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The particle size distribution should be estimated by Particle Size Distribution Estimation by Analytical Sieving 786 or by application of other methods where practical. A simple descriptive classification of powder fineness is provided in this chapter. For practical reasons, sieves are commonly used to measure powder fineness. 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. Light diffraction is also a widely used technique for measuring the size of a wide range of particles.USP35
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Sieves for Pharmacopeial Testing—
Sieves for Pharmacopeial testing are described under Particle Size Distribution Estimation by Analytical Sieving 786.USP35
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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.USP35
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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.USP35
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Classification of Powder Fineness—
Where the cumulative distribution has been determined by analytical sieving or by application of other methods, powder fineness may be classified in the following manner:

x90 = particle dimension corresponding to 90% of the cumulative undersize distribution
x50 = median particle dimension (i.e., 50% of the particles are smaller and 50% of the particles are larger)
x10 = particle dimension corresponding to 10% of the cumulative undersize distribution
It is recognized that the symbol d is also widely used to designate these values. Therefore, the symbols d90, d50, and d10 may be used.
The following parameters may be defined based on the cumulative distribution. QR(x) = cumulative distribution of particles with a dimension less than or equal to x where the subscript R reflects the distribution type.
R Distribution Type
0 Number
1 Length
2 Area
3 Volume
Therefore, by definition:

    QR(x) = 0.90 when x = x90
    QR(x) = 0.50 when x = x50
    QR(x) = 0.10 when x = x10
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
Descriptive Term X50 (µm) Cumulative Distribution
by Volume Basis,
Coarse >355 Q3(355) <0.50
Moderately Fine 180–355 Q3(180) <0.50 and Q3(355) 0.50
Fine 125–180 Q3(125) <0.50 and Q3(180) 0.50
Very Fine 125 Q3(125) 0.50
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
General Chapter Kevin T. Moore, Ph.D.
Senior Scientific Liaison
(GCPA2010) General Chapters - Physical Analysis
USP35–NF30 Page 390
Pharmacopeial Forum: Volume No. 28(2) Page 611