Liquid Particle Counter (Light Obscuration)
Counts and sizes sub-visible particles in injections by passing the liquid through a narrow light beam. Each particle casts a shadow whose size gives the particle size. Used for the USP <788> particulate matter test.
- Pharmaceutical (parenterals)
- Biotech
- Medical devices
- Industrial fluids
What is Liquid particle counter?
Injections must be practically free from particles. Particles you cannot see (sub-visible, roughly 1–100 µm) can still block small blood vessels, so pharmacopoeias set limits on how many particles of 10 µm and 25 µm and above an injection may contain.
A light obscuration counter draws a measured volume of the injection through a narrow flow cell. A light beam shines across the cell onto a detector. When a particle passes, it blocks some of the light; the size of the drop in signal is related to the particle's size. The instrument counts every pulse and sorts it into size channels.
The counter is calibrated with certified polystyrene spheres of known size, and its counting accuracy is checked with a reference suspension of known concentration. Results are expressed per container (small-volume injections) or per mL (large-volume injections).
Used for
- Particulate matter in injections (USP <788>, Ph. Eur. 2.9.19)
- Ophthalmic solutions
- Cleanliness of rinse water for components such as stoppers and vials
- Biologics development (protein aggregates)
- Hydraulic oils and industrial fluids (non-pharma)
How it works
- 01
Prepare
The sample is gently mixed and degassed so there are no bubbles.
- 02
Draw sample
A syringe sampler draws a precise volume through the sensor at a controlled flow rate.
- 03
Obscure
Each particle blocks part of the light beam, producing a voltage pulse.
- 04
Size
Pulse height is converted to size using the sphere calibration curve.
- 05
Count
Pulses are counted into channels (≥ 10 µm, ≥ 25 µm and others).
- 06
Report
Counts are averaged and converted to particles per container or per mL.
Advantages
- Fast and automated; counts thousands of particles per run
- Objective and reproducible compared with microscopy
- Pharmacopoeial first-choice method for injections
- Software applies the calculations and keeps an audit trail
Limitations
- Translucent particles (e.g. protein aggregates, silicone oil) may be undersized
- Air bubbles are counted as particles
- Viscous or coloured solutions and emulsions may not be suitable; the microscopic method is used instead
- Sizes are relative to polystyrene spheres, not true particle sizes
2 calibration procedures
Methods that use Liquid particle counter
Keep every Liquid particle counter in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.