Mass Spectrometer: Aerosol, time-of-flight, high resolution

The Quadrupole Aerosol Mass Spectrometer (QAMS) manufactured by Aerodyne Inc., was added as a capability in the EMSL user facility in 2004 and the Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS) in 2005. The AMS quantitatively measures the size and chemical composition of volatile/semi-volatile submicron aerosols. The AMS is designed to provide quantitative composition information on ensembles of particles. The QAMS combines a linear quadrupole mass spectrometer and standard vacuum techniques with recently developed aerosol sampling techniques to provide limited single particle information in addition to ensemble averages. The TOF-AMS provides much more extensive single particle information, especially with the incorporation of a new laser light scattering module that provides total scattered light characterization of both volatile and refractory aerosols. The AMS uses automated data analysis routines to classify the peaks in the mass spectrum into a finite number of classes of compounds, organics, ammonium, sulfate, nitrate, chlorides and PAH's. Concentrations for these species are given in real-time in g/m3. Using the QAMS Concentrations lower than .1 g/m3 have been measured and found to accurately reflect the true composition of the sample1. The TOF-AMS has sensitivities approaching .001 g/m3 Although the AMS can characterize the volatile and semi-volatile components of particles, it does not detect the non-volatile components of particles such as crustal oxides and elemental carbon.

Figure 1 shows a schematic diagram of the QAMS. Aerosols enter the instrument through a sampling inlet that restricts the flow with a 100m (or similar diameter) critical orifice and then through a lens which focuses the aerosols into a tight beam of approximately one millimeter using 6 apertures while removing most of the atmospheric gas. As the aerosols exit the lens, they are accelerated in a supersonic expansion caused by the difference in pressure between the sampling and sizing chambers that gives different velocities to aerosols of different sizes. After passing through the lens, the aerosols enter the particle sizing chamber. At this point there are two modes of operation, mass spectrometer (MS) mode and particle time-of-flight (PTOF) mode (not to br confused with the TOF mass spectrometer in the TOF-AMS instrument) . In MS mode particles fly unimpeded until they impact on a resistively heated surface where the volatile and Figure 1 Schematic diagram of Aerodyne Aerosol Mass Spectrometer semi-volatile portions of the aerosols are vaporized and then immediately ionized by electron impact. In the QAMS a standard quadrupole mass spectrometer detects the positive ion fragments generated by the electron impact ionization and determines the mass-to-charge (m/Z) distribution of the particle beam. When operating in PTOF mode a rotating chopper wheel with two radial slits located 180 apart intercepts the focused particle beam. As the particles travel on a straight path through the sizing chamber, they separate according to their size with the smaller particles traveling faster than the larger particles. Quick bursts of ions traveling through the mass spectrometer to a detector signal the arrival time and classify particle size for a given mass-to-charge (m/Z) ratio allowing time-averaged size distributions to be obtained for up to 25 m/Z values in seconds to minutes depending on particle concentration. Single particle events can be observed for particles larger than about 200 nm. The TOF-AMS uses a time-of-flight (TOF) mass spectrometer to provide true single particle information on size and chemical composition. More detailed descriptions of the AMS and how it works can be found in the literature.