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Aerodynamic Particle Sizer (APS) Spectrometer 3321

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The only one of its kind available on the market, the TSI Aerodynamic Particle Sizer (APS) measures the aerodynamic size of particles from 0.5 - 20 microns. Time-of-flight aerodynamic sizing determines the particle's behavior while airborne and is unaffected by index of refraction or Mie scattering.

While cascade impactors also size particles by their aerodynamic diameter, the APS offers high size resolution, one-second sampling, and real-time size distributions. Suitable for a variety of applications - inhalation toxicology, air monitoring, and drug delivery, plus many others, - the APS continues to be used in new and interesting ways.

Product Details

The Aerodynamic Particle Sizer® 3321 (APS™ spectrometer) provides high-resolution, real-time aerodynamic measurements of particles from 0.5 to 20 microns. These unique particle sizers also measures light-scattering intensity in the equivalent optical size range of 0.37 to 20 microns. By providing paired data for each particle, the APS spectrometer opens up exciting new possibilities for those interested in studying the makeup of an aerosol.

The APS spectrometer uses a patented*, double-crest optical system for unmatched sizing accuracy. It also includes a redesigned nozzle configuration and improved signal processing. The result is greater small-particle sizing efficiency, improved accuracy of mass-weighted distributions, and virtual elimination of false background counts.

Features and Benefits 

  • Double-crest optics produce high-quality measurements
  • Measures light-scattering intensity from 0.37 to 20 µm
  • Measures aerodynamic particle size from 0.5 to 20 µm


  • Inhalation toxicology
  • Drug delivery studies
  • Atmospheric studies
  • Ambient air monitoring
  • Indoor air-quality testing
  • Filter and air-cleaner testing
  • Test-aerosol characterization
  • Powder sizing

Included Items 

  • Aerosol Instrument Manager® software  

*United States Patent Number 5,561,515.   

Resource Center

  • As a single-particle counter, how does Model 3321 APS™ spectrometer resolve the coincidence problem?
    Particle coincidence is typically defined as more than one particle in the viewing volume of the particle counter, creating a signal that causes the counter to incorrectly classify the particles as a single, mis-sized particle. Coincidence typically increases proportionally with particle concentration. In the case of the Model 3321, however, although coincidence is still a problem at high concentrations, the particles are not misclassified. The double-crest signal processing technique allows the processor to determine when a signal is caused by a single low-scattering particle (event 1) or by coincidence (event 3). Coincident particles can be detected as event 3s, but cannot be sized. Therefore, they are not included in the size distribution, but are recorded for possible concentration corrections. This means that the particle size distribution during coincidence can be accurately measured. Refer to Chapter 5, “Theory of Operation,” in the Model 3321 instruction manual for more details.
  • Can I measure powder with the Model 3321?
    Yes. You can use TSI’s Model 3433 Small-Scale Power Disperser or 3400 Fluidized Bed Aerosol Generator to introduce powder samples into the APS inlet for subsequent particle analysis.
  • Can I measure the MDI (Metered Dose Inhaler) efficiency with the Model 3321?
    Yes. TSI’s Model 3306 Impactor Inlet is a specially designed accessory for the APS™ spectrometer allows pharmaceutical companies to measure the characteristic distributions of metered dose inhalers (MDI) or dry powder inhalers (DPI). It directs a small aerosol sample to the Model 3321 for size distribution and MMAD calculation.
  • Can I use the Model 3321 without a computer?
    The Model 3321 APS™ spectrometer has a display and a control knob, and it can be used as a stand-alone unit. However, it doesn’t have internal data-logging capacity. You have to use a computer to log the data.
  • Do I need an external pump?
    No external pump is needed. Two internal pumps are used for the total air flow and the sheath air flow.The sample flow rate is the difference between them. The volumetric flow rates are microprocessor-controlled and are corrected based on barometric pressure.
  • Does the APS™ spectrometer report PM2.5 or PM10?
    No, although the mass of particles in the 2.5-to-10-µm range can be calculated based on the measured size distribution and the user-input density to represent PMcoarse (the difference between PM10 and PM2.5).
  • Does the Model 3321 measure the particle mass directly? Do I need to know the material density to get mass distribution?
    The Model 3321 APS™ spectrometer does not directly measure the particle mass. But it calculates mass distributions based on the directly measured size distributions and the user-input density. The density can be input through Run > Properties > Data Settings. You have to know the particle density to obtain an accurate mass distribution. The default density is 1 g/cm3 in the software.
  • How do I clean, and how often do I clean, my inner nozzle?
    Please follow the maintenance schedule and cleaning procedure in Appendix A in the Model 3321 user manual.
  • How do I know my Model 3321 is still in calibration?
    TSI recommends that customers have a sample of known particle size available. You should introduce it periodically to the Model 3321 to see if the instrument is sizing correctly (1.0 µm PSL is common).
  • How does the time-of-flight (TOF) technique work?
    What are the advantages of TOF?Time-of-flight technique is used to measure the aerodynamic diameter of particles and to determine their airborne and deposition behavior. Particles are accelerated through a nozzle to obtain different velocities. Larger particles lag behind smaller ones due to viscous drag. At the nozzle exit, particles pass through two laser beams, one by one. The TOF between the two laser beams is measured for each particle. The TOF technique offers rapid size measurements. Because the TOF technique doesn’t convert light-scattering intensity into particle size, it has the following advantages: 1) Insensitive to dirty optics, 2) Holds calibration well, 3) Monotonic response curve (no MIE problems).
  • How long has the APS™ spectrometer been on the market?
    The Aerodynamic Particle Sizer® spectrometer has been on the market for over 20 years. It provides rapid, high-resolution, aerodynamic particle-size distributions, and it has been a great success in many applications.The first APS spectrometer, Model 3300, was manufactured by TSI in 1981. It was the first self-contained, real-time instrument to measure aerodynamic particle size in the range from 0.5 to 15 µm. It consisted of a sensor with a parallel interface to an Apple II+ computer. The sensor was based on work by Agarwal and Fingerson (1979). The Model 3310 APS spectrometer, released in 1987, had timer improvements, allowing detection of particles from 0.5 to 30 µm and used a serial interface to an IBM PC. It received a face-lift and some minor engineering changes in 1993. The Model 3320, released in 1997, featured patented, double-crest optics and improved signal processing that effectively eliminated the effect of false counts due to phantom and coincidence particles. The Model 3321 was introduced in 2001. It has improved signal processing and a redesigned eduction nozzle to eliminate recirculating small particles. It provides more accurate size and mass distributions than ever!
  • How often does the Model 3321 need calibration?
    TSI recommends yearly service and calibration for the Model 3321.
  • Is the Model 3321 portable? Can it be transported into the field?
    The Model 3321 APS™ spectrometer is very easy to transport. It weighs about 22 lb., and it measures 15 in. × 12 in. × 7 in. It is very robust and suitable for lab and field use. For example, it has been used by the military and on hog farms.
  • Is there any software to do data analysis? Do I have to buy it separately?
    The Model 3321 includes the Aerosol Instrument Manager® software, a program designed for use with the Windows® operating system. The Aerosol Instrument Manager software controls instrument operation, plus it provides impressive file management capabilities and numerous choices for data display.
  • There is a lot of data to transfer in the correlated mode with light-scattering intensity correlated to aerodynamic diameter on a particle-to-particle basis. Is there any requirement on the data transfer speed?
    In correlated mode, the data transfer speed (baud rate) should be set to 38400.
  • What if my Model 3321 is not sizing correctly?
    Incorrect aerosol or sheath flow may cause the sizing to be incorrect. Checking the flows with a Gilibrator flow cell or TSI flowmeter will tell you if these flows are correct. (Contacting a TSI technician to help with adjustment of these flows is recommended.)
  • What is the maximum concentration that the Model 3321 APS™ spectrometer can handle? What if the aerosol concentration is too high?
    The Model 3321 can handle a concentration of 1,000 particles/cm3 with the coincidence error <10%. If the concentration is too high, you can use TSI’s Model 3302A Aerosol Diluter to dilute the aerosol before it enters the APS spectrometer. The dilution ratios are 20:1 and 100:1. Two diluters can be stacked when measuring particles in the 0.5-to-15-µm range to produce dilution ratios of 400:1, 2000:1, or 10000:1. The appropriate dilution file can be selected in the Aerosol Instrument Manager® software through Run > Properties > Data Settings. For example, the file named 02000to1.e21 should be selected with a dilution ratio of 2000:1. With the dilution file selected, the concentration data displayed and recorded is the actual concentration of the tested aerosol before it is diluted.
  • What is the size range of the Model 3321? Why can’t I see any particles larger than 15 µm?
    The 3321 APS™ spectrometer detects particles in the 0.37-to-20-µm range and measures high-resolution aerodynamic size in the range from 0.5 to 20 µm. Usually, large particles will not remain airborne for a long time before they settle out. So the common problem is getting large particles into the APS spectrometer in the first place. To measure particles larger than 15 µm, you essentially have to drop the particles straight down into the inlet.
  • Why are the lower size limits different for detection and aerodynamic diameter measurements?
    For the Model 3321 APS™ spectrometer, the lower detection limit is 0.37 µm and the lower aerodynamic sizing limit is 0.5 µm. The optical system in the Model 3321 is able to detect light that is scattered from particles in the 0.37-to-0.5-µm range and even detemine their time-of-flight. However, after these particles are accelerated at the nozzle, their velocities reach airflow velocity very quickly before they enter the detection area, due to their smaller size. The time-of-flight measured for these particles is the same. Thus, particles in the 0.37 to 0.5 µm range can’t be distinguished aerodynamically.
  • Why does the Model 3321 offer light-scattering intensity measurements?
    The light-scattering intensity measured by the Model 3321 APS™ spectrometer is not used to convert to particle sizes. It offers another dimension of particle measurements in addition to aerodynamic sizing to give some insights into the composition of an aerosol. The light-scattering measurements can be made alone, in addition to aerodynamic diameter, or correlated to aerodynamic diameter on a particle-to-particle basis.
  • Why is aerodynamic diameter important?
    Aerodynamic diameter is the most significant size parameter because it determines the behavior of airborne particles. It is defined as the physical diameter of a unit density sphere that settles through the air with a velocity equal to that of the particle in question. Knowledge of a particle's aerodynamic diameter allows you to determine: 1) If and where the particle will be deposited in the human respiratory tract. 2) How long the particle will remain airborne in the atmosphere or in an aerosol. 3) Whether the particle will penetrate a filter, cyclone, or other particle-removing device. 4) Whether the particle will enter a particle-sampling system. 5) Whether the particle will penetrate a pipe, tube, duct, or channel.
  • Why is the Model 3321 APS™ spectrometer superior?
    With a redesigned eduction nozzle and improved signal processing, the Model 3321 offers greater small-particle sizing efficiency, improved accuracy of mass weighted distributions, and virtual elimination of false background counts. It also eliminates the effect of coincidence on particle size distributions by using a patented, double-crest optical system. For more details, please check the appendix on the Sales Training CD or visit our website.
  • Will the mass concentration double if I double the density that I input?
    No, this would be the case if the geometric diameter were measured. The Model 3321 APS™ spectrometer measures the aerodynamic diameter, which already accounts for the density. For a given aerodynamic size distribution, increasing the density actually decreases the mass concentration. The mass concentration is inversely proportional to the square root of the density.

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