Single Particle Optical Sensing (SPOS)

What Is Single Particle Optical Sensing (SPOS)?

Single Particle Optical Sensing (SPOS) is a highly sensitive particle counting and sizing technique that detects and measures individual particles one at a time as they pass through a narrow optical sensing zone. Unlike ensemble techniques such as laser diffraction — which measure the collective scattering behaviour of many particles simultaneously — SPOS counts and sizes each particle individually, making it exceptionally sensitive to the presence of large, rare particles in a predominantly fine-particle distribution.

SPOS is particularly valuable for detecting low-concentration tail populations of oversized particles or agglomerates that can cause failures in filtration systems, electronic devices, parenteral drug products, and precision lubricants.

Principle of Operation

A dilute particle suspension is pumped through a thin flow cell positioned in a focused laser beam. As each particle passes through the beam, it either blocks (extinction/obscuration mode) or scatters (light scattering mode) a fraction of the incident light. The resulting pulse height is proportional to the particle’s cross-sectional area or volume, from which its equivalent spherical diameter is calculated.

Modern SPOS instruments operate in the size range of approximately 0.5 µm to 400 µm, with detection sensitivity capable of counting particles at concentrations as low as a few hundred particles per millilitre.

Key Advantages of SPOS Over Other Particle Sizing Techniques

• Individual particle counting: Detects rare oversized particles that ensemble methods average out
• High resolution in the tail distribution: Provides detailed data on the coarse fraction of a particle size distribution
• Sensitivity to agglomerates: Agglomerates register as individual large particles, making SPOS ideal for stability testing of colloidal dispersions
• Absolute number concentration: Delivers particle counts per unit volume, essential for cleanliness and contamination specifications

Applications of SPOS Across Industries

Filtration and Fluid Cleanliness

SPOS is the method of choice for measuring the cleanliness of hydraulic fluids, lubricants, and coolants according to ISO 4406 particle contamination codes. It quantifies particles that can cause accelerated wear and premature failure of precision hydraulic components, valves, and bearings.

Semiconductor and Electronics Manufacturing

Ultra-pure process chemicals and deionised water used in semiconductor fabrication must be essentially particle-free. SPOS provides the sensitivity required to detect nanolitre-level contamination events that could pattern yield.

Advanced Materials and Nanomaterials

Characterising the oversize tail of nanoparticle and microparticle distributions — emulsions, pigment dispersions, and ceramic slurries — with SPOS prevents process issues during filtration, coating, and sintering.

Aerospace Hydraulic Systems

Aircraft hydraulic system cleanliness is critical for flight safety. SPOS-based particle counting to NAS 1638 or ISO 4406 codes is a routine quality assurance procedure for hydraulic fluid qualification and system flushing verification.

SPOS vs. Laser Diffraction: Choosing the Right Technique

Laser diffraction provides rapid, ensemble-averaged particle size distributions across a wide size range, but cannot detect rare large particles. SPOS complements laser diffraction by focusing specifically on the tail population. In many quality applications, both techniques are used together — laser diffraction for bulk distribution characterisation and SPOS for sensitive tail and contamination detection.

Conclusion

Single Particle Optical Sensing (SPOS) is a highly precise and sensitive technique for particle counting and sizing, offering unique advantages in detecting individual and rare oversized particles within a distribution. By measuring particles one at a time, SPOS provides critical insights into contamination levels, agglomeration, and tail-end particle populations that are often missed by ensemble methods. This makes it an essential tool for quality control, reliability, and performance assurance across industries such as filtration, electronics, aerospace, and advanced materials.

Why Choose Infinita Lab for SPOS Testing?

Infinita Lab provides Single Particle Optical Sensing analysis through our network of accredited laboratories equipped with state-of-the-art particle characterisation instruments. Whether your application involves fluid cleanliness monitoring, colloidal stability testing, or advanced materials characterisation, our team delivers accurate, actionable results.

Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.

Frequently Asked Questions (FAQs)