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PTL Instruments

Single Particle Mass Spectrometry

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The Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) (TSI Model 3800), developed by Dr. Kim Prather at University of California Riverside, yields single particle aerodynamic size and single particle chemical composition.  The ATOFMS uses an aerodynamic sizing technique, similar in concept to the TSI 3321 Aerodynamic Particle Sizer, to size individual particles in real time.  The particles are then desorbed and ionized by a pulsed laser and the composition is determined in a bipolar time-of-flight mass spectrometer.  Our instrument can be equipped with two different inlets.  The standard nozzle inlet allows particles between 0.5 m and 3 m to be analyzed, while the second inlet, an aerodynamic lens, allows particles between 100 nm and 600 nm to be efficiently analyzed.  This instrument is fully transportable allowing the possibility of doing off-site analysis.

The ATOFMS has many possible applications and has already been used for atmospheric sampling, identification of contaminant sources in industrial applications, indoor air quality measurements, powder process identification and diesel particulate matter characterization.

Particle Size Spectrometry in Gas

Scanning Mobility Particle Sizer larger image

The UM Particle Technology Laboratory has an extensive array of instruments for measuring particle size spectra in gases.  This includes Optical Particle Counters (OPCs) which measure the scattered light from individual fine or supermicron particles passing through a laser beam at atmospheric pressure or in vacuum. Aerodynamic Particle Sizer (APS) which measures the aerodynamic diameter of particles accelerated through a jet. Scanning Mobility Particle Sizers (SMPSs) which measure the electric mobility diameter of particles in crossed electric and flow fields. Nanoparticle Aerosol Surface Area Monitor which measures the charge carrying capacity of the aerosol. Together these instruments can measure the aerosol size distribution over three orders of magnitude from 3 nm to 3000 nm and higher.

Particle Size Spectrometry in Liquid

Three different liquidborne particle counters are available for use at the UM Particle Technology Laboratory: (1) a Liquidborne Particle Detection System (LPDS) (TSI, Model 7750), (2) a Laser Liquid Particle Spectrometer (LLPS), (Particle Measuring Systems, PMS Module LLPS-X) and (3) a Sub-Micron Particle Analyzer (SMPA) (Coulter, Model N4SD).

(1) The LDPS measures particles in a size range from 0.19 µm to 0.48 µm (50% detection limit at 0.2 µm) in a liquid flow.  The total flow rate can be adjusted from 4-40 ml/min, of which 0.03-0.3 ml/min are actually inspected.  The LDPS can distinguish between particles and micro bubbles in the fluid.  It uses a dual-beam interferometer to measure the phase and extinction signals for the forward scattered light from small particles.  The magnitude of the phase signal is directly proportional to the volume of the particle and the difference between the refractive index of the particles and the surrounding liquid.  The LDPS can be calibrated for any liquid, based on the refractive index.

Coulter N4SD Sub-Micron Particle Analyzer
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(2) The LLPS uses Mie scattering of particles in a liquid flow to determine size and concentration of the suspended particles.  The particle size is measured, based on the intensity of light scattered by a particle in a laser beam at a certain angle.  The LLPS can be calibrated for particle sizes ranging from 0.1 µm to 100 µm.

(3) The SMPA can size particles in liquid suspension inside a cuvette in a size range from 3 nm to 3 µm.  The measurement principle is based on the size dependent Brownian motion of the particles within the liquid suspension.  The suspension gets illuminated by a laser and the scattered light is measured by a photomultiplier tube.  The light scattered by the particles at any given instance adds together to form an interference pattern.  The intensity of light detected on the photomultiplier tube depends on this interference pattern, which in turn depends on the pattern of particles in the laser beam.  As particles move randomly through the solution, their positions relative to one another change producing a constantly changing interference pattern.  The change of both the scattered light intensity and the interference pattern are analyzed by the SMPA to determine the size distribution of the particles in the suspension.

Tandem Differential Mobility Analysis

The Tandem Differential Mobility Analyzer (TDMA) consists of a DMA, an aerosol conditioner, another DMA and an aerosol detector in series.  The first DMA selects a relatively monodisperse aerosol of known size according to electric mobility.  Aerosol conditioning (e.g. heating, relative humidity change, coating) is then applied which may change the size of the aerosol.  The second DMA followed by an aerosol detector (e.g. condensation particle counter) is used to determine the new size of the particles.

The DMA is a very accurate and precise primary standard for measuring particle size.  With proper care a TDMA system can measure a particle diameter change of less than 2%.  This high sensitivity makes it possible to measure particle coatings as thin as a monomolecular layer.  In other cases, measurements of aerosol changes with heating or humidity change can assist in determining the chemical composition of the aerosol.

Particle Classification by Mass-to-Charge Ratio

The Aerosol Particle Mass Spectrometer (APM) selects particles from an aerosol stream within a narrow band of mass-to-charge ratio.  It accomplishes this by conducting the aerosol axially through the narrow annular passage between two concentric cylinders spinning as a solid body with an applied voltage between them.  The particles successfully completing the passage are those with the appropriate mass-to-charge ratio to balance the outward mass-proportional, centrifugal force with the inward charge-proportional, electrostatic force.

When used in series with a Differential Mobility Analyzer (DMA), the APM can classify submicron spherical particles of a known density.  For non-spherical particles it selects particles with a specified effective density or a given relationship between density and shape factor.  Density information can give clues to particle composition.  For instance, the DMA-APM combination can readily distinguish between soot, oil and metal particles in engine exhaust.