Single Particle Analysis

Denis in the Portable Laboratory with RSMS2

The Baltimore Supersite RSMS3

The Pittsburgh Supersite RSMS3

 

We have developed an instrument that chemically analyzes individual aerosol particles instantaneously. This project has been supported by the National Science Foundation, the Environmental Protection Agency, the University of Delaware, and the Electric Power Partners. Research and development of the instrument is supervised by Anthony S. Wexler, UC Davis and Murray V. Johnston, University of Delaware. For a current list of our publications related to the instrument see the C.V. of Dr. Wexler. Click here for a fairly up-to-date list of all publications in the field.

Commercially available aerosol instrumentation is severely limited in its abilities to analyze the chemical composition of aerosol particles. Particle counting equipment such as Aerodynamic Particle Sizers for the larger particles and DMA/CNC (Differential Mobility Analyzer/Condensation Nucleus Counter) systems for the smaller particles do not provide chemical composition. Chemical analysis is usually done by collecting a sample of many particles on a filter or impactor stage, and using bulk analytical methods to deduce composition. These methods obscure the particle-to-particle composition variations, and the particle composition may be altered during the collection process due to condensation or evaporation of volatile compounds, or chemical reactions. Single particle analysis has been carried out with LMMS (Laser Microprobe Mass Spectrometry), but this suffers from the same condensation, evaporation, and chemical reaction problems as bulk methods since the collection process is similar. In addition, LMMS may have larger evaporation problems because the sample is exposed to a vacuum for an extended period of time. Bulk and LMMS-based methods do not give very good temporal resolution because the sample is typically collected over an extended period of time and there is substantial delay between sample collection and analysis.

The instruments we have developed perform single particle analysis on-line. Particles are drawn into a nozzle, which aerodynamically focuses them to the centerline and removes the gas. In our first generation instrument, the resulting particle beam enters the source region of a mass spectrometer where it first passes through a continuous laser beam. A photomultiplier tube detects light scattered by the particles. An excimer laser is then fired and the particle is ablated and ionized in one step. The resulting ions are then analyzed in the mass spectrometer and the spectrum is recorded on a personal computer with a 500 MHz A/D converter. The time between sampling and analysis is about 2 microseconds so minimal condensation, evaporation, or reactions can take place. The system can analyze up to about 80 particles per second, limited by the repetition rate of the excimer laser.

Our second generation instrument has been laboratory tested and is being improved for field use. Since the first generation instruments rely on light scattering to synchronize the excimer beam with particle arrival, this limits them to particles larger than a few hundred nanometers in diameter. The second-generation instrument aligns the excimer laser and particle beams colinearly and free-fires the excimer laser so that particles are hit randomly. The TOF mass spectrometer is designed to analyze ions from a 4 cm source region so that the probability of hitting particles is about 2%. In spring 1996 we tested the first incarnation of this instrument sizing particles with a DMA and were able to analyze individual particles down to 10 nm. A current version of the instrument sizes the particle aerodynamically down to below 10 nm and has much higher transmission efficiency so that it is more suitable for environmental applications.  Two third-generation instruments have been built and deployed at the Pittsburgh and Baltimore supersites.  These instruments employ the same aerodynamic sizing characteristics as the second-generation instruments but have dual time-of-flight mass spectrometers so that both positive and negative ions can be sampled from each particle.  In addition, the excimer laser management and data acquisition are now under software control.

In August 1999, RSMS2 was employed in Atlanta to sample particles ranging in size from about 15 nm to nearly 1 micron. In August and September of 2000, a similar sampling regiment was employed in Houston, Texas.  In August 2001, RSMS3 instruments were deployed in Pittsburgh, PA and Baltimore, MD.