LimsPulse
Dosage-form testing

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
WORKING PRINCIPLE

How Liquid particle counter works

Light obscuration (light extinction) measures the drop in light intensity as each particle crosses a beam in a narrow sensor cell. The drop is proportional to the particle's projected area. A calibration curve made with certified spheres converts signal height into equivalent spherical diameter.

FLOW

The path of a sample

  1. 01

    Prepare

    The sample is gently mixed and degassed so there are no bubbles.

  2. 02

    Draw sample

    A syringe sampler draws a precise volume through the sensor at a controlled flow rate.

  3. 03

    Obscure

    Each particle blocks part of the light beam, producing a voltage pulse.

  4. 04

    Size

    Pulse height is converted to size using the sphere calibration curve.

  5. 05

    Count

    Pulses are counted into channels (≥ 10 µm, ≥ 25 µm and others).

  6. 06

    Report

    Counts are averaged and converted to particles per container or per mL.

COMPONENTS

Main parts and what they do

1

Light-obscuration sensor

Narrow flow cell with a light source (often a laser diode) and photodetector.

2

Syringe sampler

Draws accurate volumes through the sensor at a set flow rate.

3

Sample stirrer

Keeps particles suspended without forming bubbles.

4

Counter electronics

Measures pulse heights and sorts them into size channels.

5

Software

Runs the pharmacopoeial test, calculates results and keeps records.

KEY CONCEPTS

Terms every analyst should know

Sub-visible particles

Particles too small to see by eye, roughly 1–100 µm. Visible particles are tested separately by visual inspection.

Equivalent spherical diameter

The diameter of a polystyrene sphere that would give the same signal.

Coincidence limit

The concentration above which two particles may pass together and be counted as one large one. Samples must be below it.

Sizing calibration

Uses certified polystyrene spheres to set the thresholds for each size channel.

Count accuracy

Checked with a reference suspension of known particle concentration (e.g. USP Particle Count RS).

In LimsPulse · Calibration Management

Keep every Liquid particle counter in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.

See how