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Single Particle Optical Sensing (SPOS)/Light Obscuration

SPOS utilizes two physical principles of detection; light extinction (also known as light obscuration) for particles larger than approximately 1.5 µm and light scattering for particles smaller than 1.5 µm. During analysis, a dilute suspension of particles is passed through a region of uniform illumination produced by a laser diode. Particles greater than approximately 1.5 µm are detected by the amount of light they obscure to the extinction detector, while smaller particles are detected by the intensity of light scattered at a range of angles towards a separate detector. The particle size is then determined by comparing the pulse heights to a calibration curve generated with standard reference materials of known size.

As a particle counting technique, particle concentration (e.g. particles/mL) can also be provided for appropriate samples.

This technique is widely accepted by numerous standards and guidance agencies such as ISO, USP, and ASTM as an appropriate method for measuring the size and number of particulate for materials made by many pharmaceutical and manufacturing industries. In addition, it is the technique of choice for USP Particulate Matter analysis per USP 729, 787, 788, and 789.  Examples of common materials analyzed include injectable suspensions, ophthalmic solutions, industrial oils and slurries, and even dilute waste/process water studies.

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  • Highest sensitivity technique available to determine outliers within emulsion, protein, aggregate, or other expected uniform populations
  • Compatible with both aqueous and several non-aqueous nonpolar liquids
  • Can handle extremely low concentration samples


  • Samples must be analyzed in a liquid medium
  • Liquid samples must be dilute or be suitable for further dilution; particles must be detected one at a time
  • Extensive dilution can result in small sample volumes analyzed
  • Not recommended for magnetic samples
  • Cannot differentiate between separate particle populations

Sample Requirements

If liquid samples are observed to have a significant amount of particulate (opaque in color or visually heavily loaded), 60 ml to 100ml of sample is sufficient. If the liquid sample is nearly transparent in appearance, 250 ml to 500 ml is preferred

For dry materials, a minimum of 200 mg is required, though, 1 to 2 grams is recommended

The above sample quantities are preferred. If sample availability is further limited, please contact us. Smaller sample quantities may be accommodated depending on sample properties, dispersion technique and required instrument configuration.

Detection Range

Primary detection range is 0.5 microns to 400 microns. The lower detection limit may be adjusted to meet guidance requirements or to improve sampling statistics.

Data Reported

A comprehensive summary including the graphical histogram of the particle size distribution on both a number and volume weighted basis will be provided, as well as the most commonly requested distribution statistics (means, 10th, 50th, 90th percentiles, etc.). Information on specific sizes can be reported on request. Concentration of particle per mL is also reported.


Particle Technology Labs uses the Entegris / Particle Sizing Systems AccuSizer instrument models A7000AD and A7000/780SIS.

AccuSizer Model A7000/780SIS

AccuSizer Model A7000AD

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