Test procedures for sieving powder materials are described under Particle Size Distribution Estimation by Analytical Sieving 786, and, where practical, the particle size distribution should be estimated by this procedure. The classification of powder fineness in this Pharmacopeia, expressed in descriptive terms, is provided in the table. For practical reasons, sieves are the preferred means of measuring powder fineness for most pharmaceutical purposes. Sieving is most suitable where a majority of the particles are larger than about 75 µm, although it can be used for some powders having smaller particle sizes where the method can be validated. Avoid processing conditions that would alter the true particle size distribution of the powder being tested.

Sieves for Pharmacopeial Testing—
Sieves for Pharmacopeial testing are described under Particle Size Distribution Estimation by Analytical Sieving 786.

Powdered Vegetable and Animal Drugs—
In determining the powder fineness of a vegetable or animal drug, no portion of the drug may be rejected during milling or sifting unless specifically permitted in the individual monograph.

Air Permeation Method for Determining Fineness of Sub-sieve Size Particles—
The average particle size measured is in the range of 0.2 to 50 µm. The test specimen is loaded into a precision bore tube and is compacted between two paper disks and porous plugs by a rack-and-pinion packing plunger. The determination of the particle size of the specimen in the uniformly packed column is based on its resistance to the flow of a closely regulated current of dried air. The liquid level of a flowmeter-manometer corresponds directly to particle size. Special handling instructions and procedures are provided in the individual monographs.

Classification of Powder Fineness—
Powder fineness may be classified by determining the smallest sieve opening through which a specified quantity of material passes. Results are typically reported as the following:

d90 = smallest sieve opening through which 90% or more of the material passes
d50 = smallest sieve opening through which 50% or more of the material passes
d10 = smallest sieve opening through which 10% or more of the material passes.
The upper and lower limit of the sieve opening values may be reported when results of two or more test lots are combined, e.g., “Lot A has a d50 value of 1000 µm with a range of 850–1180 µm.”
An alternative but less informative method of classifying powder fineness is by use of the terms in the following table.
Classification of Powders by Fineness
Classification of Powder d50 Sieve Opening (µm)
Very Coarse > 1000
Coarse 355–1000
Moderately Fine 180–355
Fine 125–180
Very Fine 90–125

Auxiliary Information—
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General Chapter Robert H. Lafaver, B.A.
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Pharmacopeial Forum: Volume No. 28(2) Page 611