Single Particle Optical Sensing (SPOS) is widely used across a variety of industries due to its excellent resolution, wide dynamic range and high accuracy. The term SPOS and Light Obscuration (LO) are often used interchangeably in the particle characterization field and international standards. The technique is also recognized by the USP and ISO as an appropriate technique for determining particle size and concentration. How does this technique compare to other common particle sizing techniques and is it right for your material?
What is SPOS/LO?
Single Particle Optical Sensing (also referred to as the Photozone or at times Light Obscuration technique) requires a particle suspension to pass through a region of uniform illumination produced by a laser diode (photozone). Each particle is counted and sized individually, allowing for the particle size distribution to be built one particle at a time. In order to operate properly and avoid coincidental effects (multiple particles in the photozone), the particle suspension must be sufficiently dilute such that one particle passes through the photozone at a time.
The SPOS system at PARTICLE TECHNOLOGY LABS utilizes two principles of detection – light extinction (also known as light obscuration or light blockage) as well as light scattering. Particles larger than approximately 1.5 µm are detected by the amount of light they obscure (block) to the extinction detector, while smaller particles are detected by the intensity of light scattered toward the scattering detector. These changes in light obscuration and scattering create a “pulse” (voltage change) which is recorded by the instrument. The particle size is then determined by comparing the pulse height (voltage change) to a calibration curve generated with reference materials of known size.
SPOS is a broader term often used to explain the measurement of single particles by this technique while the light extinction or blockage measurement is more specifically called Light Obscuration.
As a true particle counter, the primary output format is a number distribution which can also be converted to a volume distribution. As flowrate and volume sampled are known, particle concentration in terms of particles/mL can also be generated.
How does it compare to other techniques?
Each particle sizing instrument has its limitations and some may be better suited for a sample than others. Where does SPOS excel when compared to other common techniques such as Laser Diffraction or the Coulter technique? Where does it struggle?
SPOS allows for high resolution and repeatable analyses across a generous size range (0.5 to 400 µm for the sensor utilized by PARTICLE TECHNOLOGY LABS) without the need for multiple analyses or attachments. If submicron detection is not required, then the light obscuration range of approximately 2 µm to 400 µm can be used. Other common particle counting techniques, such as Coulter counter, may require multiple analyses to capture a similar size range. The Coulter technique also requires a conductive carrier, generally limiting analyses to an aqueous system. With the ability to analyze in multiple carries and capability of reporting over 1,000 channels, SPOS proves to be one of the most resolute and versatile particle counting techniques available on the market.
One of the most widely utilized particle sizing techniques is Laser Diffraction (LALLS). This technique is what is known as an ensemble technique and only reporting volume distributions is recommended. As opposed to building a distribution one particle at a time, this technique does so my measuring scatter from many particles all at once. Ensemble techniques are inherently a low-resolution technique whereas particle counters are considered high-resolution and capable of quantifying minor differences/distribution tails between samples. SPOS or Light Obscuration also does not require additional inputs such as refractive index of the sample or carrier fluid or selection of an optical model allowing for the result to not be influenced by such additional optical model factors. Laser diffraction instruments typically require more concentrated samples in order to operate properly – this can be an important factor to keep in mind while selecting a technique as sampling statistics or sample scarcity may impact the distribution.
As with many other particle sizing techniques, SPOS may not be suitable if a material is elongated or fiber like. Particle size is reported as a circular equivalent diameter – the further from a material is from a sphere, the more error is to be expected when converting to circular equivalent diameter. In this sense, image analysis is far superior as it has the ability to provide additional statistics such as length and width to better characterize the product.
When to use SPOS and Light Obscuration
SPOS and Light Obscuration are a very versatile technique and is commonly utilized if a number distribution or particle concentration (particles/mL) are required outputs. Due to the technique’s high resolution, it should be considered when small outliers/tails or differentiation of multiple peaks are of interest. Keeping in mind that the instrument requires a dilute sample preparation, more concentrated preparations will likely need to be heavily diluted. SPOS excels with low concentration samples and cleanliness test situations. Below is a list of just a few common analyses performed on the SPOS/Light Obscuration technique:
- USP<787>, <788>, <789> particulate matter in therapeutic protein injections/injections/ophthalmic solutions|
- USP <729> globule size distribution in lipid injectable emulsions (PFAT5)
- Medical device cleanliness studies
- Particle contamination in oil
- API particle size determination
- Emulsion stability studies
- Filtration studies (pre/post filtration)
- CMP Slurries
- Protein aggregation
- General waste water analyses
- Inks and paints
Learn more about Single Particle Optical Sensing and see sample output reports.
Image from: “Why Buy an AccuSizer® SPOS System?” Entegris.com, 27 Feb. 2020, https://www.entegris.com/content/dam/product-assets/accusizerspossystems/datasheet-why-buy-accusizer-system-10956.pdf.
