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Single Particle Optical Sensing (SPOS)/Light Obscuration

Light Obscuration, also referred to as Single Particle Optical Sensing (SPOS), is an analytical technique of high resolution capable of detecting a small percentage of outliers. It can also be used to obtain an overall size distribution, when operated with proper technique. An important consideration when choosing Light Obscuration is the concentration of the sample presented to the instrument. The sample must be in a dilute form (either as received or prepared by PTL) for analysis. The technology works by passing a dilute stream of particles in a liquid suspension between a light source and a detector. The light source in this case is a laser diode, which illuminates the individual particles in the stream and results in a shadow or blockage of light on the detector. This light blockage is termed obscuration. The detector measures this reduction in light intensity and using a calibration curve, processes the signal to determine the size of the particle.

Light Obscuration is another widely accepted particle sizing technique in many fields. Fields ranging from pharmaceuticals to hydraulic/lubrication applications can utilize the particle counting and sizing capabilities of Light Obscuration. The technique is recognized by the ISO, USP and ASTM for example as an appropriate method for measuring the size and number of particles in samples such as: injectables, ophthalmic solutions, industrial oil/lubricates and even dilute waste/process water studies. Numerous carrier fluids, both organic and inorganic can be used with the instrumentation. PTL has utilized Light Obscuration to solve numerous pharmaceutical and industrial problems for clients over the years.

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  • Highest sensitivity technique available to determine tails of distribution, protein aggregates, or tails of emulsions (PFAT5)
  • Provides concentration in particles per mL
  • Frequently employed for filtration design
  • Technique of choice for USP Particulate Matter analysis per USP <788>, USP<787>, and USP <789>


  • Dilute samples prepared in either an aqueous or organic solvent suspension
  • Detection range about 0.5 microns to 400 microns

Sample Requirements

If the samples have a lot of particulate, we should only need 60 mL to 100 mL. If they are mostly clear, we need more like 250 mL to 500 mL.

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

Detection Range

Primary detection range is 0.5 microns to 400 microns.

Data Reported

A comprehensive summary including the general graphical particle size distribution on both a number distribution and volume distribution will be provided, as well as the most commonly requested parameters (means 10th, 50th and 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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