A vial of bacteriostatic water that looks perfectly clear can still contain thousands of particles too small to see. The pharmacopeial test for those particles is USP General Chapter <788>, Particulate Matter, and its result appears as one line on every proper certificate of analysis. This guide explains what is counted, how, and what the limits are.
Visible vs. sub-visible
Visible particulate matter is checked by inspecting each container against black and white backgrounds under controlled lighting; a vial with visible particles is rejected outright. Sub-visible particles, roughly 2 to 100 µm, are invisible to the eye and are counted instrumentally. <788> concerns the sub-visible range and sets limits at two thresholds: particles 10 µm and larger, and particles 25 µm and larger.
Two methods
Light obscuration is the primary method. A measured volume is drawn through a sensor in which a light beam falls on a photodetector; each particle passing through casts a shadow proportional to its size, and the instrument counts and sizes particles automatically. It is fast and sensitive, but it can be confused by air bubbles, silicone droplets or immiscible liquids, and it requires the sample to be handled in a particle-free environment so that the count reflects the product rather than the laboratory.
Microscopic counting is the referee method when light obscuration is unsuitable or gives a result that needs confirmation. The sample is filtered through a gridded membrane, and particles retained on the membrane are counted and sized under a microscope. It is slower and operator-dependent, but it distinguishes true particles from bubbles and droplets.
The limits
For containers of 100 mL or less, which includes every bacteriostatic water vial, the limits per container are:
| Method | ≥ 10 µm | ≥ 25 µm |
|---|---|---|
| Light obscuration | not more than 6,000 per container | not more than 600 per container |
| Microscopic | not more than 3,000 per container | not more than 300 per container |
A well-made lot of water typically counts far below these numbers, often in the tens to low hundreds at 10 µm and single digits at 25 µm. The limits are ceilings, not targets.
Where particles come from
- The container: glass flakes (delamination) from poorly matched glass, or fibers and fragments from the stopper. Type I borosilicate and qualified stoppers keep this low; see how vials are made and sealed.
- The process: fibers from filters or clothing, metal from equipment, and environmental dust in a poorly controlled filling area.
- The product: precipitation on storage, which for a simple water product usually signals contamination or degradation rather than chemistry.
- After opening: stopper coring by needles is the largest post-manufacture source, which is why puncture count matters; see choosing a vial size.
Reading the COA line
Expect a line such as "Particulate matter (USP <788>, light obscuration): ≥10 µm: 42 per container; ≥25 µm: 3 per container; Complies." A result reported only as "pass" is acceptable but less informative; a missing line, or a result that exactly equals the limit, warrants a question to the supplier. The other lines are explained in how to read a COA; the companion tests are sterility and endotoxins.
What the laboratory can do
Inspect vials on receipt and before each use against a dark background in good light; discard any vial with visible particles, haze or fibers. Limit stopper punctures. Store in the carton to avoid the temperature swings that encourage precipitation. And keep the COA on file, so that if a particle question ever arises the lot's tested count is available. See how to inspect a vial before use.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.