Solution Turbidity Measurement

Solution Turbidity Measurement quantifies the cloudiness or haziness of a liquid caused by suspended particles, serving as a critical quality attribute (CQA) for injectables, oral liquids, and ophthalmic formulations. This test ensures compliance with pharmacopeial standards (e.g., USP <788>, <851>) and mitigates risks associated with subvisible particles, microbial growth, or instability.

Solution Turbidity

Turbidity is an optical property of a solution that reflects the scattering and absorption of light by undissolved particles (e.g., precipitates, colloids, microbial contaminants) in the nanometer to micrometer size range. It is measured in Nephelometric Turbidity Units (NTU) or Formazin Nephelometric Units (FNU).

Illustration of key factors and influences of solution turbidity (STEMart Original)

Key factors influencing turbidity:

Particle Concentration: Higher particle density increases light scattering.
Particle Size: Particles near the wavelength of light (400–800 nm) scatter most effectively.
Refractive Index: Mismatch between particles and solvent amplifies scattering.
Wavelength of Light: Shorter wavelengths enhance sensitivity to smaller particles.

In pharmaceuticals, turbidity is a proxy for:

Physical Stability: Precipitation or aggregation in liquid formulations.
Sterility Risks: Microbial contamination, bacterial growth.
Process Control: Filtration efficiency or mixing homogeneity.

Measurement Technologies

Nephelometry (ISO 7027)

Principle: Measures light scattered at 90° to the incident beam (λ = 860 nm).
Results: NTU/FNU values proportional to particle concentration.
Advantages:
- High sensitivity for low turbidity (0.01–1000 NTU).
- Minimal interference from color or absorbance.
Instrumentation: Benchtop nephelometers

Turbidimetry

Principle: Quantifies light attenuation (transmitted light) at 600–800 nm.
Results: Expressed as absorbance units (AU) or NTU via calibration curves.
Advantages:
- Cost-effective for routine screening.
- Compatible with UV-Vis spectrophotometers.

Dynamic Light Scattering (DLS)

Principle: Analyzes fluctuations in scattered light intensity to determine particle size distribution (1–1000 nm).
Results: Hydrodynamic diameter (nm) and polydispersity index (PDI).
Advantages:
- Detects subvisible particles beyond nephelometry’s range.
- Identifies aggregation trends in real-time.

Microscopic Particle Counting

Principle: Direct visualization and enumeration using light microscopy or SEM.
Results: Particle count/mL and morphology classification (e.g., fibers, crystals).
Advantages:
- Gold standard for USP <788> compliance (≥10 µm and ≥25 µm particles).
- Provides qualitative insights into particle origin.

Comprehensive Report

Illustration of collaboration with STEMart (STEMart Original)

Turbidity values (NTU/AU) with measurement uncertainty.
Particle size distribution (DLS) or microscopic images.
Compliance statement against USP/Ph. Eur. standards.
Root-cause analysis for out-of-specification (OOS) results.

For more information on our Solution Turbidity Measurement service or to discuss your specific requirements, please contact us today.

Solution Turbidity Measurement