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
LEARN & PRACTICE

Test your Liquid particle counter knowledge

Multiple-choice questions

Pick an answer to see whether you are right and why.

  1. Q1. A light obscuration counter sizes particles from:

    basic

  2. Q2. The light obscuration counter is calibrated for size with:

    basic

  3. Q3. For a small-volume injection (≤ 100 mL), the typical USP <788> light obscuration limits per container are:

    intermediate

  4. Q4. Why is the first of the four portions discarded?

    intermediate

  5. Q5. Air bubbles in the sample will:

    intermediate

  6. Q6. 15 vials of 2 mL are pooled (30 mL). Mean count ≥ 10 µm in 5 mL portions is 100. Particles per container are:

    advanced

  7. Q7. If an injection fails the light obscuration test, USP <788> typically allows:

    advanced

Viva questions

Think of your answer first, then open it to compare.

  1. 1. Why is a particle-free water blank run before testing?

    It shows that the environment, glassware and sensor are clean enough. If the blank is high, any product result would include contamination from handling.

  2. 2. When would you use the microscopic method instead of light obscuration?

    For products that are unsuitable for light obscuration, such as emulsions, colloids, highly viscous or coloured solutions, or products that form bubbles, and when a product fails the light obscuration test.

  3. 3. What is the coincidence limit and why does it matter?

    It is the concentration above which two or more particles may pass the sensor together and be counted as one larger particle. Above it, counts are too low and sizes too high, so samples must be within the sensor's stated limit; otherwise a validated dilution or the microscopic method is needed.

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