 

The ARI Aerosol Mass Spectrometer measures the size and chemical composition of volatile/semi-volatile submicron aerosols. It provides composition information on ensembles of particles, with limited single-particle information. The instrument combines standard vacuum and mass spectrometric techniques with aerosol sampling techniques.

Aerosols enter the AMS through a particle-sampling inlet that restricts the flow with a 100 mm (or similar diameter) critical orifice. They proceed through a lens that focuses the aerosols into a tight beam of approximately one millimeter in diameter, using 6 apertures. Gas is removed later by differential pumping. As the aerosols exit the lens, they are accelerated in a supersonic expansion caused by the difference in pressure between the aerosol -sampling chamber and the aerodynamic particle-sizing chambers. This expansion gives different velocities to aerosols of different sizes.

After passing through the lens, the aerosols enter the particle-sizing chamber. A rotating chopper wheel, with two radial slits located 180oapart, intercepts the focused particle beam. The chopper can be placed in any of three positions: completely blocking the beam so that no particles pass through (beam closed); not blocking the beam so that all particles pass through (beam open), and a chopping position that allows particles to pass through the radial slits only (beam chopped). The time of flight (TOF) between the chopper and the detector is the measurement of a particle's velocity; from this measurement the particle's aerodynamic diameter (Daero) can be determined.

The particles passing through the flight chamber are directed onto a resistively heated surface. Upon collision with this heated surface, non-refractory particles flash vaporize under high-vacuum conditions. The vaporization process occurs directly inside an electron impact ionizer where the vaporized constituents are converted to positive ions, which can then be detected by the mass spectrometer. The electron impact ionization process is a universal process; therefore, any species in the gas phase will be detectable. The AMS does not efficiently detect low-volatility materials such as black carbon, NaCl, crustal oxides and certain metals.  However, lower volatility species adsorbed on such material can be detected.

AMS diagram

 

The ARI Aerosol Mass Spectrometer provides real-time size resolved composition analysis of volatile and semi-volatile particulate matter. The combination of size and chemical analysis of sub-micron aerosol mass loading with fast time resolution makes the ARI AMS unique.

Aerosol particles in the size range ~ 0.04 to ~1.0 micrometers are sampled into a high-vacuum system where they are aerodynamically focused to a narrow beam (~ 1mm diameter). The particle beam is directed onto a resistively heated surface where volatile and semi-volatile chemical components are thermally vaporized and detected by electron impact ionization quadrupole mass spectrometry.