Sieve analysis uses a woven, punched or electroformed mesh, often made from stainless steel or brass, with known aperture dimensions which forms a physical barrier to the particles.

Range of Particle Sizes Measured by Sieve Analysis

The International Organization for Standardization sets a lowest sieve diameter of 45 µm and, as powders are usually defined as having a maximum diameter of 1000 µm, this could be considered to be the upper limit. In practice, sieves can be obtained for size analysis over a range from 5 µm to 125 000 µm.

Principles of Measurement for Sieve Analysis

Most sieve analyses use a series, stack or ‘nest’ of sieves, which has the smallest mesh above a collection tray, above which are meshes that become progressively coarser towards the top of the stack of sieves.

A sieve stack usually comprises six to eight sieves with an aperture progression based on a √2 change in area between adjacent sieves. Powder is loaded onto the coarsest sieve at the top of the assembled stack, and the nest is subjected to mechanical agitation. After a suitable time, the sieve diameter of a particle is the length of the side of the minimum square aperture through which it has passed.

The weight of material collected on each stage is determined and used to plot a cumulative-undersize plot. Sieving is rarely complete as some particles can take a long time to orient themselves over the sieve apertures and pass through. Thus sieving times, which are usually 5–30 minutes for dry sieving, should not be arbitrary and should be defined; hence it is recommended when standard-sized sieves (200 mm diameter) are used that sieving be continued until the mass on any sieve does not change by more than 5% or 1 g of the previous mass on that sieve.

Air-Jet Sieving

Another form of sieve analysis, called air-jet sieving, uses individual sieves rather than a complete nest of sieves.

The process starts with the finest-aperture sieve and progressively removes the undersize particle fraction by sequentially increasing the apertures of each sieve, encouraging particles to pass through each aperture under the influence of a partial vacuum applied below the sieve mesh. A reverse air jet circulates beneath the sieve mesh, blowing oversize particles away from the mesh to prevent blockages.

Air-jet sieving is often more efficient and reproducible than conventional mechanically vibrated sieve analysis, although with finer particles, agglomeration can become a problem. In the related method, sonic-sifter sieving, relatively small powder samples are lifted in a vertically oscillating column of air, such that particles are carried against a sieve mesh at a set number of pulses per minute.

Reference:

• Aulton, M. (2018). Aulton’s pharmaceutics, the design and manufacture of medicines. Edinburgh. : Elsevier